Propagation Test on 955.5 Mc,1,965 MC I and 6,730 Mc

Inherent nonlinearity of ferroelectricstorage elements prevents straightforwardanalysis when they are used in circuitapplication. I t has been shown that aferroelectric storage element may betreated as a transient current generator.The type of load used to detect binary I.'sand O's depends on the particular ap­plication of the element. The readoutmethod employed again depends on accesstime and permissible signal-to-noise ratio.

H. R. MATHWICHNONMEMBER ,AlEE

J. E. PITMANAFFILIATE MEMBER AlEE

TH E rapidly expanding need for com­munications felt by United Gas Pipe

Line Company indicated an examinationof the new communication medium,microwave relay. Work by Bullington;'Crawford and Jakes, 2 Durkee," Millar andByam.! Thompson," and others, providedvaluable information for system planningand operation in most areas of the UnitedStates. In the Gulf South area, localeof the company's operations, no suchinformation was available. Rumors}difficult to prove or disprove, were currentconcerning the propagation difficultiesencountered by early systems in the area.In this almost subtropical section of thecountry, humidity variations were knownto be high. Temperature inversion alsohad to be considered. Previous observa­tions of very-high-frequency systems anda test at 960 me (megacycles) indicatedthe possibility of unusual conditions at thehigher frequencies of 2,000 and 6;700 me.

Investigation by the Research Depart­ment of United Gas revealed the interestof the microwave manufacturers in thepropagation problems of the area. A testwas designed to gather data on three ofthe frequency bands available to pipe­line companies. It was planned to oper­ate circuits at 955.5, 1,965, and 6,730me over the same path for a period longenough to provide statistical data.United Gas, as the prime contractor, ar­ranged for Philco Corporation to furnishcertain equipment and services for the6,730-mc frequencies. By separate ar-

References1. THEORETICAL MODEL FOR BARIUM TITANATE,W. P. Mason, B. T. Matthias. Physical Review,New York, N. Y., vol. 79, Dec. 1948. pp. 1622-36.

2. THEORY OF' FERROELECTRICS, A. F. Devon­shire. Advances in Physics, London, England,vol. 3, Apr. 1954, pp. 86-130.

3. THE LORENTZ CORRECTION IN BARIUM TITAN­ATE, i. C. Slater. Physical Review, New York,N. Y., vol. 78, June 1950, pp. 748-61.

4. FERROELECTRICITY, DOMAIN STRUCTURE ANDPHASE TRANSITIONS OF' BARIUM TITANATE, A. vonHipple. Reviews of Modern Physics, New YorkN. Y., vol. 22, July 1950, pp. 221-37.

E. D. NUnALLMEMBER AlEE

A. M. RANDOLPHASSOCIATE MEMBFR AlEE

rangements with The Radio Corporationof America (RCA), equipment and serv­ices relating to the 955.5- and the 1,965­me frequency were made available. Theresul ts of the ensuing test are reported inthis paper. I ts purpose is not to judgethe relative merits of the three frequencybands involved, but rather to make theaccumulated data available to the litera­ture. It must be constantly borne inmind in the interpretation of the datapresented that the tower heights and re­sulting path clearances were not con­sidered optimum for any of the three fre­quencies being tested.

Description of Test

The test circuits were set up betweenthe company's research laboratory atShreveport, La., and a compressor sta­tion at Latex, Tex., a distance of 20.25miles. A strip map of the path is shownin Fig. 1 and a profile drawn to true earthradius is shown in Fig. 2.. The survey ofthe path was made by plotting the profilefrom geodetic maps which were availablefrom Shreveport to the Loui sian a-Texasborder. The balance of the path wasplotted from hand altimeter readingstaken along the path survey line from theLouisiana-Texas border to Latex. Allhigh points along the path were later re­checked by sensitive altimeter for ac­curacy. Tree covering along the pathranged from sparse to moderately heavy,principally deciduous with a few pine.

