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Would a disconnected wire or conductive mat below a low height antenna reduce ground losses?

A tuned length of wire under a wire antenna might be suitable for NVIS gain. But would a bunch of space blankets or unconnected random length wires or mesh (gopher wire) on or slightly above the ground help reduce ground loss of a low dipole or a vertical with far too few counterpose radials? (e.g. maybe one).

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  • $\begingroup$ What's a space blanket? I'm assuming that it's a thin layer of aluminum vacuum-deposited on a Mylar sheet. $\endgroup$ – Mike Waters May 25 at 20:37
  • $\begingroup$ Would connecting enough space blankets (aluminized mylar) to get close to 1/4 lambda by overlap (capacitive coupling) and/or wire staples help? $\endgroup$ – hotpaw2 May 25 at 21:02
  • $\begingroup$ It seems to me that galvanized chicken wire (poultry netting) or other plated fencing material would be cheaper --not to mention more effective-- than space blankets. Besides, the aluminum might be hard to connect to the earth. Both of those materials have been used in addition to ground radials. I suggest that you search the Topand Reflector archives, as this has been asked and answered there many times. And ditto to what Kevin just said. :-) $\endgroup$ – Mike Waters May 25 at 22:59
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    $\begingroup$ @tomnexus Please don't write answers — short, partial, or otherwise, in comments. Even a short answer sometimes turns out to be just right, and a comment can't be accepted — or ranked by its votes. Comments on questions are best used for asking for clarification of the question. $\endgroup$ – Kevin Reid AG6YO May 25 at 23:03
  • $\begingroup$ @kevin ok, I get grief both ways :) At least comments attract no downvotes if they're considered incomplete. $\endgroup$ – tomnexus May 26 at 11:41
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It is well-established that a wire reflector --on or just above the ground-- directly under a dipole can do just that.

I don't have a reference, but at a field day that I was present at years ago, a ham on 75m did that. Afterwards, as W8JI (who also was present) said, "He was just 'killing people'", meaning that his QSO count immediately went way up. His reflector was directly under his center-fed dipole. IIRC, it was laying directly on the ground. Although the spacing between the wire and the dipole was not optimum, it certainly seemed to work.

It was good and conductive soil, in NW Ohio near the Maumee River where Route 235 ends at a SW-NE road parallel with it. 200 years ago, that was part of the Great Black Swamp. Later, ditches were dug and tiles laid to drain it into the river, which turned all of Lucas and Wood county into fertile farmland. Still, adding that wire helped his signal.

Perhaps the band conditions improved at the same time, and the wire had little or nothing to do with it. But I've heard of too many similar experiences to discount that.


As for a vertical as you describe, I'm leaving that for someone else to answer.
See this answer by R. Fry, which seems to disprove my anecdotal statements.

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Random long wires - probably will help. Especially if they're aligned with the dipole above them.

Space blankets might not have a thick enough metal layer to be effective at HF, (though I know reflective window film is as good as wire mesh at 900 MHz).

Most importantly, things < ${1\over2}\lambda$, not connected together, will make no difference, the fields will go right through them. There is no easy way of connecting space blankets to each other. Perhaps with a substantial overlap and some sandbags on that.

Finally, note that adding metal under an antenna, as it makes it more efficient, will also push its pattern up. Better for NVIS and worse for long distances. So depending on your ground conditions, dipole height and band, you might see a loss of performance on some paths.

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    $\begingroup$ Why does higher efficiency cause an increase in takeoff angle? $\endgroup$ – Phil Frost - W8II Jun 7 at 20:36
  • $\begingroup$ Quick reply: because a dipole over lossy ground has a fair amount of low-angle radiation, especially vpol off its ends but also hpol broadside depending on height, ground; while a dipole over perfect ground has no low angle radiation, it has a null on the horizon and nothing off the ends. So for these two examples, less low-angle radiation goes with higher efficiency. I'm not as sure just how T.O.A changes though, if it's defined as the angle of peak gain. $\endgroup$ – tomnexus Jun 9 at 18:26
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Below is the result of a NEC4.2 comparison of a 75m, center-fed dipole at 10m elevation above 15 mS/m Earth, with and without a reflector on the surface of the earth directly under it.

The gain improvement with the reflector is about 0.03 dB.

As for adding a single wire on the surface of the earth or buried several inches below the surface to improve the performance of a vertical monopole -- its effect on antenna system radiation performance would be close to negligible, also.

That single conductor, no matter how long it is, will be ineffective in collecting and re-radiating the r-f currents flowing on and just below the surface of the earth within a circle of 1/2WL radius from the base of a radiating monopole.

a

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  • $\begingroup$ Thanks. What happens if the bottom wire is raised a little (~1 foot), Rich? $\endgroup$ – Mike Waters May 26 at 18:25
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    $\begingroup$ Mike - I set the elevation of that reflector at 0m AGL to match the recollection you had of that in your Answer. Elevating that reflector to 1 foot AGL increases the total, peak gain of that system to 7.4 dBi (other things equal). But maybe the reflector becomes a trip hazard? $\endgroup$ – Richard Fry May 26 at 19:53
  • $\begingroup$ I think it's worth adding that more wires also don't improve the gain much. One might expect that they could improve radiation toward the horizon, but the change is also very slight compared to the time and treasure needed to implement. $\endgroup$ – Brian K1LI Jun 7 at 12:00

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