The Initial Speech and Telegraph Emergency Networks

This topic describes both the United Kingdom Warning and Monitoring Organisation (UKWMO) and Regional Government Headquarters (RGHQ) communications from its inception in the early sixties, through the seventies and early eighties. Both network were linked together, so they are treated as a single network. A Home Office review identified the need for an upgrade which took place during the mid-eighties and replaced the systems described here by those on the 'Era 2' page tab.

Speech and Telegraph Networks in ERA 1

Overview of the Networks in the U.K.

This topic looks at the networks used by the UKWMO in their reporting of nuclear detonations and fallout warning role and the Government Control Network (GCN) operated for the local and regional government organisation. Although the UKWMO and RGHQ networks were separate entities they were joined between the UKWMO Group and the RGHQ serving the county where it was located.

UKWMO Network

UKWMO Private Network

Both the UKWMO and GCN were of a similar design and interconnected so will be treated as one. Both networks used Post Office Telephones landlines to link between nodes. A single circuit would carry both speech and telegraph signals, at each node the speech circuit terminates on a telephone switchboard and the telegraph circuit in the message centre. Many of these links had a duplicate standby radio circuit provided by the Home Office making it totally independent of the GPO network.

The UKWMO Private Network

Within the UKWMO there were two distinct networks. One linked Royal Observer Corp (ROC) Posts to their Group Headquarters for speech only, this is dealt with under the Menu Tab: Communications / ROC Post Era 1. The other linked adjacent UKWMO Group Headquarters together. This network had two sub-networks one for speech and the other telegraph. Telegraph is for sending hard copy messages between one or more places. At this stage of development, there was only a single landline with a standby VHF radio link between the adjacent groups carrying both speech and telegraph signals.

The UKWMO network only connected the adjacent Royal Observer Corp Group Headquarter. By the use of message centres it was possible to send messages between any two locations even though they were not directly connected. The message centre could also broadcast the same message to many locations if this was desired.

Of the five UKWMO Sector Headquarters, only Preston and Lincoln Sectors were directly connected as they are adjacent, therefore all other inter-sector traffic had to be relayed via another UKWMO Group. During the late 1970ís the five Sector Headquarters traffic had grown to an extent that warranted direct links. Unlike the inter-group landlines, these newly provided circuits didn't have a radio standby.

Regional Government Headquarters Network

The Government Communications Network (GCN) like the UKWMO network had two sub-networks, one for speech and the other for telegraph. This network was centred around the RGHQ, linking it with adjacent RGHQs, and the County Council Wartime Headquarters within its zone, Armed Forces headquarters (AFHQ). It had a landline with a standby UHF radio link to the UKWMO Group HQ(s) monitoring the towns and cities within its zone.

I don't have a diagram to show the GCN network during the ERA1 time period. Instead here is an extract from a declassified document, the UKCICC(H) Joint Signal Instruction. At this time the document was written the Sub-Regional Headquarters (SRHQ) was the name for the RGHQ.

UKCICC Joint Signal Instruction Part 5

The Government Control Network
119. This network provides the following facilities:

     a. A landline and radio link between adjacent SRHQs, both within and
     outside individual regions.

     b. A landline and radio link from the SRHQ to each county wartime
     headquarters (both main and standby) within the sub region. In the case
     of Greater London, the SRHQ will be linked by line and radio to the
     headquarters of the 5 London groups and, in addition, each group will have
     a "speech only" radio link to an alternative SRHQ.

     c. A tape relay centre at each SRHQ, providing duplex (two-way) teleprinter
     facilities to adjacent SRHQs and to county wartime headquarters (both main
     and standby).

     d. A landline link from each SRHQ to the Emergency Manual Switching Scheme
     (EMSS).

     e. A landline and radio link from each SRHQ to a selected Group Headquarters
     of the United Kingdom Warning and Monitoring Organisation (UKWMO).

     f. A "speech only" radio link from each SRHQ to the headquarters of the
     various police forces within the sub region.

     g. A landline link between the main wartime headquarters of each county
     and an appropriate police headquarters.

     h. In the case of metropolitan counties, a landline and radio link
     between the main or standby headquarters of the county and the
     wartime headquarters of each metropolitan district.

     j. In the case of Greater London, a landline and radio link between the
     headquarters of each London group and the wartime headquarters of each
     London borough within the group.

     k. A "speech only" radio link between the headquarters of each London
     group and neighbouring London group.

     l. A landline link from each county wartime headquarters (both main and
     standby) and each London group to the EMSS.

