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My electromagnetic waves professor said any time an electromagnetic signal is received by an antenna, a transmission also occurs.

I have not independently verified this, but I believe it is likely true.

Does someone know if this is true, and can the basic physics be explained, please?

Note: This question was inspired by my own comment on Antennas and relativity. I'm thinking of this more from a Maxwell's equation POV then relativity, however.

ANOTHER NOTE: Per comments below, transmission is usually called "scattering" when unintentional.

JosephDoggie
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    An ideal antenna behaves like an ideal optical absorber. Do ideal optical absorbers diffract light? Yes. The diffraction pattern is similar to that of an emitter of the same shape. In reality antennas are not perfectly terminated, of course, so they are not even perfectly black absorbers. – FlatterMann May 18 '23 at 18:10

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Free electrons in the antenna are made to oscillate by the oscillating electric field in the incident wave. The oscillating (and therefore usually accelerating) electrons emit electromagnetic waves. This can, of course, be treated mathematically.

Philip Wood
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If you could recreate the full electromagnetic field of a matched antenna that it creates when in transmit mode than it would not scatter in its receive mode, this is reciprocity. Note the emphasis on the full EM field that would include not just the transmitted radiation field that goes like $\propto \frac{1}{R}$ as $R \to \infty$ but also its near field $\propto \frac{1}{R^2}$ and its reactive field $\propto \frac{1}{R^3}$, as well. Also when an antenna receives, normally it is immersed in the far field plane wave region of another transmitter. That field is not the same as its transmitted field to which it is matched, hence the inevitable scattering in its receive mode. You can find more details of the scattering problem here.

hyportnex
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