Excellent thread here guys. So much so that I have switched out of
digest mode and into real time mode.
A few notes on the K9AY - some perhaps more relevant than others.
1. The far field pattern is ALL vertically polarized - consistent with
the antenna behaving as two verticals with a phasing line connecting
them.
2. The connection to ground is 100% common mode to the two verticals.
All my K9AY modeling includes what I call Rg, which is a resistance in
the ground leg. Values around 100 ohms appear to give good correlation
to field measurements I have made of the pattern here in Georgia, where
I have decent ground properties. Front-to-back is very sensitive and to
some lesser degree gain is sensitive to Rg, exactly as you would expect.
I can't figure out how one would model the antenna as two separate
verticals, as I can't see how the common mode resistance could be
included in such a model. Maybe someone has a bright idea for this?
Meanwhile, I don't really see any point in modeling the antenna as two
verticals since the model of the antenna as a loop gives good results.
3. Radials cannot be placed directly under the loop wires without
severe pattern distortion. They can be placed at right angles to the
loop. Many of us run orthogonal K9AY loops - the best answer for that
configuration appears to be radials at 45 degrees to the loops. I use
no radials - just a six foot ground rod, in part because I'm not
interested as much in F/B as I am in RDF. Meanwhile I have excellent
F/B with just a ground rod.
4. While the F/B is very dependent on the termination resistance, the
actual noise performance of the antenna, as defined by RDF, is a
different story. RDF changes only about plus or minus 1% for
termination resistances from 340 to 600 ohms, with Rg=100 ohms. I need
to pull out my model runs from a few years ago and check RDF as a
function of Rg.
It may take a day or two for me to get back to this, but I am fascinated
by the possibilities. I'll stick my neck out and take a guess: I don't
expect to see nearly any improvement in gain, but with variable delay in
a dual feed loop you should be able to adjust the tilt angle of the null
in the back which will make the antenna APPEAR to have more F/B to the
user, while in fact it probably doesn't. I'm also guessing RDF will
remain nearly unchanged by going to dual feed. With a phasing box like
the DXE or MFJ you should be able to adjust both the tilt and the depth
of the rear null. If cross-fire feeding is used, the settings for delay
and amplitude of the phasing should hold reasonably constant over a wide
frequency range.
I hope I'm wrong! Just being able to control the rear null tilt angle
however is worth running the second coax for me.
I have a movie and presentation on the K9AY loop at www.sedxc.org As
the movie shows, a properly built K9AY, without radials, can produce
phenomenal F/B under some conditions. Isolating grounds and ferrite
beads to prevent further pattern deterioration from common mode current
on the feedline are the key to getting these things working well. And,
of course, keeping them uncoupled from everything else in the yard
that's vertically polarized - like transmit verticals.
73,
Hal N4GG
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