VeryConverting to A Narrow Band

Commercial FM users are squeezing more channels into available UHfspectrum by reducing their bandwidth to 12.5 and even 6.25 kHz.Here's how we can do the same on our 440-MHz band.

By Rod Wheeler, WA6ITC*

How do you squeeze more channels into our crowded repeater subbands ? You could try thisapproach, but a narrower IF filter could be much more effective-and less expensive! (Photos

by Joel Gelfand, KD6BRI)

Editor's Note : When we printedWA6ITC's "Op-Ed" promoting nar­rower FM bandwidths in the De­cember, 1996, issue of CQ VHF, wealso challenged him to show us how todo it.He met the challenge. Here's how.

With more and more amateursjoining our ranks, and it's wellover half a million now, we're

going to have to use our precious spec­trum of frequencies more efficiently .

In reading through the 1984 RadioAmateur's Handbook (now The ARRLHandbook), I found "The Amateur'sCode," right in the front, next to a pictureof Hiram Percy Maxim. Point #3 of thecode says "The _amateur is progres­sive....He keeps his station abreast of sci­ence. It is well-built and efficient. Hisoperating practices are above reproach ."On the same page, under TechnicalDevelopments, it says that the "ever­growing Amateur Radio continuallyovercrowds its frequency assignments,spurring amateurs to the developmentand adoption of new techniques to per­mit the accommodation of more sta­tions." This need for additional channelson which we can operate will lead to evernarrower channel spacing and band­widths to more efficiently use our limit­ed radio spectrum.

*RodWheeler, WA6ITC, has been a hamsince 1959 and has been involved withrepeaters since 1964, including two12.5-kHz bandwidth machines in the LosAnge les area. Professionally, Rod is theRadio Shop Foreman at Disneyland.

24 • CQ VHF . July 1997

We've done a lot to improve this situ­ation on the HF bands, where the use ofsingle sideband transmission has given usa three-fold increase in usable "channels"over the days of AM. For some reason,we haven't done the same in the VHF andUHF spectrum. On 440 MHz, for exam­ple, we stopped at 25-kHz channel spac­ing while the commercial operators wentto l2.5-kHz and are now heading for6.25-kHz spacing.

Maybe it 's time to start converting ourold 440-MHz FM radios to take advan­tage of l2.5-kHz channel spacing. The

radios will still work fine on the currentrepeaters but will have sharper IF selec­tivity. It 's like working on the HF bands,but when the bands gets busy, you'd sim­ply switch to narrower filters to helpblock out adjacent stations .

It's Not Really DifficultA simple switch of the IF filter in your

radio and the implementation of CTCSSencode and decode will open up a lot ofnew channels for all of us. Old amateurequipment from the '70s and ' 80s was

Ham Radio Above 50 MHz

No, this won 't narrow your bandwidth , although it WILL significantly reduce your deviationlevel! Again, a narrower IF fi lter might be a better option .

Converting Surplus GERepeaters to 12.5 kHz

Many amat eurs use the GeneralElectric MASTR II, Executi ve II, andMVP series radios as the buildingblocks of their repeaters. The 150- and450-MHz versions of this radio use an11.2-MHz IF system. Narro w filters atthis frequency are difficult to find. ButCommunication Specialists (see "Re­sources") has an abundance of 10.7­MHz filters with a bandwidth of 3.75kHz, so I thought it might be possibleto modify the GE radio to a 1O.7-MHzIF system.

I replaced the 1l.2-MHz filters withthe 1O.7-MHz narrow filters, realignedthe IFs, and it worked! The receivernow had a true 12.5-kHz band widthand worked just fine-and all with afew dollars worth of filters. There willbe one additional expense: a newreceive crystal. Since the IF waschanged, I'll have to order crystals .5MHz higher than I want to listen to inorder to end up on the right frequency.Changing the 10.7- or 21.4-MH z IF fil­ters ahead of the 455-kHz filters willimprove the overall receiver operationand avoid relying on one filter for allbandpass requirements.

designed for 25 kHz channels because thecommerc ial equipment back then alsoused 25 kHz separation.Today' s amateurFM equipm ent is a lot better, but it canstill be narrow ed with little effort andcost. Turning down the tran smitter'sdeviation is even easier, and, if everyoneused ± 2.5 kHz, it would allow for a lit­tle extra adjacent channel spacing.

