Home Defence Speech and Telegraph Network mid-1980s to 1992
This topic describes the changes to the original Home Defence communications network that took place during the mid-eighties. By this time the effects of the Electro-Magnetic Pulse induced by atomic weapons were understood and reflected into the design.
The Final Speech and Telegraph Networks
This map shows the two core networks, the Government Control Network (GCN) for Regional Government Headquarters, in orange and the other for the United Kingdom Warning and Monitoring Organisation (UKWMO) in green. The two are linked between the RGHQ to its UKWMO Group Control, shown in blue. The GCN is also connected down to Local Authority Emergency Centres (LAEC) and regional Armed Forces Headquarters (AFHQ). In the telegraph subsection further down this page, the extent of the network at County level will become apparent.
In this 2nd Generation network the main transmission links for both networks were carried on BT Landlines as they had been in the 1st generation. The main landlines had planned radio backup provided by the Home Office Directorate of Telecommunications ( D-o-T ). Both the GCN and the UKWMO network were to use the same radio bearer network known as Radio Network One (RN1) for the backbone and Radio Network Two (RN2) for the spurs to operational buildings. The 1st Generation radio networks, VHF for the UKWMO and UHF for the GCN were recovered during the mid-eighties. It is unclear whether the replacement 2nd generation network on RN1 & RN2 was ever completed before stand-down in 1992. It may be that it partly worked in some areas and not others, even by 1990 there was a large hole in RN1, without radio equipment or aerials installed, around the North West of England .
2nd Generation Telephony - The E.C.N.
The Government Emergency Communications Network (ECN) was introduced during the eighties. An extensive automatically switched network replaced the previous Government Control Network (GCN). A fragmented structure which often relied on the public telephone network to connect between the fragments. Now for the first time Police, Fire, Councils, Royal Observer Corps, Regional Government Headquarters, Armed Forces and Government departments were all connected to the same emergency network. It was managed by British Telecom for the Home Office Telecommunications Branch and operated separately from the normal public telephone communications systems. This provided a trunk dial facility for ECN telephone to call any other ECN telephone in any bunker across the whole of the UK.
Installation of the SX2000 switches for the UKWMO part of the ECN started in July 1988 and was completed by March 1989. After the UKWMO stand down in 1992 their part of the network closed leaving just the RGHQ part of the network until it too closed circa 1994. A restructured ECN continued to be used long after the closure of the RGHQs, see the next topic on this page.
The photograph shows a typical ECN exchange switch cabinet on display at the Hack Green Museum, they have placed a guard fence around the unit and removed a door to show the inner cabinets. Two SX2000 exchange cabinets are installed within a strong sealed box hardened against electromagnetic impulse. The switching network is duplicated to improve fault resilience. The lines into and out of the exchange pass through Electro-Magnetic Pulse (EMP) filters, visible on the outside of the cabinet facing the camera.
A TSX50 unit was used when fewer extensions were required, such as at District Council controls. It has a capacity for up to 80 extensions, 24 exchange lines, 8 private circuits and has its own control console. The telephone instruments could be conventional desk or wall telephone (LD or MF4) as well as the TX14 featurephone.
The switch would be installed in the Telecoms apparatus room in the bunker. In the office accommodation there would be an operators console to control the extension phones. If the console was not fitted the extensions could operate in 'Night Service' mode. Any extension on the ECN could directly dial any other in the UK.
Details of the TSX50 as used in the public network may be found on the website www.telephonesuk.co.uk/tsx50.htm. The SX2000 is still in production but has been enhanced since the model featured here was installed as part of the ECN. This seems to describe the ECN version www.britishtelephones.com/sx2000s.htm
ECN Numbering Plan
A uniform numbering plan was adopted across the whole of the UK, allowing any ECN extension to call any other on the system. This facilitated the following arrangement. RGHQs and UKWMO Group HQs had 3 digit extension numbers, all other extensions had 4 digit numbers. To call the switchboard operator in your own bunker, key 100.
To call any extension within your own RGHQ building, key the 3 digit extension number.
To call any extension within your own Group HQ building, key the 3 digit extension number.
To call any extension within your own Emergency Centre, key the 4 digit extension number.
