[volt-nuts] Best way to measure micro Ohms

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 19 September 2017 at 20:32, Charles Steinmetz <csteinmetz at yandex.com>
wrote:

Looking at the pix, there appear to be lots of aluminum joints due to the
> "built-up" construction, maximizing the potential for the sort of troubles
> you are having.  I would re-make the piece in brass, doing everything
> possible to use as few pieces as possible (for example, by milling recesses
> into one piece rather than building up a compound piece from more easily
> machined sub-parts).
>

A practical problem is the tools available to me. The U-channel was
machined by someone in my radio club, and the rest I made myself using
nothing more than a drill and hand tools. It would be nice to make more out
of one piece, but it would require better tools than I have readily
available. There are certainly engineering companies that could do a better
job, but it would be quite costly.


>
> I would also plate everything in silver.


Yes, I was thinking that.

I contacted one local plating company, and asked them about silver plating.
The lady said they did not do brass, but did aluminum. This struck me as
odd, as they had plated brass for me before -  but that was gold, not
silver. From what I read, aluminum is a lot more tricky to plate than
brass. I sent them a drawing some time ago, but got no response. I will
have to chase them up.

The conductivity of aluminum is better than brass, and since I am mainly
interested in low frequencies (1.8 to 28 MHz), the skin depth will be
deeper than it is possible to silver plate. So the "RF resistance" might
well be set by the material its constructed from, rather than any plating.
Of course, if the plating stops an oxide building up, that should cut the
losses.

But certainly something is not quite right, as the RF losses are higher
than expected. I was keen to see if any of those could be explained by DC /
low frequency losses.

Best regards,

Charles


Dave, G8WRB