Copper Vs silver plated tank coil

Cool Amp definitely helps on contact resistance, we use it all the time for that. But not for RF losses. Look up skin effect and see that you would need a great thickness of silver to carry RF current at HF. At VHF and UHF it begins to play (as Carl mentioned). Copper alone is very conductive, at first. But over time it oxidizes and RF conductivity decreases at the surface. So the RF current must penetrate deeper, according to skin effect theory. If there isn't enough material there, and it is very oxidized then heating and RF loss is apparent. Q goes down in resonators and inductors. Adding a silver skin, via plating, that is of appropriate thickness (typically recommend 3-5 skin depths to carry 99% of the current) and you now have a conductive layer over base copper (or other metal). Silver sulfides are less conductive, and form in the atmosphere over time. So the trade off is having something with better long-term stable conductivity (of lower mhos) versus something with high conductivity at first that degrades over time. I design high power cavity amplifiers for work, and this is a huge consideration. If there are tuning contacts, that must wipe the walls and work with hundreds of amperes of RF, then copper alone is not suitable except in a prototype. Right now I am operating an aluminum cavity (for weight and cost) at 250 kW average power at 200 MHz, that has a thin electroplate of copper (for adhesion) followed by 0.0006" thick pure silver plate. It took several companies and $11,000 of testing expense under electron microscope just to qualify a source to make this, that didn't have voids, and other problems. I have a great plater now who is NASA qualified and do first class work, for a higher cost than the local outfits.

A. Fowler published an interesting piece, 'RF Performance of Electroplated Finishes" in Proceedings of IREE in Australia in 1970. In it, he expounds on the idea that most commercial electroplaters don't do what RF engineers want, and use various alloys and additives such as phosphorus for de-oxidizing agents. He specifically is talking about copper tubing coils for transmitters in this report. He reports a range of conductivity of a plated piece from 0.13 to 95% of the conductivity of pure electrical copper, which is reference at 100%. So following Fowler's advice, you'd be making a mistake to silver plate a coil without specifically knowing the composition of the plating bath and even testing for conductivity. He does talk about tin plate (worse resistivity) and using a coating such as lacquer on top of bare copper to protect it.

Devices like roller inductors need to have silver or gold plating, just for low contact resistance for the wiper, similar to cavity circuits for the same reason. Materials such as Rhodium top layer have been applied in some cases, but care must be taken to protect it from fracturing. My advice to hams is to use copper and cover it with a spray finish for protection, unless you must have wiping contact, which then can use Cool Amp or silver plating. If you are running kW's above 2 meters, then you should consider further electroplating of the appropriate thickness to satisfy the skin depths required for current. Skin depth may be different for each material, so its not as simple as a single value for everything.