Group to Post Communications

A detailed look at the communications network radiating out from Group Headquarters to their monitoring posts. Landlines had been used since WW2 but the Home Office never seemed to give much priority to establishing the radio networks.

1st Generation : Group HQ to Clusters of Posts by Landline

ROC Post Cluster Emergency Circuits
Landlines within a Cluster

Posts were grouped together to form cluster of generally three posts, but in practice the number could be from two to five. Each cluster was connected amongst its self with just a single landline back to Group. A typical cluster is shown above.

At either end of the Group to Post circuit dedicated pairs of wires carried the circuit to the nearest telephone exchange. The remaining part of the circuit normally consisted of one or more switched lines. In peacetime these carried ordinary public telephone calls between telephone exchanges. For Royal Observer Corp exercises, or in time of a war, these emergency circuits were switched over for ROC use. These were known as an 'Emergency Circuit' as they were to be used in an emergency. On a post's drill night they were not switched, so there was no way to communicate between Group HQ and Posts.

The circuits were designated with the letters 'EC' meaning 'Emergency Circuit' and five or six digits e.g. 'EC20221'. The Range 20000-29999 being used for the ROC. Switchboard operators employed by the GPO / Post Office Telephones / British Telecom up until the mid-eighties may recall being involved with the switching of the EC's as they were commonly known.

Switching the Emergency Circuits was a complex task when considering that in 1964 Oxford Group alone had 46 posts in 12 clusters and required switching a total of 109 switching points at 51 different telephone exchanges. This was replicated up and down the country some twenty-five times. At each staffed exchange the switching was done manually by either a switchboard operator or engineer. 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.

At manual switching points, 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 for each ROC exercise

Example : Oxford 1964 One Cluster's Emergency Circuits
Oxford Cluster

In the example above Henley-in-Arden post has a private circuit NM2619 to its local exchange where it is switched onto the Birmingham (BM) - Henley (HHX) circuit No. 101 and there it is switched into the the teeing point. Inkberrow has a private circuit NM2622 to its local exchange where it is switched onto the Evesham (EV) - Inkberrow (IBW) circuit No. 13. That circuit is switched at Evesham to the Birmingham - Evesham circuit No. 68 at the Birmingham end it is switched into the teeing point. Likewise for Barford but its intermediate switching point is Leamington Spa (LG). Bromsgrove, Redditch are teed at Worcester onto a single circuit to Birmingham.

At Birmingham the four switched circuits from Henley in Arden, Evesham, Worcester and Leamington are teed together and switched to the Birmingham (BM) - Oxford (OF) circuit No. 56. At Oxford, the Birmingham 56 circuit is switched to HC5056 a private circuit to Oxford ROC Group building. Switching of only this one Emergency Circuit EC20338 has taken 9 circuits out of service that are usually used for normal telephone calls.

1st Generation Group Headquarters to Master Posts by Radio

As a standby to the line communications between the cluster and Group HQ, the master post in each cluster was equipped with a VHF wireless set. One post at least could still pass it reading to Group. If the fault affected the line between the cluster teeing point and the Group HQ, all the posts within the cluster would still be able to talk to one another. If this happened, the master post would collect reading from all the other posts and relay these through to Group HQ.

Bedford ROC Post Radio Scheme 1980
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. 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. 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 disabled.

The master posts used an 'ATE Countryman' battery operated thermionic valve wireless. Plessey purchased Automatic Telephones and Electric Co. ( ATE ) so some posts radio sets may have been badged as Plessey. This is radio shown on the page ROC Post Communications accessed from the website menu.

The gallery above shows a number of the Group schemes in detail. The arrangement in North Wales Group based at Wrexham was much more complex as it needed to cover a larger geographic area with a hilly terrain. This involved no less than six hilltop sites and a chain of radio links back to Wrexham. The Wrexham, and more modest two hilltop schemes serving Shrewbury, Coventry and Bedford area schemes can be seen in more detail by clicking into the image on the left. The black lines are the radio links between hilltop sites with the transmit frequency at each end of the link. The red text is the Transmit and Receive frequencies from the hilltop to the Master Posts.

