Radio WC6W wrote:
>
>>>Hello,
>>>I am new to amp design / building and have
>>>a question for VHF / UHF is there any advantage
>>>to having the anode cavity silver plated ?
>>>If so what spec (i.e. ISO or BS number) for the type
>>>grade of silver plate and to what thickness.
>>>
>>>
>>>Thanks Martin
>>>
>
>Hi Martin,
> In all the discussion that evolved from your query, I didn't notice
>happen to notice the answer... unless my delete finger outran my
>eyesight. :-)
>
> Yes, silver has the lowest electrical resistance, which a couple other
>postings noted but, at high frequencies the slight apparent advantage
>over copper is enhanced due to skin effect.
>
> For radio work you want to plate with 999 fine, or better, silver, in
>plating bath without the "brighteners" that are used for plating jewelry
>or silverware.
>
> Since skin depth at 100 MHz is on the order of .0003 inches, silver
>plating of .001 or thicker will be sufficient for VHF and up.
Here's more information, from a posting by N7WS some time ago that I
retrieved from DejaNews. It emphasises my original point that you have
to be very careful about the *kind* of silver plating you have done, and
Marv's requirement for "999 fine".
N7WS wrote:
>I've sort of been following this thread while looking for a paper I
>tracked down the last time this issue came up.
>
>The paper is:
>
>"Radio Frequency Performance of Electroplated Finishes" by A. M.
>Fowler. Mr. Fowler was a contributor to some of the previous
>discussions and mentioned the paper, although I didn't ask for a copy
>of the paper since he is in VK land.
>
>The paper was presented at the Institutes 1967 Radio and Engineering
>Convention in Sydney in May, 1967 and published in the Proceedings
>I.R.E.E. Australia, May 1970. (I guess they move kinda slow down there,
>being upsidedown.)
>
>I will directly quote from the Summary.
>
>"It is often found that silver-plating a copper conductor increases the
>radio frequency losses instead of reducing them as expected. The main
>causes of this apparent anomaly are found to be:
>
>(a) the conductivity of an electroplated metal is normally lower than
>that of the pure, cast and wrought metal;
>
>(b) the way that the current is distributed between the layers in a bi-
>metallic conductor;
>
>(c) the use of unsuitable intermediate layers beneath the silver, and
>
>(d) the use of a thick medium conductivity protective layer to prevent
>the silver tarnishing.
>
>From a consideration of these various problems, it is shown that the
>most satisfactory low-loss finish can be produced by first plating with
>a high conductivity copper plate, and then following this with a very
>thin layer of low conductivity material to provide corrosion
>protection."
[That's what was in the back of my mind... ]
>
>Mr. Fowler discusses the fact that commercial platers are, for the most
>part, attempting to deliver a particular surface finish
>(brightness, hardness, etc) and not necessially a high conductivity
>surface.
>
>One interesting quote is: "The silver deposit is readily attacked by
>sulphides present in the atmosphere, with an increase in the contact
>resistance and severe discolouration. In a typical case the contact
>resistance of two silver-plated wires rose from 6 milliohms to 200
>milliohms after two hours' exposure to hydrogen sulphide."
>
I distinctly remember Mr Fowler himself posting messages on one of the
newsgroups or lists, but couldn't find any of them on DejaNews by
searching on "silver + australia" or "silver + fowler". It would be very
good if someone more experienced with DejaNews could pull out the
original messages, from the guy who did the work.
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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