On 5/11/03 8:13 AM, Guy Olinger, K2AV at olinger@bellsouth.net wrote:
>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.
I plan to add 12 more this summer. I went from 4 to 13 last year. Didn't
make a big difference on 80m, but it sure perked up the 40m 1/4 wave
sloper.
>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.
Retuning isn't a problem. The real question is, will it cause the shunt
to work "worse" on 80m?
>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.
The A3S has a current balun at the feedpoint, but I doubt it is very
effective on 80m, certainly less on 160m.
>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.
I keep the unused antenna connections grounded at the antenna switch.
>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.
I don't have a KW, just a couple of barefoot rigs, so the voltages might
not be so extreme.
The L would be grounded for 80m operation.
>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.
OK, so what do you suggest -- the L or trying to adapt the shunt to
service 160m as well as 80m?
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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