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How could one compare different paints for a UHF / microwave dish ?

Question asked by drkirkby on Dec 15, 2012
There was a discussion on a mailing list I subscribe to (moon-net) which is for radio hams interested in earth-moon-earth communication. i.e. bounce a signal off the moon, which requires fairly high powers, sensitive receivers and high gain antennas. Typically amateurs do this in the frequency range 144 MHz to 10 GHz, with most using 144, 432 or 1296MHz. 

The question was about what one should paint microwave dishes with. Obviously you don't want the dish corroding, but neither do you want the paint to impact the RF performance of the dish. Someone had reported painting their dish 30 years  had caused a major loss of signal, and so had to remove the paint. I suspect, back 30 years ago, the paint may have contained lead, which might not have been an ideal subtance. 

Someone else found out the paint he has used has 5% carbon in it, but he can't measure any conductivity of that with a DVM. 

This got me wondering how one could best evaluate protective coatings for use on dishes. I originally had an idea of using two coax to rectangular waveguide transistions, with a piece of paper in between the  two of them. Then put another piece of paper which has the paint on it. Given the paint is likely to change both S11 and S22, one would need some care to work out the attenuation of the actual paint in free space, as it wont simply be the change in S21 as measured in a bit of waveguide. Then of course to complicate matters, the paint wont be in free space, but on a dish. One can't use waveguide at lower frequencies anyway. 

To complicate matters even further, the particular dish was  fibreglass, with wires burried in it. which might cause a problem if the paint was a significant fraction of a wavelength from the wires. 

Does anyone have any ideas how one could best determine the performance of paints for dishes? I can't help feeling a VNA would be a good tool for this, but exactly how best to do it is another matter. 

I know Agilent sell the 85070E Dielectric Probe Kit. How suitable would that be for this purpose? 

Dave, G8WRB.  

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