How to Become a Radio Amateur 1974

8/13/2019 How to Become a Radio Amateur 1974

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How toBecome R DIO

M TEUR

PublishedbyTHE MERIC N R DIO REL Y LE GUENewington Connecticut 06111 U.S.A.


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COPYRIGHT 1974BY

THE AMERICAN RADIO RELAY LEAGUE INC.

Copyright secured underthePan AmericanConvention

International Copyright secured

This work is Publication No. 8 of The RadioAmateur sLibrary publishedby the League.A ll rights reserved. Nopart ofthis work may be reproduced in anyform exceptby written permission of the publisher. Al l rights of

translation are reserved. Printed in U.S.A.

ued n reservadostodoslosderechos

Library ofCongress Catalog Card Number: 55-8994

Twenty NinthEdition


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T BLEOFCONTENTS

TheLure of Amateur Radio 5Fundamentals of Radio 12ASolid State Receiverfor theBeginner 47A Simple One Tube Transmitter 62TheWorld Above50Megahertz 72The Final Steps 78

LearningtheCode icensesArrangingaStationGettingon the Air

Symbols 84


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bbreviations ommonly UsedforTechnical Termsa fc automaticf r e q u e n c y controla f s k audio f r equenc y - s h i f t keyinga ge automatic gaincontrola le- automatic load controla -m amplitude modulationa nl- automatic noise limiterB F O beat-frequency oscillatorc f m cubicf ee t per minuteco crystal oscillatorc ps cyclesper secondCP cathode ra yC R T cathode-ray tubec t centertapc w continuous w a v e(telegraphy)d c directcurrentD F direction finderd p s t double-pole single-throwd p d t double-pole double-throwdsb double sidebandEC O electron-coupled oscillatorem f electromotive force (voltage)fd f r e q u e n c y doublerfm frequency modulationfs- fie ld strengthfsk f r e q u e n cy - sh i f t keyingG D O grid-dip oscillatorhf high f r e q u e n c yht high tensionhv high voltageID inside diameteri- f intermediate frequencyI f - lowf r e q u e n c yLO local oscillatorIsb lower sidebandm f medium f r e q u e n c yM G motor-generatorM O master oscillatorn b f m narrow-bandf r e q u e n c y modulationN C - normally closed;noconnectionN O normally open

O D outside diameterP A power amplifierP EP peak envelope powerP IV peak inverse voltagep o poweroutputpp push-pullps power supplyrf radio frequencyrm s root-mean-square ( e f f e c t i v e ) valueRTTY radioteletypesb sidebandsf standard frequencyshf superhighfrequencysic straight-line capacitanceslf straight-line frequencyS N R signal-to-noise ratios p s t single-pole single-throwspdtsingle-pole double-throwssb single sidebandsw short-waveSW R standing-waveratioT-R transmit-receiveTV televisionuhf ultrahighfrequencyusb upper sidebandvc voicecoilVF O variable-frequencyoscillatorvg voltage gainvhf very high frequencyvlf very lo w frequencyV O M volt-ohm-milliammetervp velocity o f propagationVR voltage-regulatorV S W R voltage standing-waveratioV T V M vacuum-tubevoltmeterVXO variablecrystaloscillatorVOX voice-operated controlw v working voltagew w wire-wound

M etri cPrefixes Used in R a d i o WorkName

k i loM e g aG i g amillm i c r oj m i c r o m i c r oj p i c o

bbreviationkMGmMM M

ultiplier103 (1 0 0 0 )10 (1,000,000)109 (1,000,000,000)io -3(1/1000)10- 1 / 1 , 0 0 0 , 0 0 0 )10~12 (1/1,000,000 X1/1,000,000)


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Chapter 1

TheLure of mateur Radio

you A R E I N T E R E S T E D in radio and inth e magic of radio communicat ion . T hethrill of direct two-way radio conversationwith persons in foreign countries, of partici-patingin emergency communications intimeof disaster, of exploring the frontiers ofradio development with equipment youbuild yourself all these an d more may beyours through th e medium of amateur radio.

You probably knowthat there are peoplecalled radio amateurs w ho talk amongstthemselves at allhoursof the day andnight.You may have read of them in your dailynewspaper after some flood or emergency inwhich they rendered great public service.

W hoareradio amateurs?What sam ateur radio?Amateur radio is direct private experi-mental communication, from your ownhome, on appara tus you have built orassembled yourself, with other amateurssimilarly equipped.Anyone ca n become an amateur boy

orgirl, man or w o m a n almost regardlessofprevious training and experience. All that isrequired is a sincere desire to learn and alittle effort acquiring the necessary know-ledge. Boys an d girls of 8 and 10 havebecome amateursas have m en of 80 . Theycome from allwalks of life, their sole bondthe fascination that the amateur game af-fords.

You m ay have already tuned in somedistant station in a foreign land on ashortwave receiver. B ut that is only a small partof th e thrill that comes only to the radioamateur not only hearing foreign coun-tries but also throwing the switch on your

own t ransmit ter and talking with th estationsyou hear.Y ou would like to k n o w how thesepeople came to be amateurs , how they

acquired the ability and the equipment toget on the air and talk with each other. Y oumight like to become ana ma teur yoursel fat least you would l ike to k n o w how to goabout becoming one.T he purpose of this booklet is to tellyou , as simply and st ra ightforwardly as

possible, what amateur radiois, how one canbecome an am ateur , how to bui ld a s implereceiver and t ransmit ter , and how to get onth e air. B ut first let us explore some of them a n y possibilities amateur radio offers.Adventure

Eachnight's operation is a newadven tureinto space. An amateur's station some-

This modest amateur stat ion, assembled fromcommercial ly-avai lable units, requires very littlespace yet under favo rable condit ions can easilycommunicate to theother side of the world


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The LureOf

The operator of every amateur station takes pridein a neat logbook showing dates times andfrequency bands for other s tations he has worked.t imes an elaborate affair that rivals th eequipment of a big broadcast ing stat ion,more of ten an inexpensive outf i t assembledat home in spare moments becomes amodern Aladdin 's lamp. Y ou never know,when you si t down to your transmit ter andreceiver for a few hours' operat ion at theend of the day's work, what those hours willbring. Perhaps, to star t , a few friendly chatswith neighboring am ateu rs in nearby states.Some ofthesemay be contacted for the firsttime that particular night; others may beamateurs who have been worked beforeand with whom regular schedules have beenarranged once or twice a week. Followingthis there may be an opportunity to pass thet ime of day with aV irgin Islanderor, later, amissionary afar in Africa or a weatherobserver on some remote U.S. island in the

Pacific. You may suddenlybe asked to relaya messagefo r assistancefor a town even thenbeing devastatedb y ahurr icane , orhaveth eexperience, as m a n y a m a t e u r s h a v e , of ex-changing signals with some ArcticorA n t a r c -t ic expedit ion.EndlessVarietyThese are but a very few of the thingsthat you, as an ama teur , may do. The reasonthat amateur radio is often called the mostsatisfying and thrilling of allhobbies is thatit offers som ething for everyone. I t is, to usea fam iliar phrase, all thingsto all men.

For example : You may be a tinkerer you may like to play aroun d w ith gadgets,bui ld them up, make them work. Amateurradio is the ideal hobby for the t inkerer w holikes to gointo the why of the things he

A housewife or XYL ham takes time out fromi roning to op erate her amateur station.

Would you like to talk direct with the owner ofthis fine station in Japan or other amateurs in allparts of the world? It is one of the most interestingfacets of becoming a ham.

builds. It offers endless room toexpe r imen t ,an infini te variety of prob lemsto overcome.You may be a rag-chewer. Most hams are .The most enjoyment you know may comefrom gett in g together with a crowd of goodfellows and talking over everything un derth e sun. Am ateur radio is full ofcon f i rmedaddicts of the conversat ional art ; indeed,there is even a Rag-Chewer'sClub, with amembersh ip cert i f icate signed by Th e O ldSock, himself , for those who can qua l i fy .


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mateurRadio

Many amateurs find their greatest interest in thehobby to be experimentationwithnewcircuits andgadgets.

as in the smal ler contes t s , amateurs competenot only on anational scalebut loca l ly .Intothe Space ge

A s technology advances , th e scope ofam ateur radio act ivi t ies ex pan ds. Only a fewyears after the very first man-made satell i tewas thrus t in to orbi t , hams had their ow nsatell ite b ea m ing greetings to the wor ld . Awhole series of Oscar (Orbital Satell i teCarry ing A m ateur R adio) packages has s incebeen in space. The first direct satellite linkbetw een the U.S. and the Soviet Union wa saccompli shed not by a la rge com mercia lsatellite, but by Oscar4Group Activities

But all this still does not convey thewhole p icture of amateur rad io .In the U.S. , there is the Mil i ta ry Aff i l ia teRadio Sys tem (M A RS ), whereby civ il ians

CompetitionY ou m ay have th e competitive urge. Ifyour biggest kick in l i fe comes from putt ing

everything you've got into some sport orgame that requires a high order of intelli-gence and skill, amateur radio will provideplenty of act ivi t ies to test your mett le .Every day in the year thousands of am ateurscompe te to see who can relay the m ostmessages; elaborate traffic nets, with trunklines, field officials and a comprehens iveorganizat ion have been established by theCommunicat ions Department of the Ameri -can Radio Relay League.Hundreds of other amateurs competewith each other in working DX (dista nt)stat ions. DXing is actually a glorified form

of fishing; it takes endless patience and skill,but to the t rue f i sherma n i t has a zestnothing else in the world can equal andit 's a sport you can indulge in any day, anyseason of the year.Beyond these daily activities there aredozens of contests of various kinds held

annual ly . The biggest is the Sweepstakes,engaged in by amateurs al l over Canada andthe United States. Field Day brings thou-sands of ama teu rs into th e count rys ide wi thportable se l f-powered equipment . In these ,

nother field of experimental interest by amateursis th e very-high-frequency bands where simplerantenna systems will perform adequate ly butwhere elabo rate beams such as this will providecom mun icat ions over d istances unheard-of until afe w years ago.


