STATION CONSTRUCTION SECTION.
In October 1942 the only SOE Station in the United Kingdom had its receiver site located at Grendon-Underwood and its transmitter site at Charndon. The Signal Office was in a down-stairs room in the house but a new building was under construction capable of accommodating equipment for increased traffic demands. The transmitter building whose dimensions were 20' by 12', housed eighteen 250 Watt transmitters. One 20-pair cable was installed between the receiver and transmitter site for remote control. Stand-by power supply was available from a 9 kVA P/E generator at both sites.
The problem which presented itself at this time was due to ever increasing traffic. Originally the station had been designed to cater for a smaller number of channels and at that time no doubt the space available was sufficient. Now, however, the transmitter building was very overcrowded and considerable loss of efficiency and flexibility were most apparent. The original aerial installation had catered for 12 cage-dipoles, but subsequent modifications had increased this number to approximately 20. This had been done without increasing the number of masts.
Initial steps taken to increase traffic handling capacity.
Receiver Site.
The general lay-out of the Signal Office was considerably improved and there was a total of 18 operating positions of which 4 were equipped for automatic sending. A superintendant's desk was installed, where facilities existed for connecting any operating position to any transmitter and also allowed the superintendant to monitor any receiver.
The receiving aerials which consisted of 8 Rhombic and 4 cage-dipoles were not modified, but the feeder was diverted to the new building. These aerials were permanently connected to 12 of the receivers and the remaining 14 were connected to a new set of end-fed aerials erected round the Signal Office.
A new 28-pair remote control cable was now made available for service, although its main function was to be a stand-by nature. To achieve this satisfactorily this route to the transmitter site was different from the original 20-pair cable.
Transmitter site,
It had been decided that a second transmitter building was necessary to house further transmitters and a building measuring 35' by 18' had been erected. A further 6 250 Watt transmitters were installed here. Four half-wave dipoles were erected and arrangements were made so that any aerials could be connected to any transmitter. In addition each transmitter had available two end-fed aerials, one for day frequencies and one for night frequencies. Adjustments of length could be made electrically by matching units. The 28-pair remote control cable was extended to this building.
Construction of Station 53B.
The reconstruction outlined above was not sufficient to handle the increasing load and it was decided that an additional station was required. The receiver site chosen was at Poundon and that for the transmitter at Godington.
Receiver site.
In the light of the past experience a very much larger scale Signal Office was build. This measured 40' by 12'. Here 40 operating positions were built of which more than half had sufficient space to permit installation of automatic sending facilities.
A new technique was employed in this Signal office in the use of Wide-Band amplifiers. these afforded great economy in aerials because each amplifier was capable of operating simultaneously as many as 50 receivers.
Two 3-wire receiving Rhombics were built, covering between them a frequency range of 3 - 13 MHz. In addition two single-wire Rhombics were built, each capable of operating 4 receivers simultaneously without the use of the Wide-Band amplifier.
The amplifiers themselves were mounted on racks; a lay-out was designed so that any receiver could be connected to any of the 3 Wide-Band amplifiers installed or to the single-wire Rhombics. Each receiver position was wired with coaxial cable and a total quantity of some 7000 ft of cable was used. 15.000 ft lead covered pair was used for wiring the keying, High Tension switching and monitoring circuits.
A superintendant's table was installed where facilities existed for any operating position to be connected to any transmitter and monitoring facilities were also available.
A disk recorder was installed, consisting of a double turntable recording unit and a double turn table play-back unit. The output of any receiver could be recorded. At a somewhat later date the automatic equipment was improved by the addition of an undulator which again could be connected to the output of any receiver.
Subsequent additions to the facilities of the station were made and included the building of two more 3-wire Rhombics and a folded dipole. The number of Wide-Band amplifiers was increased to 5 to accommodate the additional aerial facilities.
A 9kVA P/E generator was fitted for stand-by power and a system of automatic switching was installed so that in the event of a power failure the alternative power supply was first selected and if this too failed the stand-by generator was brought into action. Voltage regulators were also installed.
