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

Sat, 09 Feb 2002 09:43:57 -0500

As a follow up to this thread, has anyone figured out a way of obtaining accurate
antenna models when using insulated wire?  I would love to build a multiband quad
using teflon coated wire in order to minimize icing during the winter.

73, Joe

Tom Scott wrote:

> 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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> Arrow Electronics
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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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