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M.J.B. Scanlan


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Chain Home Radar - A Personal Reminiscence

On a very small echo, as first picked up, the bearing would probably not be very accurate, and it might be impossible to take a height, since the signal on the lower of the height-finding dipoles would be much smaller than that on the upper, which was itself only just detectable. However, even the range information was valuable as early warning and because it might enable a range cut with an adjacent station: also, knowing the station V.P.D. and the range of pick-up, a skilled operator (or her supervisor) could estimate a height with surprising accuracy. As the target grew closer in range, the signal strength on both upper and lower dipoles would improve, and height measurements and more accurate bearings would become available.


The theory of CH direction and height-finding is simple enough, but depends heavily on the site being ideal. Many sites were obviously far from ideal, and even for those which promised well, it was difficult to be sure about surface slopes and conductivities in the critical areas in which ground reflection occurred. For this reason, CH stations were always calibrated.

Calibration began with a careful check over the receiving system, from dipoles to gonio: in particular, it was important that the feeder runs between pairs of dipoles were of the same electrical length. By injecting signals into the dipoles and adding short lengths of flexible cable in a 'phasing box', equality of electrical length was achieved. The goniometer was also carefully checked for sensitivity and a high cross-talk ratio between the pairs of coils.

Finally, calibration flights were carried out, using either an autogyro (a predecessor of the helicopter) which would hover over local landmarks such as churches, or a small aircraft, which would fly a succession of radial flights at various heights. The position of the target aircraft was often checked with a theodolite. The radar transmitter was not involved in this process, since the aircraft carried a 'squegging' oscillator, which gave an unmistakeable and relatively large signal, which could be 'D/F'd out' relatively easily and accurately by the gonio.

Needless to say, calibration was carried out, not by station personnel, but by a peripatetic band of specialists. While calibration, or re-calibration, was in progress, normal operations were in abeyance, and the station routine disrupted.

Technical Maintenance

At the heart of the RAF system of maintenance for all radar stations (not only CH) was Form 1497, a large pre-printed sheet of paper which on one side carried notes and comments on any repairs or modifications carried out, and on the other side a list, on a day-by-day basis, of the routine maintenance operation which had been carried out. A separate Form 1497 was made out each month for every major piece of equipment (receiver, transmitter etc): the form was clipped like a loose leaf in a book, the covers of which were two pieces of plywood, hinged along one edge and painted black on the outside faces. One side bore a large 'S' (serviceable), the other a large 'U/S' (unserviceable); when the boards were hung in some prominent position on the equipment to which they referred, the serviceability state was immediately obvious to all. Details of the fault (for example, 'no time base') could be looked up on the Form 1497 inside.

Every item of equipment also had a schedule of 'Daily Routine Maintenance operations. Thus, on Mondays, operations 1, 4, 7, 11 and 15 might be called for on a receiver: for detailed instructions on these, recourse must be had to the handbook, which was of course a secret document. The mechanic would therefore sign out the handbook, carry out the prescribed operations, enter them as completed on F. 1497 and return the handbook. In this way and over a period of time, every operation would be carried out, entered on the form and signed for as complete and satisfactory. Over a longer period, the senior N.C.O. mechanic and the station technical officer would also work their way through the routine, either independently or by supervising the mechanic: these tests would also be entered and signed for.

Often, a maintenance operation such as 'clean and inspect the time base panel' would be called for, which seemed to hark back to the maintenance of an aircraft engine. In that case, cleaning might have some point, and inspection might reveal an incipient fault. With a tray holding perhaps a dozen valves with their associated components, inspection was not likely to reveal much; it might detect a loose top cap or a 'soft' valve, but was unlikely to show that a valve was near its end of life, or that there was an incipient dry joint in the panel. While inspection did little harm, if not much good, cleaning did little good, and could do much harm: for instance, a displaced valve might lose contact to one of its pins, causing a difficult and quite unnecessary fault.

Test gear was provided, of course, but the bare minimum to carry out the prescribed tests. Essentially, it consisted of a signal generator and a multi-meter: there was no oscilloscope, so that for any out-of-the ordinary fault, for example, in the time-base circuits, recourse must be had to Wing headquarters, perhaps (and very desirably!) a hundred miles away: there there was an oscilloscope, unless it were already out on a panic call from elsewhere.


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Constructed by Dick Barrett

Email: editor@ban_spam_radarpages.co.uk

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ęCopyright 2000 Dick Barrett

The right of Dick Barrett to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.