5. ApPARATUS AND A METHOD FOR ELECTRO­STATIC RECORDING AND REPRODUCING, C. F.Pulvari. U. S. Patent No. 145,361, Feb. 1950.

6. FERROELECTRIC MATERIALS AS STORAGE ELE­MENTS FOR DIGITAL COMPUTERS AND SWITCHINGSYSTEMS, ]. R. Anderson. AlEE Transdctions(Electrical Engineering), vol. 71, pt. I, 1952 (Jan.1953 section), pp. 395-401.

7. DOMAIN FORMATION AND DOMAIN WALLMOTION IN FERROELECTRIC BATIOa, SINGI,BCRYSTALS, W. T. Merz. Physical Review, NewYork, N. Y., vol. 95, Aug. 1954, pp. 690--98.

8. FINAL REPORT, C. F. Pulvari. USAF Con­tract No. 18(600)-106, Wright Air DevelopmentCenter, Dayton, Ohio, Apr. 1955.

Tree heights were determined on thehigh points of the path by measuring theheight of the tallest tree in the groupunder consideration. No sizable streamsor bodies of water were under the path.From Fig. 2 it should be noted that thepath is roughly grazing at three points.

The test was conducted from May 20,1953, through September 30, 1954. Theweather for the period was generallynormal except for precipitation: Themonth of May 1954 showed the onlysignificant excess, with only 2 othermonths showing any excess. The bal­ance of the period was so dry that thetotal deficiency was 17.58 inches. Anaverage of the highest daily humidityreadings which normally occur at or nearsunrise showed an increase of about 3per cent above normal. The lowest dailyhumidity readings usually occurring inthe middle or late afternoons showed anaverage of about 6 per cent below nor­mal. Temperatures for the period aver­aged nearly normal, with the daysslightly warmer and the nights slightlycooler. Wind velocities were only slightl ygreater than usual with no significantvariation from normal.

Fig. 3 shows the antenna installationat the Shreveport terminal. The para­boloid on the right side of the tower cross­member was illuminated by the 1,965-mchorizontally polarized dipole antenna.The paraboloid on the left was illu­minated by the 955.5-mc verticallypolarized dipole. The paraboloid to theleft of the tower near ground level wasilluminated by the 6,730-mc horizontallypolarized horn-type antenna. Energy

Paper 56-968, recommended by the AlEE RadioCommunications Systems Committee and ap­proved by the AlEE Committee on TechnicalOperations for presentation at the AlEE FallGeneral Meeting, Chicago, 111., October 1-5, 1956.Manuscript submitted May 28, 1956; made a vail­able for printing August 1, 1956.

H. R. MATHWICH is with the Radio Corporation ofAmerica, Oamden, N. J.; E. D. NUTTALL and A.M. RANDOLPH are with the United Gas Corpora­tion, Shreveport, La.; and J. E. PITMAN is with thePhilco Corporation, Philadelphia, Pa.

JANUARY 1957 Mathwich, Nuttall, Pitman, Randolph-Propagation Test 685

Fig. 1. Strip map of test path

Table I. Propagation Tests

Equipment Philco CLR-6 RCA CW-20A RCA CW-5 ATransmitter location Latex Latex. . . .. . LatexTransmitter power output, watts (db referred to 1 watt) .. 1 (0) 3 (+5) 2 (+3)Receiver location Shreveport Shreveport. . .. .. ShreveportParaboloidal antenna size, feet 6 6 4Passive reflector size 6 by 8 feetNominal antenna gains, two terminals, db 79.0 59.4 39.6System loss for free-space transmission, db 66.3 77.0 90.8Over-all median loss, db 80.6 92.0 105.1Assumed obstruction loss, db 1•. 3 15.0 14.3

Technical Methods

From the data sheets the information waspunched on International Business Ma­chines Corporation (IBM) cards, one cardfor each IS-minute period for each cir­cuit, IBM machines were used for sort­ing and tabulating the data, but the math­ematical computations and curve drawingwere done by hand.

