In radar surveillance we use primary and secondary surveillance radar for different purposes. I'm wondering about the antenna that use in both system:

  • Secondary Radar uses an array antenna
  • Primary Radar uses a reflector

Why the different designs?

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1 Answer 1


With the secondary system, you are simply sending a one-way message from the radio tower to the transponder in the plane, and the transponder is transmitting a message back from the plane to the radio tower. It is a very simple two-way radio conversation between relatively high powered stations.

With primary radar I am sending out a signal and all I am receiving back is the nearly microscopic amount of energy that bounced off your aircraft. That signal may be hundreds of thousands--or even millions--of times weaker than the secondary signal being transmitted by your transponder.

From an engineering perspective, the primary receiver is designed to solve a far, far more difficult task, hence the more complicated antenna requirement.

  • 2
    $\begingroup$ If I may extend your answer from a physics perspective: the primary radar is (usually if not always) a different frequency (and thus wavelength) from secondary radar. For a certain design, antenna size is typically a factor of the wavelength. With two different frequencies, a different antenna design may have been necessary just to get the sizes in the right ballpark. With different sensitivity requirements, a less sensitive SSR antenna design that allows it to be small is likely to be selected. $\endgroup$
    – Azendale
    Jan 18, 2023 at 23:00

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