Does RF go through thin corrosion better than DC?

Question

I might have a bit of a problem with my groundplane, but I am unsure. I am using a ground screen instead of radials. The problem is the ground screen is made of galvanized metal, but I can see that a white layer of corrosion has formed. The connection to the groundplane at the base of the antenna is no problem; however, the way that each individual sheet of screen contacts another is simply by being a little overlapped and then stapled to the ground. Is this good enough? Let's say the oxide did form between them, does it even matter? I read that RF travels on the surface. Will it just permeate this layer?

I know this is a hard question to answer without seeing the situation, but maybe you could just answer in regards to if RF does permeate corrosion to any degree.

Answer 1

A layer of corrosion between two metals forms a capacitor (or, sometimes, a diode). RF will go through it better than DC, because RF can go through capacitors and DC can't. However, that doesn't mean it's an acceptable substitute for metal-to-metal contact, because the antenna wasn't designed to have an impedance (capacitance) there!

In your specific case, it isn't likely a problem in that way because, after all, both radials and ground plane work for a vertical antenna, and so does any intermediate form between those two designs. However, you must make sure that the ground plane is in fact well-attached to the feed point and that the screen is oriented so that there are single wires running radially, otherwise there may be current paths through the corroded parts.

However, there is another reason why it is a bad idea to have any such poor connection in an antenna: passive intermodulation, a.k.a. the rusty bolt effect occurs due to some corroded connections within an antenna acting in an electrically nonlinear fashion (like diodes) and therefore creating unwanted harmonics in transmission. Or, in a loose connection with sufficient transmit power, there could be arcing — another nonlinear phenomenon famous for creating harmonics.

In your case, there shouldn't usually be any significant voltage difference between two points at the same distance from the feed point horizontally, so these effects would be unlikely to be significant — unless another fault develops such as a portion of the ground plane being disconnected from the base!

In conclusion: Antennas should never contain loose or oxidized connections between conductors. You should make sure that all metal-to-metal connections are clean and clamped, crimped, or soldered so that any oxide layer that could be between the metals is removed and prevented from reforming.

Answer 2

I know StackExchange wrote "But avoid responding to other answers."

Sorry, but great answer from AG6YO.

arcing — another nonlinear phenomenon famous for creating harmonics

... or setting your lawn on fire

If the arcs are big enough, you'll hear them in an AM transistor radio while you transmit, such as on voice peaks. You'd be able to hear the staticy sound through the transistor radio's front-end overload.

And I think you'd see your SWR jump around, especially on power-peaks, if the ground plane is arcing.

Ham radio antennas are often a compromise: bent, shortened, less-than-textbook radials, not high enough, etc. If there's a voltage difference between two points that are the same distance from the feedpoint, I guess the first question to ask would be "How different?" (Not that I'd know what to do with the answer.) A difference would indicate that the groundplane isn't perfect. But if you're not arcing and sparking when you transmit, then your antenna is certainly better than many others'. Perfection is overrated.

I'm also thinking of DeoxIT, Amazon item # B00006LVEU. Deoxit dissolves corrosion, and then puts down a protective layer that prevents future corrosion. It's a nice thing to dab onto the edge-connectors of PC boards. I've never used it outdoors on the lawn, but I imagine treating connections with DeoxIT, clamping or crimping them, maybe spraying on a 2nd layer so the clamps don't corrode, and then being good for a long time.

Adding a clamp around every point on chicken fencing where the wires cross sounds like an arduous task. Maybe we could get rich inventing chicken fencing especially for ground planes where all the crossover points are electrically secure.

A few decades ago, you could have depended on neighbors complaining about TVI as an indicator that sparking and rectification is occurring in or around your antenna. But now that everyone has cable TV, you can't depend on them, anymore.