Hi, Brett.
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.
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.
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.
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?
73,
Dave AB7E
VR2BrettGraham wrote:
> N4UM had explained here to "Tom" that instead of using a longer
> radiator for a 160m inverted-L (then tuning out the inductive
> reactance with a series cap) he increases the length of the radials
> instead.
>
> I had asked "Presumably this is with elevated radials?", to which
> K9YC contributed:
>
>
>> I don't see where what you've quoted has anything to do with whether
>> the radials are elevated or not. And it is VERY tough to get radials
>> high enough to have the advantage of elevated radials on 160M -- the
>> literature says at least 1/8 wavelength above earth, which is 20M.
>> Maybe at your QTH, but not at mine. :^)
>>
>
> N4UM did not say whether the radials were elevated or not, which
> is why I asked, stating my ignorance for all to see by saying I
> thought radial length on the ground had more to do with providing
> a return than establishing resonance.
>
> Hence the hope that N4UM might clarify his post, as what he
> is saying, I suspect, one will find to contradict pretty much
> all existing literature on the subject.
>
> As for how well verticals "play" - which is not what was being
> discussed - our experience here is that 160m verticals with
> elevated radials in the area of 1/20-wave up (in my case,
> starting at 1/20-wave & descending to about 1/100-wave) do
> quite well - the often repeated "ground-mounted verticals are
> superior to elevated verticals" claim appears to completely
> disregard ground clutter... which simply driving around
> listening to MW tends to support. Perhaps if like those who
> have more open space & buildings not made of reinforced
> concrete & other far-from-RF-transparent objects are not so
> prevalent, then those of us here who are active on the low bands
> would not think that ground-mounted verticals are the worst
> possible thing someone could use.
>
> Especially when you apparently have to trim all those bloody
> radial wires in order to tune one. ;^)
>
> 73, VR2BrettGraham.
>
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