Guys,
Any older ARRL Handbook will give you information on building your own
"antenna tuner" for balanced transmission lines. Perhaps the most simple
example would be a parallel-tuned circuit resonant at the operating
frequency with a link wound around the center of the coil. You may want to
tap the link in the middle to allow using all of it or only half of it. If
you can get the hardware to make it a "swinging link" that would be better,
if not just put a high capacitance variable in series with the link - the
tuning condenser pulled out of an old 5 tube broadcast receiver would be
well suited. Connect the antenna to either end of the coil, or tap it down
on the coil symmetrically from both ends. Tune the resonant circuit
capacitor and the capacitor in series with the link and adjust the antenna
tap positions for zero reflected power on an SWR meter between the
transmitter and the link. It will take a little cut-and-try, but in a few
minutes you will have found the right combination of settings and you will
be all set to go. I use a tuner like this, implemented with a few clip
leads to change positions on the coil and the link. You don't need
expensive, unobtanium switches. My capacitors are "BC375 surplus" size, and
the tuner easily holds the 600 watt output of my Heath SB200 linear. My
coils are the larger B&W Miniductor types or a BC610 plate coil for the 20
to 10 meter range. I also have clip leads attached to each end of the coil
that allow me to short out a portion of the coil, symmetrically from each
end of course, as necessary. You can also center-tap the coil and connect
the center-tap to an earth ground to bleed off static electric charge build
up.
A little more fancy tuner is shown in my 1965 Handbook. It has three
variable capacitors connected in series across the coil with the antenna
connected across the middle capacitor of the three. It also uses a
capacitor in series with the link as above. The three caps in series allow
you to vary the tap position of the antenna electro-mechanically rather than
with clips on the coil. I've implemented this one with Hammarlund low-power
transmitting variable condensers and a coil wound on a ceramic form from a
BC375 tuning unit. It handles the 200+ watt output of my Globe King 275
with no problem.
So you don't have to go out and spend a gazillion bucks on a fancy,
commercially-made tuner. Just keep your eye out for some likely,
bread-slicer tuning caps at the next swap meet and build one of your own.
Jim, W8KGI
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