County Wide VHF Radio Schemes

In order to facilitate good quality communications across the whole of a UK County at VHF it is necessary to employ a number of hill top relays. In my home county of Northamptonshire, which is relatively flat, two sites about 30 miles apart provided the necessary coverage. Other counties that were larger or had many hills and valleys required more sites. The hill top sites were linked to the control room by radio links.

In this simplified diagram the arrangement of hilltops and links demonstrate that when the control room operator speaks, the link transmitter A turns on and sends the voice message to the hilltop. The presence of a signal on the radio link operates a relay in receiver B that turns on the main transmitter C. The audio output from link receiver B is connected to the audio in on the main transmitter. When the mobile replies, its signal is picked up at the hilltop by main receiver D which operate a relay that turns on link transmitter E. Link receiver F receiving the signal and operates a call lamp in the control room and the audio goes through to the operator’s handset. Using more than one radio site presented a number of technical difficulties that had to be solved.

In the outgoing direction when the control room operator speaks two or more transmitters will radiate the signal on the same channel. Depending on its location a mobile receiver could pick up more than one signal. Too avoid creating a heterodyne whistle distorting the spoken signal the transmitters radiated on frequencies offset from the nominal channel frequency by an amount than causes the heterodyne to be outside the 4kHz audio bandwidth of the receiver.

In schemes with two hilltops, carriers were offset by +3kHz and –3kHz causing a 6kHz heterodyne. With three hilltops offsets of –9kHz, +3kHz and +9kHz, created heterodynes of 6kHz, 12kHz and 18kHz all outside the 4kHz bandwidth of the receiver. More than three sites transmitting simultaneously was to be avoided due to the difficulty in filtering out the heterodyne whistles.

In the incoming direction the problem is not quite so easy to solve. A mobile at position A will present a good signal to hilltop site North and no signal to hilltop site South. The North hilltop only will relay the mobile user’s voice to the control room. A mobile at position B will present a good signal to the South hilltop site and a poor signal to the North hilltop. Both sites will relay the mobile user’s voice but the control room will hear the combination of the good and noisy signal so may have difficulty in hearing the mobile.

In the 1970’s Norfolk police used a scheme like the one described above but with about five hilltop sites. Northamptonshire police had a two hilltop scheme but the link transmitters at the hilltop were not switched by the base receiver but radiated a continuous carrier.

Pye Telecommunications in Cambridge, England, designed a solution to the incoming signal problem and marketed it under the name ASSORT an acronym for 'Automatic System for Selection of Receiver and Transmitter'. Their solution was to transmit a signal in the audio channel from the hilltop to the control room that represented the strength of the received signal at the base receiver. At the control room, the ASSORT equipment would examine a maximum of sixteen incoming links and dynamically select the audio from the one with the strongest signal being received. If the mobile was moving during a transmission the audio heard by the control operator could be seamlessly switched from site to site receiving the strongest signal. In their 1971 literature, Pye claimed to have installed a twelve remote site scheme in Paris.

An additional board in the base receiver measures the signal strength and generates a tone between 2700-3000Hz representing that quality of the signal. The stronger the signal the higher the frequency. The output from the receiver is a combination of the quality tone and the received audio.

 

Radio Link Assembly

Pye Telecommunications was awarded a contract by the Home Office to supply standardised Hill Top sites and control stations using their ASSORT system known as the 'Radio Link Assembly' ( RLA ).

The main features of the Radio Links Assembly are.

- Duplication of all parts of the system.
- Connection of a maximum of 5 hill top sites.
- Remote control of switching of the duplicated equipment from main to standby.
- Signalling hilltop fault conditions to control station.
- Two of the hilltop sites may be indirectly connected via another standard hilltop.
- Keying of individual hilltop main transmitters.
- Selection of the strongest received audio using the ASSORT system.

The outgoing link from the Control Station to the hilltop radiates continuously. This carries a number of high frequency tones, one per hilltop, just above the normal range of speech frequencies but audible on a scanning receiver. Each tone is shifted upwards in frequency by 60Hz to activate the selected hilltop. Normally all hilltops are activated but the control has the facility to inhibit a site. The multiple high frequency audio tones gave a distinctive signature to the link channel.

At the hilltop both link receivers operate in parallel. A filter removed the activate signals from the audio going forward to the duplicated main transmitter so they are not heard by the mobiles. The two main transmitters are configured in a 'Working' and 'Standby' mode. The currently 'Working' transmitter was keyed in response to the site's tone shifting to the activate frequency.

Failure of one or both link receivers is signalled back to the control station by using the link pilot transmitted from this site. If both of the duplicated link receivers fail to receive a signal from the control station, the outgoing link pilot changes from the usual OK; 3270Hz (nominal - 30Hz) to FAIL; 3330Hz (nominal + 30Hz). Failure of both receivers could be due a fault at the control station or an aerial or feeder fault at the hilltop, or both receivers electronics failing. Under this condition the hilltop site switches to local talk through independently of the control station. Any received mobile signal is relayed back out via the main transmitter. This would allow mobiles in the vicinity to communicate with each other. If just one incoming link receiver fails, this less serious problem is signalled to the control by switching the link pilot between the OK and Fail tones at 0.5 second intervals.

