Introduction of CD/CHL and CD Sets

November 27, 1940 - National Archives of Canada

THE WAR OFFICE

27 November, 1940

57/P.F./160(A.A.1.(d))

Sir,

1. I am commanded by the Army Council to inform you that a new method of detection and observation for fire control involving the use of radio equipment is to be introduced into Coast Artillery units for Home Defences and defended ports abroad.

2. I am to say that the equipment is being developed in two stages, namely,

1. CD/CHL Sets for detection and location only.

2. CD. Sets for greater accuracy and for transmission of fire control data.

Both equipments comprise a transmitter and receiver which are housed in a brick, or concrete building, part of which is located between the legs of a tower 17 ft. high supporting an aerial array which rotates on a turntable. The engine room is a separate brick or concrete building and should be sited so that advantage is taken of natural protection and sunk partially below ground if drainage permits.

Observations of range and bearing are made at the receiver and are transmitted by telephone for CD/CHL and by Magslip transmitter for CD Sets to the Battery or Plotting Room via a converter.

The CD Set is designed to operate in the band 1.4 to 1.6 metres (214 to 187 Mc/s) and the CD/CHL over a slightly narrower band approximately 1.43 to 1.58 (210 to 190 Mc/s). As the radiating elements on the aerial arrays have to be made for a specific frequency, these have to be changed if it is necessary to alter the operating frequency, and retuning of the equipment is a skilled operation. Frequency changing, therefore, necessitates the supply and fitting of certain parts and retuning of the equipment which will take about a day to carry out once the parts are available.

Energy is radiated in the form of a narrow beam and therefore continuous rotation of the aerial array is necessary for the detection role. In the CD/CHL the speed of rotation is variable up to 8º per second. In the case of the CD Set the speed of rotation with hand control is variable up to 6º per second and on automatic from 1º to 6º per second in fixed steps.

3. CD/CHL Sets are intended to give-

1. Early warning for beach defence, illuminated areas and defended ports and are capable of detecting and tracking MTB's from a minimum range of approximately 1000 years to a range of 6000 yards, and large barges from 1000 yards to 8000 yards when the set is sited at a height of 100 ft.

2. Detection and tracking of larger vessels up to a maximum range of 40,000 yards when sited at a height of 300 ft.

The range accuracy will be plus or minus 400 yards, bearing accuracy, plus or minus 2º.

4. CD Sets are intended for fire control role and will be available after the CD/CHL Set. They will be used as close defence and counter bombardment range finders.

The performance of the CD Set is such as to cover the role for which the CD/CHL Set is to be used.

It is hoped to attain a range accuracy of plus or minus 100 yards with a consistency of plus or minus 25 yards, and bearing accuracy of plus or minus 20 minutes with a consistency of plus or minus 5 minutes. The bearing accuracy depends on the site of the station and it may be necessary to apply corrections over certain parts of the angle of sweep covered by the equipment.

5. A subsidiary role of both types is the detection and tracking of aircraft within the potential limits of the set, but operationally this role cannot be used when surface vessels are being followed.

6. The working detachment consist of -

• One NCO. i/c Set
• One O.R. transmitter and generator
• One OR receiver range
• One OR receiver bearing.

It is proposed that 3 detachments shoud be provided for each set.

7. Siting. To produce good results it is necessary that each site should be selected by specially trained personnel. The most important consideration is that of height and probably the ideal site would be one on a promontory and at the edge of a high cliff. There must be an uninterrupted view of the sea in front of the set. The range obtained is proportional to the square root of the height of the set.

The siting requirements are most severe in the case of sets used primarily for short range detection roles. Considerable interference under these conditions occurs at short ranges resulting from echoes from local objects. These echoes are due to radiation from the transmitting array and pickup on the receiving array at certain angles on each side of the main beam. These echoes cannot be entirely eliminated and it is, therefore, essential to reduce them as much as possible by careful siting.

8. Owing to the paramount need to obtain height for the equipment and the general difficulties of siting, the sets will usually be sited away from the guns and existing battery plotting rooms. A special Displacement Corrector is being designed to enable the information supplied by the CD equipment to be accepted by the existing installations.

9. At ports abroad the local commanders should consider the following points:-

1. The possibility of producing locally made aerial towers of timber, steel or concrete.

2. Whether the CD/CHL Set should be used primarily for short or long range roles having regard to the siting problems. Specially trained Officers are being posted to Ports Abroad to advise on siting.

3. Ports Abroad will be informed as soon as possible of the number of sets allocated to them. The War Office will then call for recommendations as to siting.

4. The actual siting should be done in conjunction with the RAF.

10. Training . Technical personnel will be posted to Commands and Ports Abroad. Training will be carried out as follows -