[AMPS] Silver Plating question

[Ian White, G3SEK]

Norman Hockler wrote:
>
>Guys...
>
>The advantage in silver plating is, of course, the lowering of  resistance 
>caused by the skin effect, and as we know all the current will travel on 
>the surface of the cavity.  The silver plating does not have to be very 
>thick to get the desired effect of the additional few watts this will afford.
>
>There are a number of low cost silver plating kits available.  I used one 
>for my 8877 amp that worked fine. Cost me $12.

It's unlikely that you would see much - if any - improvement in
efficiency in a typical VHF "cavity" amp.

The places where conductivity is the most important are the parts
carrying the highest current density (amperes per square inch). This
typically means the tuned line, which is generally made of copper
anyway. Pure copper actually has a higher conductivity than silver, and
copper has a higher surface conductivity for RF if the surface is kept
clean. 

The story that "silver is good because silver oxide is a good conductor"
is largely a myth. When silver tarnishes, it usually forms silver
sulphide, which is a pretty lousy conductor. In order to keep silver-
plated surfaces conducting RF at peak efficiency you still have to clean
it. 

The only significant advantage of silver plating over bare copper is for
use in situations where you cannot conveniently clean it - for example,
I use silver-plated conductors in sleeve balun assemblies that live
outside, and would have to be unsoldered to clean them. But in a VHF
amplifier, you only need to take the bare copper tuned line out to clean
it.

The other problem with silver plating is that it has to be done right,
or else the result will be worse than copper. Decorative silver plate
contains some nickel, and its RF conductivity is significantly worse
than pure silver. If you have the plating done for you, you can specify
pure silver, but you can't guarantee that they really did empty the
tanks and change the chemicals just for you. Do-it-yourself silver
plating gives you more control over the composition, but less control
over the crystal structure of the metal that's laid down - which can
greatly affect the conductivity.

The aluminum box parts of the cavity are no problem. Generally they
carry a lower current density than the tuned line, simply because there
are more square inches involved. Since losses are proportional to
(current density) squared, the losses are much lower.

I'd recommend using bare copper and occasional elbow-grease, and keep
the silver in your pocket.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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