The receivers and transmitters for eachcircuit were calibrated as nearly as pos­sible on a weekly basis and also beforeand after maintenance. All calibra­tions were made in db referred to onewatt. Accuracy of the calibrating equip­ment was ±2 db. To provide a receivercalibration for a given period, a calibra­tion curve was plotted from values aver­aged from the calibrations made at thebeginning and end of that period. Thiscurve was corrected to the standardtransmitter output of 1 watt or 0 db byusing the average transmitter power forthe period as a correction factor. Thecorrected curve was then used to preparea chart-reading card for transcribing thedata from the charts to the data sheets.

The speed of the Esterline-Angus re­cording paper was chosen as 3 inches perhour as a practical maximum, For anyparticular chart speed, a certain mini­mum fade duration can be defined due tothe finite width of the pen's trace. Forthis chart speed, a line in a downwarddirection followed by a return in an up­ward direction 1 minute later are barelyseparable from each other. For thisreason, and because fades of shorter thanl-rninute duration were expected to con-

The received signal level of each cir­cuit was recorded continuously for thetest period. The following informationfor each l5-minute period was trans­cribed manually from each circuit's dailychart to its daily data sheet:

1. Average loss value in decibels (db).2. Minimum loss value in db.3. Maximum loss value in db.4. Number of times loss was greater than20 db above reference value.5. Duration of time loss was greater than20 db above reference value, in minutes.6. Number of times loss was greater than30 db above reference value.7. Duration of time loss was greater than30 db above reference value, in minutes.8. Number of times loss was greater than40 db above reference value.9. Duration of time loss was greater than40 db above reference value, in minutes.10. Equipment condition (one of sevensymbols to denote equipment normal, undermaintenance, etc.) ..­w

Wu,

955.51,965

Circuit Frequency, Mc

6,730

me radio-frequency equipment. The La­tex antenna and equipment installationswere similar to those at Shreveport.

The radio equipment used in the testwas of standard manufacture. Sincemodulation was not used and bandwidthwas not a consideration, the RCA equip­ment was adjusted where possible to in­crease its recording range. Modificationsmade to the Philco equipment did not in­crease the recording range over that ob­tainable from production equipment.Suitable arrangements were made for at­taching signal recording instruments tothe receiver for each test frequency.

~-._-----~---~~~ ~~-- -------------------

3 4 5 6 7 8 9 10 II

MILES

Fig. 2. Profile map of test path

PROFILE OF SHREVEPORT, LOUISIANA - LATEX, TEXAS MICROWAVE ROUTE

(-----. Major Tree Heights)

TRUE EARTH RADIUS

460420 I __~ ~"-'-"IL.-~-----'~ "'*.__"'U m ,,.~-= _

.- 380

was beamed from this parabola to thepassive reflector at the top of the mast.The paraboloid near ground level in frontof the tower and the passive reflector be­neath the tower platform were part of thediversity tests not reported at this time.I t is interesting to note that no sig­nificant loss differences were measured be­tween horizontal and vertical polarizationchecks of the 955.5-, 1,965-, and 6,730­me circuits. .A.ntenna gains and circuitlosses are a part of Table 1. Fig. 4 is aview of the radio-equipment installationat the Shreveport terminal, showing, fromleft to right. the 1,965- 6,730-, and 955.5-

686 Mathwich, Nuttall, Pitman, Randolph-Propagation Test JANUARY 1957

Fig. 5. 955.5-mc, 1,965-mc, and 6,730-mc loss distribution, May 20, 1953, through Sep­tember 30, 1954

«x

the major item of importance was thetime the loss increased to fairly highvalues. For this reason actual integra­tion of the outage time was set up onlyfor losses of 20, 30, and 40 db with re­spect to the selected reference losses.As the test progressed, however, it wasfelt that following the variations of themedian value and other values near italso would be necessary. It then wasreasoned that since the signal variationswhen the signal is near the median valueare usually slow, it would be acceptableto use the I5-minute average signal.Thus, as an example, the outage time forthe 4-db point used was the sum of thenumber of cards whose average loss wasgreater than 4 db times 1.5 minutes.

