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I want to put up a small tower and beam on my roof, but I don’t want the aluminum to “stick out” and want it to be unnoticeable as possible.

I would think black anodizing would be the best solution but it is very costly. I have access to powder coating and fully understand that you can’t coat the traps entirely and you would have to mask where aluminum parts would come in contact with each other. Same with any kind of wet paint/coating.

However I am worried that coating the antenna would affect performance or the factory would have coated it in the first place.

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Unless the coating contains metal, no it won't affect the performance.

As you've surmised, you are going to have to take steps to ensure good metal to metal contacts between sections and anywhere electrical continuity is desired. Either through masking before hand or steel wool/etc afterwards. You can paint traps, no problem. You could paint coils, though I wouldn't unless you've tuned the antenna and don't see having to mess with it again. Otherwise, you'll have to clean off a new place.

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It almost definitely won't - but there are effects that could be some coatings that would cause problems. It isn't just metal content, there's also the possibility that some coatings could absorb RF at certain frequencies. There are many amateurs who have tried this sort of thing and tested it, so the best bet would be to search for the specific product you intend to use to see if anyone has posted results online. If not, test the antenna before and after to see if the resonant frequency or gain has changed.

When using powder coating, make sure that whoever is doing it knows that the antenna may flex, otherwise the coating could become damaged too easily. Anodizing the antenna would prevent this problem, but may be more likely to alter the electrical properties - especially due to the "skin effect" causing more current to flow in and near the anodized layer than in the center of the antenna.

Other tricks include heat shrink tubing, which will form to the antenna. Some types will become stiff once activated, but will still allow enough "flex", others will retain their flexibility. All are non-conductive, so no worries about skin effect or shielding, they should have little effect on the antenna's electrical properties.

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  • $\begingroup$ I really doubt anodizing would have any effect due to skin effect. Anodizing forms a layer of aluminum oxide, a good insulator. This isn't much different than heat shrink. $\endgroup$
    – Phil Frost - W8II
    Oct 31, 2013 at 12:55
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Yes. Any coating will change the velocity factor of the conductor. However, this change is minimal, and isn't generally worth considering. The difference will be tuned out when you do your antenna tuning. You might want to try to account for it for beam antennas at higher frequencies to get the best performance, though.

The amount of insulation you're suggesting, however, isn't enough to appreciably affect the velocity factor. Choose whichever option meets your other requirements, and once coated re-tune the antenna.

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  • $\begingroup$ The link you give says that a coating wont change the velocity factor: "it doesn't work that way. The wave is only slowed down if it propagates in the dielectric material alone." Velocity factor is a property of the dielectric, not the conductor. In the case of an antenna, the dielectric is air. Saying that a thin coating changes the velocity factor is like saying a bridge constructed in China changes the tuning of your antenna. Sure, I suppose there's some degree of capacitive coupling, but it's far beyond insignificant. $\endgroup$
    – Phil Frost - W8II
    Feb 11, 2014 at 15:50
  • $\begingroup$ @PhilFrost The link includes several positions on the matter, one of which says it doesn't matter,and two further that provide correction: "Actually, using insulated wire for an antenna does change the antenna length due to the velocity factor" and "The insulation will shorten the antenna." I stand by the statement that "Any coating will change the velocity factor of the conductor. However, this change is minimal, and isn't generally worth considering. " $\endgroup$
    – Adam Davis
    Feb 11, 2014 at 16:38

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