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The Lure Of

O ne of the highlights of the amateur s year is participation in ARRL Field Day each June, whenportable emergency units are set up and teams of operatorsworkaroundtheclockas ademonstrationoftheabilityof amateursto serve in t imesof disaster.

can assist the armed forces at the same timeacquiring valuable training in military com-municat ions procedures. M A R S is operatedjointly by the three major branches of thearmed forces.

In the comprehensive field organizationof th e A R R L you may find satisfaction in anappointment as Official Observer helpingyour fellow amateurs avoid trouble withregulatory agencies or as an Official BulletinStation transm itting the latest am ateu r newsbulletins on regular schedules, or as anOfficial V H P S tation helping plumb th emysteries of the ultra-high frequencies.

N or is all of amateur radio confined tocontacts over the air or solitary experimenta -tion. There are more than 1200 activecommunity radio clubs in the country affili-ated with the A R R L and they offer pro-grams of wide general interest. Each yearseveral divisional conventions and somedozens of hamfests are he ld . Hundredsofamateurs attend these f ra ternal get-to-gethers which last from an a f te rnoon orevening to as m uch as three days. N ot onlyare they instructive not onlydo they permitamateurs to meet in person those they havetalked with over the air but they are mightygood f u n aswell.

Public ServiceIt is one of the finest aspects of amateurradio as a hobby that it is not only a source

ofdelightful fun and pleasant recreation butit is also an outs tanding oppor tuni ty fo rvoluntary public service.The commun ica t ingexperience an amateur acquires and theorganized ne tworks in which many hamsparticipate in t ime of disaster become ofuntold value to the co m m un i ty and thenation . Let a hurricane or an earth qu ake or aflood destroy normal lines of communica-tion and hun dreds of am ateu rs are ready tostep in and provide emergency circuits fo rthe Red Cross civil defense mili tary andmunic ipalagencies.Perhaps as many as one amateur out ofevery three has in addition to his station athome a complete comm unications set-up in

his car. Driving back and forth to work oron a longer weekend trip,an amateurcan bein constant touch with other hams to whileaway the t ime in pleasant conversation orto ask local highway directions of local hamsat their home stations. Here again in theevent of comm unications em ergency an ama-teur mobile unitbecom es invaluable becauseit not onlyhas its own power supplybut can


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Amateur Radiobe dispatched to key points to furnish vitalcommunications for relief work.

So important , in fac t , is the amateur ' swork in emergency communications that theFederal Communications Commission hasset up special rules for a Radio AmateurCivil Emergency Service, in which public-spirited am ateu rs can enroll a s a pa rt of civildefense efforts. These local networks engagein practice drills regu larly, hoping that disas-ter will never strike their community butdetermined to be fully prepared for anycatastrophe that might come.From Senatorsto Newsboys

This, then, is amateur radio. That it sappeal is universal is demonstrated by thetype of people that pursue it. A cross-sectionof amateur radio is a cross-section of anycommuni ty . The popular myth that allamateurs are attic experimenters has nobasis in fact . It is true, of course, that aconsiderable num ber of boys and girls un der20 do become amateurs , for it is one of theadvantages of amateur radio that it is notto o intricate fo r young people of high schoolage to master. But if you, as an am ateu r, geton the air tonight or tomorrow night andcontact other amateur stat ions you may findyourself talking with a U. S. Senator or aMiddle Eastern King or a famous radiocomedian or your newsboy or filling-stat ion owner . . . The list could go on

Another f ield of intense interest for am ateurs ismobile a simple bu t complete two way instal la-tion in anautomobile. Besides a great deal of funsuch units ar e invaluable in time of communica-tionsemergency.

Many of the 1200 or more local amateur radioclubs around the nation conduct f ree courses ofcode and theory instruction to help newcomersobtainham t i ckets.

endlessly. The point is that amateur radioisindeed a universal hobby , hav ing an app ealto professionaland factory worker alike, toyoung as well as old.Careersin Electronics

With the intense personal interest ayoung amateur develops in his hobby, i t isnot surprising to find that many radioindustry executives, and electronics engi-neers high in the profession today, were firstdrawn to radio as a career through a hamstation of their own. Nearly one-half of allamateurs are employed in communicat ionselectronics or allied fields. Some are ownersand presidents of their ow n m an ufa ctu ringcompanies; others are engineers or labora-tory technicians, or sales engineers, ormain-tenance men, or broadcast s ta t ion a t ten-dants , or government research workers onclassified projects. The rapid advan ces in theart of electronics and its daily expansioninto new phases of American indust ry makeit apparent that th e field is a cont inual ly-growing one which offers unparalleled op-portunit ies to the y o u n g man of today w hois faced with selecting his life's work. Andexperience as an amateur is the ideal step-ping-stone to a professional career, offeringnot only an easy method of early self-training, but equal ly as i m p o r t a n t , the de-velopment of a personal interest that makesone's working hours pleasant and rewardinginstead ofdrudgery .


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The public-service recordof amateur radio operators hasdrawn many words of praise from governmentand municipal agencies and from the military. But one example is this city communications controlcenter organized andmanneden t i rely by hams.

National OrganizationAmateur radio is not a spontaneousdevelopment. It is the result of five decadesof evolution.For more than 50 years it hasbeen guided in technical and operating pro-gress and defended against legislative threat

by its national organization in the U.S.andCanada the American Radio Relay League.The League which was founded in 1914is the tradit ional spokesman for amateurradio. N um ber ing in i ts ranks a majo ri ty ofthe active licensed amateurs it is operated asa mu tual non-stock corpo ration entirelyamateur-owned and directed. Through arepresentative system of gove rnment itmakes th e amateur body art iculate in repre-sentat ion domestically and at internationalradio conferences. From it s headquar ters at

Newington Connecticut wh ere visitors arealways we lcome wh ere some seventypeople are employed it publishes th emonthly journal of amateur radio QST aswell as many amateur handbooks and book-lets all available at low cost to help ama-teurs obtain th e greatest enjoyment fromtheir hobby.LicensesEssen tial

It is the law that no one can operate aradio transmitter without a license. In theUnited States al l forms of radio ar e adminis-tered by a government agency at W ashingtoncalled th e Federa l Co mm unicat ions Com mis-

sion. The Commission assigns radio frequen-cies and issues licenses to all types of radiostations and often certain servicesfeel t ha tthey require more space on the air. Thiscom petition the necessity for every class ofradio station to d e m o n s t r a t e that it isoperated in the m a x i m u m of public interestconvenience and necessi ty forces am ateu rradio through th e A R R L to mainta in aunited front in order to preserve it s rights.

Governments require that every amateurstation and opera tor be licensed. There areheavy penalties for operation of an unli-censed station a maxim um of two years in

Headquarters activities of the League include awell-equipped l aboratory picturedabove inwhichnew designs of amateur gear are developed andtested.


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Amateur Radio jail and a fine of $10,000. In C a n a d a , th eDepartment of Communica t ions i s the regu-latory agency. As you read on in thisbooklet , you will learn how the necessarylicenses can be easily acquired and the otherrequi rements met .Build or Buy?

Sooner or la ter you will have to decidewhether to construct your ownradioequip-ment or buy i t ready-made. There areadvantages to ei ther choice. Strict ly fromth e standpoint ofcost,buy ing ready-madei scheaper in the long run, principal ly becausethe m anufa ctured equipm ent w i ll have amuch higher trade-in or resale value if andwhen you decide to change your station sapparatus (for higher power, o r some otherreason). In many instances the beginner canget be t t e r pe r fo rmance f rom manufac tu redgear,since it is the result of design by skilledengineers, and of mechanical production bytechniques o fte n imp ractical to dupl ica te inth e home workshop. It is also true that youcan buy a receiver and have i t home inoperating condit ion the same day, whilecons t ruct ing your ow n would require anum ber of evenings or other spare-t imework .

T h e pr inc ipa l advantage to home con-struction is that i t can furn ish half thepleasure and sa t i s fac t ion in the h o b b y ofamateur rad io . In bui ld ing your own, yo ulearn fa r more about radio theory andtechnique than the buyer of ready-made gearever possibly can. You can thus become amuch more ski l l ed amateur . Furthermore ,when a new technica l development comesalong, you can take adva ntage of i t bymodifying y o u r h o m e b u i l t equ ipmen t ac -cordingly ; the purchaser of am a n u f a c t u r eduni t m a yhesi tate to m a k e a n ym od i f i ca ti onssince alterations might adversely affect itsresalevalue.A n u m b e r of manufac tu re rs today makea ma te u r equipment ava i lablein k it fo rm ;th i s i s one compromise between the twochoices. Much of the mechanica l work diffi-

cult to accompl i sh in the home workshopsuch as chass i s -punching and d i a l mechan-isms - is a l ready done for you . But inassembling and wir ing the components youcan acquire a certain amount of knowledge

The Headquarters station W1AW maintains exten-sive operating schedules dedicatedto sendingcodepractice fo r beginners transmission o f news bulle-tins for al l amateurs a n d general contact withother amateur stations.

of the innards of rad io equipment and ofconstruction techniques general ly. You arethu s m ore easi ly able to grasp tech nica lknowledge for your l i cense examinat ion .R e a d y - m a d e ama teur rad io equ ipmen tand ki ts a resold by num erous rad io s to res or

distributors in various parts of the country( a n d , in a few ins tances , d i rec t by them anu facture r) . Check the ye llow pages ofyour loca l te lephone d i rec tory, or the adver-t i s emen ts in this publ ication or the League'sm o n t h l y m a g a z i n e , QST you can usual lyorder by mail if there is none located in yourc i ty .