.01 0.1 1.0 10

PERCENT TIME LOSS IS GREATER THAN ORDINATE

-..c"0

Cf)Cf)0...J -I 0z<:r25l.LJ 0~

...J

...J<:r

+10Q:l.LJ>0

~ +20I.... :\

W~

Wl"~ +30~~

...J VW VQ: '"(/) "i'4a(/)0...J

I~ 0010.

would be expected to have the greatest ef­feet on the statistics of the 6,730-mc cir­cuit since these have the shortest medianfade lengths [see Figs. 8(A), (B), and (C) j.From Fig. 9(A) it can be seen that only15 per cent of the total outage time wascontributed by fades of 1 minute or lessduration at the +21-db level. Thus, ifthis total time were in error by a factorof two it would only change the totaloutage time for the +2I-db point on theover-all distribution curve by 7.5 per cent.Consulting Fig. 5, this equivalent inac­curacy of level would make these datapessimistic by about 1 db. Similar anal­yses have proved none of the rest of thedata to be in question by more than this 1db.

At the start of the test it was felt that

Fig. 3. Shreveport antenna installation

tribute very little to the total outagetime, all fades of 1 minute or less weretabulated as I-minute fades. It wasproved later that this assumption hadonly a very small effect upon the over-allstatistics. For example, this assumption

Definition of Terms

fig. 4. Shreveportradio equipment in­

stallation

"Median loss" is that loss betweentransmitter output and receiver inputsuch that 50 per cent of the time the ob­served loss was less, and 50 per cent of thetime the loss was greater. In the case of"over-all median loss," this definition ap­plies to data for the entire test.

"System loss for free-space transmis­sion" is the loss that would he measuredbetween transmitter output and receiverinput if the path were operated in freespace without obstructions between theantennas. This loss was computed bythe equipment manufacturers who usedtransmission-line losses measured at thetest site and specification gains of theantenna systems.