To call a UKWMO Group, key the 3 digit group code followed by the 3 digit extension number. The group code is created by adding a 6 in front of group number; 608 = Coventry No.8, 612 = Bristol No.12
To call a RGHQ, key the 3 digit regional code followed by the 3 digit extension number. The Regional code is created by adding 5 in front of the RGHQ number; 531 = Skendleby 3.1, 592 = Drakelow 9.2
To call any County Council or District Council Emergency centre, key the 3 digit regional code followed by the 4 digit extension number. This Regional code is the same one for the RGHQ it comes under.
County Councils are allocated 100 four digit ECN extension numbers per Emergency Centre. The first digit being a 7, then within any one RGHQ zone, the second digit (x), starting from 0, is obtained by listing the counties alphabetically and assigning firstly the main and then the standby emergency centre. The switchboard would always be 7x00. To make it easy to contact generic functions in any council some numbers were predefined, so for example the County Controller would be 7x01. Scientific Advisers 7x04.
District Councils are allocated 25 four digit ECN extension numbers per Emergency Centre. The first digit being a 4, then within any one RGHQ zone, the remaining three digits (xyz), starting from 000, are incremented by 25 for each centre in an alphabetical list by County and District. e.g. 4000 - 24;4025 - 49;4050 - 74;4075 - 99;4100 - 24; etc. The lowest number at each emergency centre is the incoming number for the switchboard. Again a series of extension numbers were reserved for generic functions, but with a 25 number block it became a little more complicated, Scientific Advisers would be one of 4x04, 4x29, 4x54, 4x79 according to the first number in the block of twenty five.
The paragraphs above describe the numbering plan, the actual number of extension telephones fitted per emergency centre would be dictated by local circumstances and most likely well short of the ultimate capacity of the telephone switch.
Home Defence Region 3, ECN Number Ranges
RGHQ 31 Skendleby 531 100-299
Derbyshire Main Emergency Centre 531 7000-99
Derbyshire Standy Emergency Centre 531 7100-99
Lincolnshire Main Emergency Centre 531 7200-99
Lincolnshire Standy Emergency Centre 531 7300-99
Nottinghamshire Main Emergency Centre 531 7400-99
Nottinghamshire Standy Emergency Centre 531 7500-99
Amber Valley District Council 531 4000-24
Bolsover District Council 531 4025-49
Chesterfield District Council 531 4050-74
Dales District Council 531 4075-99
Derby City Council 531 4100-24
Erewash District Council 531 4125-49
High Peak District 531 4150-74
North East Derbyshire District Council 531 4175-99
South Derbyshire 531 4200-24
Boston District Council 531 4225-49
East Lindsey District Council 531 4250-74
Lincoln City District Council 531 4275-99
North Kesteven District Council 531 4300-24
South Holland District Council 531 4325-49
South Kesteven District Council 531 4350-74
West Lindsay District Council 531 4375-99
Ashfield District Council 531 4400-24
Bassetlaw District Council 531 4425-49
Broxtowe District Council 531 4450-74
Gedling District Council 531 4475-99
Mansfield District Council 531 4500-24
Newark + Sherwood District Council 531 4525-49
Nottingham City Council 531 4550-74
Rushcliffe District Council 531 4575-99
RGHQ 32 Loughborough 532 100-299
Leicestershire Main Emergency Centre 532 7000-99
Leicestershire Standy Emergency Centre 532 7100-99
Northamptonshire Main Emergency Centre 532 7200-99
Northamptonshire Standy Emergency Centre 532 7300-99
Blaby District Council 532 4000-24
Charnwood District Council 532 4025-49
Harborough District Council 532 4050-74
Hinkley + Bosworth District Council 532 4075-99
Leicester City 532 4100-24
Melton District Council 532 4125-49
North West Leicestershire DC 532 4150-74
Oadby & Wigston District Council 532 4175-99
Rutland District Council 532 4200-24
Corby District Council 532 4225-49
Daventry District Council 532 4250-74
East Northamptonshire District Council 532 4275-99
Kettering District Council 532 4300-24
Northampton District Council 532 4325-49
South Northamptonshire District Council 532 4350-74
Wellingborough District Council 532 4375-99
Post Cold War use of the ECN
At the end of the Cold War, the network continued to exist for another fifteen years or so. In 1999 it was made fully ready should the millennium bug hit, disrupting the public telephone network. A government letter dated 10 August 2006, advises of the decommissioning during the following 12-15 months. This would suggest final closure around the end of 2008. A Cabinet Office Freedom of Information request FOI320131 quotes an earlier decommissioning date of 2005.