How mu ch shou ld I spend? is a com-mon ques t ion f rom the beg inn ing ama teur .Tha t is large ly a personal decision. Theamateur equipment marke t is a fairly com-pet i t ive one , h ow ever , so tha t in genera l youge t jus t abo ut wha t you pay for . The m oreexpensive receivers, for exam ple , gene ra l lygive cons ide rab ly be t t e r p e r fo rm ance un de radverse opera t ing condi t ions than cheaperones, al though under good radio condit ionsan inexpensive receiver and low-power t r ans-mi t ter wi l l do near ly as wel l .Asecond-handreceiver (there are m a n y on the m a r k e t ) isof ten a good inves tment .B u t we recomm end tha t , a t t he s t a r t , y o u

bui ld a t least o n e ma jo r i t em fo r y o u ra m a t e u r s ta t ion receiver or t r a n s m i t t e rfor th e experience a nd t ra i n ingit will a f f o r d ,no t to m e n t i o n th e satisfaction o f pe rsona laccomplishment.


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Chapter2

und ment lsof adio

\rou A R E G O I N G to run into some n e wwords in ham rad io. Don t le t thembother you. In any new hobby you have tolearn the language. We l l explain the newwords as they come up, and a f ter a whileyou l l be handling ham radio talk like aveteran .Amateur s learn how radio works becausea modest understanding is required for thewrit ten part of the l icense examination.Further , the more one knows about rad io,th e easier it is to get the most out of it, in

the way a good mechanic gets top perfor-mance f rom his car. And, of course, youhave to know a l it t le about radio to be ableto shoot trouble in equipment that goesb a d .AnAmateurStation

Y our amateur s ta t ion, be it simple o relaborate, will consist of three basic parts:the t ransmit ter , the an t enna , and the re-ceiver. You may use one antenna with thet ransmit ter and another with th e receiver, o ryou may use a single anten na for bothtransmitting and receiving, switching theantenna f rom receiver to t ransmit ter andback again. Reg ardless of these details, thethree basic parts are the t ransmitter, theantenna and the receiver.

The t ransmit ter is the unit that takeselectrical power f rom a battery or, morecom m on l y , f rom the electrical powe r line inthe home and converts it into r ad i o f r equency power . Wh e n this rad io f requencypower is fed properly to the antenna, th epower is radiated in many directions. Al-though the r (radio frequency) power iselectrical power somewhat similar to t h a t inth e house power l ine, th e house power willnot leave th e wires and travel great distancesthrough space the way rf power will. Thedirections in which th e power leaves th eantenna depends upon th e form of theantenna, and this is a study all by itself. It issufficient for now to remember that theradiation f rom an antenna is not equal in alldirections.

The power leaving th e antenna travelsthrough air and space in straight lines awayf rom the antenna at the speed of light(186,000 miles per second) . I t cont inues inthese straight lines unless it is d iverted orre f lec ted. Fortunately for long-distance com-munication on a spherical ear th , there arecondit ions above the surface of the earth (upto about 250miles) thatcause radio energyto be bent back toward the ear th , andconsequently it is possible for the energyleavingan antenna to bere turned to earthatpoints beyond the horizon. (These condi-

RADIOWAVES NORMALLY TRAVELIN STRAIGHTLINES BUTTHEREARE FACTORSTHAT CAUSE THEM TO BEND AROUND THE EARTH S CUR-VATURE A ND M A K E LON G- D IS TA N CE C OM M U N IC A TION S POSSIBLE


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13tions vary with the amateur band in whichyou are opera ting, and the time of day andyear, and they constitute a study of theirown, called prop aga tion. )

RADIOTELEGRAPH ISCALLED A I TRANSMISSION

The rf power radiated from th e antennawill generate tiny electrical currents in anywire or antenna pas t which it travels. If thiswire is connected to a suitab le rad io rece iver ,the tiny currents can be ampl i f ied to alargesignal in the receiver. The signal disap pea rswhen the transmitter is turned off.If thetransmitter power is increased, the receivedsignal increases in the same proportion.W he n th e t ransmit ter is turned on and offrapidly with a telegraph key to form th echaracters of the radiotelegraph code, theidentical characters can be heard at thereceiver from headphones or a loudspeaker ,and th e operator at the key can talk toth e operator at the receiver. Radiotelegraphcommunication l ike this is called A1trans-mission by FC C definition or , in am ate urlanguage, cw (for continuous waves ) orcode

With sui table equipment , th e powerout-put of the transmitter can be varied by thesound waves in the voice of the transmittingoperator , working through a microphone.W he n the t ransmit ter output is var ied, ormodu l a t ed in this way,the o u t p u t in thedistant receiver varies in the same way . If thereceiver output is fed to headphones or aloudspeaker , ne w sound wavesa re generatedthat duplicate the voice waves of the trans-mitting operator. There are numerous modesof transmission in use by a m ate u rs, and eachtype of emission has its own characteristics.The FCC has defined each mode of emissionwith a specific designator. Similar ly, eachform of transmission has an amateur - radioterm assigned to it. The list that followsillustrates what we have discussed here.

FDesignatorA1A2A3A4A5

F1

F2

mateur TermCWCWPhone a -m a nd s in gle - s id e ban d )F A X F a c s i m i l e )ATVS S T V(Amplitude-modulated TV) Slow-Sc an T V )RTTY R a d i o T e l e t y p e )RTTY R a d i o T e l e t y p e )

F3 FM or PMF r equency or Phasem o d u l a t i o n )F4 FAX Facs imi l e )F5 ATVSSTV

o eT e leg r aphyAmp l i tude - modu la t -ed tone te legraphyVo ice commun ica -t ionPicture t ransmiss ionPicture transmissionV i d e o )

Carr ier -sh i f t t e le -graphy Cw sen t byt y p e w r i t e r )Audio-sh i f t te le -graphy Cw s e n t bytypewr i te r )Vo ice commun ica -tionPicture t ransmiss ionby m e a n s of FMPicture t ransmiss ionby m e a n s of FMV ideo )

A n in-depth description of each of themodes listed is too detailed to include in thisbook. T he reader m ay wish to consult T heRadio Amateur s Handbook and The RadioAmateur s License Manual to obtaincom-plete definitions of these terms.

A s mentioned earl ier , there are severalfactors affecting a ny radio path , so it is notpossible to c om m unica te with a l l pa rts ofth e world a t any one t ime. H ow ever , byrecognizing the factors on e can, over aperiod of t ime, communicate with most ofthe regions of the world where there areamateur stations. And,of course, one ca ncom m unicate over short p aths within th ehorizon limit without worrying about th elong-distancefactors .

RADIOTELEPHONE.ISCALLE D A-3U TRAMSMISSIOM


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FundamentalsThe Receiver

The firs t part of your amateurstation toacqui re is the receiv er. I t w ill enab le you tolisten to a m a t e u r s and other services, youwill gain firs t-hand k n o w l e d g e of the re-ceiver's opera t ion, and you will be able tocopy signals that will help you to improveyour code speed , as outlined later. Y ou willnot need a special an tenna until you haveyour l icenseand t ransmi t te r , and even a 15-or 20 - foo t l e n g t h of wire in the room orat t i c will be sufficient with most receivers .The receiver can be one you build fromplans given later in this booklet, or you mayselect a design from The Radio Amateur sHandbook or a copy of QST magaz ine .Y oum ay elec t to assem ble one of the receiversoffered in kit form on the m a r k e t , or youm ay dec ide to inves t in a new or second-hand communications receiver. Don't hesi-ta te to inves t ig a te second-hand receiversoffered by reliab le dea lers ; these receiversshould be in good w ork in g cond ition and areexcellent values. Don't expect any bar-ga ins, how ever; receivers are rea l is t ica l lypriced and you will get what you pay for.The only bargains are receivers that can bereworked in to bet ter performance, and th isrequi res technica l know-how that you won ' tacq u i r e overnight . Usual ly the m arket va lueof a commercial receiver doesnot depreciaterap idly . Be sure to save th e shipping car tonand the instruction book; i t helps when youtu rn in the receiver on an other one.

There are two general classes of receiverstha t are useful for am ateu r wo rk: genera l

coverage and a m a t e u r - b a n d sonly. As then ames in dica te, the general-coverage receiverwill tune over a wide range, while the othercovers only the ama teur bands , w i th a littleto sp are in som e c ases. The general-c overagereceivers usually include the regular broad-cas t band (not fm ) as well , so they can beused by theother members of the family forthe recep t ion of news and e n t e r t a i n m e n t .This is a good a rgument for the potent ia la m a t e u r to use in abudget -consc ious fami ly .Amateur Bands

Rig ht here is a good place to get ac-qua in ted wi th the ama teur bands men t ionedabove. If you r only exp erienc e wi th rad ioistun ing a broadcast or TV receiver, you willhave noticed that your local s tations come ina t cer ta in points on the tun in g d ia l . Forexample, your localstations might appear at6 , 1 1 and 15 on your broadcast receiver dial .These numbers represent 600 , 1100 and1500 k i locyc le s p er second or k i l oher t z(kHz) . The unit Her t z meaning cyc les persecond, has been adopted in terna t ional lyand is now in general use. The prefix kilomeans 1000 , so akilohertz is 1000 Her tz ,or1000 cycles per second. The d i f fe ren t frequencies of transmission that these stationshave permits tuning to one or the otherwithout mutual interference. The broadcastb a n d , or g roup of frequenc ies , extends f rom535 to 160 5 kH z .

