Russ Hines wrote:
Thanks, guys, a good topic and one that always seems to come up. And
it sparks more questions and comments, of course.
The cable length issue is a brother to "if you don't like your VSWR,
change the point along the transmission line where you're measuring
it." By changing the length of the line, we're creating a
transmission line transformer (a good thing) but we're limited by its
length (not so good). It seems to me the mentioned
circulator/isolator at the output of the xmtr is a better fix, as
reflections coming back from the duplexer is absorbed by the
circulator's load, the xmtr is generally happy, and we're no longer
limited where we can put things in a rack or elsewhere.
For amateurs, coming up with usable VHF circulators seems to be
difficult and usually expensive, and coax always seems to be cheaper.
Has anyone had luck finding a source for reasonbly priced VHF
circulators, or success in rolling their own?
Also, I noted in the pamphlet Kevin referenced that the unused
duplexer port was left open (Figs. 1 & 2). I guess if the isolation
is already greater than the load's return loss, it doesn't matter, at
least at the reject frequency. But it seems to me one could possibly
create problems for oneself by not terminating the unused open port.
Just a thought.
Maybe I work better knowing there's a load there. ;-)
Your comments, please.
73, Russ WB8ZCC
I think we all agree that a real impedance matching device is the best
approach, but hams (generally speaking) are cheap. Many will spend two
days hacking on a piece of RG-214 before spending fifty or a hundred
bucks on a different (better?) solution.
Allan Crites and I are currently in discussion which will be used as the
basis of a RB web article that will explain exactly what is happening,
why it happens, and why an 'optimized' cable length can be used to
transfer power ending up with the stated loss of the duplexer and have
little reflected power toward the transmitter - so long as the duplexer
is tuned properly and exhibits good return loss on the frequency it's
designed to pass.
Kevin Custer
