On Wed, 06 Jan 2010 04:18:10 -0600, Lostgallifreyan
wrote:

Thankyou. This is good, it sounds like the basic plan will work then, and I
might be able to get some chicken wire to cover at least part of it.

There is a down-side to this and what you have revealed below:

One
possible complication I didn't mention is that the intended mounting point is
at a T junction of three wire mesh fences of equal height, about 6'. They
don't have very reliable conductivity between each zigzag strand (oriented
vertical) as at least one fence has a green plastic coating on its wires.

Both the mesh of chicken wire, and the fence crosspoints may suffer
from cross-modulation products due to corrosion at the joints AND if
the metal is galvanized. This is evidenced in a nearby transmitter
(and nearby is relative measure) exciting the wire, and rectifying at
the corroded crosspoints. This rectification creates harmonics and
you are off to the races in terms of spurious frequency generation
across a wide bandwidth.

Oh Brave New World of common mode.

I
intend mounting the whip on a concrete post at the junction of these fences.
I imagine the fences will raise (and make diffuse) the precise physical level
of the RF ground, but I don't know whether they'll be a serious problem, or
maybe even be helpful. I can try grounding them a bit better, but otherwise
there's not a lot I can do about them.

As you describe at least one fence having insulated wire (which is
good from the cross-mod point of view) this makes no difference RF
ground-wise; and being elevated only slightly shifts things.

One exception is found in proximity in that this elevated ground will
indirectly short out the lower section of your vertical. This is more
a matter of Z than sensitivity.

The solution is to elevate your vertical's feedpoint to the height of
the fence top.

One other thought... In that USMC antenna manual there is a mention of
something similar, a 15' whip tilted and also tied back so the upper part is
almost horizonatal, it's intended as a way to use short(ish) distances for
skywave propagation. It looks useful given the context of trees and buildings
within 100m of my best mounting point. What I'm not sure of is whether the
curvature of their tied antenna is relevant, or a straight tilted whip would
have no significant differences.

Well, what they (or you) call sky wave is properly NVIS (near vertical
incident _____ - I forget the last part) which is meant for local
communications, which is more what the ground forces are interested
in. The Marines in Afghanistan are not going to DX headquarters back
at Pennsylvania Avenue in DC. When I taught VHF/UHF comm in the Navy,
our equipement easily lost 10dB of transmitted/received signal levels
just getting from the shack to the antenna. We didn't care. Push
more power if necessary, as for reception, line of sight was all that
was necessary, and that was to the horizon (no more than 8 or 10
miles). I don't think the government has bought any QRP rigs since
WWII.

In fact, that tilting's mission has also been satisfied with end
loaded dipoles place directly on the ground (which was largely sand).
I and my buddy used one for field day. Another antenna is an
unterminated coax laid across the ground. Both suffer mightily in
efficiency, but they offer ease of construction and purport to enjoy
less noise problems. To this last, most local noise arrives by
vertical polarization, and signals in the sky arrive by elliptical
(both vertical and horizontal by varying degree) polarization. For a
quick and dirty test, I doubt any other test could be quicker to do.
You might want to add a short pig-tail to the unterminated coax.

Further experimentation would be to add 8 to 10 inches of ferrite
beads to the coax, half way back on it toward your shack. This would
snub your home's injection of noise into your receiver (conducting out
from the house on the coax outer shield and folding back at the far
end). The next experiment (if this first proved useful) would be to
add a local ground at the same point and tie it to the shield (after
penetrating the jacket, of course). Yes, this violates some of my
other advice about mixing grounds, but for experiment's sake, it will
add to your repertoire of learning the complexities and benefits of
ground. The purpose of this new ground is to discharge that choked
noise into ground. I have successfully done this to quiet my home's
noise generation as detected in my receivers. The ultimate proof of
this concept is being able to throw the master breaker on your home
and noting any change in the noise floor.

Throwing that breaker is probably at the extreme of your family's
tolerance of your hobby. To this end you want to plan to do it once.
This means doing a noise floor survey at hourly intervals for all
bands and keeping notes for a week or two. Then, one day when there's
the least possibility of disrupting domestic tranquility, throw the
main and do a quick survey again. I have managed to quiet my
receivers by 5 to 10 dB through tests like these.

Buried cable, or ground level run cable can snub local noise induction
too (but it is still a good idea to choke at the feed point if no
where else). This last observation is to inform you that metallic
connection to earth is not always necessary. You should be equally
informed that the proximity of earth can also negatively affect what
positive gains you are seeking too.

73's
Richard Clark, KB7QHC