Martin, Mike and Chin-Leong
I analyzed again any suitable test networks for measurement of S21 for active small loop antenna ( not just antenna amplifier ).
Martin used coupled loop antennas in the near-field, where only magnetic field present.
I guess it is only acceptable method currently at least reading literature related to this field.
Since it this measurement is involved magnetic flux density of excitation loop that is created by current flow in this antenna.
We should worry the current for a wide frequency range is maintained almost the same due to the inductance of excitation antenna.
I propose in series with this excitation loop include a fixed current limiting resistor which value is calculated as follow:
Rlim= 2 pi x f c x L a
where fc - highest broadband frequency used for S21 measurement of active loop antenna.
So, within 3 dB the current in excitation loop will be maintained the same from 0 to fc frequency.
Let's discuss any shortcoming of proposed.
Regards,
Raphael