If the broadcast receiver were a veryspecial on e tha t could cont inue to tunehigher in frequency (there are technical

S U P E R S P E C I AL W I D E R A N G E R E C EI V ER.5 I.O I.S 2 3 4

5 7IO IS 20 3O 4O so 70MC

4OM E T E R S

4 0 7 0 7.3 I4 O 14.35 2I .O 21.45 28.O

The dial of a super -special wide-range r ece i ver might look something like this.As we tune away from thebroadcast band left-hand end) we run into manydifferent radio servicesand an occasional amateurband so l i d da shes) , and we end up in Channels 2 and 3 of the TV bands. The amateur bands areexpandedbelow the scale, to show their frequency l i m i t s . The cross-hatching indicates subdi v i s i ons allocatedbetween phone and code. The ARRL icense Manual gives the latest FCC regulations on thesesubdivisions.This dial scale represents only a portion of theradio spectrum,andstill morespacewouldberequired toshowthe 2-meter 144-148 MHz) and other amateur bands.


17/102

of Radio 15reasons why this is impractical withoutswitching), you would find man y dif feren tgroups, or bands, of frequencies, used byma ny dif feren t services. Som e of these in -clude ship-to-shore communications, trans-oceanic code, voice, and teletype writer l inks;military radio, ae ron autic al radio , and over-seas propaganda broadcasting. And, whatyou are most interested in right now, theamateur bands. Unlike other services, weamateurs are permit ted to operate on anyfrequency within these am ate ur ban ds, inaccordance with th e terms of the particularlicensewe hold.

To indicate the amateur bands and thesequence in which they would be encoun-tered on this hypothetical receiver, th e dialscale is shown on p. 15. Because the kilo-hertz designation starts to become clumsy atthese high fre qu enc ies, we show the scale in MHz, for Megaher tz . A Megaher t z is1,000,000 cycles per second; 1,000kHz = 1MHz.Inam ateur workyou will of ten hear th eamateur bands refer red to as the 40-meterband (7.0 7.3 M H z ) , the 20-meterband (14.0 - 14.35 MH z), etc. This is acarry-over from past years when asignalw asidenti f ied by its wavelength rather thanby i t s f requ ency.Frequency and Wavelength

Th i s bus in es s o f f r equen c y an dwavelength is o f t en a stumbling block for anewcomerto radio,but it is sobasic and youwill be working with it so o f t en that rightnowis agoodtime to discuss it.The electricity from a battery (or other

direct curren t source) flows in one directionthrough a circuit. In other words, every timeyou turn on your flashlight, or the head-lights of your car, th e electrical current thatf lows through the lamp or lamps flows onlyin one direction. (The direction can bereversed by reversing the two connections atthe bat tery. ) The current in the electriclights in your home (and in all of the otherappliances) is called a l t e rna t ing current Inmost parts of the country it is 60-cycle or60-Hz alternating current or, to abbreviateit, 60-cycle ac. This meansthat the currentgoes first in one direction and then theother, to complete a cycle, and it goesthrough 60 of these cycles each second. It

ha s a frequency of 60 cycles per second(Hz) . Alternating current is used instead ofdirect cu rrent in homes and indu st ry becauseit has certain adva ntages over dc in conveni-ence of use and in long-distance transmissionof power .Aud i o f r e quenc i e s are those in the nor-

mal hearing range of 30 to 15,000Hz and,of course, include the speech frequencies.

DIRECT C U R R EN TALTERNATINGCURRENT

The storage battery in an automobile is acommonsource of direct current.T he currentalways travelsin one direction from the battery to the headlampor other load and back to the battery.Electricity in the home is the most commonexample of alternating current. The direction ofcurrent f low reverses every 1/120thof asecond. Acomplete cycle requires tw o reversalsor alterna-t ions,hence 60-cyclea lternating current.

As mentioned earl ier , radio frequencypower is basically the s am e as your housepower , but the f requ ency is di f ferent . Whenthe frequency of an alternating current ishigher than 15,000 or 20,000 Hz, some ofth e energy will escape into space f rom th econductor carrying the current. Frequenciesthat do this are called radio f r equenc i e s andth e effect is called r ad ia t i on As ment ionedearlier, th e radio frequency (rf) energy leav-in g the con duc tor (wire) travels with thespeed of light.

An alternating current does not flow atma x i mu m value in one direction for half ofthe cycle and at maximum value in the otherdirection for the remaining half of the cycle.Instead , i t r ises gradually to a m ax im um inone direction, falls to zero inthat direction,rises to m a x i m u m in the opposite directionand falls to zero in that direction. This canbe illustrated by the sketch, which showshow the magni tude of the current and its


18/102

16 Fundamentals\N

Currentflowfrom left to right through the lampcan be called, + and fromrighttoleftthroughthe lamp. A 60-cycle a l ternat ing current would flow in one d i rect ion for 1/120thof asecondand in theother for 1/120th of a second, asshown by the wavy l ine being above or below the 0 l ine.Thecurrentrises to a maximum and fa l ls to 0, and then rises to maximum in the opposite direction and fa l ls to 0, asindicatedby the wavy line. For aninstantthere snocurrent flowingin thelamp,and for a considerablepart of the t ime there is r e la t i ve lylittle current f lowing. The l ight does n t show this by f l i cker in g becauseth e changes are too rapid (the lamp can t go on and off that f as t and our eyes have persistence of v i s i o n ) ,and we see a steady light.A s ingle cycle iscompleted when an operat ion starts to repeat i tsel f .Here you seethat there is one cyclefromcrest to c rest , o rfromzero va lue to s imi lar zero value.direction varies with time.As you cansee, itrepea ts this operat ion over and over; eachcomplete operat ion is one cycle (If you arefamiliar with t r igonometry , the shape of thecycle is t ha t of a s i newave . )

Try to visualize arad io- f r equen cy cu r r en tflowing in an a n t e nna . It will vary inamp l i tude and direct ion of flow as shown.

C U R R E N TAND 0

DIRECTION

The al terna t ing current in an antenna is changingvery rapidly, but it can be represented in exact lythe same way that the house current was , by asine-wave curve . Thetime scale (hor i zonta l ) will bemuch di f ferent , s ince it will i nvolve mi l lionths of asecond instead of hundredths.

At time A some radio energy isjust leavingthe antenna, wi th the speed of l ight . Onecycle la ter (B ) the energ y that lef t a t ins t an tA will be a cer ta in dis tance f rom th ean tenna . This dis tance can be eas i ly comput-ed by mul t ip ly ing the speed of travelby thet ime interval. The speed of t ravelis186,000miles per second (300,000,000 mete r s persecond) . I f the f requency is , say , 4 .0 MHz(4,000,000 Hz ) , then it takes 1/4,000,000second to c om p l e t e one cycle. In that t imeth e energy will have traveled300,000,000X1/4,000,000 = 75 me ters. Thu s we say a frequency of 4.0 M H z , or a wavelengthof 75 meters, to iden t i fy the same bit ofradio energy . T he wave length des igna t ioncomes from an early concept of radio andhow i t worked, based on early technicalme thods tha t ac tua l ly m easured the w a velength.Butdirectm easurement of frequencyis more accurate , par t icular ly in the rangesused by a m a t e u r s , and now the wave lengthconcept is useful primarily in discussinga n t e nna lengths and heights. It isn't neces-sary to spell out the ar i thmet ic as was d oneabove, and the s imple form ula is

Wavelength meters) .MHz


19/102


20/102

Fundamentals

l 4V

tial and resistance are /, and R respec-tively,so the formulaisusually wr ittenR

From thisand the simplest algebra,

A 1-1/2-volt battery doesn t have the potential toforce enough current througha3-volt lamp,but a6-volt battery has more than enough Notice thatth e voltages of the flashlight cells add when theyareconnectedproperly.Oneconnectionof a celliscalled + or positive (the small electrodes when notmarked) and the other is called or negative.V oltages add when connected in series (+ to ).and this property is measured in ohms.Resistance is aproperty all substances have,to impede or resist the flow of electricalcurrent. Substances with little or no resis-tance arecalledconductors while those withconsiderable resistance are called high-resistance materials. The highest-resistancematerials are called insulators because theyprevent, or insulate against, the flow ofcurrent.

Getting back to the flashlight, the 3-voltlamphas been designed sothatitsresistancepermits the flow of just enough current toheat its filament (the fine wire in the lamp)to incandescence. A 1-1/2-volt batte rywouldn' t have the potential to push enoughcurrent through the filament to heat itsufficiently, and a 6-volt battery would pushso much current through that the filamentwould burn'out quickly, although it wouldgive a nice bright light for an instantCurrent is measured in amperes and bydefinition, 1 volt will force a current of 1ampere through 1ohm. The relationship, asyou may have sensed when th e flashlightbulb burned out, is that through a givenresistance,the current is proportional to thevoltage. Thus

Current in mperes = otential in voltsResistance in ohmsThe traditional symbols fo r current, poten-

It is a good idea to fam iliarize yourself withthese simple relations, because they areessential to many of the problems you willencounter in your practical radio work, aswell as in the technical portion of anylicense examination. The relationship iscalled Ohm's Law, in honor of the man whofirst formulated i t .

Much of the time in radio work th ecurrents involvedarevery much smaller thanan ampere. Consequently the units milli-ampere an d microampere are often used. Amill iampereis one-thousandth (.001)ampereand is abbreviated mA; a current of .005amperes iswritten5 mA and spokenof a 5milliamps or 5 mils. A microampere is oneone-millionth (.000001) of an am pere and ,of course, 1 mA equals 1000microamperes.While you probably won't run into micro-amperes very often, when you do the micro-amperes may be abbreviated M A.M etersareavailable for measuring current; dependingupon their scales they will be called am-meters, milliammeters and microammeters.Different types of meters must be used formeasuringd c, low-frequencyac an d rf.

The unit of resistance is the ohm, asmentioned earlier,and normallyany fractionof an ohm that you encounter will be givenas a fraction or decimal. In the high resis-tances, running into th e millions of ohms,the term megohm is used. One megohmisequal to a resistance of one million ohms.On the wiring diagrams of radio equipment,and sometimes in a list of parts, you mayru n into a cryptic designation like "10K."This represents 10kilohms, or 10,000 ohms,an d all you have to rem em ber is that the"K" stands for thousand ohms."Thewordkilohm is practically never used, but youmay hear amateurs speak of "a 10-kayresistor." And in speech or on wiring dia-grams,a 2-megohmresistor usually becomesa "2-meg resistor." Resistance is measuredwith an ohmmeter.


