Yes, you can probably insert some coax to create a better match, but it may
not be 1/4 or 1/2 wavelengths. First of all, is there a triplexer that
splits the 3 bands onto individual feedlines? If not you may need a
switching arrangement to add in a matching section only when using 6m. if
you can't isolate 6m from the higher bands it will be much harder to match
properly without degrading the other bands. to design a coax match you must
first measure the impedance at the point where you will be inserting the
matching line. If you are lucky the impedance will be something like
50+/-jR, in that case or ones where the real part is near 50 ohms you just
need to calculate a feedline length to cancel out the reactance. If the
real part isn't near 50 ohms then you can add a series line of either 50 or
75 ohms to get it near enough to 50 ohms to allow use of a simple reactive
stub to cancel it out. I use an old smith chart calculator from rfdude.com
that lets you add series and parallel feedline sections to see how they
change the impedance.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: telnet://k1ttt.net:7373
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Jamie WW3S
Sent: Friday, April 21, 2023 11:06
To: towertalk@contesting.com
Subject: [TowerTalk] inserting tuned coax length to help match vertical
antenna
Trying to help the local club get a better match on their 2/440/6m triband
antenna, 2 and 440 are great, the swr is a little on on 6m, 52.680, a little
more than 3:1 or so, We have access to the feedline at the base of the
tower, but no access to climb the cell tower we are co-located on. Is it
possible to insert either a 1/4 or 1/2 wave piece of coax, tuned to 52.68,
to help match the feedline a bit better?
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