County Wide Radio Schemes

The 1979 World Administrative Radio Conference (WARC) required the United Kingdom remove the Emergency Services low band mobile radio allocation from the now enlarged 100MHz public radio broadcast band.

The changes to the Emergency Service radio frequencies was phased in between 1987 and 1989. During the changeover both the old and new main station frequencies were maintained until all the mobiles converted to the new bands.

To facilitate the release of the 100MHz broadcast band, two new bands (143-144MHz and 152-153MHz) were allocated in VHF high band in addition to the two existing bands (146-148MHz and 154-156MHz) previously used for links between main stations and hilltops and the Royal Observer Corp (ROC). The ROC links were removed from these allocations. A new VHF low band (70.5-71.5MHz) was allocated for use by the fire brigade base stations. Their mobile transmitted in the existing home office band at 80MHz.

Police main station transmit and receive channels were now interspersed with link frequencies. Having a mixture of links and main stations within one band helped defeat eavesdropping by users of scanning radios. Some Counties used the two new bands, the 143-144 band was for Mobile Transmit and Control Room to hilltop links, the 152-153 band for the Main Transmitter channels and hilltop links to the control room. Those Counties not using the new bands remained with 146-148 for Mobile Transmit and Control Room to hilltop links and 154-156 at hilltops.

 

Equipment Changes

By the 1980’s most Police authorities were using more than one channel for their county wide VHF mobiles. Five or Six channels were not unusual with some channels serving dedicated functions like Police National Computer (PNC) checking or motorway patrols. The move to high-band VHF from Low-Band required an increased number of hilltop sites. To save money sites were shared between adjacent counties so as a result of this policy some hilltops had a large number of main stations and their associated links.

Many counties stopped using VHF link frequencies and moved to microwave frequencies that could carry up to 24 individual links on one system and replacing a multitude of yagi aerials with a single dish aerial. These could be arranged as point to point links between hilltop and control room in the same way as VHF links. For resilience a ring of microwave links could be used for security against system failure by sending links both ways around the ring.

A new design of transmitter could handle up to 8 channels all sharing a common power amplifier stage. Duplicated power amplifiers gave site resilience and coupled together fed a common aerial. One design of common aerial arrangement - the turnstile aerial became a distinguishing feature of Home Office sites. Another design - the skeleton slot and reflector was employed but was far less obvious. Using a common aerial avoided the expense of multiple aerials and their associated feeder system.

Along with the move to WARC highband frequencies the channel spacing was reduced to 12.5kHz this now precluded the use of offset carriers used before. Using a very stable oscillator it became possible to operate multisite schemes with all the hilltop transmitters operating within a few hertz of the nominal centre frequency. This is known as Quasi-Synchronous operation and avoided problems that would cause null spots if all the transmitters operated in perfect synchronism. Quasi-Synchronous transmissions have a characteristic rapid fading effect when heard from outside the service area.

Police Mobile Radios supplied by DTELS had 255 channels enabling the first 25 Channels to be set to own county channels and some adjacent counties. The remaining channels were allocated to a national standard setup which allowed the mobile access to all other county channels in England and Wales. If a mobile was despatched to help in another area it could contact its county control when out of range of its own. Fire Authority radios operated in a similar fashion. Northamptonshire fire became notorious for having a privately operated scheme with single channel mobiles which meant they could not speak to the adjacent county controls if they crossed the border to assist at an incident.

 

Frequencies Used

To illustrate the frequency changes post WARC upgrade, this list derived from web based information published by the Surrey Scanner Group shows the new frequencies for the same counties as I had measured in 1980 (see previous page) when they used the 100MHz band for hilltop transmitters. By the end of 2006 the UK Police migrated from these frequencies to Airwave. Work is in progress migrate the Fire and Rescue authorities to Airwave by July 2010.