Why you can't use most UHF ex Home Office radios anywhere outside 70cms amateur band

This is a commonly misunderstood concept about radio equipment which I'll try and explain here.

Back in the fifties and sixties, when commercial radios were in their infancy, the newfangled FM radios used a deviation which was 50 kHz. This means that when you keyed up, you would occupy the centre carrier frequency. When you speak into an FM radio, the carrier deviates either side of the centre frequency, hence the term frequency modulation. At 50 kHz deviation, this means that at full modulation, your signal would occupy a chunk of the spectrum which went within 25 kHz above and 25 kHz below the centre carrier frequency.

As things progressed, two things happened. Filters in the IF sections of receivers were able to be made much tighter and demands on the radio spectrum, which is after all a finite resource, meant that more and more users needed to be fitted into the same overall spectrum.

It was also acknowledged that you don't actually need all that much bandwidth to convey the human voice, and to use up a whole chunk of space just for one channel was wasteful.

FM music radio does need all this bandwidth and in fact even more in order to cover the full range of the human ear to include the a full frequency response. To portray the human voice, it was found that you only need an audio response between 300 and 3200Hz. This is what is known to communication engineers as the commercial speech bandwidth and it is this that gives the telephone like quality to speech comms.

At the inception of FM police comms in the sixties, they went for what was the then rather narrow bandwidth of 25 kHz, initially in London with the Pye PF1 Pocketfone handhelds. These in fact comprised a separate handheld transmitter and receiver unit and had a spring loaded telescopic antenna and ran off a modified PP3 battery. (Stop sniggering at the back :-)

When you had a massive fleet of many thousands of radios like this, it was near impossible to migrate to a newer system for two related reasons. Interoperability is a key concern and if a change were to take place, it would have to be rolled out on a nationwide basis within a few hours. Plus, services had an annual comms budget which just about covered ongoing running costs, repairs and replacement. To suddenly change all radios at once was seen as cost prohibitive.

So while the commercial sector were forced to migrate to 12.5kHz channel spacing almost completely by the late seventies, the Police sailed on using their wider deviation 25kHz radios right up to the present day. There was no pressure put on the Home Office to come into line with modern practice and they already had a big enough chunk of spectrum which they had complete autonomy over. Other notable legacy systems are the fire services and aviation users who still use AM. Also look at the VHF marine band. They started way back even when the Police were still using short wave in just a handful of their cars. At this time the ships were using ultra wide 50 kHz spacing but eventually this became just too wasteful and demanding on bandwidth so they had to migrate to 25 kHz. This is why the marine cannels are to this day all akimbo and it caused an absolute chaos during the transitional phase.

Now let's look at what would happen if you tried to use an HT600e just about anywhere within UHF. Referring to Ofcom publication IR2044, you will notice that within UHF1 and UHF2, it is almost exclusively 12.5 kHz channel occupancy. Where 25 kHz is in use, it is either non voice i.e. data or belongs for the moment to the Home office. If you transmit on say PMR446 channel 2, your modulation will be twice as wide as the specification permits. Leaving aside type approval issues here, when you are talking on channel two with an HT600e, your are in fact encroaching into half of channel 1 which is below and half of channel 3 which is above. You are actually transmitting on three channels at once. And not only is this antisocial when you transmit, but also when you receive a legitimate PMR446 radio, your IF filter is expecting a signal twice as wide and will therefore sound weak on your HT600e.

Incidentally, it is fairly easy to reduce the maximum transmit deviation on a wide radio to bring it into line with current specifications but to make the receive side of things work properly involves changing to a narrower ceramic or crystal filter which will probably cost more than you paid for the radio itself. As a point of interest, the new digital PMR446 system which came out a few years back uses ultra narrow 6.25 kHz occupancy. To put this into perspective, if you were to resurrect an old early 50kHz occupancy radio and try to use it on PMR446, you would actually be taking up as eight of the forthcoming digital channels and causing mayhem!

Just a couple of things left to clear up. Channel step is not the same thing as channel occupancy. It merely changes the increment of the centre receive frequency but does not change the shaping of the IF filtering. Some radios, such as some of the AOR radios do actually have three sets of IF filters. One for mega wide FM broadcast, one for narrow FM and yet another for very narrow FM. Most scanners just mug along with an IF filter shape which suits around 20kHz so 25kHz is in fact over deviated and the more current 12.5kHz signals appear under deviated.

Whew, well anyway there goes the bell and I'll expect your essays in by the beginning of next term :-)