I found CTCSS encode and decodecapability to be the single greatest help inrejecting adjacent channel interference,with different tones reducing adjacen tchannel interference by as much as40 dB.So, by using narrow filters and CTCSSencode/decode, we could double, andsomeday quadruple, our channels on the440-MHz band. Applying the same con­cepts to the 2-meter and 222-MH z bandscould help solve overcrowding there, too.

Don't Panic!Now don 't panic over the thought

changing our current 25-kHz channelspacing to 12.5 kHz on the 440 band. The

commercial users did it in the early '80sand found it worked very well. Evenchanging to lO-kHz spacing on 2 metersand 222 MHz isn't that big of a deal; it' slike changing from AM to SSB was onHF. In fact, you don' t even need aschematic to modify most FM radios.And, if you do the work yourself, the costshould be about $ 10. Now that shouldn 'twipe out anyone' s bank account.

A Bit of Theory, or...Why Bother?

What's the point in changing to a nar­rower bandwidth if none of the repeatersin your area are doing the same? First,you 'll have a tighter receiver that' s lessprone to interference. Second, you'll beable to use spectrum more efficiently. Buthow does all this work?

Let' s take a look at where selectivityin a receiver is achieved. There are threebasic areas of selectivity in the receiver.First is the fron t end, where the input fre­quency is selected. However, the filterhere is usually a bandpass type of filterand the selectivity is designed for a wholeband, not a particular frequency. Secondis the Hi IF Here we find the 10.7 MHzor similar frequency amplifiers and fil­ters. These add grea tly to the receiver' sselectivity, but only bring it down to the± 30 kHz range. In some radios (notablythe GE MVP and Executive II radios usedfor many repeaters), you can add filters

"I found CTCSS encode anddecode capability to be thesingle greatest help in rejectingadjacent channel'.1' "inter]erence....

in this area to improve selectivity. Final­ly, there 's the Low IF, generally 455 kHz,which is where most radios get their realselectivity, mainly because it' s easier andless expensive to do it at 455 kHz than athigher frequencies (see Figure 1).

The Low IF filter is located near thereceiver's discriminator or audio detec­tor (sometimes, there' s more than one fil­ter, as shown in Figure I).The vastmajor­ity of radios use filters made by muRata.These look like small black cubes witheither "455" or "55" written on the topfollowed by a letter; that letter tells youhow selective your radio is (see Figure 2).The filters are very inexpensive and read­ily available (see "Resources").

Open Up That CaseTo sharpen the selectivity of your

radio , you' ll need to find out which filteryou have and which one you need. Takeyour radio apart and find the receiverboard and locate the 455-kHz IF filters(see photo);' (Remember the letter afterthe "455" or "55" tells you the bandwidth

Ham Radio Above 50 MHz July 1997 • CO VHF. 25

Ceramic filters1 and 2 "You don 't even need a

schematic to modify most FMradios. And ifyou do the workyourself, the cost should beabout $10.00."

DETECTORIC

Figure 1. A basic IF detector circuit . Note that some use only one filter; others use more thanone, as illustrated above.

bandwidth. These filters are too wide forthe demands of l2.S-kHz channel spac­ing and should be changed.

To determine the necessary bandwidthfor a given transmitted signal you can usethe following formula (assuming amax­imum modulation frequency of 3 kHz) :

2 (deviation in kHz) x 2.4 =bandwidthin kHz3

For instance, with ±S-kHz deviation,you have (2 x S) x 2.4 = 24-kHz band­width. So even though you only deviate± S kHz, your signal takes up more actu­al spectrum. Your receiver only needs tohear the major portion of the transmittedsignal in order to give you readable copy,which is why±1O-kHzIF filters work for±S-kHz deviation. Now, if you reduceyour deviation to ± 2.S kHz, it changesthings to (2 x 2.S) x 2.4 = 12 kHz.

Again, because the receiver doesn'tneed to "copy" all ofthe transmitted sig­nal, you can use a ±3.7S-kHz IF filterwith a 7.S-kHz bandwidth to copy a sig­nal with 2.SkHz deviation and a l2-kHzbandwidth. Now, if you take two of those12-kHz-wide signals and put them nextto each other, you've got two discretechannels in a space now occupied byone-doubling the current number ofchannels (the roll-off of the filters pro­vides the necessary separation betweenthe channels).

Squelch~r--7 switch

Aud ioI--+-~out

Sque lch

Demodulator

No joking. This is what the 455-kHz IF filter in a radio typically looks like. You'll need toreplace it in order to narrow your bandwidth.

of the filter.) The most commonly usedfilters are designated A, B, C, D, E, F, andG.2Some radios have one filter and sometwo. There is also a double filter that'sabout twice as wide . They all do the samething, though, so you just need to use thechart in Table 1 to determine which fil­ter you have and what you need to get.

