I've been following this silver discussion for some time. Jon hit
the nail right in this simple explaination of copper versus silver
for RF conductors. In my own experience with broadcast transmitters,
100 Mhz 1/2 wave and 1/4 wave cavities, silver used to be common. Esp
in the sliding contacts of a 1/4 cavity, where a short is moved to
tune the tube to resonance. Copper would be a disaster over time, as
copper oxide will be a poor interface. The green stuff is not good
for IMD, nor RF connections (sparks, makes noise, burns). We made our
big cavities out of aluminum in the 1980s, as did some of the other
major manufacturers such as Continental Electronics. But we were
trying to move away from sliding contacts too, and tuned with paddles
and loops that have flex joints but not wiping contacts under high RF
current. Chromate conversion coatings are applied to the aluminum to
prevent the nasty oxides which cause a bad contact. Also looks nice
for many years. Gold or clear Irridite are some of the trade names.
We did a lot of copper prototypes, just because its easy to cut and
bend. Much too heavy to sell, and must be plated to appeal. Aluminum
with a clear coat of Alodine or Irridite looks silver and shiny for
years. And it is a reasonable conductor.
At work here, our Continental PA's that provide 3 MW peak at 200 MHz,
or 300 kW average power, are made from aluminum, copper, brass. The
copper or brass are always plated with silver. But the PA tuner is a
sliding short in a cavity made of aluminum! Otherwise it would be
extremely heavy and easy to dent when working with a crane to change
the tube. Then it is nickel flashed, and the contacts are typical
silver plated fingers. Over time, the nickel seems to hold up best,
without oxides which are going to cause changes in the RF resistance.
The general losses in the nickel are higher than silver or copper,
but thats the trade off they made.
As far as the effects on Q, I think cavity designers for very high Q
still consider silver. When you are dealing with many kW of power
loss, as in a particle accelerator, yes, it makes a big difference.
Also, in a very tightly tuned filter. But in a general purpose PA for
ham, commercial use where the loaded Q is perhaps 6-10, and unloaded
is under 100, who cares? The operating point of the tube, A, AB, B, C
makes a big change in PA efficiency, but the cavity losses might be
in the noise. Take PA tube efficiency and multiply x circuit
efficiency, right? So its 95 instead of 98% cavity efficiency.
When it means sparking and unreliable tuning as I mentioned above -
or IMD in comm cavities as Jon mentioned - take care and plate it.
Why we are still even wasting time on this subject? If you want a
pretty cavity or coil, plate it. Sometimes we even use that rub on
silver paste, that electricans use for contact repairs in switchgear,
for repair work on RF contacts. If you expect to see big
improvements, go check your math. If you have an arcing point with a
contact that has to last in humid weather and long term, then
consider silver. The oxide is not so bad, it still is conductive.
When i touch a coil that is back from the plater, and those big black
marks appear from my fingers, I don't worry as far as the RF changes
go. But the customer might not buy it, so I would wear cloth gloves.
BTW, Delta Electronics (one of my first jobs out of college in the
70's) made big roller inductors for TX antenna tuners for the
government, that were pre-oxidized silver, totally black. That way
they were not so ugly when handled, and they actually were better
thermal radiators. I don't see them on their website these days:
http://www.deltaelectronics.com/
73
K5PRO
John
At 11:46 PM -0400 9/23/99, Amps Digest wrote:
>Now, comparing silver to copper. There is not much difference in pure RF
>performance. Yeah, silver is a little better but not much. So why not
>use copper filters instead of aluminum? The reason has to do with
>corrosion and intermodulation distortion. Back in the early days of
>cellular, Allen Telecom held the market in cellular cavity filters. All
>of them were copper. However, in the UK, some of the operators started
>having problems. The company I sell for (based in the UK) did studies
>and determined that the problems in the base stations were due to the
>copper filters. The copper oxide was causing passive intermod problems.
>Silver is far better than copper in preventing the generation of passive
>IM. And silver tarnish (oxide, sulphide, whatever) while not as
>conductive as untarnished silver is a better conductor than copper oxide.
--
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