21/102

of Radio But a volt is a vol t . Whi le you mayoccasionally h ea r ab ou t ki lovol ts thou sand sof vol ts ) , mi l l ivolt s th ou san dth s of a vol t )and m icrovol t s m i l l ionths of a volt) , mostof your pract ical work will revolve aroundvolts . Your flashlight bat tery is 3vo l ts , you rcar bat te ry i s 12 vol t s , and you r house

wiring is 11 5 volts to a light f ixture and 230volts to the e lec t r ic range . Your amateurt r ansmi t t e r will probably use a m a x i m u mvoltage of 500 or 600, unless you use highpow er and vol tages of seve ra l t housand .Voltages are measured by v o l t m e t e r s , andthey will be of d i f f e r en t t ypes dep end ingupon t he f r eq uen cy . L ike the a m m ete r s , a dcm e t e r is requi red for dc m e a s u r e m e n t s , andan ac m e t e r isrequi red for acm e a s u r e m e n t s.These l a tt e r o f t en have an upp er f r eque ncyl imi t beyond which they a re no longeraccura t e , so w h e n you acquire cur rent - orvol tage-measur ing e q u i p m e n t of any k i n d , itis wise to read th e i n s t ruc t ions and a c q u a i n tyourself with th e prope r use and thel imitat ions of the i n s t r u m e n t .

There is one more bas ic t e rm and u n i t inthis family that y ou shou ld know . T h e t e rmhas been u sed o fte n in this discussion so fa r ,but with no m e n t i o n of theunit.Th e termi spower , and it is me a s u re d in watts. In anelectrical circuit, th e p o w e r in w a t t s isdef ined as the p r o d u c t of vol t s t imes am -peres in the circui t . Hence if 1.0 a m p e r e ofcurrent passes through the e lec t r ic l ight bulb

O H M U E T E R

A M M E T E R

^VOLTMETER

A voltmeter and anammeter can be connected inthe lampcircuit to read current and vo l tage. Notethat direct-current dc) meters have a polarity +and ) and that it wasobservedwhen these meterswe re connected in the circuit. In other words, the+ side of the meter goesto the + sideof the voltagesource .

M P ~}

An ohmmeter is used to measure res istance, butthe voltage and current readings of the precedingpicture would allow the resistanceto be computedfromR =E/I. An ohmmeter workson the s a meprinciple; it measures the current flowing for agiven applied voltage, but instead of indicating thecurrent it indicatesthe res istance. )

in your r ead ing l amp, connec t ed to the115-volt l ine, th e power be ing used by thel a m p is 1.0 X 1 1 5 , or 115 wat t s . Obviouslyth e 60-wat t lamp on the other side of theroom, which is a lso connec ted to the 115-volt l ine , has a smal ler value of cur ren tpassing through it . -Since P power in watts)equals E voltage) t imes / current in am-peres) or P =El your a lgebra te l ls you t h a tE =P/ I and / =P/E. From th e las t formula ,you ca n c o m p u t e that th e 60 - w a t t l amph a sa cur rent of 6 0 / 1 1 5 or 0 .52 am peres 520mil l iamperes , i f you wa nt som e pra c t ice inconver t ing) .

Later on , w h e n you ge tyour t r ansmi t t e r ,y ou will want to be able to c o m p u t e th ep o w e r i n p u t to cer ta in par t s of i t . To do th i syou jus t recal l tha t th e p o w e r P =El readth e me t e r s and do the s imple mul t ipl ica t ion.In the case of your t r ansmi t t e r , th e cur r en twi ll probably be indicated by a mi l liam-meter , so don t forge t - to conver t to a m p e r e sbefore you do the mul t i p l i c a t ion . For ex -a m p l e , if the t r a n s m i t t e r i n p u t i s 150 mA 150 mi l l iamperes) a t 500 vol t s , y ou changeth e 150 m A to 0 .15 ampere a n d m u l t i p l yb y500, to give th e correct a n s w e r of 75 w a t t s .

Y ou m a y b e w onder ing about th e m e t e rt h a t t h e m a n f rom th e p o w e r c o m p a n y r e a d seach month. This is a w a t t - h o u r m e t e r a n d ,as th e name impl i e s , it records th e p r o d u c tof th e n u m b e r of w a t t s your house usest imes th e n u m b e r ofhour s it u s e d th e m . It isa clock and c o u n t e r ; th e m o r e p o w e r youdraw th e fas ter th e clock goes and the fas terth e coun t e r changes . Watch it s o m e t i m e


22/102

20when there are few electrical appl iancesrunning in the house and then a few moreswitched on ; you'l l see. You pay the com-pany for the wa t t -hoursyou use; 100 w a t t sfo r 2 hours or 200 wa t t s for 1ho ur will addthe same am oun t to your bil l.

There is one more point in connectionwith electrical power that you should befamiliar wi th , and then w e will get back tothat h am station yo u're going to have. Sincepower P =El and you know from anearlierparagraph tha t

Fundamentalsthat wil l be f o u n d in most radio circuitswhich, because they have l i t t le or no resis-tance , dissipate l it tle or no pow er.Inductanceand Capacitance

W h e n you start digging into a piece ofradio gear you will continually run into tw otypes of components tha t have th e proper-ties of inductance and capac i tance . Don ' t le tthese words fr ighten you; you will be athome wi th them present ly, and much of thesuccessful operation of rad io equ ipmen tdepends upon them.

th e rela t ions for power can also bew r i tt enK

(The small figure 2 above the / and Emea n stha t th e quan t i ty is squared , or mul t ipl iedby itself. For example , 42 is read 4squared and mea n s 4 X 4, or 16.) If youknow algebra you can have a little funderiving these relations from those givenearlier; if you don t know algebra just tryand remember these formulasf or f u tu re use,because they will come in handy. Withthemyou can c ompu t e th e power being used in acircuit if you kn ow th e current and theresistanceor if you know th e voltageand theresistance. And one las t th ing: rem em bertha t power ca n only be used up or dissi-pated in a resistance. And now we are goingto ta lk about some imp ortant com ponents

M I L L I A M M E T E R

To measure the power input to any part of at ransmitter , it isnecessary to knowthe voltageandthecurrent;These are measured by avoltmeteranda milliammeter or ammeter),and the power input equal to the vol ts t imes amperes. If the metersshown here read 500 vol ts and 80 mA .080am per e ) , the power input would be 500 X .08 =40watts.

L O Wrequen y

I nduct or

HIGHFrequency

The current f lowing through a given coil, orinductor,dec reases as the frequency is increased. Adirect current passesto a degree limited only bythe resistanceof the winding. The core is the formonwhichthe inductor iswound.

The componen t s that have the propertyof inductance are called inductors but ifyou are l ike most other amateurs you wil lcall them coils. A nd coils describes thembecause coils they are, except in somespecial cases. Inductors are usually coils ofwire wound in a single layer on an insulatingcylindrical form or self-supporting with amin imum of insulating ma terial , al though toconserve space they may be madewith manylayers of turns. The wire must be so insu-lated or supported that adjacent turnsdon tmake electrical contact .

When you apply an al ternating voltage toan inductor, you wil l find that the currentthrough the coil doesn t follow a simple law


23/102

of Radio l ike the earlier one involving resistan ce.Ins tead , the current becomes less as youincrease the f requency, even though thevoltage r e m a i n s the same. If , for e x am p l e ,you measure 1 ampere f lowing through thecoil when you apply 100 volts at 4.0M H z ,you will find that only 1/2 am p e r e isindicated at 8.0 M H z .If you applied ad irectvoltage, as from a s to rage ba t t e ry , th ecurrent would depend only upon the resis-tance of the wind ings , but any given in -duc tor will tend to hold back th e flow ofalternating current to a degree that dependsupon the f reque ncy. This prope r ty is calledreactance. Inductors used at lowf r equencieswill usually have laminated iron cores inthem to increase th e inductance for agivennumber of tu rns , and at high f requenciespowdered- i ron cores are somet imes used forth e same reason. For a given d iameter andcore mate rial , increasingthe n u m b e ro f turnsincreases the inductance.

The basic unit of inductance is the h enryand you will run into it in power suppliesand audio- frequency equipment . At rf thesmaller units of the m i l l i h en ry thousand thof a henry ) an d m i c r oh en ry millionth of ahenry) are usual. These two are abbreviatedmH and /nH, respectively and, of course,1000AiH = 1 mH.

You will o f ten hear or read about a 10-henry choke or a 2-1/2-millihenrychoke, and you wil l probably wonder whatinductancehas to do with strangulat ion.Theword choke is a carry-over f rom days goneby, when the act ion of an inductance in

oltm t r

p c i t o r

The cur rent flow ing through a given capacitorincreases as the frequency is increased. A directcurrent cannotflow through a capacitor.

Current flows from a dc source into a capacitoruntil the capacitor is charged has a voltageequal to the source). The capacitor retains itscharge until it is discharged by an external o rinternal resistance. Good capacitors will hold acharge for minutes; leaky ones havelow internalresistanceand discharge in a few seconds .holding back the f low of high-frequencycurrent w as thought of as a choking process.Y ou will also read about filter chokes and rfchokes; the first expression w e will take uplater. An r f c hoke is an inductor used in acircuit to perm it the passage of directcurrents and audio frequencies while pre-vent ing th e f lowof radio-frequency currents.Similarly, an aud io - f requency c hoke is aninductor used to pass dc and hold backaudio f requencies .

