On Apr 20, 8:05 am, Buck wrote:


Maybe I should change the subject line, but here goes.
First of all, i am fishing for information, not challenging anyone's
intelligence.

I understand from books I have read, that a ground mounted vertical
antenna needs many radials. IIRC, the point of diminishing returns on
adding radials falls somewhere between 64-128 radials. I imagine the
best radial-based ground I could have for 20 meters would be a solid
copper disk with about 16 feet radius, give or take. However, I
recall in the ARRL Antenna handbook, not the latest version, but one
prior to this one, there is no noticeable difference between a raised
ground plane antenna with 4 elements as opposed to 128. (From here,
or another antenna forum, I heard for the first time that it holds
true for two radials.)

I am still trying to figure out why so many radials are needed on the
ground and a few feet higher so few are needed.

Actually, more important than the why, is how high is high enough to
reduce the optimum number of radials? For example, i want to build a
20 meter vertical. I understand the best place for it is on top of a
100 foot+ tower, but somewhere in between, there has to be a place
where 4 radials above ground is noticeably better than the same 4
radials on the ground.

If you used 120 radials on the ground as optimum, even raising only
1/8 wave off the ground will enable one to reduce the number of
radials
to equal the same performance.
But... You will still probably need at least 60 radials at 1/8 wave up
to equal the 120 on the ground. So even at that low elevated height,
4 radials is better than 4 on the ground. But... Still not very
good.. :/
At 1/4 wave in height, you will need about 8-10 radials to equal the
120 on the ground. Only when you approach 1/2 wave in height can
you use 2-4 radials and have the same appx ground losses as the
120 on the ground.
You *must* think in terms of wavelength off the ground, not just feet
in general. A 160m vertical will need to be about 250 ft off the
ground
to be able to use 2-4 radials with optimum results.
A 10m vertical can be 16 ft off the ground with 2-4 radials and have
the same performance.
If you had the 20m vertical at 16 ft, "1/4 wave", and used 4 radials,
it would be equal to a ground mount using about 60 radials.
Pretty decent antenna.

In the case of your 20m vertical, it will need to be 32 ft high to
be able to use 2-4 radials and appx equal 120 on the ground.
But a 20m vertical at 8 ft off the ground with 4 radials will
be better than the same vertical on the ground with 4 radials.
But if you want that 8 ft high vertical to equal 120 on the ground,
you will need about 60 or so, being it is only 1/8 wave up at that
frequency. This would give pretty decent performance.
Much better than the 4 radials at 1/8 wave up where 4 radials
is equal to about 8-10 on the ground. Neither one of those
is going to set the woods on fire..

Obviously, an elevated multi band vertical with radials for
each band will have varying degrees of ground loss depending
on the band in use at the time.
If you had a multi band 1/4 wave vertical "GP" at 32 ft, and had 4
radials for each band, you would have much less ground loss on
10m, than on 80m.
On 10m, it's 1 wavelength, and just 1 radial will be enough to
make an efficient antenna, except you have a dipole.
If you use two radials 180 degrees apart, that should actually
be a tad lower ground loss than 120 radials on the ground
being it's at 2 wavelengths up.
On 20m, it's at 1/2 wave up, and still very low loss.
At this point the 4 radials should be very close to
the 120 on the ground mount.
On 40m, it's at 1/4 wave up, and the 4 radials will
be equal to about 50-60 on the ground.
The antenna will still work quite well.
On 80m, it's at 1/8 wave up, and the 4 radials
would be about equal to about 8-10 on the ground.
A good bit of loss on that band. You will be able to
operate, but not with the gusto of the higher bands.

Maybe this will give you an idea of the appx level of loss
for a given number of radials at certain heights, vs the 120
on the ground. The most important thing to remember is
to think in terms of wavelength off the ground for the band
to be used.


Another point I have heard in the forums, but not confirmed, is that a
reduced size vertical element doesn't gain much by adding radials
longer than the antenna is high.

Nope, I don't really agree. In fact, I think using the shorter
radiator makes the use of the lower ground loss radial set
even more important and worthwhile if you are trying to
approach full size performance.
You would see a difference I'm fairly sure.
BTW, a lot of the info I just wrote a novel about came
from the Bill Orr handbooks. He has sections on the subject,
and also graphs that match the levels of loss I mentioned
at the various heights.
In testing various verticals, including a full size 40m ground
plane which I could vary the height, I've never seen anything
to show his data is incorrect. It's from one of those blasted
books Art has problems with, but I happen to trust it as fairly
accurate.
All this pertains to the usual 1/4 wave elevated ground planes
vs a ground mount.