On Aug 5, 12:06 pm, "Richard Fry" wrote:
"Chuck" It would be more accurate to bill this as "Mediocre radial system vs 20 m
x 30 m solid aluminum ground screen ".

I suspect much the same results would obtain for the roof mount, even if
the roof were at ground level.

_____________

When the comparison is made between the roof-mounted version as shown in my
PDF and an earth-mounted vertical using a 2-ohm "broadcast quality" r-f
ground system of 120 buried radials each about 1/4-wave long, then the
advantage of the roof-mount version drops to about 5 dB.

N.B. these radiators are electrically very short. Using taller radiators
would reduce the performance difference.

RF

I think that comparison would be flawed though. That is not an optimum
"elevated" radial system, and I can see it losing to a 120 radial
ground system. For one thing, the radial system is not resonant.
This is quite important for an elevated system. Fairly critical
really.
Also, the manner of the wires being laid out like chicken wire mesh
is not an optimum use of wire. You would have better results using
the usual "spoke radial" system, due to more wire being concentrated
at the feedpoint, vs farther out.
If you compared a ground mount with 120 radials vs a roof mount
with 120 radials, the roof mount would smoke the ground mount
handily. For sure, as far as the local "ground/space" wave.
Also, if this antenna was for MW, and roof mounted, which is
very low height in wavelength, you will need a lot of radials to
equal the 120 on the ground. But if you had the antenna at 1/4
wave up, about 10 radials would equal the 120 on the ground.
I often see and hear about MW comparisons using ground mount
vs elevated, and almost to a tee, most ruin the comparison by
applying substandard radial systems on the elevated antenna.
Either that, or they ignore the number of elevated radials needed
to equal the 120 on the ground. That of course varies with height
in wavelength.
At 1/2 wave, 3-4 radials will equal 120 on the ground.
At 1/4 wave, about 8-12 will be needed.
At 1/8 wave, about 60 or so at least..
And it just gets worse from there as you get lower and
lower. Less than 1/8 wave, and you will need a load of radials
to equal the 120 on the ground. Probably 80-100... ??
Anyway, I've done quite a bit of comparing elevated vs ground
mount on 40m, and there is absolutely no doubt in my mind
that elevated is the best way to go. Totally smokes a ground
mount, as long as the number of needed radials per wavelength
is not ignored.
Even if you equal the ground losses between the two, the
elevated still wins due to more clearance of ground clutter,
and a much better local ground/space wave signal, which
I assume is due to the radiator being high and in the clear.
More line of sight so to speak.. This also greatly improves
the lower angle DX coverage, being both use the same fairly
low angles of radiation.
MK