120. Where both landline and radio links are provided, they are complementary,
and terminate on common switchboards. The landline circuits carry simultaneous
teleprinter working, with the radio link providing an alternative circuit if the
landline circuit fails.

121. UHF radios are also provided between SRHQs and the aircraft and helicopters
of the regional air squadron to facilitate the passing of reconnaissance reports.
If the squadron are reinforced by other light civil aircraft, other telecommuni-
cations might have to be improvised.

122. As a further standby measure, in each sub region there will be a reserve
of up to 10 HF radio sets. These are transportable, have easily erected aerials
and are omnidirectional. They have facilities to recharge their batteries or to
operate off standby generators. Their range is about 20-30 miles and consequently
they could be used in the last resort to plug the gaps in the GCN. They will be held
under Home Office control within sub regions.

Note. This document refers to a number of topics covered elsewhere on this website. These topics are indexed on the top tabs or linked at the bottom of this page.

Para 120, The system for simultaneous speech and teleprinter working, is S+DX, found under the 'Related Topics' tab. There is a brief overview further down this page in the 'Telegraph and Speech over the same Landline' topic

Para 121, refers to a topic within 'To Military' found in this 'Communication' tab group.

Para 122, is believed to be LDN found under the 'Post Strike' tab.

Para 119, sub-paras D & L, the EMSS, found under the 'Related Topics' tab.

ERA 1 Telephony

From the 1960ís until the communications upgrade in the 1980ís the UKWMO Group, RGHQ and County Council bunkers had manual switchboards with a mixture of public telephone lines and private circuits for use by the extension telephones in the building.

Cord Manual Switchboard

Cord Operated Manual Telephone Switchboard

Private Manual Branch Exchange

Private Manual Branch Exchange

The small Private Manual Branch Exchange shown here has the capacity of 10 exchange lines and 50 extensions, is of the type used at small control centres, such as those operated by County Councils. The PMBX10+50 was a standard item used on the public network. An extension would have to ask the switchboard operator to ring another extension. For outside calls the operator connected the extension to an exchange line and may have dialled the external number too.

The larger multiple position switchboard may have been used on its own as a manually operated switchboard or in conjunction with an automatic switch. The automatic switches of this era were of the electromechanical type. The operator would connect incoming calls. Extensions could usually dial an external number by prefixing it with a nine but some establishments required the switchboard operator to connect the call. Calls to other bunkers over private circuits were usually connected by the operator.

The UKWMO and the RGHQ had their own private networks of landlines and radio links. An extension wishing to call another elsewhere in the private network would ask the switchboard operator to connect them. The operator would place the cord into the jack on the switchboard face and operate the 'ring' key. This would call the attention of the distant operator who would ring the distant extension. This could be quite a protracted process as each operator may be dealing other calls. There may be only one or two lines between switchboards so extensions may have to queue to get connected.

Post Office Emergency Network

During the nineteen sixties and the early seventies, long distance calls over 25 miles, were connected by the public telephone operator. In wartime that service would be compromised so the Post Office set up a small network of special switchboards in protected areas of telephone exchanges to maintain a long distance connection for emergency services. This network was known as the Emergency Manual Switchboard Service (EMSS).

Although automatic dialling of long distance calls became universal by the end of the seventies the EMSS network was retained throughout the Cold War period as it was assumed the public network would suffer badly in an attack. Before the digitalisation of the UK network in the late eighties the automatic long distance dialling network provided no alternative routing of calls should part of the network fail. Therefore it was important to retain a manual network which could adapt to a wartime situation.

