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It's a bit counter-intuitive that for an HF vertical antenna (such as a quarter wave monopole) the best advice is to put as many radials and as close to the proper length as possible.

Sample image search results showing dozens of radials out across the fields around vertical antennas

The typical VHF/UHF "ground plane" antenna works on basically the same principle: a quarter wave monopole plus an artificial ground plane. But in the case of these builds, usually only four radials are ever used!

Sample image search results showing five-element antennas usually constructed around coaxial sockets

Usually the allowable "gap" when converting a theoretical flat surface to a works-in-practice build (e.g. discones and parabolic meshes and such) gets smaller in proportion to the wavelength. Yet here it's just the opposite — for a 160m vertical monopole I'd still want a zillion radials, yet for a 23cm vertical monopole somehow only one every 90º is totally fine?

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I believe the answer is provided within Jim Brown K9YC's Vertical Antenna Mounting Height (PDF) presentation. In it he builds a case for treating radials not so much as a "better ground" but as as a shield from the actual ground, where the lossy earth soil eats signals and turns them into heat.

This argument comes to a climax on page 82 of the linked PDF, where he explains both that

  1. More radials means less power ends up in the soil:

Radial currents couple into the lossy earth beneath them, which burns transmitter power [and since] power is I2R, dividing current between more radials reduces that lost power

and that

  1. Less radials are needed when far (wavelength-wise) from the soil:

That's why only 2 or 4 radials work for elevated “ground plane” antennas

So I think that this ends up being a very practical matter.

Because VHF/UHF ground plane antennas are small, they are easily (and commonly) mounted a large number of wavelengths above the earth's soil they can get away with only two radials. (Why we don't just build them as 2-element dipoles instead of 5-element "ground planes" is left as an exercise ;-)

Whereas HF monopoles tend to be large and difficult to support (and add radials to) unless they are installed on the ground itself — but that earth which is so useful mechanically ends up being a liability electrically!

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  • $\begingroup$ Yup. Elevated HF verticals frequently have a small number of resonant radials, but when they're at ground height you need many more of them to reduce loss. $\endgroup$ May 14 at 2:30
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    $\begingroup$ Re: "Why we don't just build them as 2-element dipoles instead of 5-element 'ground planes' is left as an exercise." Is it because then the feedpoint would need to be perpendicular to the antenna, which is very inconvenient for mounting the antenna since it's in the center? $\endgroup$ May 14 at 22:52
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Technically, you only need one radial, and that one radial needs to be about a quarter wavelength long. With a VHF/UHF antenna, that length is pretty easy.

However, the more radials you have, the shorter they can be, and the less loss you will have against the ground. This is even more important for an HF antenna, where long radials might be a pain, and near the ground,where loss will be higher.

Also, if you only have one or two radials, your radiation pattern will be distorted rather than omnidirectional, which is one reason you typically see four radials.

On an elevated antenna, the angle of the radials also affects impedance.

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Nate.

The short answer is that you don't need dozens of ground radials for HF monopoles.

There is no evidence anywhere that shows that all those ground radials are required.

Four is enough, adding more doesn't do anything.

A half wave dipole only has one ground radial and that works ok.

Standard ground plane antennas for HF and above with 3 or 4 radials mounted high up on a pole work very well, adding more than four radials doesn't make any difference.

In fact the entire subject of grounding for antennas is vastly over-rated by hams and i think comes from the days where valve equipment was grounded for safety reasons due to the HV present, and using lots of radials just makes them feel safer.

Having said that, ground radials need to be the right length, more radials doesn't mean that they can be shorter, a ground plane antenna is just a dipole on it's side with one fat element.

Incidentally a ground plane antenna has much greater bandwidth than a standard dipole due to the one ground element comprising multiple radials.

Hope that helps !

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  • $\begingroup$ The 2nd page of this 2009 paper by Rudy Severns, N6LF, shows that radials have a major affect on vertical antennas: astrosurf.com/luxorion/Radio/qex-ground-systems-part-4.pdf (in my opinion, based on that data, the sweet spot is around 8-10 radials though if you can put down 30 you'll get nearly maximum benefit). And the 3rd page of this 1978 paper by Jerry Sevick, W2FMI, shows you can get really good results from even 1/16 wavelength radials (but you can get a little bit more benefit from longer ones): py2nfe.com.br/Arquivos-pdf/radiais-W2FMI.pdf $\endgroup$ May 14 at 23:23
  • $\begingroup$ @User5754448 I read these, the articles are flawed for a variety of reasons, one being that the signal strength tests for the different numbers of elements were taken 1.8 wavelength away from the antenna, and the change in level seen is simply due to the fact the more of the antenna is closer to the test point with more elements. Also, most of the testing was done using short antennas, and even with these flawed tests, you can see that more radials makes very little difference for 1/4 length monopoles. The descriptions given for radiation resistance and efficiency are dubious also. $\endgroup$
    – Andrew
    May 14 at 23:45
  • $\begingroup$ @User5754448 Look at the graphs in the first article, even according to these flawed tests the difference in signal between full length antennas with 4 radials and 70 radials is about 0.8 dB which is negligible and not worth the effort, and my answer above is correct. $\endgroup$
    – Andrew
    May 14 at 23:55
  • $\begingroup$ good point for 1/4 wavelength monopoles, the data in the first article suggests that lots of radials are needed for modest db gains. But for short compromise antennas (which are likely at 80m, 160m, etc.) the first graph holds, in that radials can help significantly. So if that paper is accurate, a rule of thumb might be that if you have a full 1/4 wave vertical you can probably skimp on radials without much hindrance, but if your antenna is short put down at least 8 short radials. $\endgroup$ May 15 at 15:23
  • $\begingroup$ @User5754448 The tests are flawed because the levels were measured in the induction field, the tests need to be performed let's say 1 km away to be accurate. The only reason that the graphs show a higher reading for short antennas with more radials is because there is more of the antenna closer to the point where the readings were taken, not because the radials are somehow making the antenna more efficient. The authors are contradicting themselves and don't seem to understand this fact. An article in a magazine with lots of equations and pretty graphs doesn't make it true. $\endgroup$
    – Andrew
    May 15 at 23:10

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