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FAQ (Frequently Asked Questions)

The Chain Home radar system

As stated earlier, it was originally intended that each station should have the choice of any one of four allocated frequencies as an anti-jamming measure; it was intended, therefore, that each mast should carry a set of dipole stacks which had been carefully matched phased and calibrated for one specific frequency. This plan was later abandoned.

To preserve the electrical symmetry and low cross-talk between elements necessary for accurate direction and heightfinding, great care had to be taken in the bonding and alignment of the feeders connecting the dipoles to the remote goniometer and receiver system. Low loss, 72 ohm, solid copper cables pressurized with dry air to keep out moisture, were used as feeders. Phasing of the X and Y elements was achieved by inserting finely trimmed short lengths of flexible 72 ohm twin 'television' cable (BL7) between the co-axials and the goniometer, these links being folded up in a screened copper 'phasing' box and trimmed to length during the calibration process.


The transmitter, designated T.3026, figures 9 and 10, was derived from a basic design by Metropolitan-Vickers for the short-wave station at Rugby. It was very unusual, probably unique at that time, in that it used continuously evacuated, demountable tetrodes as the output and driver stages, figure 11. Originally intended for c.w., the design was extensively modified to allow operation in a pulse mode at a very high peak power and low duty cycle (5 x 10-4). It was also required to have the capability of being switched to any one of four spot frequencies in the band 20-50 MHz within 15 seconds to meet the four frequency concept of the original anti-jamming plan. The specification for pulse shape and stability was very stringent for those days; an r.f. pulse of 300 kW was completely outside the transmitter designers' experience. An additional requirement was that during the quiescent or interpulse period, the power radiated from the transmitter was not to exceed a few microwatts to avoid masking the nearby, highly sensitive receiver during the 'listening' period following the pulse.


(This article is taken from "The GEC Journal of Research", Vol. 3 No.2 1985 pages 73-83 and has been reproduced with the kind permission of the Editor. The copyright of the material remains with the owner.)


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