I have a StackMatch II (to be used on the 6M band) that I've modified to add a
BOP capability by switching in a 1/2-wavelength segment of coax in the lower
antenna's feedline when in the BOP position. This segment is permanently
connected to the lower antenna's feedline through a T connector, but since it
is 1/2 wavelength, it doesn't affect the perceived impedance of the lower
antenna when it is open-circuited at the other end. This all checks out and
works as expected.
However, because this stack is fairly low to the ground and the antennas are
spaced only 1/2 wavelength apart (imposed design constraints), the HFTA model
shows the main BOP lobe at too high a take-off angle to be very useful (around
40 degrees or so). I want to move this lobe down into the 12 to 20 degree range
where I really need it, and I think I should be able to do this by changing the
phase delay from 180 degrees to something less than that, maybe 90 degrees or
so. Assuming my assumptions are in order, I have some questions:
1) What's the most practical way to accomplish a phase shift of other than 180
degrees? If I use coaxial delay segments, they would need to be relay-switched
at both ends, not just one end as for the 1/2-wavelength case. Right? Is there
a better way to do it?
2) How can I model the take-off angle created by different phase delays in this
system over my actual terrain? HFTA only models a 180-degree delay. Can EZNEC
do this? I have it, but have yet to set it up and get into it.
Bill / W5WVO
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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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