S-parameters of waveguide without using VNA
I am trying to find dielectric properties of liquids using a rectangular waveguide. I do not have access to VNA but am having an oscilloscope and a microwave signal generator. I want to calculate S parameters of the waveguide. Can someone help me with a possibe circuit diagram and formulae ?
Thanks
Without a VNA you can use a simple waveguide reflectometer. In addition to the oscilloscope you will have to use a waveguide detector and a calibrated attenuator.
The refectometer consists of a 10-dB waveguide directional coupler with the detector connected to the coupled port of the coupler. To determine S11, you send a signal through the coupler to your device. For calibration, connect the calibrated attenuator and a matched termination or a waveguide short. The termination response should be "nothing", or the highest S11 achievable with your setup. With a short you will get "0 dB" S11 response, and with the attenuator set to 10 dB, 20 dB, etc., it will give you 10 and 20 dB S11 responses.
A good signal generator or a "flat" sweeper is necessary.
By reversing the directional coupler plus signal generator, you can get similar S22 responses, reflections by device output.
To measure S21 or gain/loss response, use the attenuator and detector without the coupler, only signal generator through the attenuator to the detector.
S12 is to be measured like S21 only from the device output back to its input.
The above is the way a VNA operates but all switching and calibration is done in VNA case.
Sxx formulas are available in any microwave techniques textbook.
waveguide is pretty much a LOSSLESS thing. So you can measure reflection coefficient, and theoretically compute transmission coefficient from it, and vice versa.