Hello all
I’ve synthesized recently different magnetic nanomaterials, and as a trial, i pressed the powder into rectangular shapes and put them inside a rectangular waveguide with similar dimensions to study their electromagnetic properties in the microwave region.
Then using the Nicholson-Ross-Wier method i calculated Epsilon, which should be nearly constant for such systems.
Finally, i calculated the return loss RL using 20*log((Zin-Z0)/(Zin+Z0)) where Zin is Mu and Epsilon dependent and Z0 = 50 ohm
RL should be negative and dipping below -10 dB once with a large bandwidth
Images are available below
The sample dimensions are 2.3x1 cm, and they have a thickness around 2.4 mm. the 2.3 cm side is 0.1 mm to big for the waveguide so it requires some quick polishing, which sometimes leaves a small air gap due to human error
My main question is, what is causing such massive peaks and jumps in the calculated parameters?
It’s either from the setup, the small gap or some hidden error in the matlab NRW code? which after importing the real and imaginary S11 and S21 is:
S11 = s11r + 1i.*s11i;
S21 = s21r + 1i.*s21i;
X = (S11.^2-S21.^2+1)./(2.*S11);
G = X + sqrt(X.^2-1)
G(abs(G)>1) = X(abs(G)>1) - sqrt(X(abs(G)>1).^2-1) %%this is to guarantee that abs(Gamma)
You probably have a (even small) air gap around you metamatrial sample and that makes it tricky.If you really want to get the eps only make a paper thin sample and use the well known resonant perturbation method and place it in the magentic minimum of a say H103 resonator.
Fritz Caspers
Dear Dr. Caspers,
Thank you so much for your clear reply
During this time i was trying to go with your suggestions regarding the thinning of the sample, up to 2 mm, i even mixed some ferrite powder with paraffin wax to make a paste and block the air gaps, which reduced the jumps in values of permittivity, but the pulsating behavior still remains. I obtained the S11 and S21 parameters for an empty waveguide to maybe figure out a way to cancel the effect of air in some manner.
The thing to note is that S11 (real and imag) plotted in a linear scale behave in an out of phase manner, for the empty waveguide and for the sample, S11 (empty)
Hello,
In microwave characterizations, you must make sure that you followed the right procedures for characterization such as calibrations and sample preparations. Many factors could have affected your results.
I have the following advice for you:
1- Re-calibrate the VNA more than one time and characterize the sample each time to check the difference between results.
2- If you have any air gap in the sample, the results will never be accepted or OK. You must make sure to TOTALLY avoid the air gap. In my opinion, the best way to avoid air-gap is to fabricate your sample inside the sample holder (the one which you use during characterization) you can refer to my article in IET
Article Experimental and Computational Study on Epoxy Resin Reinforc...
@ One more thing, are you sure that there are not any air bubbles inside your sample? Having an air bubble inside the sample will also affect your results.
@ Have you checked the homogeneity of your samples? It is crucial to ensure that your sample is homogeneous.
3- You can also check the air gap by simply directing a strong light (for example, a mobile torch) to one side of the sample and observing the light on the other side of the sample (this method could help but it is not always effective).
4- Both sides of the sample should be smooth as possible.
5- If possible, try to characterize any other samples apart from your samples and check the results.
6- If you followed all mentioned advice and the results are still the same, then, I advise you to increase the thickness of your sample and also fabricate more than 5 samples and characterize all of them.
Any tiny or very small air gap will destroy your results. The air gap can never be accepted in such characterizations.
All the best!
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