The EMSS is described in its own topic if you wish to read more. Top of page, Menu Tab: Post Strike / EMSS

ERA 1 Torn Tape Telegraph Message Centre

This describes the functioning of a torn tape message switching centre like those used in the UKWMO Group Headquarters and Regional Government Headquarters (RGHQ) prior to their communications upgrade in the mid 1980's.

Their use was widespread beyond the UKWMO, as a child I recall a conducted tour at the USAF base at Croughton, Oxon. (Possibly 1967) Being shown the tape centre and the new 80-hole punch card system that had just been installed. They gave us a punched card that I kept for many years saying something like 'Welcome to RAF Croughton from the officers and servicemen of Autodin.' Some large companies had their own tape message centre too.

Each UKWMO Group headquarters and each RGHQ had its own message centre. They were connected to a small number of adjacent centres in their own network with a link at each node between the UKWMO Group and its RGHQ. The links normally worked over the landline but could be switched to an alternative radio link and would send messages in either direction. Each link had a paper tape reader connected to the 'send' and a paper tape perforator connected to the 'receive'. A teleprinter connected to a tape reader would convert paper tape messages to printed copies. A Teleprinter with a paper tape perforator or a punch unit created outgoing message tapes.

Message Tape

The telegraph network used a 5-bit baudot code sent at 50 baud, which is very slow by todayís standards but suited the mechanical devices it was designed for. The five hole code for each character was punched in a row across the width of a paper tape.

Perforated Tape

5 hole perforated paper tape

This type of paper tape was used by business telex and small scale use. For message centres it was desirable to have the message printed down the side of the tape to assist the operators in handling the messages. The small piece of paper punched out of the hole known as a chad is undesirable in a message centre. They create disposal problems and tend ingress into mechanisms causing faults. To overcome the problems a chadless tape was used which had a square hole cut on three sides and attached on the other.

Sending a Message

Torn Tape Centre

Torn Tape Relay Centre

A typist would prepare a message by typing on a punch producing a paper tape or a teleprinter with a tape perforator. The message would start with a header indicating the destination(s) of the message, the message itself and a run out of blank tape. This was torn from the perforator and carried to the tape reader on the link to the desired recipient.

The tape was placed in the reader and the start button pressed. The teleprinter signals would be sent to the distant end, if that were a teleprinter the message would be printed on the paper roll. If it were another message centre, the signals would cause the associated tape perforator to reproduce the sent message tape. If the message were a broadcast it would be transferred to the next reader and sent again to that destination.

Receiving a message

An incoming message would be perforated onto paper tape and a lamp illuminated to indicate the arrival of the message. The operator would tear the tape from the punch and visually inspect the header. The tape would be transferred to a reader associated with a local teleprinter. This would print the message on a paper roll. The paper was torn from the machine and placed in the out-tray to be taken to the recipient.

Indirect and through messages

If the originating message was destined for recipient at a location without a direct link, or the direct link was faulty or destroyed by enemy action the tape would be sent to another message centre. On receipt the operator would inspect the header and see that it was not for their centre. If the message were for a destination which centre had a working direct link the tape would be placed on the reader of that link and sent. Otherwise it would be sent to another centre with a possible link. The choice of alternative centres is known as the 'routing'.

Flexibility and Economy

A torn tape message centre was a very efficient way to achieve full message routing throughout a network of locations without having to connect every one to the others. It is very flexible too as the operators can use their intelligence to work around a fault or non functioning centre.

When the UKWMO telegraph network was introduced in 1968 it was piggy backed on the existing speech network using Speech plus Duplex technology ( S+DX ). The actual network connecting the 25 Group Headquarters in the UK had 53 links. This economy of scale was possible as only adjacent headquarters were connected together. It still enabled a message to be sent from any point to another via one or more tandem relay points. Whereas a fully interconnected system would have required 300 links costing six times more, even though many links between remote points may never have carried a message.

