[Antennas] [Antennas]A bit of advice: Use insulated wire...

Thu, 7 Feb 2002 21:45:56 -0800

Rather than speculate on this issue, it has been very carefully tested by
Rudy Severn in QEX last year (I can supply the article details if you are
interested, it has been reported here before). In summary, it is true that
in most amateur cases, corrosion has fairly little effect on signal
radiation (usually less than 1 dB), but it is nevertheless quite measurable,
and there are some cases where the effect can get into the realm of several
dB (depending on antenna configuration). It is also quite demonstrable that
modern insulation with good dielectric characteristics substantially
eliminates corrosion and performs measurably better than corroded bare wire.

What Rudy did (VERY clever I thought, and I have personally duplicated his
results), is to create identical coils on a standard form with standard
turns and so on and then test the Q at frequency on a Boonton 260A Q-Meter.
My dad just happens to have one of those, so I was able to duplicate his
tests on samples similar to his as well as others that I was interested in.
He and I both used high-quality large ceramic coil form to achieve high Q
coils. It can be shown that the Q of a coil is directly proportional to the
series loss resistance of the wire at a given frequency when you hold the
other coil parameters constant. Rudy's data demonstrated that there was a
significant series loss resistance increase in corroded wire compared to
un-corroded wire. Since this test is at frequency under completely
controlled circumstances, I don't believe there is any reason to doubt the
calculated effect on series loss resistance. The effect was even greater on
stranded wire presumably due to it's greater surface area. I believe that it
is reasonable to assume that this effect is due to the skin effect forcing
some of the current into the partially conducting corrosion. Think of it as
a coating of carbon resistor on the outside of your wire. Since RF is forced
by the interior eddy currents to flow in the outer skin of the wire (skin
effect), this corrosion layer has a much larger effect than it would have at
DC.

The other part of the issue is to analyze the impact of an increase in
series loss resistance on signal radiation for a given antenna design. Rudy
was concerned because he was designing an antenna (I don't recall the type
now but could look it up) in which the currents were very high at some nodes
which meant that an increase in series loss resistance at those points would
be considerable. This is the case where you can find a several dB loss of
efficiency due to wire corrosion. A simple dipole on the other hand will
exhibit very small effect. You can demonstrate for yourself the effect of an
increase in resistance on a given antenna type by simply using a smaller
gauge wire on an antenna simulator. My recollection is that corroded wire is
about equivalent to one or two wire sizes smaller (ie: 14ga or 16ga instead
of 12ga).

One should of course be very skeptical of any analysis that attempts to
measure these kinds of effects by simply reporting results on the air, or
other subjective measures. While it is true that this is what we ultimately
care about, the measurement error inherent in such subjective analysis
probably becomes swamped by our personal preconceived expectations. If
improvements in receiver design were only made this way, we wouldn't have
made many of the advances in radio design that we have. I am impressed by
the technique that Rudy used because of the objective repeatable nature of
the analysis. I was able to replicate his tests with essentially identical
results.

To put all this in perspective, can we operate with corroded wire antennas
without noticeable effect, of course we can. But by the same token would
most of us notice the effect of a small sensitivity difference between one
receiver and another? Probably not. But how much do we pay for that last
little bit of performance in a run of coax or in the specs of a radio?
Within reasonable limits we should properly make all of our system design
choices to optimize performance, knowing that all these choices cumulatively
effect the ultimate performance of our system. Each soldered joint, isolated
feed line, clean connector, etc. may only contribute a little benefit, but
added together, we know they make a difference in our system. Given an
informed choice, if I am going to put up an antenna and there is no impact
on cost or other issues, I will choose to use insulated wire. I have no
doubt that you may achieve DXCC with bare copperweld or even galvanized
fence wire, but there is good data to show that there are better choices.
(Let's see now, where did I put that 12 GA silver plated solid copperweld
with the teflon insulation...?)

- Tom Scott, Field Applications Engineering
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-----Original Message-----
From:	[email protected] [mailto:[email protected]]
On Behalf Of George, W5YR
Sent:	Thursday, February 07, 2002 12:50 PM
To:	[email protected]
Cc:	Lefczik, Larry D; David W Sher; [email protected]; [email protected]
Subject:	Re: [Antennas] Need a bit of wire advice

Interesting . . .

Seems to me, and I think that Mother Nature concurs, that the layer of bad
crud would be no different than a layer of insulation (good crud) on the
wire. The current flow will always be on the surface of the lower
resistance conducting path which will be the uncorroded copper beneath the
crud.

I suspect that a negligible amount of r-f current would try to make its way
through the poorly conducting copper oxide. So, its higher resistance is
immaterial. I seriously doubt that any measurable difference in
driving-point impedance can be seen between a shiny new copper antenna when
it is first erected and a few weeks or months later when it has built up
its layer of crud.

Connections, IF made through the crud, would have higher resistance, of
course. But all r-f connections to the antenna will sensibly be made to
clean copper with solder so that there is intimate contact between the
feeder copper and the antenna copper, even though both may acquire a layer
of crud over time.

As to corrosion, many copper wires - antennas and otherwise - have managed
to stay up for years without succumbing to deterioration from copper oxide.
My limited understanding is that the oxide layer actually does insulate the
wire from further corrosion by other agents and acts as a protective layer.
And, that once the oxide layer is formed, its chemical reaction with the
copper stops and no further crud develops.

Don, I guess we just don't agree on this one . . .

72/73/oo, George W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771
Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437

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Don Havlicek wrote:
>
> The 'built-up crud' is oxidation.
> Copper Oxide and/or copper sulfate [IF sulfur is absorbed from the
> atmosphere], are both corrosive and have higher resistance to electron
> flow .. at DC or RF.  Connections with 'crud' will be higher resistance
> .. affecting the impedance of the antenna.
> Don
> N8DE

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