The main receiver incorporated the ASSORT signal generator which added a signal quality tone between 2700Hz and 3000Hz to the received audio. The stronger the received signal from the mobile the higher the ASSORT quality tone frequency. This quality tone is removed when no mobile is being received. In addition a link pilot frequency of a nominal 3300Hz is added to the audio and transmitted over the continuously operated link transmitter. When no mobile is transmitting only the pilot is heard on the link channel.

At the control station each link receiver is fed into the ASSORT voting comparator which uses the quality tone to select the audio from the hilltop with the strongest signal. This voting process is repeated 10 times a second to allow for changes in signal as the mobile moves. A low pass filter in the comparator removes the quality tone and link pilot from the audio sent to the control room so the operators do not hear it.

To enable fault conditions to be isolated all duplicated equipment can be individually changed from Bay-A to Bay-B. This may be done from the control rack in the radio room at the control station or from within the control room. The remote control of the hilltops is done using dual frequency in-band signals resembling telephone keypad tones. Combinations of tones are used to switch over the duplicated Main Receivers, Main Transmitters and Link Receivers. The hilltop Link Receivers can't be changed over as potentially a faulty standby link receiver would result in the loss ability to switch back again if it were selected as the 'Worker' therefore the Link Receivers run in parallel. Each incoming link pilot is monitored at the control rack to detect remote site faults.

Should the control station fail all hilltop sites switch to local talkthrough enabling mobiles to talk to each other. The control station is able to join the Net as a mobile using a separate transceiver provided for this emergency situation. When lightning struck the tower at West Midlands Police HQ during a snow storm in 1991 knocking out all the radio links the control room had to join the net as a mobile. The computers were affected too so they had to rely on scraps of paper to record the position of the vehicles. The heavy snow blocked the M6 through the Midlands and people were trapped in their cars all night - it was chaos!

Police Controls normally operated their radios in piptone mode. When the mobile started transmitting, pip tones were sent out from the control. These would be removed when the controller spoke and restored when the press to talk switch was released. At the end of the message exchange the control would sign off and remove the pip tones. If two mobiles wished to talk to each other, the operator would select talk-through mode. The voted audio signal from the mobile would be sent back out to all the hilltops. Fire authorities preferred to operate their radio's with talk through permanently switched on.

The diagram shows an example that illustrates how Lincolnshire Fire used the Radio Link Assembly in 1981. The frequencies shown in the diagram are those I noted down during my tracing of the ROC radio links.

At that time the fire control was in the heart of Lincoln, the mast was not very high and I have to assume couldn’t get a good radio link to the Kirkby Underwood hilltop site. Therefore it was indirectly connected using Fulletby as a relay.

 

Link Frequency Planning

It was normal practice for control room transmit links to be in 146-148MHz band and hilltops to transmit in the 154-156MHz band. Any indirectly hilltop would be an exception having its links transmitted in the 146-148MHz band. This was done so the relay hilltop would receive all signals in the 146-148MHz band and transmit both to the control and indirect hilltop in the 154-156MHz band.

The ROC Group Headquarters links also used a small group of channels in both the 146-148 and 154-156 bands plus two exclusive Civil Defence (CD) bands 168.325-168.800 and 174.025-174.500. If the ROC Group transmitted in the 146-148MHz band and its local hilltops in 154-156MHz this is the same way around as the Emergency Service links so there would be no problem of having transmitters and receivers operating in the same band. The problem could occur at the next hilltop down the ROC communications chain as it would have to receive in the 154-156 band where its Emergency Service link transmitters operated. To avoid desensitisation at these hilltops, the link to and from it would use frequencies in the exclusive paired CD bands at 168 and 174MHz. Sometimes it seems that it was unavoidable that ROC links were reversed relative to the Emergency Service links and those sites generally appeared to have two masts. There were some exceptions, Old Porse's Gorse being one I discovered.

Hilltop sites with twin masts use one mast for Emergency Service transmitter aerials and the other for their receiver aerials. The UKWMO links used just one aerial for both transmit and receive which was on the transmit mast. The physical spacing between aerials avoids the desensitisation of receivers by transmitters operating in the same band. At Bedford ROC the links were transmitted in the 155MHz band towards Streatley and Twin Woods. Both of these sites had two masts as they transmitted towards Bedford ROC on 147MHz and not the usual hilltop transmit band of 154-156MHz. Bedford used links in the exclusive CD bands to the Reed site where there was only one mast.

 

Main Channel Frequencies

Prior to the conversion program to align Emergency Service radio frequencies with the 1979 World Administrative Radio Conference (WARC) decision the Main Station transmitters at the hilltop site used the bands 97-103MHz and the Mobiles 80-85MHz. The list below shows a number of schemes where I happened to discover both the fixed and mobile frequencies.

 

Radio Equipment

This photograph shows two channels worth of Main Station link equipment (2 Racks per channel) each pair of rack serves three remote hilltop sites. The right-hand rack in each assembly has two transmitters and six receivers to provide the duplication that was described in an earlier Radio Link Assembly topic.

The Police and Fire Main Transmitter at hilltop sites were often 250 watt AM transmitters like the one shown below that I recently photographed at Hack Green Museum. These would be duplicated to guard against failure. In contrast Private Mobile Radio base stations were usually limited to 5 or 10 Watt transmitters.

 

The photo taken at Hack Green Museum shows a valve operated UHF link receiver (top) and link transmitter (below). Each is mounted on its own chassis with space for 4 chassis per 6 foot rack and predate the RLA. These are the type used for the RGHQ links at UHF and some fire services who used links in the UHF bands rather than the normal VHF links.