A capacitor is basically two parallelplates of con du cting material sheet metal,me tal foil) separated by an insulator air,mica, ceramic, plastic). To conserve space,the plates are often stacked alternately, andthe insulat ing material is made as thin aspossible for the voltage at which the capaci-to r will be used. High voltages can breakthrough insulators that are qui te adequatefo r lower voltages.) Or, the plates may betwo pieces of metal foil rolled with insulat-ing material to forma tubular capacitor.Theinsulator prevents th e f low of any directcurrent through the capacitor , but ac canflow through the capacitor, to a degreedepe nde nt upo n the f requency of the ac andthe capacitance.

It isn't strictly true to saythatno dc canflow through a capacitor. If you connect acapacitor to a source of dc a bat te ry, forexample ) , current will f low from the dcsource for an ins tant. This ene rgy is flow inginto the capacitor to charge it; if the


24/102

capacitor is removed from across the dcsource, a voltmeter co nnected across thecapacitor will show that a voltage appearedthere equal to th at of dc supply. W e say appeared because the voltmeter mightdraw enough current to discharge thecapacitor, and the voltm eter w ould onlyindicate the voltage for a short time.The ease with which acapacitor passesagiven f requency of ac and the a m o u n t ofcharge that it will hold at a given voltagedepends on its proper ty of capacitance. Thisis measured in farads, an electrical quanti tythat is too big for any practicaluse and onlycomes abou t to make th e ari thmetic workout in certain formulas. The capacitors youwill encounter will be measured in micro-farads (millionths of a fa rad) and picofarads(million millionths of a farad). These areabbreviated fi F and pF respectively. Whenyou hear an am ateu r talking abou t 10mikes, you will know he means a 10-juFcapacitor and not an Irish invasion. Capaci-tance increases with the plate area anddecreases with the spacing, for any giveninsulating material between the plates.

Y ou will encounter th e expression bypass capacitor or, simp ly bypass. Abypass capacitor is used to permit thepassage of ac or rf while preventingany flowof direct cu rrent. Wheny ou become familiarwith radio circuits you will see why chokesand bypasses are useful devices.

Inductors and capacitors are not usedsolely for choke and bypass applications.Probably their greatest use is in combinationto form a tuned circuit . A tuned c i r cu i t hasth e ability to accept signals of one f re-quency, or a narrow band of f requencies ,and not others. When you tune a receiveryou are changing inductor-and -capacitor cir-cuits within the receiver, to select thesignalor signalsyou w a n tto listen to .

undamentals ircuits

And speakingof circuits, it isabo u t timeyou were int roduce d to some of thesymbolsthat are used in radio workto showhow thevarious component s are connected together .Y ou have probably seen themin radiobooksor magazines and wondered how a n y o n ecould follow all that stuff. Actually, it iseasy. Symbols are the words of circuitlanguage. The sentences describe the wayin which th e component s are connectedtogether electrically. These are formed bydrawing lines, representing wires or otherkinds of conductors , between the appropri-ate connection points on the symbols. Thisis a much more compac t form of repre-sentation than a picture diagram or pic-torial, as the accom panying i l lus t ra tionshows. Here we have the samecircuitdrawnin both styles. The schematic at the leftcan be understood at a glance by anyonehaving a little familiarity with th e signlanguage of circuit diagrams. The pictorialat the right would require some study beforeone could be sure jus t w hat th e collection ofparts is supposed to do, even when th ereader is fairly experienced.Its only virtueisthat someone with no electrical backgroundwhatsoever could assemble and wire it .Unfortunately, blind copying of this kindadds nothing to one's fund of knowledge .

The schematic circuit diagram sacrificesany a t t empt at pictorial representation. Itdoes no t show where parts are physicallylocated in the equipment nor does it try toshow which leads must be short and whichmay be long This information must beobtained from supplem entary mater ia l , suchas photographs and the writ ten text of anarticle. Together, these will give a reason-ably experienced reader all heneedsto knowto produce a workable piece of equ ipm ent .


25/102

of adio 23Wiring

A line between two symbols, or parts ofsymbols, representsanelectr ical connection.If it is a simple line, no particular type ofwire or other conductor is implied. In theactual piece of equipment , one terminal ofone component might be soldered directlyto a terminal on the other component . Orthe two might be separated by severalinches, or feet,andconnected by wire, metalstrip, or tubing. In most actual construction,of course, th ew iring willbe done either withth e wires or leads furnished on the com-ponent or with hook-up wire which m ayrange in size from No. 12 to No. 22 gauge,dependingon thecurrentto be carried.

There are times when it is impossible toavoid having one connection line cross overanother in the drawing. When it mus t bedone it is simply done as shown at A. A B O D E

Although in nearly all other cases a linetouching a symbol means that there iselectrical contact, here o co ntac t is indi-cated. If aconnection is to be made betweentwo wires in a diagram it is usually sho wn bya dot, as at B. (Ho we ver , the dot is notactually require in such a case;the connec-tion can be shown as at C.) A four-wayconnection preferably should not be drawnas in D, because of the similarity to a plaincross-over; confusion is avoided by showingsuch aconnectionas in E.Nearly every circuit has an array of common connections; examples are theconnections to one side (usually the negativeside) of the plate-supply source used for thevarious vacuum-tube circuits in a mult i tubearrangement. These common connectionsusually are m ade to the m etal chassis, as amat ter of convenience and sometimes as amatter of specific design.In drawing circuitsit is customary to show such chassis

ground connect ions by the chassis symbolshown at A below. When you see acollec-tion of such symbols in a diagram yo uappreciate immediately that all of them areactually one multiple electrical connection.Using the chassis sym bol in this way invari-

ably makes the diagram easier to read,because without th e symbol it would benecessary to show line connections betweenall those samepoints.A B

Usually, the chassis symbol also indicatesa part of the circuit that could be connecteddirectly to earth without any effect on thecircuit's operation. If an actual earth connec-tion is called for, th e ground symbolshown at B will be used. In some olderdiagrams you will find this symbol, toindicate a conn ection to chassis with outreferenceto an actual earth connection.Special cases in wiring occasionally comeup. Sometimes a shielded wire or cable iscalled for. Such wire consists of one or moreinsulated conductors inside a metal tubeusually made by braiding fine bare wire sothe whole assembly will be flexible. Whengrounded, this tube shields the conductor sfrom electrical fields which otherwise mightinduce unwanted currents in them. Thepresence of the shield is indicated by abroken, somewhat-el l ipt ical symbol aroundthe wire, as in A and B in the followingillustration. Usually the shield will beground ed or con nec ted to the chassis, inwhich case the symbol fo r this isaddeda s inC. Coaxial cable, which isbasically shieldedwire but is used wh ere rf curre nt is to becarried, has a special symbol of its own,shown at D. Here, too, th e shield symbolusually will be shown grounded.

A B C 0aFinally, although it is not a part of theactual wiring, you should recognize thesymbol for a shield or shielding. It is simplya dashed line, often formed in the shape of arectangle, around the symbol for a compo-

nent, or set of components, which actuallyareenclosed in a shielding container.With the hooking up out of the w ay ,we can now get down to the componentsymbols themselves.


26/102

24

Examples of shielded wire and coaxial cable. Thewire left ) is the type with a single inner conduc-tor. Multiwire cable of similar construction isoften used. The coa xial cable shown r ight) is asmall type RG-58/U. Cable of this general con-struction is available in several d i f ferent d iameters ,for h andl ing various power levels.

ResistorsYou rarely meet a circuit that doesn't

have at least oneresistor in it. W hile resistorscome in a wide variety of sizes and shapes,the same basic symbol, shown at A, is usedforall oft he m . In its plain form, this symbolrepresents a fixed resistor one havingjust asingle value . If the resistor is tapped,having a connection made somewhere in itsbody that permits another value of resis-tance to be secured from the same resistor,the presence of the tap is indica ted as sho wnin B . More than one tap, when needed,m aybeaddedto the basic symbol. A )

Fundamentalstw o terminals, al though th e actual compo-nent frequently wil l have three; one endconnectionisleftunusedinthatcase.

The solid arrowhead in the symbol at Cindicates that the resistanceis adjustable invalue. Other than this, it does not give anyindication of the physica l const ruct ion ofth e resistor. The ad jus tment might be bymeans of a slider on a wire-wound resistor,fo r example . Or it might mean th e movingcontact on a wire-wound or composit ion control. Note that withthis symbol thereare three terminals , th e adjustable tap andthe two outside ends of the resistor, so thissymbol can be used for an adjustab lev ol tagedivider or potent iometer . On the o the rhand , the symbol at D, with the arrowdrawn through the basic resistor symbol,simply indicates that the total resistance iscontinuously v ariable. This sym bol has only

Commonly used types of resistors. Th e resistorsgrouped at the left have fixed va lues. In general ,th e larger th e resistor th e higher itspower rat ing.Th e four types shown in this group include a10-watt wire-wound, and 2-watt, 1-watt, and1/2-watt composi t ion resistors. A tapped wire-wound resistor is in the center, with a slider-typead justable) wire-wound at its r ight . On the farright is a composit ion control, or variableresistor.Capacitors

The basic capaci tor sym boli sshown at Ain the nex t i l lustrat ion, an d jus t as in thecase of the resistor, this symbol as it standsimplies tha t th e capacitor is fixed tha tis, it has just a single value of capacitance.Again, the symbol stands for al l sorts offixed capacitors, from t iny ceramic disks tobulk y potted high-voltage types, with di-electrics ranging f rom vacuum to oil-filledpaper.

Electrolytic capacitors are polarized that is, in dc circuits one terminal mustbeconnected to the positive side of thevoltagesource and the other to the negative side.The proper polari ty is f requent ly shown onth e circuit diagram by put t ing a + sign nearthe side of the capacitor that should beconnected to posit ive. Frequently, also, th eother sideislabeled

A special type of f ixed-capaci tor symbolis shown at C. This is the feedthroughcapacitor, used particularly in high- frequ en-cy radio circuits fo r bypassing. In this typethe circuit being bypassed goes into oneterminal , indicated by one of the smallcircles, and out the second terminal . The rfbypassing takes place internally to thecurved capacitor plate, which is a lwaysgrounded to the chassis. This typeofcapaci-tor is especially useful where th e circuit goesthrough the chassis, orother metallic sheet,f rom oneside to the other.


