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20 kHz Spacing on UHF and Motorola Radios Compiled by Mike Morris WA6ILQ

In some parts of the country, especially southern California, the UHF band has been "full"... i.e. there's at least one coordinated system on every pair, and has been for years. Southern California is probably the most complex radio frequency environment in the world where all useable frequencies and channels are in use by someone, somewhere. An unofficial repeater list is here. Even six meters is pretty full. Decision makers in many areas watch what goes on in SoCal as a predictor of their own area by anywhere from 5 to 15 years.

The UHF control and linking part of the band is narrow and was filled for years long before the repeater portion filled up (SoCal UHF bandplans and more are here). To "make room" a number of linked systems did an experiment and switched to link channel spacing of 20 kHz in the 420-422 MHz and the 439-440 MHz range, and there were very few problems, none of which could not be overcome. It's common knowledge in the 2-way business that low band IF filters (20 kHz spacing) can usually be transplanted into older UHF (25 kHz spacing) and into High Band (30 kHz spacing) radios of the same family to tighten things up... all that is important when swapping IF filters is the insertion loss (less than or equal is good), IF frequency and the physical mounting (and in most cases the mounting can be adapted).

After the outstanding success of the link experiment a number of privately owned systems that were on adjacent pairs in the repeater range did a second stage test and moved their systems to 20 kHz spacing (note that SoCal repeaters listen from 440‑445 and talk on 445‑450). Again, the 20 kHz spacing in the repeater spectrum provided major success, providing one new channel for every 100 kHz.

At the May 1, 1999 meeting of the UHF coordination group the major topic of discussion was that the band was full, had been full for a decade, and new systems were waiting and waiting for pairs. A motion was made and a vote was taken about moving to 12.5 kHz spacing and the result was a total defeat of the idea - that of all the user radios "out there" maybe 10% could do 12.5 kHz spacing (remember, this was 1999) ‑ the major problem was not VFO steps (that was a consideration), but IF bandwidth (i.e. hearing the adjacent newly coordinated system 12.5 kHz away). So the discussion moved to 20 kHz spacing and the experimental test results were presented to the membership. It was pointed out that there were areas of the band were all the systems in a 100 kHz block were privately owned, and had been running on 20 kHz steps for quite a while (just like when UHF channel spacing went from 50 kHz (and "wideband" 15 kHz deviation) to 25 kHz (and "narrowband" 5 kHz deviation) back in the 1960s). Again, concern was expressed about user radios that would and would not work on 20 kHz spacing... at this point this chart was shown on the overhead projector (the list was last updated in 2003) at the the meeting, and copies were handed out. To make a very long story very short, once the assembled system owners (over 350 were there) were assured that they would not have to go out and buy all new radios they voted to re-engineer the entire 440‑450 spectrum to 20 kHz spacing.

Over the next few months all of the systems on 44x.y00 channels stayed where they were, all the systems on 44x.y25 moved down to 44x.y20, those on 44x.y50 moved down to 44x.y40, and those on 44x.y75 moved up to 44x.y80... in 99.5% of the cases it was a minor tweak of the screwdriver on the channel element or Icom. A few folks whose adjustments were already at their limit had to order new crystals. Purists tweaked the duplexers as well. Once each 100 kHz group moved, the 44x.y60 channel was available for new system coordination, providing ten new channels per megahertz. Only one problem showed up ‑ and it was one that was totally unexpected ‑ news crews and other scanner listeners monitoring 470-471 MHz public safety channels (for example, Los Angeles County Fire Dispatch) on cheap single conversion scanners were hearing amateur systems along with the fire department dispatcher ‑ read the whole story here.

The Motorola radios that work just fine on 20 kHz steps are the newer ones: GM300, GP300/350, HT50/600/600e/1000, JT1000, M100/120/200/1225, MaraTrac, MaxTrac, MCX100/1000, MSF5000/MSF10000 Analog/Digital, MT1000/2000, MTR2000, MTS2000, MX300-S, P100/110/200, Quantar, R100, Saber, Astro Saber, SM50/120, SP50, Spectra, Astro Spectra, Syntor X & X9000, Visar, XTS.

There are some that depend on the individual radio: for example, both the GM300 mobile and the the GP-300 handheld have two different RF boards, one does 20/25 kHz steps, the other does 12.5/25 kHz steps (the GM300 is one radio where the bandwidth information is buried in the model number).

The ones that are known NOT TO WORK are quite older models: Mostar, Syntor (the original Syntor, the Syntor-X works fine). And of course, the crystal based radios work just fine...

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

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