The Teleprinter for the ERA 1 Telegraph Network

A teleprinter of the cold war era was a large Electro-mechanical device resembling an electric typewriter. There was a large public network of machines in the UK, which due to the high cost was almost exclusively for business use. The UK public network was dial up in a similar way to the telephone network but the two networks were not connected. Some large companies had the own independent network which more than likely used Post Office leased private circuits. The Post Office Telegrams division had its own 'TASS' private network up until the 1970's, used for sending public telegrams. This used Creed 47 designated as British Post Office No.11 machines that printed on a thin strip of gummed paper for sticking onto the telegram document.

Before the FAX machine became popular with businesses during the late 1980's, Teleprinters were favoured as an 'instant' communications medium as they produced a hard copy. The data rate, which was considered fast at the time, was 50 baud, with a 5-bit code plus 2.5 start and stop bits. The signalling was by sending 80 Volts Positive and 80 Volts Negative along the two wires. A good typist could type faster than the machine could send the characters, to prevent this happening the keyboard was mechanically locked to prevent a second key being pressed until the previous character had been sent.

Teleprinter No 7

Creed 7B for the TELEX Network

Creed 7B for the TELEX Network

The original Creed 7B here on the right is shown in a configuration for the British Post Office TELEX network. The 7B could be used on point to point links and terminal points on the network but as it had no paper tape facilities. Separate tape perforators and tape readers were required. The large box under the desk but normally located in a more convenient place is the power unit providing the 80 volts DC signalling.

The No.7 designed in 1931 is ingenious consisting of hundreds of moving parts. This was the mechanical equivalent of the UART found in computer modems and decoded the 5-bit code and printed the character on the paper. Only capital letters, numbers and a limited number of special symbols were available.

Creed 7ERP with cover removed

Creed 7ERP teleprinter

An improved version of the No.7 the 7E used a different clutch arrangement overcame the problem where the 7B printed one character behind the one being received. In the photograph below this version of the 7E is the 7ERP as it includes a tape re-perforator (RP). The electric drive motor and speed governor (the black a white round box) which powered both the printing mechanism and keyboard can be seen at the front left.

Teleprinter No 15

PO Teleprinter No15

PO Teleprinter No15 / Creed 444

The Creed 444 introduced in 1966 and designated the British Post Office No.15 replaced the old noisy No.7 printers for business use. The new machines had two colour printing, a built in paper tape perforator and reader. They were much easier to use as they had an improved keyboard and could store keystrokes. Later on screen based telex machines were introduced, but the No.15 gave good service until the closure of the telex network.

Although the Teleprinter No15 became well established in the public network during the seventies I have little evidence it was adopted for the UKWMO networks by 1980. The reliance on a good mains supply may have made it unsuitable for use with standby generators.

Printing Perforators

Typical Cabinet

Printing Perforator

Cabinets containing three printing perforator like the one here were installed in UKWMO group and RGHQ tape relay centres. The bottom unit is open with the printing perforator pulled forward into the maintenance position. A printed version of the punched hole message is printed along the outside of the tape. The punched tape exited through the front of the soundproof door ready to be torn off by the centre operators. Longer messages collected in a floor level tray.

At the top of the cabinet are three sets of supervisory lamps and control switches, one set for each tape perforator. As there is no message store in this system it was essential for the operator to ensure there was sufficient paper tape. An alarm was raised when the paper was low. The circuit had to be blocked to the distant end when the paper or print ribbon was changed.

Each link from a remote location required an individual printing perforator, necessitating a number of these cabinets. Somewhere like Coventry UKWMO Group HQ would have required three or four cabinets, for the links from seven adjacent Groups and two RGHQ's.

Telegraph and Speech over the same Landline / Radio Circuit

Economy Measures - Speech and Telegraph

Speech and Telegraph on one line

At this time, the rental of landlines was very expensive, so to avoid having separate landline for Speech and Telegraph a very clever system known as Speech plus Duplex Teleprinter, S+D or S+DX was used in the UK Civil Defence network to carry duplex teleprinter signals over the same wires as duplex verbal communication. This system was not exclusive to Civil Defence, but was used in the public network too.

