|[AMRadio] Vertical antennas|
jcandela at prodigy.net
Sun Nov 1 21:53:13 EST 2015
Here is something that seems to work, and does not take a lot of space or a lot of investment:
I suppose a good "text book" vertical would outperform this one so long as directivity was not necessary.
On Saturday, October 31, 2015 6:37 PM, Donald Chester <k4kyv at charter.net> wrote:
> A ground mounted vertical uses buried radials. Is the primary function to provide some sort of reflecting effect or a low
> resistance counterpoise to the driven element?
The radials can be laid on the surface; they don't have to be buried. The only reason to bury them is to physically protect them from surface traffic like human and animal feet and lawn mowers.
You can think of radials from two perspectives; take your pick. (1) The radials provide a low resistance return path for ground currents, compared to bare earth, thus reducing ground losses. (2) The radial ground plane acts as a shield between the radiating vertical element and the lossy earth, thus reducing ground losses.
Another perspective is that the ground plane acts as a reflective mirror, providing a virtual image of the missing half of the 180 degree resonant antenna you have with a quarter wave monopole.
> What happens when it rains?
Ground losses may decrease, particularly if the radial system is marginal. The more radials and the more optimum their length, the less factors like ground moisture will affect antenna performance.
> In either case, why specify the length of a radial if it cannot function in a resonant manner being bare wire and buried with
The ground-mounted or buried radial doesn't have to be resonant length. Up to a point of diminishing returns, the longer each radial, the better. But it's more complicated than that. For a given limited amount of wire, it's better to lay down a large number of short radials than a small number of long ones. The more radials, the more making each one longer improves antenna performance. IOW, the more radials you lay down, the longer you want each one to be. With only a few radials, making each one more than a small fraction of a wavelength long, would make little difference in their effectiveness . Better to use the extra wire to make a few more short radials.
> How predictable is the radiation resistance of buried wire?
The buried wire has no radiation resistance since it does not radiate.
> What losses exist in the radials?
Very little loss in the radial wires themselves. But the fewer the radials, the more the ground loss from the bare earth adjacent to the radial wires.
> If it’s to provide a counterpoise, why not use a tuned artificial ground i.e. series resonant shunt to a good ground?
The ground system is more than just a counterpoise to make the vertical conductor self-resonant. For example, with AM broadcast verticals, even when the vertical tower is a self-resonant 180 degrees (half wavelength) tall, mounted on an insulator, an extensive ground radial system is deployed. If the resonant conductor were sufficient and the ground radials not necessary, given the cost of copper and real estate, why wouldn't a half wavelength base-insulated vertical tower with no ground radials be the standard broadcast antenna? The proximity of the bottom end of the ground-mounted half wave tower to the earth, with no radial system, would induce significant loss.
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