It won't show any resonance. All you can hope to see is a small change in the S11 of the antenna. Have you worked out the coupling between the two circuits?

You need to have one antenna on the VNA and one on the tag. The way you have drawn your circuit looks as if you might be going to use 2 antennas on each in a series circuit.

Thank you for your reply.

I do use 2 antennas on each side. And the energy flow should be:

VNA-->Antenna_1 Antenna_2-->resonator-->Antenna_3 Antenna_4-->VNA

There is a resonance in resonator's S21, and I need to check this resonance on VNA via wireless transmission. Do you think it is not possible to achieve this?

Regarding the coupling, I guess the coupling between antennas is quite good.

I am also wondering is it possible to use near-field antenna because the space between antenna is quite small which falls in the near field range.

I would propose to use ferrite cored transformers to couple the VNA to the split ring resonators. This would be a near field coupling. In order to decouple the transformers they must be perpendicularly oriented.

One other solution which may require more electronic effort is to input the output of your circuit on a peak detector using Schottky diodes and acquire this DC voltage and send it to nearby computer to store it and post process the data. You can also build the oscillator on the same board.

Best wishes

It depends how close your antennas are to each other. If they are 10 cm apart you will lose about 20 dB between one pair of antennas. I suggest you try the antennas first without the resonator to see what your losses are. You may also get a strong direct signal across the two VNA antennas, unless you design it so that doesn't happen - like using crossed polarization or a null in the polar diagram to isolate the two sets of antennas. Are your VNA antennas close to each other?

Patch antennas radiate (and receive) strongly at 90 degrees in the E plane.

Thanks for your suggestion. Abdelhalim Zekry

I am very interested in the near field coupling. I found that nfc antenna is actually an inductor, which I planned to work on. Two inductors can form a transformer even though there is no core.

Usually, a resonant circuit is needed to maximise energy transfer between two inductors, as those designed in near field RFID tag. But I am not sure is it possible to build those inductors on the microstrip and achieve the same resonant frequency. It would be great if you can explain more about this.

Peak detector is a good idea, but by far, I don't need it as I am still using VNA.

Folow

Thanks for your advice, Malcolm White

The antennas are quite close, about 3 cm apart so it is near field, which corresponds to 5 dB loss in my CST simulation. The cross talk between two VNA antennas is about -50 dB. I thought patch antenna has very good polarization, so I put it 90 degrees apart, but it seems that it is not the case.

Do you have any suggestion of simple microstrip antenna that has good polarization and directivity? Meanwhile, I am a little bit worried whether the VNA antenna would couple the resonator directly and change the property of the resonator?

If you have 5 dB loss and 50 dB cross-talk that is good enough. The cross-polar isolation can be quite good - partly because there is a null in the H-plane at 90 degrees. I don't know why you can't see the resonator dip.

Can you put a stronger resonator in the simulation instead - to find out what you can see! A lower Q and deeper dip would be more visible.

In real life reflections may couple back in as well, so be careful about that.

I think that 5 cm is not really near-field. It is just under half a wavelength, and the antenna is half a wavelength across so the full far-field distance is about 6 cm and the inductive near-field will be half that or less.

The formulae I use are for large antennas, so are probably not quite right for these small ones.