ROC Post Radio Frequencies
Group NameGroup TX MHzPost TX MHz
Bedford147.000155.000
BelfastNo First Generation Post Radio
Coventry155.025147.025
Shrewsbury147.050155.050
Wrexham155.200147.200

This table shows just five of twenty five group frequencies. While it has been possible to find the "Second Generation" low band frequencies for England, Wales and Northern Ireland shown below it has proved more difficult for "First Generation" and any information for other groups would be most welcome via the home page.

Gallery

Placement Across Country
North Wales Wrexham No.17
Shrewsbury No.16
Coventry No.8
Bedford No.7

2nd Generation : Group HQ to Clusters of Posts by Landline

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. This led to an improved quality of communications around the cluster and back to Group. During modernisation many of the very vulnerable overhead lines were replaced with an underground cable.

2nd Generation : Group Control to Master Post Radio RN4

Second Generation Post Radio Scheme
Diagram of scheme

This second generation equipment operated as single frequency simplex, meaning all the master posts and the group control transmitted and received on the same frequency, allowing master posts to talk amongst themselves as well as with group and they could overhear other post's conversations with group.

RN4 Rack at Group
RN4 Rack

At Group Control, a radio rack houses the interface with the internal telephone system on the upper shelf and the Burndept BE458 radio transceiver on the lower shelf. The plotter in Group uses the internal telephone system and a special control box to speak to master posts. Radio contact could only be initiated by the plotter. If posts called while the plotter was off-line their call would go unheard.

Should the internal telephone systems fail, the plotter could move into the radio room and use the handset plugged into the lower shelf to communicate with the master posts.

ROC Post RN4 Radio Channels
GroupCodeFrequency MHz
AberdeenABE80.2500
BedfordBED80.2500
MaidstoneMAI80.2625
WrexhamNWA80.2625
DundeeDUN80.2750
OxfordOXF80.2750
HorshamHOR80.2875
InvernessINV80.2875
YorkYOR80.2875
BristolBRI80.3000
DurhamDUR80.3000
PrestonPRE80.3125
WinchesterWIN80.3125
CarlisleCAR80.3250
CarmarthenSWA80.3250
LincolnLIN80.3375
YeovilYEO80.3375
CoventryCOV80.3500
ExeterEXE80.3625
NorwichNOR80.3625
ShrewsburySHR80.3750
BelfastBEL80.3875
ColchesterCOL80.3875
AyrAYR?? help ??
EdinburghEDI?? help ??

Feedback via 'Contact Me' on the Home Page would be most welcome if you know the Ayr or Edinburgh Group radio frequencies.

The first phase of the radio modernisation provided a BE458 base station operating on the 80MHz band at Group. Designated by the Home Office Directorate of Telecommunications as Radio Network 4 ( RN4 ). This allowed communication with posts local to the Group HQ but in hilly or larger areas the distant posts were out of range. As all worked on the same frequency it was possible for messages to be relayed from the distant post via nearer ones. While this might have been acceptable for exercises it wasn't suitable for real action

Phase two of the radio modernisation program rolled out hilltop sites connected via radio links to Group much the same way as in the 1st Generation post radio. The links used Burndept BE458 working in the 168 and 174 Home Office bands. The rollout of Phase 2 was protracted and not all Group were equipped before stand-down in 1992.

Post Radio to ECN Interface
Interface Unit

The Post Radio interfaced with the Group Control's PABX switch, part of the ECN Network. The Post Controller's TX14 Telephone has a button to connect it via the PABX to the radio system interface. After connecting to the radio, indicated by the yellow button glowing on the interface ( photographed at the Dundee museum ), the post controller used the green push-to-talk button and other buttons to control the radio.

The Post Controller can then set up a conference call with any Post Display Plotter(s) with a cluster operating by radio. Each plotter looks after two clusters of posts as explained in the Group Control page on this website.