This drawing shows the principle of the arrangement. At each end of the Private Circuit (Landline) a box of electronics combines the inter-switchboard speech line with a bothway link to a teleprinter or torn tape message centre. A more detailed explanation is given in the topic S+DX listed in the Further Reading section at the foot of this page, or via Menu Tab: Related Topics / S+DX at the top of this page.

In both networks, the S+DX could be switched to the radio circuit should the common landline fail. A special jack panel was provided to switch from landline to radio. During weekend ROC exercises, a test of switching to radio standby circuits was usually undertaken for about an hour.

ERA 1 Radio Backup to Landlines

Within both the UKWMO and RGHQ networks radio systems were provided to act as a standby for the landline network.

The UKWMO network utilised single channel radio links in the VHF band. The more extensive RGHQ network radio links operated on UHF with both single channel per carrier and multiplexed links. The UKWMO radio links used Home Office Hilltop wireless stations equipped with standby generators already used for Police and Fire Brigade radio schemes. All three groups of users were Home Office departments so it made sense using the same hilltop sites. The radio equipment was supplied and maintained by the Home Office Department of Telecommunications (DTELS)

In this first era from the sixties to mid-eighties the frequency bands were 147.000 - 147.300 MHz paired with 155.000 - 155.300 MHz which were a sub band of the larger 146 - 148 MHz and 154 - 156 MHz frequency assignment for Emergency Service use. An exclusive Civil Defence band 168.325 - 168.800 MHz paired with 174.025 - 174.500MHz was used exclusively for the UKWMO in accordance with notes N27 N29 in UK Table of Radio Frequency Allocations 1985, ISBN 0-11-513819-6

Click to Show - The Detailed VHF Channel Plan

VHF Frequencies allocated for UKWMO Royal Observer Corp.

   SHARED BAND                 CIVIL DEFENCE EXCLUSIVE BAND
  CHANNEL PAIRS           CHANNEL PAIRS           CHANNEL PAIRS
 147.000 155.000         168.325 174.025         168.625 174.325
 147.025 155.025         168.350 174.050         168.650 174.350
 147.050 155.050         168.375 174.075         168.675 174.375
 147.075 155.075         168.400 174.100         168.700 174.400
 147.100 155.100         168.425 174.125         168.725 174.425
 147.125 155.125         168.450 174.150         168.750 174.450
 147.150 155.150         168.475 174.175         168.775 174.475
 147.175 155.175         168.500 174.200         168.800 174.500
 147.200 155.200         168.525 174.225
 147.225 155.225         168.550 174.250
 147.250 155.250         168.575 174.275
 147.275 155.275         168.600 174.300
 147.950 155.300
 

UKWMO Radio Links

Inter UKWMO Group Radio

Inter UKWMO Group Radio circuits

VHF radio links were provided to supplement the landlines. These provided a duplicate speech circuit between switchboards and additionally the telegraph S+DX could be switched over to use this reserve radio path should the landline fail.

The range of a single VHF radio link is limited to 30-40 miles, so a number of tandem links were required to connect the long distances between Group controls. The Belfast to Preston link contains an exceptional hop across the Irish Sea from Slieve Croob to Cefn Du of approximately 118 miles, this only being possible with the use of a power amplifier and phased yagis, over an unobstructed sea path.

At the Group HQ bunker, a windup mast carries a number of VHF 3 or 6 element yagi aerials, pointing at the nearby DTELS hilltop sites. Link on less physically obstructed routes might be mounted directly on the building's roof. The chain of hilltop sites use 6 element Yagi aerials on the links, a single aerial is used for both the transmitter and receiver. Some Hilltop sites would have more than one link between Group Headquarters passing through them.

The aerials are either mounted with their elements (rods) in a horizontal or vertical direction. Aerials at each end of the link will have the same element polarisation. An aerial of the opposite polarisation only receives a weak signal. This can be exploited to allow the same radio frequency to be reused in the order of fifty miles away if the element polarisation is different.

