On 7/17/18 10:43 AM, Mark Spencer via TowerTalk wrote:
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.)
I support your idea..
Having the tuner at the feedpoint is the ideal. And whether the
circuitry inside the box is symmetric or not makes no difference. It's
two terminals.
Yes, you need a choke on the coax at the tuner (which is what forces the
above statement - you don't want currents going down the outside of the
coax).
I'm not actually sure you need a transformer at the antenna side tuner,
unless you're putting the tuner some distance away, in which case, the
higher Z balanced line *might* reduce the loss. But not necessarily,
unless it's "many wavelengths" long. That is, the loss through the
transformer, and a short <1/4 wavelength length of transmission line at
a different Z might actually be more than just the short length of
transmission line.
(in some ways, this isn't much different than an off center fed dipole:
being off center is effectively a transformer to a different impedance
than at the center)
You'd have to run the math on it.
The key thing you'll need is the "feedpoint Z vs frequency" of the
antenna and to know what frequencies you want to operate at.
The other thing you'll want to know is what's the loss in the tuner at
various mismatches. I believe this has actually been measured and
reported in QST and QEX.
Do you want to have X1 dB of loss in the tuner without a transformer, or
Y db of loss and Z dB of loss in the transformer, where you choose X or
(Y+Z). Again, it's something you need to calculate.
How long is your feeder? How long is your dipole? what bands are you
operating on?
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