AB7E added to my desire to understand how it is possible for
the length of radials on the ground to establish resonance of a
vertical:
>I think you are correct on all counts, even though I still think
>radials close to the ground (in terms of a wavelength) tend to as if
>they were actually on the ground. That sounds contradictory (and
>may in fact be so since I'm not an expert), but here is how I see it
>based upon my own experiences.
Oh, there is no question that proximity to ground has an effect
on low elevated radials - this is evident when you try to trim them
(by temporarily using opposite pairs as dipoles)... but they still
behave like a wire up in the air, only just not so far up in the air.
>1. My friend Bob, K7ZB, and I operated several 160m contests from a
>cabin in northern Arizona with tall Ponderosa Pine trees all around us.
>Using a slingshot and a fishing reel to run a support line over a
>tree top, we would string up a full length wire vertical with a
>couple of radials about 8 feet above the ground running through the
>trees (using the exact same SO-239 scheme that K3LR has written
>about). We were definitely able to tune the feedpoint impedance by
>trimming the length of the radials (longer radials move the
>feedpoint "off center" and raises the impedance), but I still
>believe that for loss purposes those two radials were totally
>insufficient and acted more like two ground radials when there
>should have been many. We ran about 300 watts, worked lots of
>stations and twice worked all states during a weekend, but we worked
>almost no DX mults no matter how hard we tried.
Definitely not enough radial there to keep the vertical from
"seeing" too much of the ground (I failed to mention that
radials - I believe for both elevated & ground-mounted
verticals - also serve to decouple the antenna from its
surroundings... that coupling in the ground-mounted case
then becomes ground loss).
>2. Due to neighbor issues, for many years the only antenna I had
>was a 40m ground plane vertical on the flat roof of my one story
>house in Phoenix, fed with a homebrew tuner for the higher
>bands. My "radial system" was simply a bunch of aluminum wire
>strung as best I could across the roof. I had about 8 wires ...
>some short, some longer, several cross connected ... but changing
>the lengths of any of them (or even deleting one or two of them as I
>was once forced to do) had no effect on the tuning. So even though
>elevated, the radials really weren't "tuned" but the antenna worked
>very well for DX, at least on 40m. I'm convinced that having a
>clear shot over the neighboring trees and houses was the key. The
>mass of wires provided a current mirror but not in the manner of a
>tuned counterpoise.
Again, there is a possibility of not being decoupled enough
from the house - also, was the feeder decoupled? Before I
went QRT, my LF antenna was an HF2V on the roof of a
three floor reinforced concrete building & it was very clear
as I added more resonant 40m radials, at some point the
antenna stopped "seeing" the rather electrically obvious house
underneath it. On 160m, I have also found that the antenna
played significantly better when all the radials for the higher
bands were connected - JAs would CQ in my face until I
hooked up all the radials - so those extra (short or whatever
length) radials do seem to contribute to the return for elevated
verticals.
Perhaps I was wrong above & that decoupling function of
radials also translates into reducing "ground" losses for
elevated verticals - whether it is a reinforced concrete roof
three floors or forty floors above real dirt.
>So I wouldn't expect anyone to be able to tune a large radial system
>on or near the ground and have much effect on the feedpoint
>impedance, but I would expect that it would be possible to tune a
>couple of radials near ground (or a lot of radials well above
>ground) and be able to adjust the feedpoint impedance. Having only
>a couple of tuned radials near ground will result in higher ground
>losses and poor low angle signals, though, just as if you had a
>vertical antenna with only two ground radials.
>
>As far as obstructions go, the modeling I've done seems to indicate
>that even for average ground conductivity verticals tend to work
>best near ground, but if you're surrounding by trees, houses,
>fences, and other clutter that sucks the life out your signal, it
>would make sense that an elevated ground plane would do a better job.
>
>So ... does any of that make sense or am I just creating more
>superstition here?
No, it all jives with my experience, which is why I would really
love to be certain that N4UM said applied to just elevated
radials, as my only ground-mounted vertical simply didn't get
out with short radials... and it didn't significantly shift in frequency
when longer radials were added, it just worked better as more
of the RF apparently went to something other than keeping the
worms warm.
As for ground-vs-elevated, that is a bit of a tangent, but the
point I think to keep in mind is that antennas are really more
of a system & you have to step back & consider the big
picture: yes, ground-mounted, provided "ground" is good &
ground clutter isn't an issue, then that's probably better. I
think in practice, if folks worry about what trees might do, then
elevated is the best way to go.
But elevated means tuning everything & attention to things
that probably don't get the attention they deserve, hence
beliefs such as "verticals are antennas that get out poorly
in all directions".
73, VR2BrettGraham.
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