Hi Mickey

Copper is GREAT for Antenna's!
It's only slightly heavier than Aluminum.
But their are MAJOR advantages to using copper.
Solid corrosion free joints is only one of them.

There are three (without getting technical) basic copper tubing types
available. Soft Copper (easily bendable), Hard Copper (water pipes,
which can be swedged under heat), and Annealed Copper (refrigeration
tubing).
Regular Hard Copper water tubing is sufficient for most antenna work.
But if your building something like a long Yagi, you definately will
want to use Anneled Copper for the boom or you will wake up with
U-shaped Yagi one morning.
You can also often work in smaller pipe sizes when using the Annealed
Copper as well. Such as 1/2 inch Annealed instead of 3/4 inch Hard.
Even the soft copper comes in handy for some non-supporting components
such as coils and the like.

When joining copper using fittings or swedged joints use either
standard plumbing methods or preferrably silver solder in these
joints. When building things like Yagi's where you may be passing the
elements through the boom without using fittings, copper phosphorus
bronze brazing rods work at propane torch temperatures and make a more
secure joint than silver solder on these types of connections.

Where electrical connections need to be made (and often may need
changing or maintenance) I found that adding a soft copper tube or
heavy copper wire, affixing it to the antenna with copper phosphorus
bronze, allows a good soldering gun and rosin core solder to make the
electrical connection to the pigtail installed for this purpose.

TTUL - 73+ de Gary - KGØZP

On 29 Aug 2004 12:28:31 EDT, am (Gary V.
Deutschmann, Sr.) wrote:

Copper is GREAT for Antenna's!
It's only slightly heavier than Aluminum.
But their are MAJOR advantages to using copper.
Solid corrosion free joints is only one of them.

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).

However, if the antenna is not protected by a radome, the weather will
within a few years affect the copper surface, creating oxides and the
copper becomes green or dark brown. Now the question is, what is the
conductivity of these surface layers. If these layers are isolators,
the RF current will flow in the clean copper below, but if the layers
have some mediocre conductivity, part of the RF current will flow in
these oxide layers, increasing the total resistance.

This should not be areal problem with VHF antennas made of tubes,
since the length to surface ratio is quite small.

Paul OH3LWR

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

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