Making antenna elements with copper wire lashed to a steel rod

Question

For making stealthy antennas with rigid elements, I want the elements to be thin but stiff. Steel is stiff and is a conductor, but not a great one. I was wondering about using thin steel rod (say 1/8 inch or 3mm) with thin copper wire lashed or glued to it.

Let's assume

• The element is for a 100W antenna.
• The wire is not necessarily insulated but could be (I suppose non-insulated is an advantage to reduce the dimension).
• HF frequencies (plus maybe 50 MHz).
• Antenna is temporary so doesn't need to be super-rugged.

It would be sort of like copper coated steel wire, but the copper is lumped.

What would be the effect of this, in terms of efficiency? Would much of the RF energy couple into the steel, and then experience I^2*R loss? Or might the RF largely "ignore" the steel?

Alternatively I could just use steel and not worry about the power loss, but if I can easily improve upon that I might try it.

I'm aware there are endless antennas and ways to build them, but am specifically interested in this technique.

Answer 1

Without simulating it... my gut feeling would be that the steel and copper will share the currents approximately by the ratio of their projected surface area. With some modification for the way the fields won't reach into cracks. Because the two wires are in contact, or have good capacitive coupling, they will be at essentially the same electrical potential all the way up.

So a thin copper wire on a thicker steel rod will make no difference (even though the thin wire alone may have been a better antenna.

Two equal diameter wires may reduce the losses to half those of steel.

Once the steel is mostly obscured by the copper, the losses will be similar to plain copper.

The best strength/price/performance ratio would be several fine wires around the steel rod, so the copper "coating" is thin.

Note that the copper wires must run along the length, not be wound tightly around the steel like a spring.

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You will find many questions and answers on this site, discussing the losses from switching from copper to steel. It depends strongly on the type of steel, diameter and the frequency. For example on a steel 108" whip the loss is neglegible. But on a 40 m long stealth end-fed halfwave with 0.3 mm stainless wire, it's more than 10 dB.

Answer 2

You will have capacitance issues if you are planning to attach the copper to the rod with insulators. If you plan to make the steel rod part of the antenna, you have differing velocities, which to me will make the frequency calculations harder. Beyond that, when you use very thin elements you reduce the bandwidth and power that can be handled by the antenna. There is an additional concern with using dissimilar metals; if you plan to have the antenna up for a long time and the steel and copper are both antenna elements, the connecting surfaces will tend to corrode more quickly over time leading to all kinds of weirdness. I know you say the antenna is temporary.. but in my experience solutions are only temporary if they don't work. :)

My recommendation is to model it in EZNEC and experiment! While the modeling will help you to get close, the experimentation is where the fun is and where reality meets theory.

If I could suggest, make everything a bit longer than your calculations say it needs to be. It's reasonable to expect that whatever you make will not match the ideal antenna that you model, and it's always easier to make things shorter than it is to make them longer. :)