MOULD Home Defence Radio

MOULD first came to my notice in January 1987 when an article in the RSGB Radcom warned radio amateurs that links between MOULD hilltops were using the 70 cm band that we shared on a secondary basis with the Ministry of Defence (MOD). The article explained that MOULD would provide communications between Regular Army, TA Battalions and the Army District and Regional Headquarters. Phase 1 cost £7m to install and used 18-channel Pye Pegasus radios operating at 66-88MHz. A network of hilltop sites were linked together to give area coverage. These bearer links used 140-150MHz and 420-450MHz in a formation giving a maximum of 7 radio hops between users.

Having been alerted to the presence of the system, I was able to find the frequencies being used locally for the area coverage and the inter site links. I never had the inclination to investigate military communications and back then did not appreciate MOULD's connection with home defence. Only after the scheme closed down has this become apparent, in June 2009 I obtained a copy of a technical article published in the mid-eighties that has filled in the gaps in my knowledge and is the basis for this page. Further information was forthcoming in 2010 and 2011 allowing more specific details to be added.

MOULD closed during the two thousands and the users have been transferred to AirWave the common system for UK Emergency Services' communications used by the Police, Fire and Ambulance authorities. The MOULD equipment was being recovered from hilltop sites during 2008.

Phase 1 of the UK Army MOULD radio system was introduced in 1976 - 77 in England, Scotland and Wales, but not Northern Ireland, as a communications system was already in place there. It was designed to give area coverage in each home defence region, these align with the civilian Regional Government Headquarters boundaries. In the mid-eighties, Phase 2 provided additional schemes giving area coverage in each county.

In order to give area coverage a number of hilltop sites with rebroadcasters working in the VHF low band, were connected together with radio links, operating in VHF high band or UHF. In this drawing of the network in Home Defence Region 7, there are two sub networks. In the Sub-Region 7.1 under control of the RGHQ at Chilmark, there are nine hilltop sites, and nine in Sub-Region 7.2 controlled by Bolt Head RGHQ. Three sites Middleton Court, Cranmore Tower and Newton Barrow are common to both networks and have rebroadcasters on both networks. The schemes were carefully designed so there are no more than five links in between any two sites. In Sub Net 7.2 for example, the extremities at Four Lanes or Collaton Cross, are only five links away from Newton Barrow.

Countrywide, Phase 1 employed 120 hilltop sites. This increased to 150 when Phase 2 was implemented. Most of the English and Welsh hilltop sites chosen for MOULD were those in use by the emergency services and UKWMO / GCN. Some MOD, Civil Aviation and Broadcasting sites were used too. All the hilltop radio equipment was maintained by the No.2 Signals Brigade.

In Phase 1, the MOULD mobile and fixed sets were 18 Channel crystal controlled Pye M252 Pegasus transceivers, outputting 5 to 15 Watts in the 79MHz band and receiving in the 74MHz band. In Phase 2, Pye FM914, 25 Watt synthesized 250 channel transceivers were used.

The hilltop radio equipment was all mains operated. The rebroadcaster equipment was Pye 400 Series, 60 Watt transmitter with a separate receiver, operating in the VHF low band. VHF high band links, using a Pye T404, 25 Watt transmitter and separate R404 receiver. UHF links using a Pye T414, 5 Watt transmitter and separate R414 receiver.

The hilltop rebroadcasters and mobiles, use frequencies in military bands, shared with other fixed and mobile military services. I don't know the exact upper and lower limits of the bands used for the VHF and UHF links, but the approximate band edges are given below.

For example, in my notes recorded during an exercise in October 1988, 74.050MHz, 74.125MHz, 74.200MHz, 74.525MHz, hilltop transmit frequencies were all carrying the same traffic. I now find out in 2011 these were four of the five transmitters on the Region 9 Principal Net., which covers the area where I live.

The design of the MOULD network is very different from the wide area schemes used by the emergency services. Emergency services have a number of hilltop locations all transmitting and receiving on the same frequency and linked back to a single control point. A full description is given in the 'Emergency Services Radio / Pre 1987' in the website menu. In contrast each of the MOULD hilltop sites use different pairs of transmit (TX) and receive (RX) frequencies to give area coverage. The MOULD network did not have a central control point so adjacent sites were linked together in an open network. A limit of five serial links was imposed to ensure reliable communications.

On any particular network, each hilltop was permitted to have a maximum of five links to other sites. Cranmore Tower, one of the sites shown on the Region 7 map has five links in sub-network 7.1. There are also two links forming part of network 7.2, but this has a separate set of equipment. In Phase 2, and therefore not shown in the drawing above, Cranmore Tower got a third set of equipment working on the Somerset County network.

Referring to the diagram above this text, each mobile would select the channel used at the nearest hilltop. For mobiles M1 and M2 this would be Channel 5. If M1 called M2, its signal would be received at the hilltop and be rebroadcast so M2 could hear it. As well as simply rebroadcasting the signal, it would be sent on the link to the next hilltop and broadcast on Channel 4, this site would relay the signal on the link to the next site. Here it would be broadcast on Channel 2, so mobile M4 could hear the signal. This site is linked to two others, where it would be broadcast on Channels 1 and 3. Mobile M3 would also hear the signal on Channel 1. If mobile M3 called mobile M2, it would transmit on Channel 1, its message would be similarly broadcast by all sites on the network, so the other three mobiles including M2 would hear the message as its receiving Channel 5. When M2 replied on channel 5, all the other mobiles including M3 to whom the reply is directed would hear the message.