After the 1992 closure of RGHQs the network was rearranged, moving top tier switches to new locations around the UK and linking them together. The new switch locations with their former RGHQ zone numbers in square brackets were: Civil Defence College at Easingwold . Six police training colleges at Durham ; Ryton in Dunsmore [31, 32, 91, 92]; Ashford ; Bramshill, Basingstoke ; Cwmbran [81, 82]; Bruche, Warrington [101, 102]; Police HQ at Halesworth [41, 42] Home Office, Queen Anne's Gate , Yeovil [71, 72]. SubBrit states the Yeovil switch node on the map above, was located in the former ROC Group HQ which had closed 31.12.1973 as part of the ROC rationalisation plan.
Top tier nodes acted as tandems as only adjacent nodes were directly connected, shown by the black lines in this diagram representing 2 Megabit / 30 speech channel links. The numbering scheme for the counties remained unchanged from the cold war period.
For the first time, a bridge was introduced giving access to the ECN from the public fixed and mobile network. A national number allocated to each top tier switch could be dialled, when ringing tone changed to dialling tone, a pass code was entered, if this was correct, dial tone returned and the required full ECN number was keyed in. There was also a bridge from most top tier nodes into the Army Telephone Network (ATN).
2nd Generation MSX Telegraph Switch
Modern store and forward automatic switches similar to those in the civilian world replaced the old torn tape telegraph centres. All messages have a header as well as the text of the message, the header contains a routing code or addressing information that the switch recognises.
After an initial trial in Dundee UKWMO Group Headquarters the rollout of the 'Case MSX' message switch started at Maidstone and was completed with Belfast in June 1985. I have no detailed information about the RGHQ switch rollout. The MSX switch worked with Visual Display Units ( VDU ) and modern electronic teleprinters. These were much quicker and easier to use than the old mechanical teleprinters, paper tape perforators and torn tape centres used in the previous era.
The data switch is programmed with a routing table that determines for any particular routing code which of it's outgoing ports to send the message. There are three types of ports, those connected to other message switches in the network via landline or radio link through a Line Interface, ports to a Visual Display Unit (VDU) and ports to local printers.
As each node is connected to a limited number of nearby nodes, it is often necessary to route messages via a tandem node to get to the destination. If the message is destined for a recipient at a location without a direct link, the routing table contains the port number of a link going to a distant location, whose MSX can forward the message. It may be necessary to pass the message through a number of tandem MSX before it reaches the destination. In each switch, messages are stored in a queue and sent when the outgoing port becomes free. The message header will contain a Priority Indicator, showing the importance of the message, this governs where it is placed within the queue.
If the data link's land line or radio circuit connected to a port becomes faulty or is destroyed by enemy action, the system operator can divert all traffic from that port onto another working port, the messages will find a different path to their destination.
Outgoing messages were prepared using a VDU and sent to the destination, a local copy could be printed on the journal printer. The VDU operator can also perform management commands on the switching system, such as redirecting failed messages, adding and deleting routing codes. Incoming messages for recipients in the bunker are printed onto paper, to be delivered by hand.
Each function within the RGHQ & UKWMO telegraph network is allocated a short address code used in the message header to indicate the desired recipient(s). The message header may contain more than one address or one routing code can be used to broadcast a message to a number of destinations. This was clever stuff for 1980 and another 20 years would pass before EMAIL became commonplace - its what we know as an address list. One particular printer or VDU may be the destination for many routing codes. In EMAIL terms - having many email addresses drop into one Inbox.
The table shows a few examples of the Telegraph Network destination routing codes. A list of MSX codes are itemised in the Royal Observer Corp, Standard Operating Procedures, Annex AM. 'Identification Codes (Address of Units) including Multi-Address Codes'. The table gives examples of the format of codes for various kinds of users.
