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[ai.xuexi]

In free space, the TEM wave can transmit.

In a pair of parallel plates (transimision line), the mode is also TEM.

Why can not we use the parallel plates to receive the wave?
Why?

you may say the impedance is not match. but how can they be matched?

I find there is no any context about this in textbook.

Thank you very much.

[arka]

Hi..A normal transmission line doesn't radiate TEM waves. It guides TEM waves. Two conditions that are vital for radiation to occur are:
1. There must be a time-changing excitation, which we already have we suppose.

2. The above condition only states a possibility of radiation. Another vital condition which must act is : spatial imbalance.

in normal tx lines, there is no spatial imbalance. currents in both conductors flow in opposite direction. If the conductors are closely spaced, the fields at a far off distance cancel out each other, due to equal and oppositely directed currents on the tx line. (Spatial BALANCE)..

However, when we flare the ends of a transmission line to form a dipole antenna say for instance, the currents at two arms are directed along same directions.. Hence, the fields by the two conductors no more cancel out each other, and adds constructively for radiation (Spatial IMBALANCE)..

Thus, by reciprocity theorem, if we have a structure which can provide spatial imbalance, surely it can receive EM waves as well. But normal parallel plates, or 2-wire tx lines etc., when the conductors are nearly spaced, there is no spatial imbalance. hence cant be used as EM waves receiver.

[ai.xuexi]