Jinze,
Unfortunately, your response is giberish because it can be made two independent measurements of S11 parameter regarding the frequency range of interest and related either to the antenna under the test or S11 parameter related to the input of antenna amplifier using VNA. Yes, since VNA's port 0 has constant 50 ohm resisistive impedance value, we measure how good the input impedance of amplifier matches this 50 ohm value or the antenna under the test matching the same 50 ohm value.
In this test we are not assessing true loop antenna reception but weighing whether it pays to match antenna impedance to the input of selected amplifier.? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? However, using VNA with a transmit antenna connected to port 0 and receive antenna to port 1 you can measure S21 scattering parameter, observing balance to unbalance conditions between your antennae and VNA unbalance ports. There is a possibility even indirectly calculate Gain of? antenna in the frequency range of interest.
Only one difficulty is to? observe? - the measurements shall be done in the far field of your transmit antenna ( for electrically small antenna ): d > wavelength of interest/ 2 x pi, where d - distance in meters
The calculated then?G antenna = 4 x pi x d x sqrt | S 21 | / wavelength of interest.
Also, there is known relation between [S11] scattering parameter parts Re [S11] + Im [S11] and? impedance Z value = R + j X .
You can convert/transform? them like Re [S11] = ( R ^ 2 - 50 ^ 2 + X ^ 2 ) / [ ( R + 50 0 ^ 2 + X ^ 2 ]? and Im [ S11] = 2 x 50 x X / [ ( R + 50 ) ^ 2 + X ^ 2 ].
Raphael
