You are into some interesting stuff. Can of worms...
1) 60 is more the point of diminishing returns for 1/4 wavelength
radials. Depending on your ground, 13 radials can be anywhere from
just so-so to awful. ADD radials.
2) the inverted L CAN mess up the shunt tuning. A rule about loading a
tower active for other bands/uses, ANYTHING you change on the tower
means you need to recheck the shunt tuning.
3) All wires on or near the tower are coupled by shunt-feeding the
tower. The effect they have depends on lengths, connections. The piece
of coax from the A3S into the shack is part of the antenna. Unless the
feedline has longitudinal current blocking where it enters the antenna
field, miscellaneous rf and noise floating around on the house third
wire electrical ground is coupled to the antenna and heard on RX.
This can happen even if there is a DC ground where the coax enters the
shack, as ground rods often have a significant RF impedance.
4) The inv L for 160 is no different. If the L is grounded during 80m
operation, something like 10% of the power can be on the L. It does
not affect the pattern. There is very little current in the horizontal
portion.
If the L is disconnected from ground and feedline during 80m
operations, then it is a resonant wire on 80m and is closely coupled.
Integrating the currents on the L and the tower will show that the
majority of the power winds up in the L.
For all practical purposes the 80 meter pattern of the
tower/disconnected L combo is the same as if you had endfed the 160 L
on 80m and the tower wasn't there, wire magically supported somehow.
It is still omnidirectional but with an equal high angle component.
Warning, the disconnected end of the L will produce high voltage RF
that with a KW on the antenna can jump across the connections in
typical coaxial antenna switches like the Ameritron RCS series, etc.
They are designed for the voltages found on 50 ohm systems. If you
switch it open for 80m operation you will need to use something that
has a large air gap in the open position. At the extreme, I have seen
such RF jump across an open SO239 coax jack.
This is not to discourage use of such a configuration. In fact it
might make the shunt fed tower the one-size-fits-all antenna you need,
since it will now cover high angle.
If you leave coax/tuning devices on the L coax while operating 80m,
the results will be somewhere between the extremes described above,
and the 160 stuff will be hot with RF while operating 80 meters. You
will have to pick devices and coax lengths accordingly.
This can be seen very easily running the following simple model in
EZNEC and using "view" to observe the currents along the wires. Units
in meters.
wire 1 0,0,0 0,0,20.4216 #12 67
wire 2 0.3048,0,0.1524 0.3048,0,13.716 #12 45
wire 3 0.3048,0,13.716 24.6888,0,13.716 #12 80
This has the L disconnected from ground. Change the Z value of wire 2
end 1 to zero to ground it. Note the difference in patterns.
Vary the ground to see what you are doing to yourself with your
radials.
73, Guy.
----- Original Message -----
From: "Bill Coleman" <aa4lr@arrl.net>
To: <towertalk@contesting.com>
Sent: Sunday, May 11, 2003 12:46 AM
Subject: [TowerTalk] Inverted L, Sloper or Shunt Feed?
>
> Now that it is summer, I need to make some improvements to my
station on
> the low bands - particularly 160m and 80m.
>
> Right now, I've got a 15m tower with an A3S on top. I have a shunt
feed
> made of two #14 wires spaced 10" apart that goes from the base to
the 12m
> level at a spacing of roughly 5". I have 13 60 foot (.22 wl on 80m)
> radials installed, and feed it with an omega match on 80m.
>
> So far, the shunt works, although not noticably different than my
low 80m
> doublet at 10m. I plan to add 12 more radials, but I don't know if
that
> will improve the antenna that much.
>
> For 160m I've considered putting up an inverted L, which I can run
to the
> 12m level and then down into the front yard. The advantage is that I
can
> share the radial field of the shunt. The inverted L would run on the
> opposite side of the tower from the shunt, but would be only 1/2m
from
> the tower.
>
> I'm worried that the 160m inverted L may seriously detune the 80m
shunt.
>
> Another idea is to put a 1/4 wave sloper at the 12m level. I already
have
> a 40m 1/4 wave sloper at the 11m level.
>
> Another ham had suggested I just put in a matching network to run
the
> shunt feed on 160m. I've experimented with a few different capacitor
> values in the omega match, but I haven't been able to achieve at
matched
> condition.
>
> One limitation I have is that the tower is the highest support on my
> property, so just about any antenna has to be mounted to the tower,
or be
> substantially lower.
>
> Anyone had any good advise, suggestions? Will the Inverted L work,
should
> I try a sloper, or how to I figure the matching network for the
shunt?
>
>
>
> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
> Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 1901
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless Weather Stations", and lot's more. Call Toll Free,
1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
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