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of Radio A )

25 B ) C )

T ^ UVT XTw o common types of cont inuously-

ad justab le or variable capaci tors used fortuning rf circuits are shown symbolical ly atA and B. The first sy m b o l may s t and for anyof several physical ly-different types . One ofthese is the air type , which has a set ofmeta lplateson arota table shaf t , interleavingwith a similar set of sta t ionary or fixedplates. A second is the compression t ype ,in which th e spacing be tween tw o sets ofleaf-spring pla tes is changed b y screw adjus t -ment. The two sets of plates are insulatedf rom each other by thin mica wafers. A thirdis th e piston type , in which a meta lcylinder ismoved in or out of aconduc t ingtube b y sc rew ad jus tment .T he cylinderandtube are insula ted from each other . Sma l lcapacitors are often used as trimmers that is, set to a desired ca pa citan ce valueexper imenta l ly in the equ ipment and there-after left alone . A capacitor used fo r thispurposemay be (but doesnot h veto be) soind ica ted by put t ing a smal l T a longside thestraight l ine repres entingthe fixed p l a t e .

When the arrow is added to the basiccapaci tor symbol to show that the capacitoris variable the curved l ine represents themovable plates.

Fixed-va lue capaci tors come in var ious shapes,sizes, and types o f const ruct ion . At the le f t are apaper tubular capacitor , two s izes of molded micacapacitors, and three type s o f ceram ic capaci tors tw o sizes of d isks , and atubular with a x i a l l eads) .Th e large met a l can is an electrolyt ic f i l ter capaci -tor in this case, several ca pacitors in one can,which is a common connect ion for all u n i t s ) .Beside it is a smal l e l ec t r o ly t icwith wire leads. T h esmall capaci tor at the far r ight is a feed throughtype useful at v h f . )

A )

The split-stator capaci tor symbol isshown at B. This is an air -type capaci torhaving tw o sets of rotor plates and two setsof stator plates. The fo rmer a remounted onthe sa me sha ft ; in effect, there are twoidentical variable capacitors operat ing to -gether. Capacitors of this type a re used in balanced or push-pull circuits.

Whatever th e capaci tor symbol , it iscus tomary to use the curved line to indicateth e side of the capacitor that goes to thegrounded or lower-potent ia l s ide of thecircuit, except that in the case of theelectrolytic capacitor th e curved l ine usuallyindicates the side connected to negative d cvoltage, which is not a lways a t groundpotential .

A f e w samples of var iable capacitors; there a reinnumerab le styles of these, and only a few areshown here.From le f t to r ight , a m ica compressiontrimmer, a tubular trimmer, a s ing le-sect ion var i -able, and a dual-sect ion or split-stator var iab le .The lat ter can be used ei ther as a balancedcapacitor or as two separate sect ions dr iven by asingle control shaft .Inductors and Transformers

T he basic inductor symbol , l ike th e onesfo r resistors an d ca pa citors, gives no infor -mat ion abou t th e type of inductor . It isshown at A be low. I t represents an induc-tance of fixed value , and it can s t and for asimple rf coil w o u n d on a f o rm , for amulti layer coil , a universal -wound coi l , oreven for one w o u n d on an iron core. Likethe resistor symbol, it can be shown t apped(B), or ad jus t ab le (C and D). C is used fo r A ) B ) C ) E )


28/102


29/102

of Radio 27Resonance

Earlier i t was me nt ioned tha t perh aps th egreatest use of inductors and capacitors is incombinat ion to form tuned circuits. To f o r ma tuned circuit , th e inductor and capacitorca n be connected e i ther in series or inparal lel ; th e m o r e c om m on in radio work isth e pa ra l le l connect ion. Connected in pa ra l -lel the combinat ion behaves l ike ( lookslike ) a high res is tance a t one f requency,called it s r e sonant f r equency or f r equency ofresonance, except that it does no t dissipatepower. (You will recall that power can onlybe dissipated in a resistance.) To c larify thisa little, le t us suppose we have a source ofvariablef r e qu e nc y r f energy, and we connec tits o u t p u t to a parallel-conn ected tunedcircuit through a resistor, so:

jns Generator

The box represents the source of rf energy,with a dial for set t ing the freq ue nc y, and theou tpu t runs to the parallel-conne cted tun edcircuit (C and L) t h rough th e resistor R . Anrf vol tmeter , V, i s connected across thetuned c i rcu i t , to make some vol tage mea-surements . W e will assum e that the source ofrf , o r signal gene ra tor , has a cons tan t o u tpu tat all f requencies, th e f requency be ing de te r-mined by the setting of the dial. If we t u rnth e dial to change th e f r equency f rom th esignalg enera tor , we wi l l find that the voltagemeasured across the tune d circuit (L and C)would fol low a cu rve like this:

Vo l ta j e crossLC

w ResonantFREQUENCY

Hi h

It would be low at low f requencies , thenbuild up to a m a x i m u m a t the r e sonantf requency, and t hen become low again a t

higher frequencies. The resonant f requ encydepends upon th e produ t of L and C I.e.;L mult iplied by C), so a circuit with a10-mH inductance and 50-pF capacitorwould reso nate at the same frequen cy as a10-pF capacitor and a 50-mH i nduc t ance (or20 mH and 25 pF, or 25 mH and 20 pF,etc.).A series-connected circuit behaves insomewhat the same way, except that itbehaves like a very lo w resistance at there sonant f r equency:

CurrMl routL RondC

1 0 RtsountFREQUENCY

Hi|h

The basic points to r emember are (1)aparallel- tuned circuit acts like a high resis-tance at its resonant f requen cy (wi thoutconsuming power) , and a series-tuned circuitacts like a low-resistance connection at itsresonant f requency. (2) The resonant fre-quency depends upon the LC p roduc t (Lt imes C ).B y the way , did you notice that you'rereading the schematic symbols? It isn't too

diff icult , is it?Vacuum Tubes

U p to now we have talked about electriccurrents wi thout any detailed description o fwhat they really are. This is sufficient whenyou re dealing with curren ts in wires, bu tnot when you get into vacuum tubes . Tounde rs t and vacuum tubes you have t o knowsomething about these currents and to knowabou t currents you have to be on speakingterms with th e e lectron

N ow e lect rons can get to be p re t t ycomplica ted, as any physicist will testify,but for our purpose it is suff ic ient to know


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D i r e c t l y Hea l e dC a t h o d e I n d i r e c t l y H e i t e dC a t h o d e

F i l am e n t Ca thode H e a t e r

S y m b o l f o rC a t h o d e a n dH e a t e r

that they are elementary particles having thesmallest known electr ical charge. Togetherwith protons and neutrons electrons arepart of the atom and it s reasonable toassume you know that word and the f actthat all matter is made up of atoms. Theatoms of the various basic elements aredi f ferent because they contain dif ferentnumbers of electrons and the other ingredi-ents .A s mention ed above, the e lectrons havean electric charge (called negative to tell itapart f rom the opposite kind o f charge,called posit ive that p ro tons have) . A basicrule in electr icity is that opposite chargesattract and lik charges repel Each atomcontains a n u m b e r o f electrons, togetherwith a nuc leus (p ro tons and n eu t rons ) ; the

electrons are believed to ro ta te about thenucleus in shells at varying distances f romth e nucleus . The positively charged nucleusattracts the negatively charged electrons andholds them in their orbits around the nu-cleus. Ho weve r, in some s ubs tance s, such asmetals , an electron in the outer-most shell isnot t igh t ly bound to the a tom and canreadily be dislodged. In others, even the onesfar thest f rom the nucleus cannot be made toleave th e a t o m .If an electron is d is lodged f rom an atomin a conductor, this atom in turn attracts anew electron f rom a neighbor, and theneighbor f rom its neighbor down the line,

Fundamen t a l san d so a regular chain of m o t io n is set up.This motion of the e lectrons along th e line iscalled electric cu rrent.

W h e n a s trong source of d i f fe rence inpoten t ia l , like a b a t t e r y , is connected to alength of wire a large current f lows th roughthe wire, and the electrons work overtimet ry ing to equalize the large d i f fe rence incharges that exists at the two ends of thewire. But i t mus t be r e m e m b e r e d t h a t thecu rrent through a wire is only a bala ncing ofcharges maintaine d by the m ovem ent ofelectrons f rom a tom to ato m , a re lat ivelyshort hop. In other words , any single elec-tron doesn t f low the length of the wire orany th ing like i t.

The sub stance s in which it is easy todislodge an electron are the conductorsbecause they conduct electr icity easily. Theyinclud e m ost of the m etals , with silver beingthe best , fo l lowed by copper and aluminumamong the common metals . Substances thatconduc t little or no cur rent are calledinsulators; d ry air , ceramics, some plastics,mica and quar tz a re a m o n g th e b e t te rinsulators.

Getting back to the v a c uum tube, i t wasf ound some time ago that some mater ia lswhen heated in a vacu um wil l emitelectrons.The emi t t ing mate r ia l is called a cathode.Two methods a re generally used to heat th ecathode: a cur rent can be passed through th ematerial to raise it s t empera ture , in whichcase it is called a f i l a m e n t or d irect ly -heatedcathode: or a thimble of ca thode mate r ia lcan be heated by a coil inside th e t h imb le , inwhich case it is called an indirect ly-heatedcathode. Simple pictures of these are shownhere, together with th e symbols used torepresent them in schematic diagrams. Itshould b e app aren t, of course, t ha t it isn tnecessary to use a b a t te ry to h ea t theca thode ; ac i s commonly used in radioe q u ip m e n t , and the b at ter ies are shown herefor s implicity. In many schematics the heat-er connections may not even be shown, sinceany radio m an wor thy of the na m e willknow that the cathode must be heated, andthe heater-connection details only clutter u pth e d iagram.

