A Simple and Effective High Power Antenna Tuner
Submitted By Tom K1JJ
A Bulletin Board Chat with Jeff KA1OGM - - -
I've been using an off the shelf roller inductor tuner with its "internal
balun" for output to balanced line for quite a while, now. But I wonder if
there's anything to be gained with a different scheme...: Jeff
Unless you go to the next power level and start heating up coils and blowing baluns, there's nothing to be gained... Unless you desire your own homebrew tuner.. :-) (Good reason)
If you do, Here's a simple design that I've used for years and now Clegg uses with his 813 rig to tune all bands. It has no balun and power limits defined only by the diameter of the copper tubing in the main coil!
50 Ohm Unbalanced to Balanced (any impedance) Super Tuner:
Use a single capacitor in the output, preferably a >350pf vacuum variable that is clip-on tapped across a 40 turn 3/8" copper tubing, 5" diameter coil. (Home Depot tubing) Mount BOTH capacitor and coil floating above ground.
A vacuum variable cap is nice but any single section 350pf or greater, 4kv or greater bread slicer cap will do too. (the more power the bigger spacing needed, and depends on antenna match too)
Then wind five turns of (#14 or greater) teflon wire on the outside of the center of this big coil as an input link. Tie wrap it into place. One end of this link goes to the coax inner conductor and the other end to ground.
The balanced open wire gets tapped on each side of the big coil with BIG clip ons.. Big copper alligator clips work fine...file the openings rounded so that contact with the coil is maximum.
By changing the open wire taps, the capacitor taps and the capacitor tuning you can cover about any impedance or band situation. No switches needed. Use colored tape for each band to pre-mark the tap positions for quick band changes.
A ~1400 pf input tuning cap (large 4 section broadcast receiver type) in series with the link to ground is sometimes used, but I've found it is seldom needed and usually leave it out - Clegg didn't need it in his.
The tuner is simple and effective...and needs only one standard (unbalanced) cap. The rest is easily made.
Then put the "MFJ" balun tuner crap on eBay for cash and buy some good 811A for Hopkinton! :-)