Mathwich, Nuttall, Pitman, Randolph-Propagation Test 687

J J A SM J J

DASHED LINES ARE EXTRAPOLATED FREE SPACE -14.3 db-IOr--...,...---...--...,...----r--~---r--~-.......,r---....,-IO

J A S +40

DASHED LINES ARE EXTRAPOLATED-2or----.---r-----r---r-----r----r--~---r-....,-20

Free Space

+40 M J J A SON

1953

A-6,730 me C-955.5 me

DASHED LINES ARE EXTRAPOLATED-IO,....---r--~--~---"T"--_r_-___r--_r_-___r-___,

g~wU>~g<t

~~+20~~~~~~~~~~~~~~~~~~~~+ro R~ 6. Seasonal proba~l"y ~ay 20, 1953/~rough September 30,~~ 19549-1

~~~+40t---+----+-+--+--+---+--t--+----+--+----+~~+40

M J J

8-1,965 me

"Assumed obstruction loss" is the dif­ference between over-all median loss andsystem loss for free-space transmissionand represents the loss nominally attrib­uted to the obstructions in the path.

Data

The primary information desired fromthe test was the loss distributions of thethree frequencies involved operated overthe same path. In setting up the test,consideration was given to providing otherdata such as seasonal and diurnal varia­tions, fade-duration and faded-time dis­tributions, rate-of-fading, and weathercorrelations. Since the difficulty of cor­relating fading and weather effects waswell known, no specific and detailed ef­fort was made in the primary phase of thetest to make such correlations. AWeather Bureau station located on thepath provided a source of weather dataalready punched on IBM cards on anhourly basis (Weather Bureau WBAN-lcards). With the test data also on IBMcards, it is expected that this aspect of thetest will be completed as time permits.

Loss Distribution Curves

The curve shown in Fig. 5 is perhaps ofthe greatest significance. In this curvethe loss distributions for the three fre­quencies are shown. In general, it showsthat as the frequency increases, theamount of time the loss is greater than aselected value increases. I t must becautioned, however, that data from thisfigure should not be used alone to judgethe relative quality of the different fre­quencies as regards their use in micro­wave relay systems or their use in dif­ferent geographical areas. Other itemswhich must be considered include antennagain, transmission-line or wave-guide loss,radio-frequency output availability, andattainable receiver noise figures.

Seasonal Probability Curves

In the early phases of the test it washoped that the seasonal variations noted,Figs. 6(A), (B), and (C), could be corre­lated with seasonal changes in K, the ef­fective earth's radius, since K does varyon a rather periodic basis. Computa-

tions of the expected variation in signalwere made assuming grazing smoothsphere diffraction. These computationspredicted that the greatest losses wouldbe present in February and the least inAugust. The calculated difference be­tween the February and August medianlosses was 1.0 db for the 955.5-mc circuit,1.4 db for the 1,965-mc circuit, and 2.3db for the 6,730-mc circuit. It can beseen from inspecting the data that sea­sonal effects of a random nature are pres­ent to such a degree that even if thecalculated effect were present, it wouldtake many years of data to enable its ex­traction from the random data. Forexample, both the summers of 1953 and1954 were covered by this test, but themean signals of the summers were quitedifferent, as shown in the median losses.

It can be noted, however, that thevariation in median loss from one monthto the next increases as the frequencyincreases. This accords with theoreticalexpectations that a given change in pathobstruction height will cause an increas­ing change in the value of the receivedsignal as the frequency becomes higher.

688 Mathwich, Nuttall, Pitman, Randolph-Propagation Test JANUARY 1957

10

0.199.9 99.9910 30 50 60 90 99

PERCENTAGE OF FADES FOR WHICHDURATION IS GREATER THAN ORDINATE

A--6,730 me, Dec. 1, 1953, through Sept. 30, 1954

\\

I -........ \\ II d~ \

1-21dbl;>Y \(\ 6730 me

i \/1 \\ Iidb

21db'\ USE RIGHT HAND SCALE

I \~ ....--:.--[\.',~

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USE LEFT-HAND SCALE

r------t-~ \I

'\ db values are loss relative

Ito overall median loss

I I

1000

10001 01

(/')Wf­:::>z~

6 100~<X:c::::>owo<X:u,

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o

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20 2412 16 16 20