The hilltop radio equipment was controlled by a 'Control Switching and Signalling Unit' (CSSU). The CSSU routed the signal from the receivers to the appropriate transmitter. The logic diagram shows that any signal received on the low band area channel was rebroadcast by the area channel transmitter and all the link transmitters. A signal received on a particular link would be broadcast by the area channel transmitter and sent out on all the other links except the originating link.

Where two or more networks shared the same hilltop, the CSSU may be configured to link together two networks for operational reasons, usually a 'Principle' Regional Net. and a County Net.

Whereas the emergency services radio schemes had all their transmitters and receivers duplicated, this was not the norm for MOULD, except at the more remote hilltop sites where access could be difficult in adverse weather conditions. To guard against the loss of a hilltop site, due to wartime action or a fault, mobile units known as 'Insertion Vehicles' were kept on standby, so they could be driven to the site to replace it, while repairs were undertaken.

The 'Insertion Vehicle' is a hardtop Landrover, with a supporting stores vehicle, and a generator. It contains the same equipment used in the hilltop installation. The radios are channelled to allow them to replace any of the MOULD hilltops within the Region it supports.

Both Vehicle Photographs Courtesy of Martin Swift

In Wales there were two Principal Networks, Region 81 in the North and Region 82 in the South, corresponding with Brackla RGHQ 82 and a nonexistent RGHQ 81. There were eight County Networks in Wales, in Region 81, Clwyd and Gwynedd; in Region 82, Dyfed, Mid, South and West Glamorgan, Gwent and Powys.

The County Networks consisted of one or more hilltop sites, Mid Glamorgan had one rebroadcaster whereas Powys had a network of four. If desired, county networks could be coupled in with the regional Principal Network. Abergavenny was the coupling point for both Gwent and Powys, into Principal for Region 82. Dyfed could be coupled at Mynydd Sylen into Region 82 Principal net. Network control including coupling was performed remotely by the use of SELCALL signals. The hill top site at Old Pale is located in Region 10, but also serves Region 81, with rebroadcasters on networks for Region 10 Principal, Cheshire and Merseyside; Region 81 Principle and Clywd. Similarly Brown Clee is located in the West Midlands Region 9, but also serves Region 81 Principle Net.

The Pye Pegasus M252 wireless sets used in Phase 1 of MOULD were able to select one of eighteen channels, restricting their use to one defence region. Some regions required two patterns of channel variants. In Region 2, Set No.1 could operate through all nine sites in the 'Principal Net', four on the north and five on the south. A choice of two sites for 'North Yorkshire County Net' but only a single site for the remaining 'County Nets'. Channel 17 is allocated to the Mould Insertion Vehicle (MIV) should it need to be deployed. The Region 2, No.2 type set had a different variation of radio channels, with the emphasis on 'County Nets' as can be seen from the tables below. Some defence regions were divided into two sub-regions aligned with the civilian sub-regional government areas of the RGHQ, an example being Region 8, Wales, divided into subregions 81 & 82.

The limitation of only 18 channels meant army home defence forces could not be deployed outside their own region. This shortcoming was addressed in Phase 2 by the introduction of sets with 250 channels.

A 'Net' or network consists of one or more hilltop sites all broadcasting the same messages. The map shows the radio links between hilltop sites used to create these networks. Links between hilltop sites were either single channel at VHF or UHF frequencies or two channels per link frequency using a 1 + 1 carrier system with a 'Pye RTU' at each end of the link. VHF links being used across longer hops or difficult terrain where UHF couldn't provide a reliable path. These links are shown on each Regional map according to the Key here. In some places, two separate radio links operating on different frequencies connected hilltops together, these appear as two parallel lines on the map.

A detailed explanation of 1 + 1 speech multiplexing used by MOULD links is provided in a separate chapter on this website. Found in the Miscellaneous Topics part of the Chapter Menu.

The last two columns give the frequencies that were in use by MOULD rebroadcasters, TX frequency is transmitted from the hilltop and RX is transmitted by the mobile (or fixed mobile stations).

Regional channels were allocated for direct vehicle to vehicle operation when they were in range of each other. The set's transmitter and receiver both operate on the same frequency.

The Pye CAT 80 Low Band 80 MHz aerial shown to the right, were used for MOULD as well as commercial Public Mobile Radio (PMR). They were made by Telecommunications Limited (TCL) of Dublin Ireland, which eventually became Sigma prior to its closure, following a buyout of the antenna business of Sigma by American PCTel. TCL / Sigma was a feeder factory for Pye Telecom.

Regional Government Headquarters (RGHQ) had access to MOULD. Local Authority Emergency Control (LAEC) centres had MOULD aerials fitted, either the CAT80 shown here on Dudley Borough LAEC or a folded dipole. The LAEC also had access to the council's own radio scheme, generally used countywide by their highways departments. Post cold war in Cambridgeshire, they tried and tested an interlink between the council's radio scheme and MOULD, allowing direct communication between the two systems. This was demonstrated at a civil protection conference at Scarborough in June 1995.

In the nineties a program of fitting MOULD aerials to civilian buildings was undertaken. Aerials were fitted to larger telephone exchanges and wired down to the protected accommodation in their basement, but no radio equipment was connected. As CAT 80 aerials have been spotted on other public buildings as well as Police and Fire stations, these may have been wired for MOULD too. I can only speculate that this would allow military liaison at those places in a civil emergency.

During the eighties a number of local authorities, predominantly Labour Party controlled, declared themselves as 'Nuclear Free Zones'. The Army had some of their MOULD vehicles painted in brighter colours and bore civilian number plates for use in those sensitive local authority areas.