If nothing else is put in the v a c u u mexcept the hot ca thode, th e f irst few millionorbillion electrons set free will cloud aroundth e cathode a nd, having negative charges,


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of Radio repel any other electrons that are t rying toescape f rom th e ca thode. However , ifanother element, called the p la t e or anodeis installed in the v a c u u m , and if thiselement has a posit ive charge, some of theelectrons will be attracted, in proportion tothe magnitude of the posit ive charge. Youcan think of it like this:

Diodel a t e

Catho(Hot )

The e lectrons flow from cathode to plate toequal ize the charge; this constitutes an elec-tric current. The d i f fe rence be tween thiscurrent f low, and the current f low in thewires in the externalcircuit,lies only in thedistances th e e lectrons travel; it is still th esame basic business of the electrons trying toequalize th e charge around th e circuit, andhnot being able to do it until th e ba t t e ryruns ou t of current and can t prod uce thedifference in charge s, or until the c athod eruns out of emission or becomes cold be-cause the heater is turned off or bu rns out.A mil l iammeter-connected in the circuit willindicate the current flowing around thecircuit.

A tube l ike this with two elements (plateand cathode) is called a diode.Current canbe made to pass through it in only onedi rect ion; no current would f low throughth e d iode if the negative ) t e rm inal of thebat te ry were connected to the plate, becausea negative charge on the plate would repelth e e lec trons coming f rom th e ca thode .Diodes and Power Supplies

So far we have shown batteries as thesources of direct currents , but in mos t ofyour work you wil l probably use an acpower supply. This is a device that takes the60-Hertz ac f rom your house wiring an deventual ly delivers dc that is a lmost as goodas you would get f rom a bat te ry . (Later onyou will see why we said almost as good. )An ac power supply consists of threebasic sections: th e t r a n s fo rm e r , the d iode

INDUCTOR TR NSFORMER

rect i f ier , and the f i l ter. Let's start with thet r ans former . Earlier w e ment ioned induc-tors, or coils, and said that they were usedwith laminated iron cores at low-frequencyac . The sketch shown here illustrates thepractical form such an inductor might take.The laminated iron core is usually built upf rom L- or U-shaped stampings, to facil i tatewinding the coil first and then assemblingtheunit.Anactual unit might have hundredsor even thousands of t u rns of wire in it; th esketch is greatly sim plified.

The second draw ing shows th e physicalconstruction of a t r a n s fo rm e r . As you cansee, two separate coils of wire are wound onthe same core, and, l ike the s imple inductor ,i t may have hundreds or thousands of turnsin the windings, although in some trans-fo rmers one of the windings m ay have only afe wturns.Transformers a re designed for specificf requencies or frequency ranges, l ike 60Hertz , 300 H ertz , and the audio-f requ encyrange. The lat ter are called audio trans-fo rmers and the fo rmer come unde r theheading of power t ransform ers . A 400-Hertzt r ans fo rmer wil l get very hot when used at60 He rtz , but a 60-Hertz t ransfo rm er wi l lwork at 400 Her t z , for reasons beyond th e

scope ofthisdiscussion.(Butwe didwant towarn you not to buy surplus 400-Hertzt ransformers and expec t to use them at 60Her t z . )The input, or p r ima r y winding of at r ans fo rmer connects to the power source .Let s say we have a 60-Hertz power trans-f o r m e r designed to work with 115 volts onth e p r imary . (Some are designed for 230 and

other voltages.) W e connect this p r imary tothe 115-volt line by means of a p lug an dlength of lamp cord. Measuring the voltageacross the secondary ( the o ther) w indingwith an ac voltmeter, we find that thevoltage is something other than 115 volts.We could predict what this voltage would beif w e knew th e n u m b e r of turns on bothpr imary and secondary, because th e volt getr nsform tion epen s upon the turns r tio


32/102

o f th e transformer. Take a specif ic case. Thepr imary has 300 turns and the secondary has900 turns. This is a ratio of 1 to 3, and thesecondary will measure 3 X 115 , or 345,volts. If the secondary had only 30 turns,th e rat io would be 10 to 1 and the secon-dary voltage would be 115 H 10, or 11.5volts.You can see what a useful device at rans former can be, because i t permi ts us tostart with an ac power source at 115 voltsand go up or down in voltage level veryconveniently, which isn t t rue of a dc powersource. The power level remains the s ame,however ; th e t ransformer that delivers 345volts at 0.1 ampere P = EL, 345 X 0.1 =34.5 watts) draws 0.3 a m p e r e at 115 volts (/=P E, 34.5 f 115 = 0.3 amp) f rom th eline (plus a little bit more because there aresome slight losses in the transformer itself).Similarly, th e t ransformer that delivers 11.5volts a t 3 amperes draws 0.3 ampere f romth e 115-volt line. In other words, th e powerdrawn by the primary must equal the powerin watts delivered by the secondary (neglect-ingthe slight losses in the transformer).

M a n y power transformers will have sever-al secondary windings, to give several differ-ent voltages. For example, the power trans-former in a receiver or small transmittermight have three secondaries: one delivering6.3 volts, one 5 volts, and one delivering 800volts. This would be shown in a schemat icdiagram likethis:

und ment ls

I I 5V . -

6 .3V .

5 V .800V.

P O W E R T R A N S F O R M E R

The symbol for a transforme r with several wind-ings, such as a power transformer. Th e numbers onthe right-hand side secondaries) represent thevoltages that will be obtained when 115 vol ts isapplied to theprimary lef t-hand)winding.Noticethat a lead is shown from the center of the800-volt winding; this is usually a center tap thatdivides th ewindinginto tw o equal parts.

The 6.3 volts is used to heat th e cathodes ofthe many vacuum tubes in the receiver or

Filament

E00C U R R E N T 0 r T I M E

Schematic diagram of a power tran sform er c on-nected to adiode. In this case the cathode of thediode is a filament of wire heated by the currentfrom the 5-voltwindingof the transformer. Diodesused in power supplies often use directly-heatedf i laments instead of the indirectly-heated cathodescommon to receiving tubes, but the action ofemitting e lectronsand theper formanceof thetubeis the same asdescribed earl ie r.Since the diode can conduct current in only onedirection the current through R is al ternate halfcycles of the ac, asshown by the solid l ines in thebottom sketch. Thedotted l ines representthe halfcycles when noconduction takes place.

t ransmi t ter , th e 5-volt winding heats th efilament or cathode of the diode rectifier,and the 800-volt winding will supply th epower that is to be converted to dc by therect i f ier an d f i l ter. Power trans form ers areusually com pletely enclosed in a case, sothey won t look like the earlier sketch unlessth e case is removed.

To un ders tand diode rect i f ier act ion, let sconnect a diode to the 800-volt secondary ofour t ransformer us ing th e 5-volt winding to .heat the f i lament . This is shown in the nex tsketch. (A plug has been connected to theprimary, and the 6.3-volt secondary wasomitted because we aren t considering itright now .) You w ill notice that w e hav eadded a resistor R and that only half of the800-volt secondary is being used. The resis-tor is necessary to comple te the c i rcuit , andwe are going to see what the current flowthrough R looks like. Since only half of theturns of the 800-volt secondary are beingused, w e know tha t 400 volts ac appearsbetween the t rans form er center tap and thediode plate.During the half of the ac cycle that theapplied voltage makes the plate posi t ive with


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of Radio

When diodes ar e used to conduct onboth halves ofthe ac cycle, full-wav e rec tifier circuit is ob-tained. The bottom sketch showsthe current flowthrough R . Note the use of the chassissymbol fo rthe return path in this schematic and compare itwith th e preceding circui t where no chassis connec-tionw asshow n. )

respect to the cathode, the diode rectifierwill conduct and current will pass throughR. During th e other half of the cycle, nocurrent can pass through th e diode andconsequently nocurrent canpass through R.The current passing through R varies withtime as shown in the sketch.The action ofth e diode in permit t ing th e current to flowin one direction only is called r e c t i f i c a t i on .

If now an other diode is conn ected to theunused port ion of the secondary winding,this second diode will conduct during theportions of the ac cycle that the first diodecannot . The current through R varies asshown. The rectifier circuit with diodes-working on both halvesof the cycleiscalleda full-wave rectifier, asopposed to the singlediode half-wave r ectifier that only uses halfof the cycle.

The vacuum diode is not the only devicethat can rectify al ternating current tha t is ,conduct current inonlyonedirection.Thereare many others that possess this sameproperty. Some have long since dropped outof use. In fac t , th e vacuum diode , too, isbecoming obsolescent, and is now almostsuperseded except in older equipment) byth e semico ndu ctor r ectifiers discussed later.The operation of the v acuu m diode is easyto understand, however, and serves very w ellto illustrate th e p h e n o m e n o n of rectifica-t ion.

TheFilterThe o utpu t from the rec tifier, as repre-sented by the current flowing through theresistor R, is not dc like that you wouldgetfrom a bat tery. Actual ly it iswha t isk n o wn

spuls ting dc; it flows in only one directionbut not at a steady value.If you were to usethisto replace a battery supply in a transmit-ter or receiver, it would introduce a stronghum on all signals, of the type you some-times hear from smallacbroadcast receivers.To smooth out the pulsations and el iminatethe hum and its effects , a fi l ter is usedbetween th e rectifier and the load R.

The simplest type of filter is a largecapacitor across the load R. If the capaci-tance is high enough and the resistanceof Ris not too low, the capacitor will chargeduring the t ime th e diodes conduct but i twill not completely discharge during

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How to Become a Radio Amateur 1974