PERIOD04 08 08 12

DIURNAL00 04

Free 6730rnc~Space ,----",\

~----~____J

\

~ 99% j ~'f',\. 90 P/o

50%

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1%,."r': 1\0.1%

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'----\~I : I~----,

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% Percent Time LossGreater Than Ordinate.

I I- - - I - I - -

+40

_-100..0~~

~~ 0-0~--J

~za::~ +10

o(J)W(/):Eo...J...J +20

--JIc:x:~a::«W0..>

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DIURNAL PERIOD

o

(f)w~~z~

Izo~~a::::>owa~

0199.9 99.9910 30 50 60 90 99

PERCENTAGE OF FADES FOR WHICHDURATION IS GREATER THAN ORDINATE

I

\\ \\ \\ ~

\ , -, ISdb

.DSdb\.. ~ USE RIGHTHAND SCALE

\.. Y

\\'\

'\f'\\..~ 1965mc

2Sdb '..".I ........... "'"I --

, 1\\ \j \

2S~b \ \.> \ db values are loss relativeUSE LEFT·

HAN~ SCALE1\ to overall median lossI I10

001 0.1

8-1,965 me; May 20, 1953, through Sept. 30, 1954

1000

enw~::>z~Z 100o~<X:a::::>owo<X:u,

o

-10

+40

+30

+10

+20

A-6,730 mc

90% 1965mc ~.,,I(

50% ~ J

10%

-I( "'

1% j t ,)rr: 1\

J~ % Percent Time Loss

0.1% J Greater Than Ordinate.

I I00-04 04-08 08-12 I 12-16 I 16-20 20-24

+40

FREE SPACE -15_0 db-10

0..0~~ 0

W(/)>(/)-0~...J +10...J..,w'0::«o~~+20o...J...J

--JI«~a:: +30a..~

o

8-1,965 mc

woit

99

db values are loss relativeto overall median loss

I I

-, ~ USE RIGHT-, ..-----fHANDSCAl E

" ....

'\ '\", ~12.9db 955.5 me

22.9db~ [\

\

10 30 50 60 90PERCENTAGE OF FADES FOR WHICH

DURATION IS GREATER THAN ORDINATE

\\

\

0.1

C-955.5 mc; May 20, 1953, through Sept. 30, 1954

~US~ LEFT \HAND SCAL~4\-+----+--+-+-----+-----t----+----i

I I

1---_+-_-+----\,Z_I2-+.9_d_b-+---l--+---+----t-----1!-------i\

1000,....--.,...---......,.....-~--,r-----,--.,--,,---.-----r--~---,10

+40

0 enw~:::>z~+10~ 100t=<X:c::

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I ~.5mc90% , ..J

~~,

50%

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~--~~,

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~

: 1 L....--, ____.J

_~1!~~' i! i % Percent Time Loss

Ii Greoter Thon Ordinate.i

I I!

00-04 I 04-08 I 08-12 I 12-16 I 16-20 20-24+40

DIURNAL PERIOD

C-955.5 mc

DASHED LINES ARE EXTRAPOLATED-10

Fig. 7. Diurnal probability May 20, 1953, through September 30, 1954 Fig. 8. Fade-duration distribution

JANUARY 1957 Mathsoich, Nuttall, Pitman, Randolph-Propagation Test 689

enwI­::>zs

oI',-..",

'" \

" \-, \\

~,

22~

\ I 7'2.9 db, I\.

\ ", -.,' ....... I

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12.9db

USE LEFTHAND;;rt~ 955.5me

r·9db \db values are loss relolive

"'--'~" I Ito overall median loss

I I10 0.10.01 0.1 10 30 50 60 90 99 99.9 99.99

PERCENTAGE OF FADED TIMEDUE TO FADES OF GREATER LENGTH THAN ORDINATE

1000

z0100

~a:::>o

o

00wr­::>z~z

10~<ta:::>owa<tu,

'l.----'-r-:---~.!

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>----- 21db \ \' I~SE LEFT HAND SCALLJ-/~\ \ USE RIGHT HAND SCALE

\ ....:.----Iidb

2ldb--" .\.\ ~

\\

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\0 0.1001 0 I \0 30 50 60 90 99 99.9 9999

PERCENTAGE OF FADED TIMEDUE TO FADES OF GREATER LENGTH THAN ORDINATE

1000

(j)Wf­::::>z~zo 100~«a::::::>owo«u,

A-6,730 merDec. 1, 1953, through Sept. 30, 1954 C-955.5 me, May 20, 1953, through Sept. 30, 1954

Fig. 9. Faded-time distribution

(i)WI­::>z~zo~<ta:::>awa<tLL

aI

\\

1\ I \\ , \ ISdb

I~~.JS ~ '\ 8SE RIGHT HAND SCALE~

\ \ '\\2Sdb I\.

I. "- "-..... "'" "'\

~1

\ 1965me

2~bT \ db values are loss relative

USE LEFT HiND SCALE

I~ ~to overall median loss

I 'i- I I

1000

10 0.1001 0.1 10 30 50 60 90 99 99.9 99.99

PERCENTAGE OF FADED TIMEDUE TO FADES OF GREATER LENGTH THAN ORDINATE

(i)wr­::>z~

~ 100~~a:::>awa<tu,

8-1,965 mc; May 20, 1953, through Sept. 3D, 1954

Diurnal Probability Curves

In Fig. 7, loss distributions collectedinto 4-hour diurnal blocks are presented.From these it can be seen that the mostfavorable part of the day (for minimumfaded time) is in the afternoon; also, itcan be seen that the most unfavorabletime is in the 4-hour interval betweenmidnight and 4 a.m. Presumably theafternoon is most favorable because, onan average day, the sun has been warm­ing the earth for some time. This causesvertical turbulence which does, not sup­port stratification. Conversely, duringthe early morning hours winds and at­mospheric turbulence tend to be low andstratification is much more liable to causefading.

