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So I'm still QRT; apart from a brown-out on the rig, the support on the inverted-vee toppled over in high winds.

Thanks to the topple-over I discovered the conductors (SWG 16 bare copper) had started to turn greenish-black; a sure sign of oxidation. This may be because I was QRT for a while before the mast came down. It could probably happen of it's own accord eventually even if the station were on the band with any regularity.

My antenna was an inverted-vee with exposed conductors. It may possibly happen on other wire antennas (sp. antennae??) as well.

Which brings me to my questions

  • Does oxidation of the antenna elements have any impact on the signal quality of the station?
  • Is there a simple mechanism to avoid/mitigate such oxidation on the antenna elements?
    • E.g. Use a laminated conductor instead of bare
  • Is any regular maintenance activity necessary on an antenna?
    • E.g. In my case, how much should the conductors be allowed to darken before replacing/abrading/scoring the conductors?
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If the oxidation is superficial, that is it hasn't penetrated to a significant depth, it will not affect the antenna. Most metal oxides are good insulators. The antenna is already surrounded by a good insulator: air. Adding a thin layer of another insulator (metal oxides) doesn't change the electrical picture in any significant way.

You should look for corrosion on any connectors or mechanical contacts. The corrosion might work its way into a connection, and since the corrosion is not a good conductor, the contact resistance will be increased.

Antennas can be made to be nearly maintenance-free if you design the connections to avoid corrosion. Protect any connectors from the elements with self-amalgamating tape, or pot them in epoxy, or somehow avoid environmental exposure. Welded connections are great, where they are practical. Be mindful of galvanic corrosion.

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I don't know if a little oxidation affects the radiation of the elements. I guess it could, but I suspect, not enough to matter in the big picture.

I have an 80m inverted-vee that I soldered up out of spare parts and it's exposed, but hasn't been in the air all that long.

If there is still good, solid electrical contact on the connectors, then it's probably fine. If you wanted to be sure though, clean them off and then coat them with spray paint or lacquer. As long as it's non-metallic paint, it will be fine and keep the connectors from oxidizing. Seems like cheap insurance since the antenna is down already.

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Similar to anodizing and powder coating antenna elements, oxidation will affect the velocity factor of the antenna element, which in turn will affect the tuning of the antenna. The change will be very slight, though for the typically very long antennas in the HF band it might be noticeable and measurable. But that also means it's correctable. You can use an antenna tuner to mitigate the effects, but you'll probably want to retune your antenna yearly. Even with the oxidation, the antenna is perfectly fine as long as it's treated like any other bare antenna, and is well insulated from nearby conductors.

The green is typical copper oxidation, the black may indicate Copper(II) oxide, which typically requires higher temperatures to form, so it might be something else. It can be a skin irritant, though, so wear gloves when handling it.

As mentioned in WPrecht's answer, the oxidation will get in the way of transferring power to the antenna at any point where you want conductors to contact. Make sure they are clean, consider using a good anti-oxidation conductive joint compound, and sealing the joint to make it airtight so you don't have to perform maintenance frequently. You'll want to check it yearly as well.

I wouldn't recommend spending much time replacing the wire with something different. The few problems of oxidation aren't significant enough to warrant replacement, or the effort removing the oxidation and coating it.

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  • $\begingroup$ Conductors don't have a velocity factor: dielectrics do. For an antenna, the dielectric is mostly air. You'd need an absurd thickness of oxidation to make any significant change. $\endgroup$ – Phil Frost - W8II Feb 11 '14 at 16:08
  • $\begingroup$ @PhilFrost Yes, which is why I state, "The change will be very slight, though for the typically very long antennas in the HF band it might be noticeable and measurable." Just because the change is inconsequential for most applications doesn't mean it shouldn't be pointed out, particularly for those few situations where it will matter. $\endgroup$ – Adam Davis Feb 11 '14 at 16:41
  • $\begingroup$ Whether you think it's a significant change or not, saying that it will affect the velocity factor of the conductor is wrong. Conductors don't have a velocity factor, dielectrics do. Velocity factor is a function of permittivity: $1/\sqrt{\varepsilon_r}$. Permittivity is a measure of how much a material aligns its electric dipoles with an external field. Conductors, by the definition of conductor, hardly have an electric field within them, so permittivity of a metal, and thus velocity factory is not a useful thing to consider in this context. $\endgroup$ – Phil Frost - W8II Feb 11 '14 at 19:38
  • $\begingroup$ I've changed the word from "conductor" to "antenna element". Is that more clear? $\endgroup$ – Adam Davis Feb 11 '14 at 21:21
  • $\begingroup$ No, not really. The problem is not that it's unclear, it's that it's misleading. You need two things to have a velocity factor, because the velocity factor is a property of the wave, which travels in the stuff between the two things: the dielectric. Transmission lines have specified velocity factors because they contain two conductors and guide a wave between them through a specified dielectric. An antenna element does not have a velocity factor on its own because it's only one thing. We are changing the VF of the waves in the field between the element...and what? $\endgroup$ – Phil Frost - W8II Feb 11 '14 at 21:59
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Surface oxidation will lead to two effects impacting antenna performance

1) building a dielectric layer between radiating (or passive element) and air - here I concur with @Phil Frost that the effect is neglectible (except maybe at microwave frequencies)

2) an oxide layer will change the surface of the radiator, i.e. the surface will become more rough. Knowing that the current density in a conductor is maximum near its surface (-> skin effect) this can be noticed - especially on higher frequencies. I have seen myself on frequencies of >= 430MHz that polishing aluminium rods (and let the polished surface passivate by air oxidation which is a good protection) did improve the performance of my self built antennas. For HF antennas (<= 30 MHz) I couldn't find any significant effect though.

3) Corroded connection points of course always impose a danger and such connection points should be regularly inspected, maintained and protected ... not only cable/radiator joints, but also soldering spots within baluns etc. as everywhere, where two different metals meet in moist conditions (air humidity, etc.) local elements may form and acellerate corrosion up to the point of permanent damage.

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