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qrp-gaijin
PostPosted: Mon May 14, 2012 10:33 pm 
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For my short, muitl-band, loaded dipole project, I'm studying up on how to wind high-Q loading coils, which is pretty interesting stuff (coils are complicated!). One thing that concerns me is protecting the copper coil from oxidization which could reduce the Q.

On the one hand, I want the copper coil wire to be bare, so I can tap it for multi-band use with an alligator clip. On the other hand, an insulated wire (maybe polyurethane?) would provide protection against oxidization but would require removing the insulation for the taps, and might reduce the Q as well. Silver plating is one option, as silver oxidization is still conductive, but seems expensive and difficult. I do have 2 percent silver solder - would it help at all to tin the entire length of the wire with silver solder?

Any ideas or suggestions? After I've would the high-Q coil, I imagine it will be very difficult after-the-fact to clean off any later oxidization, so I'm looking for good preventative measures.
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GL1000
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PostPosted: Tue May 15, 2012 9:04 am 
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Location: The Netherlands, JO21TQ
The multiband base loaded verticals i made have polyurethane coating on it and are 0,75mm thick. I made a groove on a piece of PVC pipe using my lathe so that the windings are appprox 1mm from eachother.

Every band has it`s own "coil" and going to another I use the alligator clip, connected to a piece I made blank.
bandwith is perfect, and enough to cover the band, even good results using the 817. High Q also narrows the bandwith and you have to adjust you coil again. Don`t try to get a very high Q coilyour antenna.

Option,..no coil at all, just twin lead wire going to a tuner also works OK.
Got my vertical now here the base coil completely by-passed, and twin lead connected to a MFJ tuner (MFJ-971).
As long as the SWR is beneath 1:2 it`s OK, better to get an antenna with a nice flat radiation pattern for the DX.

for 20M, around 5,15meters allu pipe vertical, and 3 groundplane wires makes a good DX antenna. (search for 20meters groundplane)

73
Frank

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exray
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PostPosted: Thu May 17, 2012 11:43 pm 
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I'm just guessing but I feel that a tapped coil will kill Q faster than the surface oxidation.

Something I read way back, I forget the source, suggested that due to skin effect the RF simply travels deeper into the copper. So its not like the oxidation is raising the RF resistance...its more like the wire is a tiny bit smaller diameter which is not an issue.

An analogy would be enamelled wire. The RF resistance doesn't change by the 'surface effect' of the enamel. The path is that of least resistance.
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qrp-gaijin
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PostPosted: Fri May 18, 2012 4:22 am 
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exray wrote:
I'm just guessing but I feel that a tapped coil will kill Q faster than the surface oxidation.

Heh, very interesting that the Q might be ruined already even before oxidization. Oh well, at least the antenna bandwidth will be wider :). Anyway, what is the deal with taps and low Q? Is the unused part of the inductor somehow causing loss? There was a discussion here a while back about how shorting turns kills Q, but the mechanism still isn't clear to me, and anyway shorting turns is different than choosing a tap point on a coil (where the unused part of the coil "floats" - is that the problem?).

exray wrote:
Something I read way back, I forget the source, suggested that due to skin effect the RF simply travels deeper into the copper. So its not like the oxidation is raising the RF resistance...its more like the wire is a tiny bit smaller diameter which is not an issue.

An analogy would be enamelled wire. The RF resistance doesn't change by the 'surface effect' of the enamel. The path is that of least resistance.

Sounds nice, and I hope it's true. I recall reading that copper oxidization is semi-conductive, which throws a monkey wrench into my ability to intuitively reason about its effect on RF resistance.

But if the Q is killed anyway through tapping, then I guess I'll just not worry about the oxidization for now.
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Bob Weaver
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PostPosted: Fri May 18, 2012 8:18 am 
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Location: Saskatoon
Time to examine some popular myths.
Components and Materials: Part 1 - G3YNH wrote:
Having praised silver however, we should state that there is some controversy over the matter of whether or not the silver plating of copper conductors is worthwhile [2][12], especially since the reduction in surface resistivity is only about 7.8%. The issue here is that silver plating definitely reduces RF resistance initially; but if the silver is allowed to tarnish, the RF resistance will eventually exceed that which can be achieved using plain-copper which has been subjected to the same environmental conditions. We can understand what is happening here by noting that both silver and copper will grow a surface layer on exposure to the atmosphere. In the case of copper this will be composed of copper oxides (and will include a mixture of copper hydroxide and carbonate if the wire goes green), whereas in the case of silver; the surface layer is composed mainly of silver sulphide. If the tarnish layer is moderately conductive, either throughout, or in the partially-formed boundary layer between it and the metal, some of the conduction current will flow in the tarnish layer, and this process will be lossy.

The above excerpt is from the following page:
http://www.g3ynh.info/zdocs/comps/part_1.html
The discussion of oxidation effects starts about halfway down the page, and is quite interesting reading.
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homebrew
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PostPosted: Sun Aug 19, 2012 8:58 pm 
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Mil specs on silver plated wire are very tight as far as tolerances on thickness so making your own probably will not be worthwhile.
Eyelett taps will lower the coil Q but smooth access points, bare wire, or with just the insulation stripped off should not. I prefer bare wire as it gives a infinite selection of tap points.
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battradio
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PostPosted: Sun Aug 19, 2012 9:45 pm 
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Location: near St. Louis MO.
Buy a rollar inductor , that woud be the easy way to do it .

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exray
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PostPosted: Tue Aug 21, 2012 12:55 am 
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Kinda unwieldy on a trap dipole!

Where have you gone with this, gaijan? In thinking it over I don't think Q is a huge factor in loading coils/dipole traps. All of the antennas like that I have ever used were pretty plain jane when it came to coils. I built one with typical coils wound on a pvc pipe and coax stubs as capacitors and didn't perceive any shortcomings.
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qrp-gaijin
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PostPosted: Sat Aug 25, 2012 1:20 pm 
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exray wrote:
Kinda unwieldy on a trap dipole!

Where have you gone with this, gaijan? In thinking it over I don't think Q is a huge factor in loading coils/dipole traps. All of the antennas like that I have ever used were pretty plain jane when it came to coils. I built one with typical coils wound on a pvc pipe and coax stubs as capacitors and didn't perceive any shortcomings.

I think (correct me if I'm wrong) that as the dipole is shortened severely, radiation resistance goes down, making coil loss more significant as it is then a larger percentage of the radiation resistance.

Anyway, I suspended this project (the short loaded dipole) because some NEC-2 simulations indicated that at a mounting height of 3m a 7MHz horizontal dipole with 5m length wastes most of its energy by radiating almost straight up. I've gone back to trying to build a good magnetic loop antenna, which has better low-angle radiation than a dipole at low heights.

However, even with a magnetic loop antenna, the surface of the loop conductor is still subject to the loss-inducing oxidation that prompted this thread. :)
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seanvn
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PostPosted: Sat Sep 01, 2012 6:23 am 
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Polyurethane for sure would cause very high dielectric losses at RF. Coat the coil with polystyrene dissolved in a solvent such as acetone.
The main plastic to avoid at RF is PVC. The best plastics are Polyethylene, Polypropylene and Polystyrene.
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