R.O.C. Observer Post Communications - ERA 1
This topic investigates the early communications systems used by ROC Posts. The TeleTalk and Warning receiver were installed in the early 1960's and in the 1970's master posts were equipped with a radio too.
Landline link to Group
Telephone, Observer AD163
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. This device, 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.
Teletalk Unit AD3460
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.
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.
WB400 Front View
Battery Dry No. 27
In order to receive warning of an Air Attack the Post was fitted with a Carrier Receiver. 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 topics on this site. The observers would respond to messages sent via the Carrier Receiver in the same way as another other warning point using the hand operated siren or maroons to warn the public.
This version of receiver is power by 6 volt dry-cell battery in the base of the unit. A stock of batteries [Battery, Dry No. 27] were kept in the post for this and the TeleTalk featured the previous sub-section.
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, 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. Unused batteries had their terminals covered with a wax paper strip to prevent short circuits.
Master Post Wireless Sets
ROC Post Radio ERA1
Click image to see connection panel
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.
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.
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 knocked out. These issue were addressed with the second generation radios.
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.
Bedford ROC Post Radio Scheme
Due to the limited range of the posts VHF radios, Home Office hilltop sites relayed the signals to and from group headquarters via VHF radio links. All Master Post radios would listen to the hilltop site(s) on one frequency and transmit back to the hilltop on another frequency. Due to this frequency split the posts could not speak to one another. This was the same technique employed on the Police and Fire Countywide Networks and whilst it meant Group was always in control of the radio waves there would be no post to post intercommunication if Group was destroyed.
During the time of my involvement, Bedford Group usually operated their radio scheme with talkthrough (they called it 'Bounce Back') turned on, allowing posts to hear both sides of the conversation. The diagram shows the actual frequencies used by the Bedford Group during the 1970's. Bedford Posts transmitted on 155.000MHz. The two Home Office hilltop sites at Old Poor's Gorse and Streetley relayed the Post's transmission to Bedford Group on 168.425MHz. Bedford's messages were transmitted on a 174.125MHz link to the hilltops which rebroadcast it to Posts on 147.000MHz.
ROC Post Radio Frequencies
|Post TX |
While it has been possible to find the "Era 2" low band frequencies shown on the next page it has proved more difficult for "Era 1" and any information for other groups would be most welcome. This table shows four of twenty five group frequencies.