Vic wrote:
you could make a
doublet with the horizontal wire coming off the top of the pipe. Sort
of an inverted L fed at the top.
feed here :----------------
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That would have a little lower angle from the horizontal part and less
ground loss. I'll have to try these in EZNEC and see what happens.
> Yes, the arrangement would introduce some unbalance in the feeders.
If it's severe, then the line can be quite lossy. Possibly looking at
the element currents computed by EZNEC will give me a clue.
-----------------------------------------
Yes! The challenge with the DeMaw design is getting the whole system up high
enough so that the horizontal radiator is at a decent height.
The issue of feedline balance is one I've got on my to-do list to
investigate. Few "balanced" antennas are really balanced. We tend to tie
antennas to trees or buildings, so the part of the antenna that is most
sensitive to surrounding objects - the high-impedance ends - is closest to
such objects.
But what is the real world difference? I can't answer that today. I can tell
you that the RF currents in the feeders on my doublet are within 5% of each
other, but phase is important too, if we don't want the feeder to radiate. I
haven't done phase measurements.
Some early ATU designs used an RF voltmeter across the open wire feeders
(using large resistors to reduce the voltage and provide isolation) instead
of two RF ammeters in series with the feeders. The idea was that maximum
voltage meant maximum out-of-phase currents in the lines. Balanced ATU's
like I use can be adjusted to alter the phase and balance of currents in the
feeders, at least at the shack end where unbalance is the greatest problem.
It's done by changing the tap points on the main coil.
Ron AC7AC
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