Thanks Mark and Nick for getting back to me with really valuable help.
Despite appearances I really would like to make this as simple as possible.
My thought was that in Mark's document it is noted that the null pot works
best at the antenna (your assessment of my thought on this was right, Mark),
or not more than 100 feet away if connected by CAT5. No problem, but in
order to make it reversible I needed the ability to switch the null pot from
end-to-end, so I thought why not extend both ends to the middle, which would
be under 100 feet, place the null pot there, and use a relay to swap ends.
Since both ends were extended to that point anyway I figured I could also
swap the receiver termination end at the same time, using one relay. I
thought that this was a cheaper solution than having two of everything in
order to have a resistance available at each end to switch in and out.
What I hadn't considered was that I needed to transform the impedance down
to 100 ohms BEFORE I extended them. I apparently hadn't considered a whack
of other things as well, there goes my nascent engineering career.
It's starting to look like the best approach may be to pick a null direction
and get that set up, and worry about reversibility at another time. I think
the FLG100 would be the best choice for an amp because of the native
impedance match. I would try and have it at the antenna rather than in the
house. Does this make sense? As I say I'm going to be running about 250
feet of feedline and am wondering if I need an RG8 type or if a smaller
diameter will do. Intuitively I would go for a bury-flex solution because of
weatherability, but it's not cheap. And the whole cost thing is what was
driving me to try and design something that would not require redundant
components in the first place - after all I'm only going to be listening to
one direction on a particular antenna at one time, and so having a bunch of
expensive kit sitting there doing nothing and waiting for me to change null
direction seemed like a waste of resources.
It looks from the diagrams like the beam width is about 60 degrees at about
3dB down and so I think I'll try and do 2 antennas, one at 75 degrees true
which will give me from Scotland to West Africa in that 60 degrees and null
North America, and one at 195 degrees true which will cover western Brazil
to Belize.
What is the pattern of an unterminated DKAZ? I just noticed that there on
the back-side of an antenna at 195 degrees is northern Labrador through to
Scandinavia and there probably isn't really much worth nulling up there. If
the pattern is figure of 8 then maybe I'll just go with an unterminated
version.
Thanks again for all your help, guys.
Regards,
Michael
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