This set of three images shows the three types of antennas used on the SCR-540 radar set as installed on an A-20, the P-70 prototype. This was the US copy of the UK's AI Mk. IV radar, the first airborne radar system for fighters, and the second airborne radar to enter service. It is predated only by months by the ASV Mk. I, developed using the same electronics as this set. There is a possibility that this is in fact a "real" AI Mk. IV, and not the -540, as one example is described by Bowen on page 179 of <i>Radar Days</i> as being fit to an A-20 for testing.
<p>The radar signal was sent from the arrowhead antenna seen in the center frame, filling the space in front of the aircraft like a floodlight. Reflections from targets were receive on a set of four antennas, two of which are shown in the left and right frames. The left frame is one of the two vertical antennas mounted on top of the wing, the other would normally be located directly below it. The right frame shows one of two horizontal antennas mounted to the left side of fuselage, its partner would be directly opposite it on the right side.
</p><p>A motorised switch, inside the aircraft, quickly sent the signal from each of these antennas to the display screens. The left and right antennas to one, the upper and lower to another. By comparing the signal strength between the two antennas in a pair, the operator could determine if the target was more aligned with one than the other. For instance, if the signal was stronger on the left side of the horizontal display, the target was located to the left.
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Each antenna consists of a half-wave dipole with an associated director or reflector. The passive directors are easily seen as the white rods in these images. The angle of the vertical receivers matches the bend in the transmitter, which was introduced to improve the vertical sensitivity of the otherwise vertically oriented signal.
Originally from U.S. Radar Survey: Section 1 Airborne Radar. Washington, DC: National Defense Research Committee, 1944.
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