[TenTec] 450 Ohm feedline tuners

[JAMES HANLON]

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