[TowerTalk] Back to the XQuad

[L. B. Cebik]

Please discard the previous note on the XQ.  And thanks to Pete Soper,
KS4XG, for catching my gapless model, which is simply wrong.

Restoring the required gaps at the top and bottom of the beam loops tells
quite a different story.  Again, free space.  12 dBi gain, or about 10 dB
over a dipole similarly placed.  However, no great shakes on F-B, best I
could do was between 7 and 8 dB.  Quite narrow beamwidth.  Very high
impedance (both R and X), which is likely why Orr recommends a quarter
wavelength parallel line matching section.  Something of the order of 600
ohms for a line will give a 10:1 ratio to the feed Z and hence a 10:1
ratio at the bottom for something near 60 ohms.  Make it just a little
long and feel for the lowest SWR tap.

Over ground, a dipole will have 7.5-8 dBi gain, so the equivalently placed
X-Quad might reach 18 dBi gain over ground.  However, two items concern
me.  One is that low F-B; the other is the low current readings on
numerous of the directors.  So John appears on the right track when he
suggests that fewer directors placed at better spots would help the array.
I suspect that looking at a few long Yagi and normal quad designs would
put the directors in more optimal positions.

Then, again, except for local communications, I have not heard many
complaints about QRM off the rear at 6 meters.  But the F-B of the models
suggests that there is more gain off the rear of the XQ than from a
dipole, so that might be reason enough to improve it.

I apologize for the initial haste that led to the erroneous modeling.

Here is a little trick that will give ballpark results in modeling when
you want to see what a gap in a wire will do:  insert a load with only a
resistance (10 Meg works fine) at the gap point.  Did this to check
gapping with only a 0.25 dB difference in gain, 1 dB difference in F-B,
and under 15% difference in Feed Z.  These differences could also result
from changing the gap size.  So the result told me to go do the real
model, since I was now on the right track.  The big load R is not precise,
but it is useful to see what might happen with a gap.  (Do not make "gap"
into capitals, since this is not a comment on a vertical antenna.)

-73-

LB, W4RNL


L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
1434 High Mesa Drive      /  \/  \/\     ----/\---     (Hm) (423) 938-6335
Knoxville, Tennessee     /\   \   \ \   /  / || /      (FAX)(423) 974-3509
37938-4443     USA      /  \   \   \ \       ||              cebik@utk.edu
         URL:  http://funnelweb.utcc.utk.edu/~cebik/radio.html




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