Transmitter site.
A transmitter building 100 ft long by 24 ft wide was designed with special facilities for bringing in open wire feeder route. Thirty four (34) 250 Watt transmitters, together with their remote control apparatus were installed, the wiring of which necessitated the use of some 600 ft of lead covered pair cable. All transmitters were mounted on platforms, and facilities were made so that in the event of a failure, any transmitter could be removed and quickly replaced by one in a working condition.
Six 20 ft, one 80 ft and fifteen 100 ft masts were erected, so that there were 32 dipoles and 2 Rhombics. Some 10.000 ft of wire was used in their construction, and approximately 3000 spreaders were used in the down leads. Open-wire feeder routes were employed erected on approximately 100 Post office telegraph poles and employing some 75.000 ft of copper wire.
A 40 kVA D/E generator was installed for stand-by power. Automatic change-over in the event of power failure and automatic voltage regulation were fitted. Owing to the somewhat unusual requirements of SOE communications with the field, it is essential that transmitting stations should be capable of providing reasonable field strength over a wide area. Moreover, it is highly desirable that an operator should be able to change frequency on any transmitter with great speed. A standard 250 Watt transmitter requires on an average some four to five minutes to accomplish this accurately.
Investigations were made in order to meet these requirements and a Wide-Band transmitting amplifier working with three-wire Rhombic aerial was developed. Such an installation provided not only a good signal over a wide area, but would also transmit on twelve channels simultaneously, each channel giving a field strength equal to that from a250 Watt transmitter connected to a resonant half-wave dipole.. Ant frequency between 3 MHz and 8 MHz could be transmitted on the first type of amplifier produced, but at a later date, a second model was brought into use, covering a range of 8 MHz to 13 MHz. The rapid frequency changing facility was most satisfactorily met: an average operator could do this in 30 seconds. One of the 3/8 MHz and one of the 8/13 MHz models were installed and were thus the first of their kind ever to operate in this country. Subsequently the total number of aerials in use was brought up to a total of 36 dipoles and 5 Rhombics.
Re-design and building of Station 53A.
This station in spite of earlier modification could not handle the still growing traffic and it was decided that the signal office should be modified to incorporate its Band Receiving Amplifiers with consequent alteration of the receiving aerials. Further that a transmitting building on the lines of the ones built for station 53B should also be provided. This would at once centralise all the equipment in one building and be capable of housing a greater number of transmitters.
The problem was somewhat complex in that operations had to be maintained at all times in spite of the fact that eventually all the existing equipment would be modified.
The initial stage was to build and equip the transmitting buildings. Thirty-nine 250 Watt transmitters and two 12 channel 3/8 Mc Wide Band Amplifiers were installed. Two 3-wire Rhombics were built to operate with the Wide Band Amplifiers. 24 dipole aerials were erected and test runs carried out.
In the meantime a temporary signal office had been built in the house at the receiver site because the modifications to the signal office envisaged were of such a nature that it was impossible that the operational traffic could be carried en whilst these were being installed. In the temporary signal office 26 operating positions were installed, together with automatic sending equipment and this temporary signal office operated with the transmitters in the existing building. In the permanent signal office all existing wiring was removed. The bench lay-out was considerably modified so that a total of 30 operating positions were available with sufficient space for automatic sending on all positions. In addition facilities were made for installation of 2 double turn-table recording and play-back units. Two undulator positions were also installed and arrangements were made so that the output of any position could be recorded on either piece of equipment.
A different and more trouble-free system of remote control was installed which necessitated the use of some 10.000 ft of twin lead covered wires.
Five Wide Band Amplifiers and the ancillary patching equipment were installed and these operated in conjunction with an entirely new aerial layout, consisting of five 3-wire Rhombics. All the existing aerials were scrapped. This entailed the dropping of 24 masts and the erection of 17 masts in new positions. Some 72.000 ft of copper wire was used in the new aerials and the feeder route, consisting of coaxial feeder, some of which was buried and some run in overhead route, employed about 4500 ft of coaxial cable.
