[TowerTalk] 80 Meter beam
jimlux at earthlink.net
Sat Apr 1 12:59:50 EDT 2017
On 4/1/17 7:07 AM, David Aslin G3WGN wrote:
> Grant, did you happen to also do a Pro/4 analysis of a 4-square over poor ground?
> At my new QTH I'm not going to be able to get a beam higher than around 125ft for 80m, and that would be a 2-el wire inverted vee style since a Yagi is out of the question from a neighbour/town planning approval perspective.
> Although the conductivity map of this part of UK suggests average ground, I'm getting some empirical data from the installation of electrical safety grounds at 3 points on the property. UK regulations require that these show less than 200 ohms resistance (!) and the electrician is finding he needs 5/8" ground rods 8ft or longer to meet that test (most homes in this country have 3/8" 4ft rods for safety ground at the service entrance). This suggests that locally I may have poor ground so a 4-square may not be my best choice. It would be good to compare the wire 2-el with a 4-square over poor ground. The downside of the wire beam of course, is that it will be fixed in direction.
> All thoughts/comments welcome as I will only get one shot at building this station!
> 73, David G3WGN M6O
you may want to use your antenna analyzer to measure the soil
properties.. There's several ways - one is to drive a pair of rods in
with the right separation to make a balanced transmission line and
measure the Z.
The ARRL Antenna book (and maybe the handbook) describe the technique.
electrical safety grounds are measured at line frequencies and that can
be quite different. For one thing, they're really just measuring
"sigma" (conductivity) not "epsilon", and for antennas, both make a
But to your desire for gain and different directions. With the same
number of supports (4), you could build 4 doublet (between the supports)
and switch among them with some simple phasing and get gain in 4, if not
8, directions. The dipoles don't have to be a full 1/2 wavelength
long.. the gain difference between a 1/4 wavelength long doublet and a
1/2 wavelength is small. You could also do a drooping 1/2 wavelength
(1/4 wave flat between supports, 1/8 wave sloping off the end) to get
the match better (although I think bulk loading at the feed point would
be just as good.)
You could also make a sort of hybrid V dipole system with 4 supports.
Feed point at the top of the support (so the post holds the coax up). A
90 degree V, with the two feedpoints 1.414*90 =127 degrees apart -
actually, for a V dipole, the "phase center" (apparent feed point
location, given the far field phase and extrapolated back) is somewhat
"within" the V anyway.
You'd probably need some guys to hold the wires, and you'd need some
switching at the top of each post (or very cleverly at the bottom, with
openwire line coming down) to share a given wire between two dipoles
(the switch has to take fairly high voltage)
And then there's some clever schemes where you have multiple posts with
two inverted Vs supported from the post (and serving as guys), and
switch among the V's to get gain in the right direction. I've looked at
schemes with 3 and 4 posts.
However, all these schemes do require some fiddling around to get good
F/B (just like a 4 square) in the face of non ideal, non identical
elements. Getting gain is pretty easy and you can be sloppy with
tolerances, but getting a good null is harder.
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