Breakdown of Code Types
RGHQ destination codes in England and Wales were three numbers made by padding the RGHQ number with zeros. Scotland and Northern Ireland Zones had a longer code. As well as the civilian part of the RGHQ, the Military liaison comcen had a separate code in the NATO range RBDxxx
Local Authority Emergency Controls have an identity made up of a three letter County code e.g. Gwent 'GWE' suffixed with 'MN' for the County Main Control and 'SB' for the County Standby Control, addressed as 'GWEMN' and GWESB' respectively. In counties with District controls, their address used the county code with a two letter suffix in the form of an abbreviation of the District Council name. Districts in Gwent, Monmouth 'GWEMO'; Newport 'GWENO'. County Police Headquarters routing codes followed a similar convention with a 'PO' suffix following the county code, 'GWEPO' for Gwent Police. County Fire Brigade Headquarters used a 'FB' suffix, 'GWEFB' for Gwent Fire.
All the UKWMO Group Controls have a suffix of 'WM' added to their Royal Observer Corp location three character code, e.g. 'BEDWM' for Bedford. The Group's admin address is 'ROC' and the Group number e.g. 'ROC07' for Bedford. The five UKWMO Sector Headquarters a three letter abbreviation of the Sector name, the METropolitan Sector at Horsham, along with CAL MID SOU WES to form a code like 'METWM'.
In addition to direct point to point codes, special broadcast codes were allocated to allow a single message to be sent at once to all recipients in a sector. These use 'MA' then a 2 character abbreviation for the sector name CL MT MD SU WS and a single digit. For example, addressing a message to MAMT2 would deliver it to all Groups, NRC and Local Authority Emergency Centres in the Metropolitan sector area.
Examples of Broadcast Codes for Non-Routine Messages
Sector Control to all Groups in their sector
Sector Control to all Groups, NRCs and Customers in their sector
Sector Control to all other Sector controls
From CAL (Caledonian) to MET MID SOU WES
From MET (Metropolitan) to CAL MID SOU WES
From MID (Midland) to CAL MET SOU WES
From SOU (Southern) to CAL MET MID WES
From WES (Western) to CAL MET MID SOU
UKWMO Routine Messages
During the attack and post-attack period, Group and Sector controls would originate many routine messages in a fixed format. Some message formats being copies of ROC's paper forms containing vital information. To facilitate sending these routine messages, broadcast codes in the format xxYYY were created, where xx was the form code and YYY the originating group or sector. For example BB (bomb burst) messages originate from Groups while TT (fallout threat) messages originate from Sectors. Two examples are shown in the Broadcast Code table.
Using an originator code as the destination address may seem strange, but it facilitates a way of directing messages only to those authorities that need to receive them. For example, fallout warning from groups are broadcast throughout the network but may only need to be printed at County emergency centres whose territory is part of that group. Leicestershire Emergency Centre, would need to receive messages from COV, BED and LIN, to cover the whole of the county.
Broadcast Codes for Routine Message Formats
Broadcast BB (Bomb Burst) Messages : One per Group, BBxxx
Broadcast TT (Fallout Threat) Messages, one per Sector TTyyy
From Southern Sector
From Western Sector
Military Liaison In RGHQ
The U.K. Armed Forces routing codes begin 'RBD' these are a sub-set of ACP121 Military routing codes used by all three services and our NATO allies, all those based in the UK are in the format RBDxxx.
The military were tasked in supporting the Regional Government and County Councils in the post-strike recovery phase by providing manpower and logistics. The MSX network extended beyond the civilian organisation to include the parts of the military who would provide that assistance.
Within each Home Defence Region, one Armed Forces Headquarters (AFHQ) served the Region and was linked into both the RGHQ and UKWMO telegraph networks and possessed its own routing code. In the case of Region 2 the code for its AFHQ is RBDAPM. The two RGHQs in each Region contained a room designated 'Military Liaison' containing military personnel and communication equipment. There were printers and telephones connected into the RGHQ network. Each Military Liaison office having its own separate routing code from that used by the Civilian part of the RGHQ. At Shipton the Civilian routing address is 021 and its military counterpart RBDPAP.
Armed Forces HQ
Armed Forces Headquarters ( AFHQ ) were also linked into the MSX network. The list below is a snapshot derived from the ROC Standard Operating Procedure, Annex AM. Anyone who has studied UK military history will appreciate it seems to be an endless set of reorganisations, every document I see has a different set of codes reflecting the snapshot in time when the document was written. The ECN Directory has a similar set of codes with a few variations. The locations are derived from a Declassified Joint Signalling Instruction, where the codes match. Some physical locations of codes in this list are currently unknown.
