On Mon, 30 Aug 2004 09:28:06 +0300, Paul Keinanen
wrote:

Due to the skin effect, RF currents only flow on the surface of the
conductor. At VHF frequencies, the skin depth in copper is only about
5 um, in which nearly all RF currents flow, thus the low resistivity
of this thin surface layer is critical. From RF point of view, it does
not matter much what is below this surface, so it might be empty (a
tube) or it might be something with lower conductivity, such as
stainless steel (with much greater mechanical strength).

Another plus of this is used in some high-powered industrial equipment
that uses RF, and I believe some broadcast equipment. For cooling,
copper tubing is used as the RF conductor (skin effect only) and water
is pumped through the inside. A few tricks of the trade, like using
pure water without dissolved ions, and connecting the plumbing with
non-metal parts to isolate the RF, are involved.

Sometimes the copper is plated with silver, not so much for better
conductivity as the better oxidation properties of exposed silver.
Gold would be a bit better, even if a lot tougher on the budget.

Happy trails,
Gary (net.yogi.bear)
------------------------------------------------
at the 51st percentile of ursine intelligence

Gary D. Schwartz, Needham, MA, USA
Please reply to: garyDOTschwartzATpoboxDOTcom

Gary Schwartz wrote:
"For cooling, copper tubing is used as the RF conductor (skin effect
only) and water is pumped through the inside."

That`s exactly the structure of the final amplifier tank coil of the
100KW GE watercooled shortwave transmitters used at RFE. The coil was
silver plated. We distilled our own cooling water and the water
circulation system was connected by Saran tubing. Though many KV were
applied to the plates, d-c leakage was insignificant.

Best regards, Richard Harrison, KB5WZI

On Mon, 30 Aug 2004 08:42:27 -0500, (Richard
Harrison) wrote:

Gary Schwartz wrote:
"For cooling, copper tubing is used as the RF conductor (skin effect
only) and water is pumped through the inside."

That`s exactly the structure of the final amplifier tank coil of the
100KW GE watercooled shortwave transmitters used at RFE. The coil was
silver plated. We distilled our own cooling water and the water
circulation system was connected by Saran tubing. Though many KV were
applied to the plates, d-c leakage was insignificant.

Best regards, Richard Harrison, KB5WZI

Almost like I have seen high powered RF coils like this. ;-)

I've seen them, used them, and fixed them. I think the biggest system
I worked on was a mere 25 or 30 kV. Not broadcast, but plasma
generating process equipment.

Only so many reasoanble ways to solve this problem.

Your water cooling does need to be able to handle the heat generated
by significant mismatches, not just ideal operation. It is bad luck to
melt parts of your cooling system.

Happy trails,
Gary (net.yogi.bear)
------------------------------------------------
at the 51st percentile of ursine intelligence

Gary D. Schwartz, Needham, MA, USA
Please reply to: garyDOTschwartzATpoboxDOTcom