Topband: Radial system gain
w8ji at contesting.com
Sat Sep 30 15:20:53 EDT 2006
> Aren't all these figures ground wave? That's quite useful
> for broadcasters, as that's where the consumer radios are,
> but do we know by measurement of some kind what the
> results would be at even 5 degrees, so we are not dealing
> with any possible ground proximity effect that does not
> map to the useful (for us) skywave?
Yuri brought up the point that the large ground system aided
low wave angles, giving four square gain from an omni
antenna. The fact is there just isn't that much power being
wasted close to the antenna once we have anywhere close to
The Fresnel zone of a vertical extends out several
wavelengths. What we do to the ground in the near field,
assuming we don't make the ground system a radiating
element, only affects the overall efficiency of the system.
Even 1/2 wave out is not enough to affect the low angle
problems associated with a vertical. It's the conductivity
over the first few miles that changes low angle skywave
propagation on 160, not what we do within a few hundred
feet. Within a few hundred feet we just change efficiency.
I believe N6RK did some measurements, and there have been
professional measurements that support this. Certainly
geometry and Method of Moments programs do.
> Anyone ever do that or something similar with 160 or high
> broadcast verticals to see if the modeled efficiencies
> hold for skywave?
This has always been of interest Guy, even with AM
broadcast. The large clear channel 50kW and larger AM
transmitters were always concerned about laying down maximum
signal on skywave. Smaller local stations with directional
patterns also have to protect skywave. It was never
neglected in AM service. It simply became unimportant
because it is meaningless.
We have to go out a very large distance in wavelengths to
affect low angle signals, and higher angle signals aren't
affected much by any change once a minimum number or radials
are used. For skywave at high angles our local conductivity
doesn't mean much. All the radials do is change the overall
efficiency. To change the wave angles that earth losses
affect the ground has to be changed several wavelengths from
the antenna. That's why all through history no one has
worried about changing what they cannot change. At upper HF
with a tight pattern vertically polarized directional array
it becomes practical to go out several wavelengths with a
grid of parallel wires and affect skywave pattern and gain.
On 160 meters it isn't reasonably possible, and it certainly
can't occur just by adding more wire where it already isn't
We already know, or should know, that once radials of
reasonable length are spaced .025 to .05 wavelengths at the
open ends adding more wire in between those wires is a total
waste of copper. It already looks like a sheet. Our only
choice for pattern enhancement, which can only occur at very
low angles, is going out thousands of feet with wires spaced
ten feet apart or moving next to a large body of salt water.
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