I personally use the "G" filters or ±4.SkHz bandwidth at -6dB. This gives methe best adjacent channel rejection andstill maintains the necessary bandwidthfor all current repeaters . For instance,most radios have a "D" filter, or ±lO-kHz

2ndoscillator

00'1"" I

HIIF >--+---1In

CIRCLE 77 ON READER SERVICE CARD

26 • CQ VHF . July 1997 Ham Radio Above 50 MHz

Table. The Basic FilterBandwidths (at -6 dB)

I' ve found that , in practice, a tighter fil­ter than calculated can be used withoutnoticeable deterioration of signal quali­ty. This is probabl y because most con­versat ions are not at full deviation, andthe filters don' t have straight slopes on

27

2 Meter/440 MHz Mobile withoptional 3rd Band

160-10m HF Transceiver withbuilt-in Antenna tuner

YAESUFT-920

AI/-Mode HF/6M Transceiver

• High Performance • Built-In Auto33 MIPS DSP Antenna Tuner

• 1DOW Output On HF • Dual Display Withand 50MHz Twin VFO Knobs

• •••" I I I . I I I .

I I • • , I

I I I I

Narrow- bandwidth 455-kHz IF fil­ters are availabl e from :

muRata , 2200 Lake Park Dr., Smyrna,GA 30080: Phone: (770) 436-1300

Done and DoneSo that' s all there is to it: find the 455 ­

kHz filter and change it. Be careful , andif you' re not confident doing the work ,find a technician who can do it for you.

We need to continu e experimenting tosee what else we can work out and im­prove upon. If we all changed to the nar­rower filters, we could greatly increasethe number of usable FM channels in ourVHF and UHF bands- and we could alluse that. Don 't wait , let ' s all get on thenarrow bandw agon now! •

Com munication Specialists has alarge inventory of IF filters for certainother frequencies , such as 3.75 kH z,10.7 MHz, 21.4 MHz, and 2 1.8 MHz).If you can' t find these parts locally, callthem directly at (800) 854-0547.

Resources

Can You SqueezeIt Tighter?

Now, if you want to experiment withreally narrow operation, you can use the"I" filter at ± 2.0 kHz. Using this filter,you could start approaching the 6.25-kHzchannel spacing the commercial opera­tors are trying to attain . If you run 6.25kHz through the formul a, the deviationgets really narrow-l.25 kHz. I believeit' s possible to use ± 2kHz at 6.25-kHzchannel spacing and still have the radiowork fine. One side note : when youreduce the deviation, the recovered audiowill be reduced as well. So you' ll need toturn up the volume on your receiver.

there bandpa ss. In addition, as noted ear­lier, using CTCSS encode and decode allthe time further eliminates adjacent , andeven some co-channel, interference .

Notes1. If your radio doesn 't have a 455-kHz[F (as is the case in some commercialequipment), contact the manufacturerf orposs ible fac tory modifications for 12.5­kHz operation.2. Filter data courtesy ofmukata.3. FM and repeaters for the Radio Ama­teur, ARRL, 1972.

F= ±6.0 kHzG=±4.5 kHzH= ±3.75 kHz1= ±2.0 kHz

A= ±17.5 kHzB= ±15 .0 kHzC= ±12 .5 kHzD= ±1O.0 kHzE= ±7.5 kHz

Figure 2. Typical response curvesfo rmukata455-kHz IF f ilte rs. Note the differencebetween the bandwidth ofthefi lters in Figure2a (bandwidths A-E; most 70-centimeterham rigs use "D" fi lters) and those in Figure2b (bandwidths F-I; the author recommendsgoing to ±3.75 kHz " H " filt ers for reduced

bandwidth. (Courtesy mulia ta)

0

/'l ~~10

f \20

~. i \1\CFR455ACFR455B

ill 30 I----\~ /' CFR455C

c: CFR455D0

40~ I '.--' CFR455E Ic:CD

~ 50f

60 f----

70~ ~V 'N'. vY1-' II(

420 430 440 450 460 470 480490

Frequency (KHZ)

0 7110

~ CFR455F

20CFR455G

VCFR455H

30CFR4551

ill~c0 40~cCD

~ 50

60 r---

~t\ ~Vl---70 r y

420 430 440 450 460 470 480 490

Frequency (KHz)