antenna-theory.com

[eak]

Hello,
I'm playing with solenoid antennas and i'm trying to understand how does it work. My question is how does the voltage across an antenna port at reception is related to its gain ? When i double the number of turns of the solenoid my voltage at its port double (+ 6 dB in HFSS simulation and measurement), BUT the simulated gain does not increase by 6 dB when antenna is excited. Why ?
Another question, is it possible do get the gain of an antenna when voltage is taken from non 50 Ohm port but some complex value?

[helix]

When you say "solenoid antenna" I assume you're referring to a very electrically small loop or coil of wire that behaves quasi-statically, i.e., the open circuit voltage at across the output terminals is Vout = j*omega*B*Aeff, where omega is the radian frequency and B is the magnetic flux density (i.e., Teslas) and Aeff is an effective area.

For an electrically small antenna (either electric of magnetic type), the directivity is basically fixed at ~2dBi. And the gain differs from this only by the efficiency. (Realized gain includes mismatch loss as well.) Antenna gain has to do with power (or power density per unit solid angle -- check the definitions in an antenna book or IEEE standard), and power is related to voltage (or current) AND impedance.

Now, when you add more turns, you indeed will get more voltage for a constant impressed B field, but the efficiency and antenna impedance (and therefore the mismatch loss to a constant, real impedance such as 50 Ohms) of the antenna also change. In reality, the gain (and realized gain) will only decrease as you add more turns. If you use non-PEC materials in your simulation, you'll see the effect on efficiency.