[BC] RF gasket in CCA

[John Lyles]

I have always had the best results using beryllium copper finger contacts, like Instrument Specialties, Tech Etch, Laird, make. These can be obtained in clip in versions that will fit against the edge of a panel like a door, and compress when it is shut. The companies have formulae that enable you to calculate the force that a certain contact linear surface would have using their product, and are compressed to about 1/2 of the free standing height of the fingers when under contact pressure in a door. Nice thing about the clip in versions, is that they can be easily replaced if they get crushed or fall off. The finish can be bright tin or just copper finish on your aluminum as long as you don't have a corrosive or damp salty atmosphere.  

I've been nervous for years about using the knitted gaskets that look like steel wool woven around or in a rubber compound or without. For one thing, they wouldn't seem to have the contact surface to carry many amps of circulating RF current. And second, they might crush and little wires begin to fall out, into your cavity. But if H used them, they must be OK. At BE on the original FM3.5 and 5 i used Spira, with the rubber strip gasket beside it (outside the RF circuit). Not sure if later BE 5's continued with that or went to Instrument Specialties. 

Somehow I am not surprised that the old CCA missed that detail. When I left the RF gasket off an early BE 5kW prototype with a rectangular cavity, in the lab in Quincy, and fired up, the DMMs and VOMs (Simpson 260 of course) in the lab were acting crazy. It only took a few minutes for me to figure out that I had a nice slotted radiator, when I rested my arm against the working transmitter, and smelled burning flesh where I bridged that seam on the door.....Ouch. 

John L.