What would be a good way of measuring the return loss of fabrics in the microwave frequencies? I see papers published where they put the fabric between two waveguides (not clear if it is inside the waveguide or just sandwiched between the two waveguides. Any suggestion? Can one just use the N1501A Dielectric Probe Kit since the dielectric constant of the fabrics are low to begin with? Thank you for any suggestion.
Dear @Somnath, you can do it two ways.
1. Bistatic method using two horn antennas inside an anechoic chamber. In this case use antenna #1 as the transmitter and antenna #2 as the receiver. They should be separated by the minimum electrical angle. What you are receiving at antenna #2 can be taken as s11.
2. Use the two waveguide method. Use a material holder ( small waveguide) in between the waveguide for better results. Or else you can sandwich the material between the waveguide. But don't allow the material to overlap the size of waveguides.
The N1501A kit is entirely unsuitable for this. Its measurement is an average over a thickness that is almost certainly more than the thickness of your fabric.
If you measure fabric in a waveguide then you are measuring it at oblique incidence because the waveguide modes are not the same as plane waves, but can be thought of as two sets of plane waves making an interference pattern in the guide If you use horns the wave can be more nearly plane at the end of the horn, but the horn has to be long enough to eliminate the spherical phase error.
In any case, be careful about the way you mount the fabric, because I expect the return loss will be low and anything like gaps in the walls may change the return loss significantly. It is possible to put fabric between the ends of two guides if there are choked flanges on the guides, which can give very low return loss even if the guides don't touch each other.
The material of radomes is often similar to fabric and gives low return loss.
of course the comment by Malcom is correct in principle; but still if you can put your fabric between two waveguide flanges (i do not refer to the N1501A kit specifically) can be a good starting point when you measure both in reflection and also in transmission; just doing this in the frequency domain at a single frequency may be prone to errors thus using the time domain transform can give additional cross- check possibilities. It also depends on the ballpark of losses of your fabric(perhaps you have in mind to test fabrics with woven in metallic wires (for EM shielding).then the waveguide flange method is not bad to start with. And for real low loss "foils" or layers you can apply the waveguide resonator method.
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