Home Office : Last Ditch Network 'LDN'

From the Archives

This topic is based on information held by the National Archives at Kew, Civil Defence Planning in the late sixties, kindly supplied by Dave of 'radiohistory.uk'. Hitherto the LDN had been the source of much speculation.

The minutes of a meeting on 18th April 1966, for the purchase of H/F transportable Wireless Sets to be used by Civil Defence sets the scene. Note: In the sixties, RSG's and Sub-Regional controls were the names given to Regional Government Headquarters (RGHQs) this final name is used throughout this website.

The meeting was called to consider the specification of the transportable H/F wireless sets recommended by the 1965 Working party on Communications for the Control System, and prepare a purchasing programme for the 1966/67 financial year.

The essential function of the equipment is to give skeleton and flexible communications in the immediate pre and post attack phases between R.S.G. Groups, Sub-regional Controls and selected local authorities. To do this it must work independent of relay stations, be transportable from base to base, be capable of use independent of mains power and give links between groups and Sub-Regions located at distances apart varying from 10 to 125 miles. An important point is that the Group centres will not be capable of receiving equipment in peacetime. It must, therefore, be easy to install and operate without special skills.

Mr. Morley informed the meeting . . . . Typical 100 watt portable equipment was in commercial use for communications over considerable ranges in peacetime (when ionosphere conditions were normal) but in the attack phase when only ground wave propagation might be possible we could rely on no more than 20 to 30 miles effective range . . .

Home Office Communications Branch, produced Outline Specification No. 58 (April 1966) for the tendering process. Five manufacturers offered their products. G.E.C. RC420TR; Marconi H4000; Pye SSB125T; Racal TRA355; Redifon GR410

These tenders were considered at a meeting on 5th August 1966, where is was decided to purchase the Pye SSB125T although they were unable to decide whether to purchase the battery or mains version. The minutes of this meeting refer to the network as 'Last Ditch'

Pye SSB125T Transceiver — Pye SSB 125 Transceiver used for the LDN

. . . The equipment is quite commonly produced to give switch tuning between 4 working frequencies and it has been accepted that this would be adequate to meet the prime need for a cheap, simple, reliable, and readily obtainable "last ditch" means of communication.

An addendum dated 12th August 1966 was added to the minutes by R. L. Jones. This gives an insight into the availability of standby mains power at that time.

3. As regards powering of the sets, I am now reasonably satisfied that we need at least 100 battery operated sets whatever the relative cost may be, because we could not count during the next calendar year on having more than 13 S.R.C.s, or than about 30 of the selected local authority controls equipped with emergency standby power, and the number with standby power will grow fairly slowly. . . .

At a subsequent meeting on 15th August, it was decided to purchase 100 battery sets and 50 mains operated ones to be used where standby power already existed. The minutes of this meeting also give an insight into the frequencies used by the LDN. Note: m/cycles or Megacycles has been superseded by Megahertz ( MHz )

   It is expected that 30 frequencies will be obtained, 10 between 2.5 and 3.5 m/cycles and 20 between 3.0 and 5 m/cycles, 10 of the frequencies between 3.0 and 5 m/cycles would be useable for peacetime training, the others would become available after declaration of a war emergency. A final decision will be given during the course of the week.

   Mr. Morley said that the frequencies could be used at least twice up and down the country so giving not less than six frequencies per Region (compared with the 4 frequencies previously stated to be the acceptable minimum).

   In discussion about the desirability of keeping in touch with the Army. H/F transmissions, Mr. Morley pointed out that the Army D.11 sets would work over long distances and would use different frequencies at different times of day according to the conditions. In civil defence however, it would not be possible to allow more than one channel for the Army contact. A solution, later, might be to listen in by means of Army equipment.

The L.D.N. Field Equipment

The radio set supplied by the Home Office is a Pye SSB125T, a single sideband transceiver intended for marine operation but used here for point to multi-point communication. The transceiver has a choice of powering options. The A.C. Power unit operates from either 100-150 or 190-240 Volts 50-60 Hz for use with mains electricity or standby generators. A 12 or 24 Volt D.C. Power unit for operation from vehicle or ship batteries or rechargeable batteries. The receiver is transistorised with a valve transmitter section capable of an output power of 125 Watts of S.S.B. on the HF Bands.

Channel Frequencies

Documents suggest during the transition to war, that frequencies used by the National Air Traffic Service ( NATS ) would be released to the Home Office. Ten 6 kHz channels in the band 3400 - 3500 kHz and ten more 6 kHz channels in the band 4650 - 4700 kHz. These are shown in the U.K.F.A.T. as civilian aircraft bands.

