Posted by
Guy, K2AV on
Dec 20, 2010; 1:18am
URL: http://elecraft.85.s1.nabble.com/Semi-OT-vertical-wire-antennas-tp5851823p5851866.html
See interspersed.
On Sun, Dec 19, 2010 at 7:21 PM, Lew Phelps K6LMP <
[hidden email]> wrote:
> I'm currently running my K3-10 into a 40 meter horizontal loop antenna, mounted on my roof about 35 feet above ground. It's impractical to use on 80, and has a very high angle of radiation on 40 and 20 meters.
>
> So, I'm thinking of replacing it with a 43' wire vertical. Yes, I know it needs a wide-range tuner, because it's non-resonant on any ham bands. I already have that. And I know that it needs a 4:1 balun. I can make that.
>
> I have three questions for the group.
>
> 1. Is there any reason to expect that a wire vertical will perform significantly differently than one made from aluminum tube (e.g. 2" OD at base)? EZNEC modeling shows a slightly lower gain for a wire antenna, but not significant. Is this borne out in real life?
The DX will never know the difference. The succinct reason for
aluminum is when the wire has to support itself. With aluminum it is
possible to make a self-supporting 43' vertical. This is really
important in situations where there are no other suitable means for
support. It's a choice based on common sense mechanical issues that
relate to your particular back yard. The only performance issue in
your design is radials. That will be entirely responsible for wow, or
why did I bother.
> 2. The available grassy yard space where the antenna would be installed would permit a maximum straight-line radial run of approximately 30 feet, well under the desired length of 58 feet for operation on 80 meters. Would it affect antenna performance if the radials were laid out in a series of Z jogs rather than in straight lines?
Radials are self-terminating. The higher the frequency, the less
length actually carries any significant current. Ground radials are
NOT resonant devices. Do them straight.
Individual radials on/in the ground are severely attenuated by the
ground around them, and will exhibit a per wire ground-absorbed
resistance of upwards of a hundred ohms each for some kinds of dirt.
The secret is to divide that resistance by a large number in parallel
so that the COLLECTION of radials APPEARS to be a SMALL resistance to
the antenna system. This concept is the huge gorilla in the room.
Just about nothing else counts until you deal with this.
Use bare wire and notch each one well down into the sod. Keep the angles even.
BECAUSE you intend to use this on the higher bands, notch a minimum of
60 bare radials into the sod. The resistance is divided by sixty, and
the density NEAR the center is MANDATORY for the higher bands. DO NOT
SCRIMP on ground radials. What you do with the radials is about
ninety-five percent of the multiband performance. Without this the
power loss in the ground around the vertical on the high bands can be
astonishing, so much so that lossy ground induction completely
controls the impedance and turns a 100 watt transceiver into a QRP rig
AND WORSE. This situation is responsible for the old saw that "A
vertical radiates equally poorly in all directions."
If you don't have the patience or time for this, don't do radials
on/in the ground. Ground radials are a special case with brutal
penalties for partial implementations.
What those buried bare radials have going for them though, is once
sufficient numbers are notched into the ground under the sod, and the
grass has recovered and been mowed a couple of times, you have the
best you can do, and they are completely invisible.
> 3. The antenna would be suspended from a large sycamore tree. Will it make any difference in performance if I run the vertical right up the side of the trunk, as contrasted with suspending it from a limb at some distance (e.g. 5 - 10 feet) from the trunk?
How much loss tacking a wire to a tree trunk will give you is
controversial. But there is no point in tempting all the physics in
that unresolved question. It may vary by species of tree and time of
year. Just don't go there unless you have to.
Pulling a vertical wire up through a tree, ten feet or more from the
trunk, has been a wonderful stealth vertical in developments. What
that has going for it is that a radial center out ten or 15 feet from
a trunk will allow the radials to be kept even and still pass around
the tree. It is important on the high bands to have a dense uniform
center coverage of the radials without any gaps.
73, Guy.
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