Mark-
For many years I have used an 80M dipole strung between two fir trees and
fed with open wire line (not ladder line). A tuner allows me to use it from
10M through 160M.
Running 100W, I used a little MFJ 941 antenna tuner with a built-in balun.
I bored holes through the wall and installed two ceramic insulator tubes
(from the old post-and-tube electrical technology). I aimed the holes at a
slight downward angle so rain water won't run in. Inside I brought the open
wire line right to the back of the MFJ tuner's balanced line input
terminals.
For 10M through 80M it works as a dipole. For 160M I short the open wire
line together at the tuner input and match it as a random wire. I had to
add a fixed capacitor in parallel with the tuner's cap to get a decent
match. But it acts like a poor-man's T-top vertical in that configuration.
It's not great as there is no ground system underneath for the vertical
return currents, but I worked Peter I Island with 100W from near San
Francisco, a pretty long path. Only problem was sometimes having the RF get
onto my radio chassis causing either RF burns or a terrible tone report!
Moving the open wire away from the power supply solved that.
It's certainly a simple set up and should be very useful at your cottage.
Your idea of putting the auto tuner outside and bringing coax into the shack
sounds good, and cleaner than my ceramic tubes. Guess the only issue there
will be weatherproofing the tuner and balun.
73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Mark Spencer
via TowerTalk
Sent: Tuesday, July 17, 2018 10:44 AM
To: Tower Talk <towertalk@contesting.com>
Subject: [TowerTalk] Auto tuners, Baluns and open wire feeders
Hi:
I spent some time trying out various wire antennas at the summer cottage and
have more or less settled upon a dipole antenna fed with a short length of
open wire feeder that runs from the antenna feed point to slightly above
ground level, the open wire feeder is in turn connected to a Johnson match
box (a tuner with a balanced line connection), which in turn feeds a coaxial
cable that runs to the radio. The tuner sat on top of a short step ladder
during my tests.
The coaxial cable had various mix 31 and mix 43 ferrite chokes applied. It
all more or less worked as I expected it would on various bands.
Moving forwards I'd like to replace the Johnson Matchbox with an auto tuner.
At this point I'm leaning towards purchasing a suitable Balun from DX
engineering, connecting that to the output of my SGC231 tuner and replacing
the Johnson Match box with the Balun and the auto tuner. I'll have to do
some modelling (to figure out the likely range of feed point impedances for
the dipole antenna at various frequencies) before deciding on what ratio
Balun to buy. I expect I'll install the auto tuner and Balun on top of a
wooden pole perhaps 10 feet or so from the ground.
I am curious if anyone else has done something along these lines and if
there are any issues with doing so ? Any comments would be welcome.
I'd like to move away from using a commercially made trap dipole at that
location. (The appearance, lower weight and wind loading of a simple wire
dipole with a short open wire feeder is attractive to me, plus I need to use
a tuner to match the existing trap dipole.)
73
Mark S
VE7AFZ
mspencer12345@yahoo.ca
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