Additionally, the military had their own private automatic telegraph network 'Telegraph Automatic Switching System' ( TASS ) working in a similar way the to the now defunct UK Public TELEX network but with four digit numbers. Lines from this network terminated in the RGHQ military liaison offices.
Message exchanges operated by humans, such as the Torn Tape Relay centres are fairly tolerant of messages that don't conform to the specified formats. For example if the recipients address is invalid because the clerk has typed 'COVMW' instead of 'COVWM' they can easily make the correction and pass it to the correct destination.
Computerised message exchanges are unable to tolerate even the slightest mis-formatting. It was planned that county controls would compose their messages on a teleprinter and then send it off to the RGHQ message switch but it was found to create lots of work for the switch supervisors to correct the message format. Smaller message switches were rolled out to county and district controls so they could use a Visual Display Unit (VDU) to enforce the correct format.
RGHQ / Council Messages and ROC Non-Routine Messages
These free format messages adopted the protocol used by the UK Military and NATO, known as ACP126 protocol. Also known as the 16 line message format although not confined to 16 lines, as the message itself may take up a variable number of lines. Dependant on the message type certain of the 16 lines may not be required.
UKWMO Fixed Format Routine Messages
Many fixed format messages are broadcast around the network from the Warning and Monitoring part of the organisation. These are composed on VDU's in the UKWMO Group or Sector Headquarters and their format reflects the paper forms used to prepare the data, in general the text of the form is not sent but only the data. There are 19 formats used by Groups and a further 6 by Sectors The example here is a message informing customers of a nuclear bomb detonation in France, a Bomb Burst BX form is used.
2nd Generation County Telegraph Network
The 'Case MSX' telegraph switches were used in the RGHQ and UKWMO Control bunkers, but in Local Authority Emergency Controls (LAEC) a smaller capacity switch was needed, the 'Autex 1600 Telex Manager' with a maximum of 24 ports were used instead.
As the Autex 1600 has only a single processor, a backup 'Autex 100 Telex Manager' with just 5 ports and 128 Kilobyte of memory, allowed a very minimum service to be maintained in the event of a failure of the main Autex 1600 switch. Only two external links are provided to the backup Autex 100, one to the RGHQ and the other linking the Main LAEC to the Standby LAEC. One VDU and two printers are supported by the backup switch. Both the main and backup message switches are mounted in the same rack.
Standardised Autex 1600 Port Allocation
Incoming Message Printer A
Incoming Message Printer B
VDU B (spare)
Line data link to UKWMO Group Control
Line data link to RGHQ/ZONE
Radio data link to RGHQ/ZONE
Line data link between Main & Standby
Line data link to Police HQ
10 ... 22
Line data links to Districts as required by the Local Authority to a maximum of 13 District controls
Input from a portable Disk Drive
A standardised configuration of ports on the Autex 1600 switch was adopted as shown in the table, for the County Main and Standby LAEC. Flexibility in the use of ports 10 to 22 allowed for Counties with a number of district controls. Note the connection to the RGHQ or Scottish Zone HQ, has two data links, one by landline and the other over the radio circuit. The installation consisted of three Siemens PT85 printers and two Zentec ADM3E Visual Display Unit (VDU).
District council controls and county police headquarters were fitted with a 'Autex 100 Telex Manager' having only 4 ports and 64 Kilobytes of memory, like the one shown in the gallery. These small units connected into the national telegraph network via the switches in the county's Main and Standby LAEC. The unit has two Single Channel Voice Frequency Telegraph (SCVFT) modem cards for the external line connections to the Main and Standby LAEC, the rear panel houses D-Type connectors for a single MT80 printer and a dual purpose Zentec ADM3E. This VDU would be used to prepare outgoing messages as well as manage the switch's supervisory functions.
The AUTEX 100 switches used at District level and Police HQ's also had a standardised port allocation for their four ports to ease with network administration.
