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

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). . .

You're correct that most of the current flows in the first skin depth,
but a signficant fraction, 37%, does flow at greater depths. (This
number is both the fraction of the surface current density at one skin
depth and the fraction of the total current that's below that depth.)
The current density at 3 skin depths is 1/e^3 or about 5% of the density
at the surface, and at 5 skin depths, 1/e^5 = 0.7% of the density at the
surface. So it's reasonable to state that the material below a depth of
a *few* skin depths isn't important. At RF with metallic conductors, a
tube with any reasonable wall thickness is at least several skin depths
thick, so I have no disagreement at all with the conclusion.

Regarding the importance of the material resistivity, it can be argued
that it's actually less important at RF than at DC. The reason is that
the skin depth is greater when the resistivity is greater. If a material
is twice as resistive as, say, copper, the skin depth will be sqrt(2)
times as great, resulting in an RF resistance which is sqrt(2) -- about
1.4 times -- greater than copper rather than twice as great.

I hope none of this has detracted the original poster's attention from
the good advice given early on in the thread -- that virtually any metal
is just fine for his receiving antenna. He won't be able to notice the
difference between one and the other. There are two reasons for this:

1. The efficiency of most common antennas is so high that making them
out of even quite poor conductors won't reduce the efficiency enough to
be able to notice or even measure; and more importantly,

2. Efficiency doesn't matter anyway for HF receiving antennas, except in
very special cases where the antenna is extremely inefficient and the
receiver noise figure is extraordinarily high.

There are situations where it's important to pay attention to material
conductivity and to understand skin effect. This isn't one of them.

Roy Lewallen, W7EL

The RF resistance of a wire is the same as the DC resistance of a tube of
the same outer diameter and a wall thickness equal to skin depth.

This allows a pictorial representation of what takes place.
----
Reg, G4FGQ

Reg Edwards wrote:

The RF resistance of a wire is the same as the DC resistance of a tube of
the same outer diameter and a wall thickness equal to skin depth.

That is correct.

This allows a pictorial representation of what takes place.

Yes, but not an accurate one.

----
Reg, G4FGQ


Roy Lewallen, W7EL