An addi tional observation is that themedian loss on all circuits was found tobe lowest near midnight. The probableexplanation of this is as follows: On atypical night the evening starts with the

ground relatively warm from the sun'sradiation. As night falls, the groundradiates its heat energy into space, be­coming cooler; however, the higher at­mosphere does not do this. Therefore, alayer of air in immediate contact with theground becomes cooler than the air aboveit and thus has a dielectric constant higherthan that of the air above by a definiteamount. This causes the effective radiusof the earth (K) to become larger and,in a grazing path such as this, causes thesignal levels to rise. This effect wasnoticed on many evenings to a markeddegree.

Fade-Duration and Faded-TimeDistribution

Two informative types of presentationof the observed fades which occurredduring the test are the fade-duration dis­tribution curves, Fig. 8, and the faded­time distribution curves, Fig. 9. It

should be pointed out that in Figs. 8(A}and 9(A) the period of time covered isnot the entire test period.

Subjective Observations

One of the important subjective obser- .vations made during the course of thistest was that it takes a great deal of in­formation to describe accurately thestatistics of even one specific propagationpath. This was illustrated by severaloccurrences. First, the summer of 1953was observed to have rather differentcharacteristics as regards median loss andtype of fading than the summer of 1954.Second, the substandard fades were in­frequent but due to their great length hada sizable effect upon the over-all statis­tics. For example, in the entire 16­months of the test there were only sixfades of 30 minutes or longer durationfor the 1,965-mc channel at the -18-dblevel. These fades, however, contrib-

690 Mathwich, Nuttall, Pitman, Randolph-Propagation Test JANUARY 1957

uted 52 per cent of the total faded timeat that level. Six is hardly an adequatestatistical sample, which illustrates thelength of time needed to obtain accuratedata, especially on the long-durationsubstandard fades.

Conclusions

On the basis of the data taken duringthis test, the loss is greater than a givenvalue relative to median loss for a largerpercentage of time as frequency in­creases. In general, fades become morefrequent but shorter in duration as thefrequency increases. It can be seen thatgrazing paths of the type tested suffer asizable obstruction loss, in this case 14to 15 db. The 1,965-mc data have beencompared for a 4-month period withotherpropagation data taken on 2,000 me onpaths of similar length in the same locale,but with adequate clearance to insuretheir median signals being near the free-

space value. This comparison revealedthat the grazing path was 30 db or morebelow free-space value three times longerthan the clear path, and 40 db or morenearly five times longer. Caution shouldbe used in considering the actual value ofthe reliability advantage. A comparisonof the circuit outage time for the two hopsshowed the time advantage on the clearpath to be approximately 3 hours for the4-month period. This advantage waspurchased for nearly three times the an­tenna cost. Land required for guyingwould cost about four times as much andthe tower itself twice as much. A com­plete economic evaluation of the cost ofadditional propagation reliability is re­quired to ascertain its actual value forany particular hop or system.

The importance of good propagationreliability for systems using telemetering,remote control, and supervisory func­tions, is apparent from an examination ofthe diurnal curves which show that

periods of maximum loss occur during theperiods of greatest need for control func­tions. It should be emphasized againthat, in addition to the important factorof propagation reliability, other factorssuch as antenna gain, transmission-lineloss, and economic aspects must be con­sidered in the design of a system.

References

1. PROPAGATION OF VHF AND SHF WAVESBEYOND THE HORIZON, K. Bullington. Proceedings,Institute of Radio Engineers, New York, N. Y.,vol. 38, Oct. 1950, pp. 1221-22.