This arrangement at Coventry is typical of a group headquarter setup during this era. It has VHF radio standby links to seven other groups. The image gallery contains a larger version of this image marked with route, frequency and hilltop information and a route map. As well as the VHF yagis for the inter-group links and master post radio hilltop repeater link, there are two UHF yagis linking it to the two RGHQs in its area.

UKWMO VHF Link Details and Frequencies

Links between UKWMO Groups

The enlarged map shows all the individual radio links emanating from Wrexham, Shrewsbury, Coventry, Bedford, Lincoln and Norwich. And a partial set of information for the others. Where the routing of a radio circuit isn't known it is shown as a squiggly gray line. Appealing to ex-DTels staff - If you know the routing for any missing home defence radio links just a small part would be most helpful in building a bigger picture.

It has been suggested that not all the inter group radio links actually worked and some worked but had not been formally commissioned. All the VHF links were switched off as part of the W.A.R.C. project which changed the emergency service radio frequencies during the mid to late 1980's. The old UKWMO VHF link frequencies in the 147 and 155 bands were reallocated to the emergency services when they migrated from 100 MHz, and the circuits migrated to the RN1 and RN2 networks that are described on the topic page for Era 2 communications.

RGHQ Radio Links

In this first era the bands used were 452.000 - 452.225 MHz paired with 466.000 - 466.225 MHz and some additional channels in the larger 451 - 453 MHz and 465 - 467 MHz Emergency Services band which was mainly used for Police personal radios. There were two other bands 455.875 - 456.000 MHz and 468.875 - 470.000 MHz exclusively used for RGHQ links.

Click to Show - The Detailed UHF Channel Plan

UHF Frequencies allocated for RGHQ UHF Network
LOWER CHANNEL PAIRS   UPPER CHANNELS   OTHER CHANNELS HEARD
 452.000 466.000    455.8875 469.8875    450.850 464.850
 452.025 466.025    455.9125 469.9125    451.150 465.150
 452.050 466.050    455.9375 469.9375
 452.075 466.075    455.9625 469.9625
 452.100 466.100    455.9875 469.9875
 452.125 466.125
 452.150 466.150
 452.175 466.175
 452.200 466.200
 452.225 466.225

Some of the Links to RGHQ 4.2

Map of RGHQ 4.2 UHF radio links

UHF 12 Element Yagi

UHF 12 Element Yagi

The shorter wavelength of these UHF bands lends itself to the use of high gain aerial with many elements. This photograph of a local council control centre shows a 12 element UHF yagi aerial, typical of the type used on RGHQ links to councils and other users.

Compared with a UKWMO Group HQ, there were a large number of circuits radiating out from a typical RGHQ, which dictated a different linking arrangement. If they operated a single channel per carrier like the UKWMO there would be problems due to the large number of individual transmitters, receivers and aerials required at the RGHQ as well as the hilltop sites. A technique known as multiplexing allows a single radio channel to carry seven circuits. One circuit is carried at normal speech frequencies and six are translated to higher frequencies, the resulting combined output is used to modulate the radio transmitter. The radio receiver output is fed into a demultiplexer to separate the individual speech circuits. A pair of multiplexer / demultiplexer equipment is needed at each end of the radio system.

In this link map, the circuits from individual sites to the Thurfield mast are single channel links but the last hop to the RGHQ is a multiplexed link. A more detailed description of analogue speech multiplex process may be found on the further reading link at the foot of this page.

The Master Post to Group Headquarters Radio Scheme

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. 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

Bedford ROC Post Radios 1980

In the era, 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 Communications / Posts Era 1 accessed from the tabs at the top of this page.

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.

Group Coverage Radio Schemes

Post Radios Schemes

Click here to see full details and frequencies

During the time that I listened to exercises, 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
GroupGroup TX
MHz
Post TX
MHz
Bedford147.000155.000
BelfastNo Era 1 Post Radio
Coventry155.025147.025
Shrewsbury147.050155.050
Wrexham155.200147.200

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 are the Transmit and receive frequencies from the hilltop to and from the Group's Master Posts.

While it has been possible to find the "Era 2" low band frequencies for England, Wales and Northern Ireland 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 five of twenty five group frequencies.