How the early radars could know the angle of a basic radar rotating antenna?
How the direction of the time base on radar screen is synchronized with the direction of the radar rotating antenna?

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    – user103380
    Sep 28, 2018 at 18:47
  • \$\begingroup\$ Conventional solutions applicable to 80 yrs ago would be to use the rotor voltage commutation enabled once per revolution in a time window to get the phase voltage of the rotor with some gear ratio and an index switch and some flywheel for stability. THat's my guess. The same is done in all disk drives except using a Hall index sensor mark and for CRT's use H sync for blanking from flyback per raster sweep. \$\endgroup\$ Sep 28, 2018 at 19:06
  • \$\begingroup\$ @KingDuken this is not a homework, i have already searched for the answer and found that selsyn is used for that purpose,but did not remember how it works \$\endgroup\$
    – AAEM
    Sep 29, 2018 at 20:59

1 Answer 1


Early radar sets used a pair Selsyn motors to transfer the rotary motion of the antenna platform to a sine-cosine potentiometer.

The terminals of the potentiometer were driven with complementary copies of the sawtooth waveform generated by the radar timebase, and the taps of the potentiometer fed the horizontal and vertical deflection amplifiers of the display tube, creating a radial scan at the correct angle that's synchronized to the receiver output.

Note that this system even allows the operator to interrupt the normal periodic scan in order to sweep the radar beam across a specific target. The display remains synchronized to the antenna at all times. Note also that "pan & zoom" of the display is possible by varying the gain and DC offsets of the deflection amplifiers.

Modern digital systems simply use a shaft encoder on the antenna.

  • \$\begingroup\$ DO you know this for a fact is is this just a your guess since it was used by US in 1900. I don't think the Hungarians use that when they designed for the Germans in 1939. Citation, please. Latest Radar by Lockheed Martin uses a revolving electrified rail with magneto gears and a rotating dual radius rotor on an angle pivoting around \$\endgroup\$ Sep 28, 2018 at 19:45
  • \$\begingroup\$ @TonyEErocketscientist. Custom shaft encoders can be as accurate as 16 bits or more, far beyond the price range of Ebay. With Klystron tubes the echo-delay is very precise. \$\endgroup\$
    – user105652
    Sep 28, 2018 at 20:14
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    \$\begingroup\$ It could detect molecular steel tube crystal flaw well before a leak occurred if managed properly that changed probe impedance in ppm. We used a 1mm calibration thru hole for full scale 0deg and and change in wall thickness of 50% as 90 deg and use the Sin/Cos Pot with servo to rotate the XY screen and null impedance to like a radar dot in the centre. Then linear 12 bit ADC gave me over 60dB down res so the angular resolution was in theory infinite limited only by servo noise, which was unmeasureable for theta impedance angle error. \$\endgroup\$ Sep 28, 2018 at 20:47
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