A number of ex-UKWMO SSB125 receivers have made their way into Air Cadet training centres. These could be crystalled for the ATC channels by now, but if the set's original frequencies are known, please get in touch.

L.D.N. Locations

Regional Government Headquarters and Local Authority emergency centres were intended to have L.D.N. radios, we don't know whether these were actually fitted, or stored safely in H.O. Regional Depots for issuing when necessary. A nineteen seventies photograph of Drakelow RGHQ9.2, Kidderminster lattice aerial mast has a long wire attached. This may have been for LDN.

Although not mentioned in the archived documents, the UKWMO Sector Controls did have LDN radios, but website visitor's feedback, suggests UKWMO R.O.C. Group Controls were not issued with LDN radios as Adrian confirms that Belfast, Carlisle or Shrewsbury didn't have them. The visitor's guide at RAF Neatishead Radar Museum, mentions the LDN aerial for the Nuclear Reporting Cell, which was staffed by ROC personnel.

RN5 - A Replacement HF Radio Scheme

The National Archives File HO332/1170 opened in December 2017, kindly furnished by Dave McKay of 'pmrconversions.info', reveals plans to replace the LDN with an improved system. However the file ends before the decision has been taken on the specification for the replacement radio. It seems unlikely that RN5 was ever rolled out, but we need to wait for further files to be opened for public scrutiny to confirm this.

The Home Office Director of Telecommunications, Mr P L T Owen held a series of quarterly review meetings recorded in this file. Extract from the Director's Review of Civil Defense Telecommunications Projects, first meeting held on 28 January 1983 in Rm 115 Horseferry House.

4. On the subject of RN5 - HF Radio Circuits - Mr Eaton stated that because of reorganisations within the Royal Observer Corps and Local Authorities, the distance between group controls have increased since the original plans were formulated. As a result the present equipment does not provide adequate cover. He proposed that a study should be carried out by a specialist organisation to identify the system parameters and the type of equipment suitable for use by non-communications personnel. The Director indicated that he would welcome such a study and would be interested in the ensuing report.

It was noted that a suitable frequency for use in peacetime was required.

A frequency for peacetime exercises was something that had been lacking with LDN radios. We get a clearer idea of what was being proposed from the Director's Fourth Review Meeting on 19 March 1985.

. . . . The Director asked Dr Luetchford if he could confirm the operational requirement had been agreed.

Mr Stanton confirmed this and said that F6 requirement was for speech transmission. Mr Lawrie said that UKWMO would like to see data transmission available.

Mr Eaton stated that the study was covering both requirements. . . . .

In an Annex to the meeting notes from the Director's Fifth Progress Meeting on 19 September 1985

RN5. The study contract has been placed with MEL. They commenced work in August and are having discussions with F6 and UKWMO to ascertain the operational requirements. The study should be finished by December with the demonstration of feasibility taking place in January or February. [1986]

In the notes from the last meeting in this file, The Director's Sixth Review Meeting, on 24 March 1986.

6.1 Mr Eaton said that the aerial system designed by the contractors, MEL, had been deemed unsuitable at an in house meeting. MEL had agreed to reexamine the requirements and hence the specification. It was hoped they would report back within a matter of a few weeks, bearing in mind the constraints on them that obtained.

Home Defence Radio Communications Network HDRCN

Lacking any further information in the National Archives about RN5 beyond that in the previous section, it is believed the HDRCN is the official name given to RN5. Any information on this subject will be gratefully received via this websites home page.

The Home Defence Radio Communications Network (HDRCN) survivable radio kit provides single sideband communications on preset frequencies within the frequency range 2 to 8 MHz, with a maximum power output of 100W PEP. Two versions of this kit are in use, either the Racal Messenger VRM4555M or the Decca Messenger DTR 2002M-3 with their respective power supply units.

The aerials are specified as: A single frequency helical whip antenna, precut to the HDRCN common frequency, complete with fixings, earth radials and 30 metres of coaxial cable. Also a separate wire dipole kit, adjustable 2 to 8 MHz and 24ft fibreglass mast.

Radio standing on top of power unit — Decca DTR 2002

This radio contained crystals for three channels, 2480 kHz, 3156 kHz and 4605 kHz which may be the training frequencies. The previous sections highlight one of the problems with the old 'Last Ditch Network' was a lack of suitable peacetime training frequencies.

Post Nuclear Strike, Last Ditch Network 'LDN'