material absorption test

hi
I'm going to test the absorption of some material.I designed some test set.The test set is as described bellow:
In antenna chamber, I situated the material in one side and two antenna and vector network analyzer in the other side.Imagine that I have some rectangular plate that one side is covered by a material and the other face was leaved free(there is no material covering this face) and the plate is metallic.In the first step the uncovered face will be directed to 2 antennas(one transmitting and the other receiving).I measure the value of S21 and record this result as a reference for different frequencies.Then I will turn the plate and the covered face will be directed to antennas.Now I will record the data for different frequencies.Final step is the comparison of the result for covered and uncovered data considering the uncovered data as reference.so the difference between reference and covered face data.
Some points in the test set is that:
1)Both antennas are horn antenna(wide band one:2-18 GHz)
2)Antennas are situated in the same polarization
3)The measurment method of VNA is TDR(Time Domain Reflectometery):I meant I can cancel the coupling between antennas using this method and see the plate in time domain
So what do yo think?Is it an efective and true method?If no Do you have any suggestion?
Thanks in advance

hi again
any suggestion please?

It is possible that your VNA have internal calibration functions for measurement setup like this one: Free Space Measurement.
Each VNA vendor often have additional information about how to calibrate for this type of setup.

E Kafeman said:

It is possible that your VNA have internal calibration functions for measurement setup like this one: Free Space Measurement.
Each VNA vendor often have additional information about how to calibrate for this type of setup.

Click to expand...

If you use VNA, you can determine the complex permittivity of material samples.
If you need to determine material absorption or loss (the imaginary part of permittivity), you can as well use only a CW signal transmitter and receiver (detector), with a variable calibrated attenuator inserted anywhere ( before the detector may be preferred). And directional antennas, to focus all RF or microwave power along the line passing through the tested sample.
Without the tested sample, adjust the attenuator to a suitable reading, e.g. 10 or 20 dB approx. Then insert the tested sample and adjust the same reading by the attenuator. From both settings, the difference equals the loss of the material sample.
Various materials may be grainy, irregular shaped, etc. Then the orientation of a sample between the antennas is important. For complex materials, I can recommend using RF noise which does not create interference field and the tested samples can even move during the test.

jiripolivka said:

If you use VNA, you can determine the complex permittivity of material samples.
If you need to determine material absorption or loss (the imaginary part of permittivity), you can as well use only a CW signal transmitter and receiver (detector), with a variable calibrated attenuator inserted anywhere ( before the detector may be preferred). And directional antennas, to focus all RF or microwave power along the line passing through the tested sample.
Without the tested sample, adjust the attenuator to a suitable reading, e.g. 10 or 20 dB approx. Then insert the tested sample and adjust the same reading by the attenuator. From both settings, the difference equals the loss of the material sample.
Various materials may be grainy, irregular shaped, etc. Then the orientation of a sample between the antennas is important. For complex materials, I can recommend using RF noise which does not create interference field and the tested samples can even move during the test.

Click to expand...

hi
thanks for your replies
there are some unknown items in your reply for me
would you please clarify me or introduce me some tutorial??
unknown parts are:
1)how can I measure permittivity using VNA?you meant the method that kafeman pointed out in his replied link??
2)would you please explain about the RF noise?
3)and the same please explain me the interface field??
THANKS FOR YOUR REPLIES

It is a little tricky to do such a measurement in free space. You have to take into account the reflected wave from the first surface (air to material), internal reflection and resonance ripple from the 2nd surface (material to air), and of course the free space loss and antenna gains. It is easy to make the measurement, but difficult to interpret the results with any accuracy. Accuracy improves with thicker materials and higher loss materials.

If it were a low loss material, I would probably chose a different method, such as a resonant circuit with the material filling the cavity, and measure resonant unloaded Q.

In my view, bulk material and free space reflection are rather different measurements, although related to the same parameters. The reflection measurement also refers to surface structure and similar properties. If it's basically a homogenous lossy dielectricum, you can expect better accuracy from the bulk measurement, I think.

I would go for using two calibrated near field probes and doing a relative measurement, instead using antennas, to minimize reflections, refractions, or other multipath interferer.

**broken link removed**

vfone said:

I would go for using two calibrated near field probes and doing a relative measurement, instead using antennas, to minimize reflections, refractions, or other multipath interferer.

**broken link removed**

Click to expand...

There are MANY various methods for microwave material testing. I would recommend Agilent booklet (try google) on some methods, also using a VNA. Damaskos and other companies utilize special arrangements for the bulk samples, flat samples, etc. I used noise for moving sand, for low-loss irregular samples, and for reflectivity, too. I used the normal-incidence method at ~50 GHz to measure the complex permittivity of concrete, with better results than others using a special radar polarimeter for the same sample.