This drawing centred on Oxfordshire, shows the connections between the various controls and two adjacent Counties. The message routing codes are shown (in Green) for each node. The RGHQ at Basingstoke (RGHQ 6.2) also has links to the main and standby controls in Hampshire and the Isle of White, also to Winchester UKWMO Group Headquarters and AFHQ for Region 6, omitted for clarity.
Basingstoke was connected into the RGHQ core network with links to Hertford: 4.2, Kelvedon Hatch: 5.1, Crowborough: 6.1, Chilmark 7.1, Swynnerton: 9.1
The UKWMO Oxford Group Headquarters at the ROC No.3 Group at Cowley had links into the UKWMO core network with connections to Bedford, Bristol, Coventry, Horsham, Shrewsbury, Winchester and Yeovil.
Using this example of a small area it can be seen that the Civil Defence telegraph network in the UK was very extensive. Telegraph messages could be passed between any node throughout the countrywide network. Should links within the network be broken due to enemy action the messages could be rerouted via working sections.
2nd Generation Radio Networks RN1, RN2 and RN4
During the later part of the eighties the emergency services were required to change their main operating frequencies to comply with newly introduced World Administrative Radio Council ( WARC ) rules. This heralded the introduction of a new radio scheme for the UKWMO and RGHQ network. This network moved away from the older single circuit per carrier at VHF and UHF frequencies towards a Microwave backbone where a higher bandwidth enabled many circuits to be carried on the same radio carrier.
Radio Frequency Allocations
The WARC changes removed the Home Office frequency allocations for Emergency Services at 97.6-102.2MHz and these moved into the existing 146-148MHz and 154-156MHz frequency assignment, which was increased by the addition of 143.000-144.000MHz and 152.000-153.000MHz. The sub-bands 147.000-147.300MHz paired with 155.000-155.300MHz previously used for the ROC Network were withdrawn and reassigned for Emergency Service use. The exclusive Civil Defence band 168.325-168.800MHz paired with 174.025-174.500MHz was retained for use by RN4. At the same time the Emergency Services UHF allocations were expanded in size by adding the 450-451MHz to the 451-453MHz band and adding 464-465MHz to the 465-467MHz band. The UHF frequencies used for RGHQ links within the Emergency Service allocation 452.000-452.225MHz paired with 466.000-466.225MHz remained in use and the exclusive 455.875-456.000MHz paired with 469.875-470.000MHz continued to be used.
Two frequency assignments, 30 Megahertz apart and 2 Megahertz wide at 1668-1670 MHz and 1698-1700 MHz were allocated for RN1 and RN2 high capacity links as they needed a greater bandwidth than was available on the UHF channels.
RN1 and RN2 Structure
The new national radio network consisted of a highway designated RN1 and spurs to users bunkers designated as RN2. RN1 was a high capacity radio network where lots of individual circuits could be combined on to one radio system operating at microwave frequencies. RN2 was similar but with lower capacity links to RGHQ or UKWMO ROC Group HQ operating at either UHF or microwave frequencies. Smaller bunkers, such as Local Authority Emergency Centres had one or more RN2 single channel links. Both RN1 and RN2 high capacity links used Frequency Division Multiplex, a technique also used on landlines to cram a number of speech circuits onto one bearer. Speech Multiplex is described in a separate topic.
This diagram shows the extent of the RN1 spine network passing between hilltop radio sites and including six RGHQ bunkers and two UKWMO Group HQs too. All the other user locations were connected into the spine by RN2 links. The individual circuits are cross connected between multiplexes at the hilltop sites in order to achieve a point to point link spanning between bunkers. To allow circuits to take a diverse routing, some of the multiplexed links in RN1 were formed into a ring allowing some circuits to route in one direction around the ring and others in the opposite direction. Multiplexes varied in size, with 8, 12 and 24 channels, depending on the capacity needed in that section of the RN1 network.
The RN1 network diagram is only a small part of the story and doesn't show which circuits were carried on individual RN1 links. The RN2 network must have been quite extensive considering how many locations had to be connected, but I have very little specific information on the RN2 part of the network. Probably the majority of Home Office hilltop sites ( Green spots on the diagram) were used to convey RN2 links to bunkers. The Green spots are masts identified from various web based sources and HO Circular 155 of 1963.
It has been suggested that the RN1 spine never actually carried traffic. Apparently the interconnection of multiplex channels at hilltop sites was never completed. I would be interested in feedback to know whether this was a local problem or the national picture.
