Hi all,

a question that has made me confused lately:

From transmitting point of view an antenna looks like an impedance - it has a reactive part and a resistive part, and the resistive part is divided into ohmic resistance (converts power to heat) and radiation resistance (radiates power as EM waves).

When we have a transmitter with certain properties and a transmission line with given impedance, we can select an antenna or add a matching circuit to a given antenna to optimize power that is transfered to antenna (and make sure that the transmitter is not damaged).

But how does an antenna look like from receiving point of view? This is not really addressed in many antenna related materials.

I found one source on the internet saying that it could be modeled as a voltage source with the aforementioned impedance in series. But that was not a very professional source.

If that is correct, we can say that the antenna impedance is the same as in the transmitting case, the radiation resistance scatters power back to the environment and the ohmic resistance and reactance work in the same way as in the transmitting case - is this correct?

With a receiver with given impedance, if we have an antenna that is a voltage source in series with an impedance, it would be useful to minimize the resistive part of the antenna impedance to increase the power that is transfered to the receiver. If there is reactance in series with the receiver, canceling it with opposite reactance would be beneficial.

But what happens if we choose an antenna with much smaller radiation resistance compared to a matching antenna? Less power scattered vs absorbed by the receiver... And let's neglect ohmic resistance now, let's think that the antenna is a superconductor.

Can we still say that it's always the best idea to match the antenna to the receiver? Or are there cases in which we could have an antenna with more suitable impedance and it would transfer more power to the receiver? Let's neglect antenna directivity too.

Or does it always hold that choosing an antenna with lower radiation resistance will either change its reactance in a non-optimal way, or decrease the voltage that is induced by the EM field, and thus it will provide less power to the load compared to a matching antenna? Even if we neglect ohmic resistance?

Writing a general equation for antenna radiation resistance does not work since it depends on the antenna type and shape (thus the resulting EM field distributions).

Writing an equation for the induced voltage is also more or less a case-by-case problem.

And thus we cannot write a general equation in closed form that would give us the voltage of the voltage source when we input the EM field strength, antenna impedances, frequency?

But can we generalize this matter?