When the new signal office was completed and the aforementioned number of aerials installed at the transmitter site, a change over was made from the temporary signal office and old transmitting site and at the same time a number of essential channels were kept working from the old transmitting building. By careful coordination a further nine dipole aerials were brought into service which permitted all the old transmitters and their aerials to be withdrawn from service.
The transmitting site was now complete. This had involved the erection of five 120 feet, twelve 100 feet, four 80 feet and two 60 feet masts, giving a total of 15 dipoles and 2 Rhombics, and the lowering of sixteen 100 feet and three 80 feet masts. Some 120 G.P.O. poles were used on the new feeder route and approximately 100.000 feet of copper wire. A further 16.000 feet of wire was used for making the aerials.
At the transmitter site a special building was constructed to house a stang-by-power equipment and a 40 kVA D/E generator was installed. Automatic change-over in the event of power failure and automatic voltage control was fitted a both receiver and transmitting stations.
Station 53C.
The construction section acted in a technical liaison capacity in the design and construction of this station. It was responsible for the aerial lay-out and design and for the running of the remote control lines. The internal installation, with the exception of the transmitting Wide Band Equipment, was carried out by the U.S. Forces.
The Wide Band equipment consisted of two 3/8 Mc twelve channel amplifiers and 24 rack mounted Drive Units together with the ancillary equipment.
The receiving aerial layout consisted of five 3-wire Rhombic; Wide Band receiving amplifiers were installed.
The transmitting aerial layout employed 35 dipole aerials and four 3-wire Rhombics.
Remote Control System.
Two 28-pair cables were run by different routes from each of the three receiving stations to their respective transmitting stations. In addition, separate tie cables were run between receiving stations and also between transmitting stations. Thus it was possible for each receiving station to be connected by four different routes to its transmitting station; in this way cable failure was never likely to interrupt communications. Some idea of the extent of such a network can be grasped when it is realised that some eight million feet of conductor was used.
Building of 5kW Broadcast Station.
A decision was made shortly before D-Day that a 5kW broadcast station would be required and that it should be ready to go on the air in a fortnight. At this time the only part of the equipment which was available from our own resources was the transmitter itself. The requirements were for two microphones and a double turn-table equipment for playing records up to 16" and at a speed of either 80 or 33 1/3 revs/min. A turn-table recorder for cutting records was also required.
All the control room equipment such as the four channel-mixer. line amplifiers, etc were purchased. Rather naturally there was some difficulty in obtaining the precise equipment required at such short notice so that various modifications had to be made.
Three rooms were allocated for the installation and a little modification was necessary for those in which the transmitter was installed and the control room equipped/ The third room to be used as the studio, was made sound proof and the walls treated to prevent sound reflection. A double-glass partition was let into the wall between the studio and the control room.
A 100 kVA 3-phase delta connected transformer was installed, the primary winding of which was wired to the 2000 Volt supply. 230 Volt and 110 Volt supplies were wired up in the control room. A 40 kVA P/E stand-by generator was also installed.
An aerial array was built on four Steward and Lloyds' 100 feet steel masts and an open wire feeder route erected on telegraph poles.
The whole station was ready to go on the air in the time stipulated, although final trimmings were carried out afterwards.
Modification to Di-pole Aerials.
A new allotment of frequencies was made for D-Day and afterwards. Whilst these frequencies were in most cases not widely different from those in use before, there was sufficient reason to require aerials which would cover rather a wider band of frequencies than those in present use. It was decided therefore that the use of folded dipoles rather than the single wire type would be advantageous. Trials had previously been made with the folded dipole and the results were satisfactory. In three weeks no less than 96 aerials were re-rigged to this design. Some measure of the magnitude of the task may be gained from the knowledge that 45.000 feet of copper wire, 2000 spreaders and 700 strain insulators were involved in their construction.
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