Royal Observer Corp Monitoring Post Communications

This topic investigates the development of ROC Posts communications systems. It maps the gradual improvements from when the underground posts were built through to closure.

Landline Link to Group

WW2 into Cold War Telephone

After taking on the nuclear reporting role, underground posts were built around 1960-62 to house the observers well away from the hazards of blast and radiation. The original above ground posts were often left in situ and may be found at some posts today.

Telephone, Observer AD163
Magneto Telephone with Headset

First Generation TeleTalk

In the early 1960's the original magneto type telephone with a headset and breastplate microphone worn by the observer, "Telephone, Observer AD 163C" used since WW2, was replaced with a loud speaking unit, known as a TeleTalk by the ROC, allowed the observer freedom to move around in the bunker and not be constrained by the breastplate cord with a plug that locked into a covered socket in the rear of the telephone.

The TeleTalk or "Units Intercom LB AD 3460" to give its proper description is battery powered, hence LB in its description for Local Battery, utilising the same design of 6 volt dry battery [Battery, Dry No. 27] fitted in the Warning Receiver. Additionally a 67.5 volt battery [Battery, Dry No. 18] is used to send the call signal to the post display plotter. AD3460 is the diagram number, the letters AD designate a series of G.P.O. diagrams for Air Defence equipment.

There is a photograph of the Battery Dry No 18 in the gallery, it is marked as a radio battery, in days before transistor radios, this type of battery would provide the high tension for thermionic valves in a portable radio. This particular battery was manufactured in 1971.

Normally in receive mode any conversation on the cluster's omnibus line may be heard. Before speaking into the loudspeaker which acts as microphone too, a small lever switch on the right hand side of the unit must be pressed downwards. Posts within a cluster can communicate amongst themselves by voice calling. But to attract the attention of the plotter in Group HQ, the lever switch is momentarily pushed upwards to the CALL position and then released.

As posts were usually located on remote hilltops the pair of wires feeding them were often carried on poles along field boundaries making them very vulnerable to blast damage. The same wires fed both the Warning Receiver and the TeleTalk unit so both would be out of action if the line was broken.

Telephone engineers regarded these overhead lines as a bit of a joke. In the film Hole in the Ground, the featured post sent an observer out in the fallout to clear the line faults caused by the bomb exploding. This was not an easy task in peacetime yet alone a post nuclear holocaust.

ROC Post Cluster Emergency Circuits
Landlines within a Cluster

A dedicated pair of wires carried the circuit from the post to the nearest telephone exchange. The remaining part of the circuit back to Group HQ normally consisted of one or more switched lines. In peacetime these carried normal public telephone calls between telephone exchanges. For Royal Observer Corp exercises, or in time of war these circuits were switched over for ROC use. Each circuit was designated with the letters 'EC' meaning 'Emergency Circuit' and four or five digits e.g. 'EC1234'. Switchboard operators employed by the GPO / Post Office Telephones / British Telecom may recall being involved with the switching of the EC's as they were commonly known. On a post's drill night there was no way to communicate with Group HQ as switching only happened for a ROC exercise.

Switching the Emergency Circuits was a complex task when considering that York alone had Forty posts in Twelve separate clusters. This was replicated up and down the country some Twenty Five times. At each staffed switching point, firstly a 'Busy' key stopped the line being used for telephone calls, if a call was in progress on the line to be switched, a small lamp flashed until it completed. When the lamp glowed steadily, coloured pegs were moved from the 'normal' to the 'switched' position. This was fraught with problems as it was very easy to misoperate the switching. At unmanned remote exchanges, the switching clerk in the main exchange had to initiate three calls to two different telephone numbers in a set sequence and time interval.

Second Generation TeleTalk

Starting in 1981, the landlines to Group Headquarters were converted from switched Emergency Circuits to permanently connected Private Circuits (Private Wires 'PW'). The previous arrangement of switching the lines meant this could only happen for main exercises. Therefore the TeleTalk couldn't be used on a normal drill night. The change to permanent circuits made it possible to communicate with the other posts in the cluster and Group HQ at any time.

Cluster Private Wires
Permanent Landlines

The new Private Circuit to Group HQ operated over four wires, one pair conveying the both speech to the post TeleTalk and the carrier plus power 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.

2nd Generation Teletalk: Loudspeaking Telephone AD8010

The original grey 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.

The 'TRANSMIT' button must be depressed to switch to talk mode. 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 could be angled to suit the user, who speaks about 9-12 inches away from the microphone.


Loudspeaker Unit LB AD3460
Speaker / Microphone Volume and Call Button
Top View showing Attenuator Strapping Panel
Side View showing Battery Box and Amplifier
67.5 Volt Signalling Battery [Battery Dry No 18]

Warning Receiver

First Generation Carrier Receiver

Post was fitted with a Carrier Receiver in order to fulfill its role as a Warning Point for the public and be aware of an Air Attack to start the monitoring process. The single pair of wires used by the TeleTalk also conveyed the carrier to the Post. The operation of warning system and HANDEL are fully described in other pages, use the website menu to select the topic of interest. The observers would respond to messages sent via the Carrier Receiver in the same way as any other warning point by using the hand operated siren or maroons to warn the public.

Carrier Receiver WB400

This version of receiver is power by 6 volt dry-cell battery in the base of the unit. The Monitoring Post kept a stock of spare batteries for use in both the Receiver and the 1st Generation of TeleTalk.

