[Moon-net] Question regarding Loss due to Oxidation

[Chris Bartram]

I can't answer Charlie's question definitively, but I don't think that mild 
oxide tarnishing of the copper of a feed will make a very big difference to 
performance. The surface current densities in most feed designs tend to be 
fairly low, and the presence of a very small amount of an insulating (or 
semi-insulating) material on the surface of the material shouldn't add too 
much to the insertion loss. After all, we're all (I assume) comfortable with 
the use of aluminium as a construction material for feeds: a thin layer of 
aluminium oxide forms on the surface of that metal when it is cut.

There is some published work (Smith and Sokolwski) on corrosion from sulphur 
and chlorine based compounds, but I couldn't find any definitive information 
on oxide-based tarnishing. 

I also tried to find a figure for the resistivity of copper oxide in my 
reference books, but it isn't there. Neither is it in the on-line Kay and 
Laby reference. It's probably in the the CRC 'Rubber Handbook' but I don't 
have a copy. With that information, it would be possible to make a realistic 
estimate of the losses.

The major reason that waveguide runs are pressurised with dry air is to keep 
water vapour out. That's a real problem, but with short runs, a soluble one - 
I hope!!

Don't forget that although we tend to think of waveguides as essentially 
lossless, they are still lossy! At 10GHz WR90/WG16/R100 has a loss of about 
1dB/6m. That's a lot less than coax, but it is still finite! A 1m length of 
WG ahead of the preamp could do serious damage to a carefully optimised 
receive system!

Vy 73

Chris
GW4DGU

Christopher Bartram MIET
Christopher Bartram RF Design
Blaenffos
Pen-y-Bont
CARMARTHEN
SA33 6QG

phone:  	+44 1994 484650
		+44 1994 438165
voip/sip:	3012 3279
mobile:		+44 7968 193555
email:  	chris at chris-bartram.co.uk
URL:		www.christopherbartramrfdesign.com