The RN2 extension from RN1 sites consisted of a either high capacity MUX links to the ROC UKWMO headquarters and any RGHQ that weren't RN1 sites in their own right. The majority of RN2 are single channel links backing up Local Authority Emergency Centres (LAEC) landlines to their RGHQ. In municipal counties and London, RN2 radio links also backed up the landlines between the county and borough emergency centres. The gallery shows part of the London area RN2. In the Midlands the links radiating out of Brown Clee.
RN4 Group to Master Post Radio
The network designated as RN4 comprised of the ROC Post radios and the radio at the group control centre operating in the 80 MHz band. Due to the large geographical area covered by a Group HQ the more distant posts may not be able to communicate directly with the HQ. The RN4 network also included hilltop sites linked at VHF to the headquarters to extend the coverage of the radio scheme.
The spine of the Radio Network RN1 was carried on microwave shrouded yagi aerials as they were thought to be more resilient to the effects of a nuclear blast pressure wave than the much larger dish aerials. Operating on frequencies in the 1 - 2GHz bands allocated to the Home Office, a 1.2 metre dish would have been necessary to achieve a similar aerial gain. Dishes of this size or larger would have been likely to suffer blast damage from a nuclear blast than the shrouded yagi.
Most RN2 access spurs from the RN1 spine to the control bunkers used either single channel per carrier or multiplexed UHF links, according to the quantity of circuits needed. UKWMO Group Controls and RGHQ, warranted a UHF multiplexed link due to the number of circuits terminating there.
Some higher capacity RN2 links, such as the one to Hack Green RGHQ from Old Pale hilltop used the same frequency bands as their RN1 counterparts.
Local Council bunkers were candidates for single channel per UHF carrier links. A UHF link whether it be for single channel or a multiplexed system would use a 12 element yagi similar to the one shown on the right fixed to my local council premises.
Radio Link Equipment
This photograph shows a Plessey PRD1100 with the Orange front panels, serving a single RN2 multiplexed link. The slightly taller racks on the right, house two Pye L700 microwave links for RN1, and their associated analogue speech multiplex cards. Looking at the RN1 network spine drawing further up this page, it can be seen that most sites like this one, had two L700 assemblies providing a link in each direction. In this photograph, many of the black L700 front panels have been removed for commissioning.
The Pye L700 functioning in the 1.6GHz band, delivers a baseband signal to a separate multiplex which can provide 8, 12, 24 or 36 speech channels. To fit the signal bandwidth into the 200 kHz channel spacing in the 1.6 GHz bands the peak deviation on RN1 was adjusted to 28.3, 49.5, 70.7 and 35.4 kHz respectively.
The Plessey PRD 1100 Multiplex for the RN2 network worked in the UHF bands and carried 8 or 12 multiplexed speech channels. The RN2 network used UHF frequencies of 452.000-452.275MHz paired with 466.000-466.275MHz and an exclusive band at 455.875-456.000MHz paired with 469.875-470.000 MHz. The first band pair was shared with emergency service communications having channels spaced 25kHz apart. To fit in with this spacing, the PRD1100 capability for 12 channels was reduced to 8 channels, and still required two channels bandwidth when operating with 14.2kHz peak deviation.
When a single RN2 radio channel is required, as a spur to the RN1 network, or as a link to another bunker. One or more Pye F496 wall mounted radios were used. The F496 is normally mains powered but has the ability to have a standby battery supply too. These radios are most likely, but not exclusively, to be found at LAEC.
RN3 Onsite Security
The onsite security at Belfast UKWMO Group Headquarters, consisted of a pair of Pye PF9 Pocketfones working into a Storno CQM 5662 base station, running 5 watts into a Jaybeam UHF folded dipole. These worked on Home Office Channel 13 (451.475 MHz Tx / Rx) allowing all three units to communicate with one another.
As well as Belfast No.31 Group, RN3 was installed at Maidstone No.1 Group, Shrewsbury No.16 Group, Preston No.21 Sector, Carlisle No.22 Group. Many others had it fitted too, although it is confirmed that Coventry No.8 didn't have RN3.
It would be interesting to know if similar arrangements were used at Regional Government Headquarters too ?
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