Battery Dry No.27 used in the WB400 and Teletalk
Battery Dry No. 27

The Battery Dry No.27 contains eight 'C' size cells wired as two parallel groups of four cells in series enclosed in a waxed paper box. Referring to the photograph above, the number 170020, is the (GPO / Post Office Telephones / BT) Item Code number, for ordering purposes. The smaller number at the bottom 4/72, indicates this battery was manufactured in April 1972. New batteries came with their terminals covered with a wax paper strip to prevent short circuits.

Second Generation Carrier Receiver

As part of the modernisation of the UKWMO communications system during the 1980's the older early warning receiver was replaced with line powered units removing the reliance on dry batteries. The power for the warning receiver and the voice carrier signal was sent over the pair of wires to transmit speech to the TeleTalk.

Loud Speaker Unit WB1401
Loud Speaker

A Receiver Carrier WB1401 was used by the ROC as their new early warning receiver was as it was designed to respond to a 'SELECT' call from the Carrier Contol Point. This meant the Police could issue messages during exercises without disturbing non ROC Warning Point receiver owners. In damp environments, such as ROC Posts, the early warning receiver had a ruggedised and damp proof speaker, the "Loud Speaker Unit WB1401". However ROC Group Headquarters would employ the same receiver but with a plastic cased speaker more suitable for an office. The operation of early warning system code named HANDEL is fully described in other pages accessed from website menu.

Master Post Wireless Sets

First Generation Post Radio

The first trials of radio as a backup to the TeleTalk commenced in the autumn of 1961 in Winchester Group and completed in 1975 with the exception of Belfast, who had to wait for the second generation radios in the 1980's.

Only the master post of the cluster was equipped with a single channel VHF radio allowing it to contact the Group independently of the TeleTalk landline. If it were still possible to communicate with the other posts in the cluster the master post would relay their readings to Group. Dual frequency simplex operation meant master posts could not communicate with each other. Adjacent Groups worked on different frequencies and as posts radios had only a single channel no other Group could be contacted if their own Group was disabled. These issue were addressed with the second generation radios.

Post Radio, Plessey / AT&E Countryman

The wireless set shown here is an 'ATE Countryman'. Plessey purchased Automatic Telephones and Electric Co. ( ATE ) some posts radio sets may have been badged as Plessey.

The radio set was to be mounted vertically with the connections facing down. The sets were wired to a small box under the shelf which comprised of the headset socket and Push-To-Talk switch. There was no volume control as this was preset on the radio. The AF Gain control may be seen in the large photograph of the connection panel.

ROC Post Radio Frequencies
GroupGroup TX
Post TX
BelfastNo First Generation Post Radio

I would appreciate any feedback giving the frequencies for other groups missing from this table. Please contact me via the Home Page using the website menu.

Second Generation of Post Radio RN4

The Burndept BE525 radio was housed in a locked cabinet shown here. It was equipped with three channels, so as well as its own Group's channel the set could be switched to two other adjacent Groups, which would be helpful if their own Group was destroyed by enemy action. 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 was not possible. This second generation wireless scheme was denoted as Radio Network No4. (RN4) by the Home Office Directorate of Telecommunications.

Burndept ROC Post Radio
Radio in cabinet
ROC Post RN4 Radio Channels

This table shows the radio frequency used by each group. Posts belonging to that group had its frequency as Channel 1 in their sets. Radio 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.

A block of twelve frequencies were shared amongst twenty five UKWMO Groups. These twelve channels were in part of the Home Office allocation of 80.000-81.500 MHz also used by the UK Fire and Rescue authorities for mobile to base communications, before they switched to Airwave. The limited range of VHF radio signals allowed more than one Group to use a particular radio frequency without causing interference.

The RN4 Primary scheme which was installed as Phase 1 of the communications upgrade had Group and Posts all transmitting and receiving on the same frequency and CTCSS Tone 1. While this worked for master posts in close proximity to Group HQ, the more distant posts and those in difficult terrain could not contact headquarters directly.

The RN4 secondary scheme which was only partially rolled out before stand-down, used hilltop repeaters in areas where posts could not make direct contact with Group. Up to three hilltop repeaters could be linked back to group using VHF highband radio links. Posts out of direct contact range of their group headquarters would select a CTCSS tone number for their nearest hilltop. All the hilltops used the same group frequency but with a different CTCSS tone for discrimination.

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 ROC Post Radio Channels

In England, Wales and Northern Ireland, the Home Office Directorate 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 nowadays for the five Groups under their control. ROC Documentation from England appears to suggest there were no cross border communications and omits the Scottish Groups from the interconnection diagram. The channels marked on the only Scottish radio that I have seen suggests that inter-group communication was only possible within Scotland.

The table above was obtained from the designation plate on one post radio. I would greatly appreciate any feedback to complete the missing parts of the table. Feedback via my Home Page please.

Pump Up Mast and 80 MHz aerial
80 MHz aerial

The aerial chosen for the monitoring post radio is a dipole encased in a green glass reinforced resin tube. This is mounted on a guyed pump up mast. The inflation of the mast is performed from the safety of the monitoring room via a pipe running from the aerial connection box. This photograph of a print taken by Martin Cooke, Chief Observer at Stoke Golding, shows the mast and antenna during an exercise. The FSM dome and GZI can be clearly distinguished too.