GBPPR Tech Bulletin #3 - Why Silver Plate RF Components?

Excerpt from Carl, KM1H

Silver plating is used to preserve performance and RF conductivity in widely varying enviroments such as the military.  For average ham use it probably is not important.  The output of my 432 MHz cavity amp has not changed in 20 years and it is all copper and brass.  The only true silver plate is the industrial style hard plate and there are variations there also.  Best to consult a professional plater for details.  The brush-on soft plate is a sham and has been described as such by many VHF/UHF authors.

Excerpt from Gary, KE4ZV

... Perhaps, or perhaps not.  Since a lot of RF conductors are silver plated, and silver isn't cheap, it seems a bit hard to swallow that those hard headed manufacturing engineers are doing it just for decoration, especially since most of that silver plate is never seen by the customer.  Even Detroit doesn't chrome plate the inside of its engines without a sound technical reason.

So perhaps we're overlooking a factor here.  I think Jack may have given us a clue.  Lets see what the oxides of the two metals look like.  Right away we notice something striking.  CuO (copper oxide) has a magnetic susceptability of +267 while AgO (silver oxide) has a magnetic susceptability of -19.  Hmmm... now that seems to put us on the footing where we found ourselves when discussing steel wire for antennas a few months back.  (By way of comparison, the magnetic susceptability of FeO (ferric oxide, rust) is 293.)  The much higher magnetic susceptability of CuO means the skin depth will be *very* thin, and net skin resistivity very high.  Meanwhile, the negative value for AgO means that the skin depth will be greater than anticipated, so the margin between the two for RF seems much greater than 3%.

Figures are from the CRC Handbook, 47th edition, table starting on page E-103.

As with steel versus copper for antenna wires, we have to look beyond DC resistance to find out which material is better suited to the task.  It turns out that oxidized copper is almost as bad as oxidized steel as an RF conductor.  That shouldn't surprise us, corroded coax (from water wicking into the braid) is a very lossy medium.  Now we know why.

Excerpt from Rudy, N6LF

Sometimes silver plating is suggested for conductors.  The conductivity of silver is only 6% better than copper, but when the surface oxidizes, silver oxide is a much better conductor than copper oxide.

Excerpt from PolyPhaser Technical Document PTD1010

Silver oxide is the only oxide (that we know of) that is conductive.  This is one reason why PolyPhaser's N-type coax connectors are all silver with gold center pins.  Copper oxide is not conductive and the proper application of joint compound will prevent oxidation.

Excerpt from the ARRL Handbook

... Silver plating the tubing further reduces losses.  This is especially true as the tubing ages and oxidizes.  Silver oxide is a much better conductor than copper oxides, so silver-plated tank coils maintain their low-loss characteristics even after years of use.