I have an antenna. I want to measure S-parameters of it using a network analyzer. I am not clear how to do it. Please explain.
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Connect the antenna port to the VNA port, measure S11.
You will want to ensure that there are no RF reflectors within sight of the antenna. This could be done by pointing it at the sky, out of a window, or at an RF absorber. Any signal reflected back to the antenna will appear as a returned signal at the antenna port, and so alter your true S11 measurement.
Set a suitable frequency range for the antenna, connect the antenna to the Port 1 of the VNA and measure the S11. If the antenna is working, you should see the resonance dip on the graph:
The lowest point of the dip tells the frequency in which the antenna radiates most of the power (least power returns to VNA port). Many antennas have multiple resonances to achieve a wider bandwidth, which will be visible in the measurement.
If you disturb the antenna while measuring, you will affect the performance:
In my example, the antenna is a near-field antenna with a single resonance. By disturbing it with my hand I changed the impedance of the antenna and it changed the resonance frequency. If you are working with antenna using far-field, then a suitable environment (like an anechoid chamber or pointing the antenna to the sky) is needed. Otherwise the performance might be affected by reflections caused by the environment.