At 12:53 PM 12/27/2005, Bill VanAlstyne W5WVO wrote:
>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?
A single DPDT relay does it nicely. But, as you noticed, you can't switch
just one end. For a narrow band application, an LC network might
work. You'd have to try some combinations on paper to see if you get
enough range. Something like XLZIZL makes it fairly easy, or, you can just
model the L and C in NEC (as loads in appropriately configured stuff) and
let NEC generate the patterns. See below.
>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.
NEC can't model the terrain, but it can model the phasing effects. It's
pretty easy to model two antennas interconnected with a transmission line,
etc.. You just create a little (10cm) segment somewhere a long way from
the antennas, and then use NEC transmission line or network cards to
connect the little segment to the antenna feed points. The advantage of
using an NT card is that you can model the loss in the transmission
line. NEC TLs are lossless.
If someone were really ambitious, they could talk to Dean Straw about
offering to modify HFTA so that it would take pattern output from NEC to
add to the existing canned antenna patterns.
If your terrain is a sort of uniformly sloping surface, then you could
model it in NEC by tilting the antenna appropriately.
If you do this sort of thing for a living, there are a variety of
antenna/propagation modeling programs out there, but they are neither cheap
nor easy to use.
>Bill / W5WVO
>_______________________________________________
>
>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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_______________________________________________
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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