Landline link to Group

Starting in 1981, the landlines to Groups were being converted from switched Emergency Circuits to permanently connected Private Circuits (Private Wires 'PW'). The previous arrangement of switching the lines meant this only happened for exercises so the TeleTalk couldn't be used on normal drill nights. Now it was possible to communicate with the other posts in the cluster at any time.

The Private Circuit to Group HQ operated over four wires, one pair conveying the speech to the post teletalk and the power and carrier for the warning receiver. The other pair conveyed the teletalk speech from the post and supplied the power to the teletalk unit. This led to an improved quality of communications around the cluster and back to Group. The very vulnerable overhead lines were replaced with an underground cable. At some sites the poles were not actually removed by BT and in some cases are still present now.

The original TeleTalk was replaced with the more modern AD8010. This derived its electrical power from the public telephone exchange 50 volt battery via the private circuit thus removing the need for a stock of batteries at the post. This new design of TeleTalk still retained the simple controls of the earlier model. The 'CALL' button calls the attention of Group. The device is normally in receive mode and monitors any conversations taking place on the Private Circuit. 'TRANSMIT' button must be depressed to switch to talk. This is very much like a operating a radio transceiver. The loudspeaker and microphone are in the hinged lid that is normally kept closed to press the 'ON' button down to switch it off. When in use, the lid which included the microphone as well as the speaker would normally be angled to suit the user, who would have to speak about 9-12 inches away from the microphone.

 

Warning Receiver

As part of the modernisation of the warning system during the 1980’s the earlier receivers were replaced with line powered units removing the reliance on batteries. The power for the receiver was sent over the pair of wires to transmit speech to the TeleTalk. The TeleTalk itself being powered over the pair used to receive speech and signalling. More information about the warning receivers can be found in the HANDEL topic.

In order to receive warning of an Air Attack the Post was fitted with a Carrier Receiver. This unit was a Receiver Carrier WB1401 as it was setup to respond to a 'SELECT' call from the Carrier Control Point. This meant the Police could issue messages during exercises without them disturbing non ROC Warning Point receiver owners. The operation of Warning system and HANDEL are fully described in other topics on this site.

 

Master Post Wireless RN4

As a backup to the private circuit, from 1972 onwards the master post of the cluster was equipped with a VHF radio allowing it to contact the Group independently of the TeleTalk landline. If only part of the private circuit was damaged and it was still possible to communicate with the other posts in the cluster by TeleTalk the master post would relay their readings to Group.

Master post radios were modernised from single channel valve sets to three channel transistorised equipment. The Burndept BE525 radio was housed in a locked cabinet shown here. As well as its own Group channel the set could be switched to two other adjacent Groups which would be helpful if their own Group was destroyed. Using only a single frequency simplex channel meant that all master posts could make contact with others within range. This opened up the prospect of relaying messages to Group should radio conditions mean that direct contact wasn’t possible. This second generation wireless scheme was denoted as Radio Network No4. (RN4)

This table shows the radio frequency used by each group. Posts belonging to that group had this frequency as Channel 1 in their sets. Channels 2 and 3 in Post radios were allocated to frequencies of adjacent groups. All radios used FM modulation with Code Tone Control Squelch System (CTCSS). Tone 1 is used on all radios when working on the Primary Scheme with tones 3 to 5 for the secondary scheme.

Due to the limited range of VHF radio signals it was possible for some Groups to share a channel so twelve frequencies were allocated to twenty five UKWMO Groups. These twelve channels were in part of the Home Office allocation of 80.000-81.500MHz used by the UK Fire and Rescue authorities for mobile to base communications.

The RN4 Primary scheme which was installed as Phase 1 of the communications upgrade had Group and Posts all working on the same frequency and CTCSS Tone 1. While this worked for posts in close proximity to Group more distant posts and those in difficult terrain could not contact Group directly. During the late 1980's when monitoring Coventry Group exercises the posts frequently were unable to contact Group.

It was planned to use hilltop repeaters like those in the earlier scheme in areas where posts could not make direct contact with Group. This was known as the RN4 secondary scheme. The hilltop repeaters linked back to group using VHF highband links. A document 'Operating Instructions for UKWMO Group to Post Radio Scheme' dated March 1990 suggested the repeater hilltop sites had not been completed at that date, so perhaps it never came to fruition before the UKWMO was stood down.

I can't imagine how the Secondary scheme would have proved successful as each hilltop repeater would use the same channel as the primary scheme but with one of three different CTCSS tones. To use the secondary scheme Posts would switch their radio's tone switch to the appropriate tone number. The situation could arise when the channel may appear free when in fact it was being used with a different tone.

In the Group control, the radio scheme was connected into the internal telephone system. The Post Supervisor had a small control unit associated with their TX14 telephone. This had a Press To Talk (PTT) button to control the radio. If necessary the post display plotter could join in the radio messages by setting up a conference call. However the post supervisor would have to operate the PTT button.

Scottish Groups

In England, Wales and Northern Ireland, the Home Office department of Telecommunications was responsible for the radio scheme. In Scotland this was the function of the Scottish Office and little information appears to be available for the five Groups under their control. ROC Documentation from England appears to suggest there were no cross border communications and omit the Scottish Groups from the interconnection diagram. The channels marked on radio suggest inter-group communication was only possible within Scotland.

The table on the left was obtained from the designation plate on one post radio. I would greatly appreciate feedback from anyone owning a set from a Scottish Group to complete the missing parts of the table. Feedback via home page please <Link>