On Sep 30, 1:24 am, Owen wrote:
On 30/09/10 12:05, wrote:
...

So being as the increase is fairly small at such low heights
in wavelength, it is probably practical to just bury them so
people won't trip over them.. :/
If tripping is no issue, then it might be worthwhile to get
the slight edge in performance. But the increase over buried
will be fairly small with them only 100mm up.

I am interested in the electrical performance rather than trip hazard.
Once electrical performance is known, issues like trip hazard, mowing,
rock etc can be dealt with for each application scenario.

So, back to the electrical performance, do you have measurement data, or
can you refer me to articles that contain sound objective measurement
data that would suggest that my NEC4 model is not valid.

I haven't done any careful measurements, but one of the books
I have has an article and graph that pretty much matches the
figures I gave as far as amount of elevated radials needed to
match a certain amount on the ground.
I'm not sure which book it is..
Maybe one of the Bill Orr radio or antenna handbooks..
I don't think it was the ARRL handbooks.
But I have tried exactly what you are proposing. And it didn't
really pan out too well. I could hardly tell the difference between
having them on the ground, and slightly elevated.
Sure, it worked OK, but it didn't mimic a large number of radials
by any means. I also tried the elevated ground plane at various
heights using the same four radials. You could tell an obvious
difference between 1/4 wave up, and 1/8 wave up using the same
four radials. The real world results I've seen seemed to fairly
closely match the graph I have in that book.

BTW, I know it was one of the Bill Orr handbooks which
suggesting trying the elevated low radials.. I think the
same one that had the graph. He seemed to suggest it
was better than having them in the ground. But on the
other hand his graph dealing with elevated radials
suggested the results would be fairly lackluster.
Which they were at this QTH.
Now that I think about it, I think it was the Bill Orr
antenna handbook that had that article and graph
pertaining to the radials.

Being as you are talking about radials which are very low,
I assume the base of the radiator will be low also.
Myself, I think a large part of the lack of success I saw
was due to not enough radial density at the base of the low
vertical. I think one would probably be better off taking
the three or four 1/4 wave radials and chopping them into
many short radials and laying them on the ground.
And this is indeed a fairly common practice when laying
them on the ground.

I think having the higher density of wire at the base works
better than have just a few radials elevated. Most of the
ground loss seems to be in the area of the base.
I know metal density under the radiator is quite critical as
far as short mobile whips.
I saw that when I tried mounting my mobile whip on a
piece of angle iron that was running across the bed of
my truck. It didn't work too well, and being I was used
to using that antenna on various mounts, it was obvious
something was wrong. And the angle iron was very
well grounded as far as bonding to the truck.
That wasn't the problem.
I then moved it over to the top of the side mounted
utility bed box, which is a part of the truck body and
a good bit wider, and my usual performance was back.
Just by adding more metal under the base of the whip
did the trick.

I think this is one reason why I always had so much
better results with elevated vs ground mount verticals.
At 1/4 wave up, the base of the antenna was much
farther from ground, and the antenna started to
perform more like a vertical dipole, than a grounded
monopole.
With the ground mount, I was up to 32 radials at
one point. And the performance was still mediocre,
and barely better than my dipole at 36 ft on longer
paths.
Only when I got it up at 1/4 wave did it really come
alive. After that, it ate the dipole for lunch on longer
paths, and with only four radials.
All of my comparisons were done using a full 1/4 wave
radiator.

Anyway, maybe your model is more accurate than I
think. But I've already tried doing that, and wasn't too
impressed with the results.
I've heard others that tried it also, with the same lackluster
results. I think elevated or not, it's just too few a number
of radials to really be effective at that low height in WL.
Too much lossy dirt between the few radials is my
theory.

I just don't believe in too much of a free lunch when it
comes to just a few radials at low heights in wavelength.
My stance comes from actually trying it, vs modeling
it. I don't totally trust the modeling programs in
this area. I've seen too many differences comparing
the models vs the actual vertical antennas.
For instance, using "average" ground, most of the
programs underestimated the performance of my 1/4
wave high ground plane. To get a model that more closely
matched real life vs my dipole, I had to set the ground at
a much higher conductivity.
And in the case you are modeling, it seems to be
over estimating the performance vs real life.
Or at least what I have seen after trying it.

I wished it would have panned out better. Would save
a lot of wire. But for me, the improvement was minimal.
But.. You are welcome to try it. Maybe you will have
better luck than I did.