2. SELECTIVE FADING OF MICROWAVES, A. B·Crawford, W. F. Jakes. Belt System TechnicalJournal, New York, N. Y., vol. 31, Jan. 1952, pp.68-90.

3. RESULTS OF MICROWAVE PROPAGATION TESTSON A 40 MILE OVERLAND PATH, A. L. Durkee.Proceedings, Institute of Radio Engineers, vol. 36,Feb. 1948, pp. 197-205.

4. A MICROWAVE PROPAGATION' TEST, J. Z.Millar, W. F. Byam. tu«, vol. 38, June 1950.p. 619.

5. MICROWAVE PROPAGATION EXPERIMENTS, L.E. Thompson. iue.. vol. 36, May 1948, pp.671-76.

G. D. WALLENSTEINNONMEMBER AlEE

Telephone Carrier Frequency Networkssome means of subscriber identification andof calling the selected party.3. Local, regional, and national networksmust have points of connection for the inter­change of electric energy representing theintelligence to be transmitted..

TH E GROWTH of carrier systems inthe United States may be likened to

the growth of the country's highways.Highways originally were built betweenthe outskirts of cities to cope with theintercity traffic; so were carrier systems.When he traveled through a city, amotorist had to leave the highway andbecome submerged in local traffic un tilhe could make connection to the nexthighway heading out of the city. Like­wise, if carrier channels were to passthrough a city, they had to become localvoice drops first before they could be re­inserted into another carrier system andpassed on to the next city.

Highway engineers now not only buildsuperhighways between cities, but alsoprovide by-passes and through-connec­tions by way of interchanges. The samecan now be done with carrier systems,'with interconnections and dropouts to

Paper 56-783, recommended by the AlEE WireCommunications Systems Committee and approvedby the AlEE Committee on Technical Operationsfor presentation at the AlEE Summer and PacificGeneral Meeting, San Francisco, Calif., June 25-29,1956. Manuscript submitted March 26, 1956;made available for printing April 30, 1956.

G. D. WALLENSTEIN is with the Lenkurt ElectricCompany, San Carlos, Calif.

suit the traffic. It is done with 45-classcarrier networks.

Voice Channel Networks

In telephony, the ability to establishelectrical connection between any twotelephone instruments is the basic com­modity-of-value to the subscriber. Fromlocal networks for a privileged few J theindustry has grown to the point whereglobal interconnection is taken forgranted. In 1954, about 96 per cent ofthe world's registered telephones werepotentially able to be connected one toanother.

In general, such connections betweenany two of 90 million stations are randomin character, unscheduled, and of tem­porary duration. This means that theconnection has to be flexible and anony­mous and that common standards mustbe employed in all participating telephonesystems. Three basic features are re­quired to accomplish these objectives.

1. Any two telephones must be able to forma working bridge for talker and listener, re­gardless of the instrument's origin and ofthe type of speech employed.

2. Telephone companies must provide

During the first 40-odd years of teleph­ony, telephone transmission relied onthe use of direct voice frequencies as con­tained in speech. It confined itself totranslating the acoustical energy intoequivalent electrical phenomena and viceversa. Networks permitting connectionbetween subscribers were required tohandle and recognize these voice fre­quencies. Difficulties in maintainingstandards of intelligibility over long andmultisection connections, however, led tothe need for definition of transmission re­quirements. Thus it is now possible torefer to the voice channel as a unit of in­formation exchange upon which all tele­phone connection schemes are based.

I t is noteworthy that discussion is stillactive regarding the minimum require­ments for the voice channel although aninternational committee, Comite Con­sultatif International Telephonique(CCIF), founded in 1923, has deliberatedon such questions for about 30 years.Nevertheless, within some margins oflatitude on actual circuit quality, it isnow standard practice to request anddeliver voice channels fit for interconnec­tion between a great number of separatelycontrolled telephone companies. So muchhas the voice channel become an accepted

JANUARY 1957 Wallenstein-Telephone Carrier Frequency N etworks 691