[TenTec] 160 meter antenna matching

[JAMES HANLON]

Mike,
 
I've been reading the mail on the Ten Tec reflector about your 160 meter antenna problem.  I use a similar antenna, 268 foot center fed with 600 ohm open wire, on all bands from 160 through 10 meters.  My feedline is long enough to reach from the antenna out in the yard to the shack.  It goes through some trees, up over the house roof, and then down two stories.  It's probably somewhere around the length of yours, I really don't know.  I use a home-brew antenna tuner consisting of a parallel tuned circuit (coil and capacitor).  I tap the antenna onto the coil, and I short out equal portions of the coil, from the outside, to adjust the tuner for higher frequencies as needed.  I link couple from the center of the coil to my rig with a variable capacitor in series with the link.  I adjust the antenna and tuning coil taps and both condensers for zero reflected power, 1:1 swr, into my 50 ohm coax feeding my rigs.  I do not use a balun of any type between the feedline and the tuner.  
 
On 40 meters I was not able to achieve a satisfactory match without creating such a high voltage across the main tuning capacitor that it would arc over with very much power applied.  I hung a 40meter,  1/4 wavelength section of 450 ohm line up on the side of my house and put it in series with the feedline from the main antenna just outside of the shack.  That changes the feedline impedance that I see in the shack so that I can now make a satisfactory match with my tuner.
 
I notice that you were advised to lengthen or shorten your feedline by about 10 feet to see if that would change the impedance by enough to make your tuner happy.  For 160 meters, 10 feet amounts to about 0.019 wavelengths, so that length of line would not change the feedpoint impedance very much.  
 
You might ask Marsh to run his transmission line calculator for you to determine what kind of impedance change you might expect to see for some practical length of transmission line change.
 
Another interesting possibility would be to put an inductance in series with the transmission line before it enters your tuner.  Marsh calculated the impedance at the shack end of the line as 45.32 - j746.71 ohms.  If you put an inductance representing +j746.71 ohms in series with the line at that point, the resultant would be a resistive load of 45.32 ohms, something your tuner should be able to match easily or that your rig could load into without a tuner.  I would suggest that you split the inductance into two parts and put each part in series with each side of the transmission line, just to keep things balanced.  764 ohms of reactance at 1.83 MHz amounts to 66.5 microhenries of inductance.  A little calculation with a formula from an old ARRL Handbook shows that for a 33.25 microhenry coil, one could wind a coil with a 3 inch diameter, 5 inches long, and 31.5 turns.  That would require about 31.5/5 = 6.3 turns per inch (of coil length).  The diameter of #16 wire is 50.07 mils, meaning you could get as many as 1000/50.07 or 19.97 turns per inch of #16 into a coil if it was close-wound.  6.3 turns per inch would be easy for a space-wound coil.  
 
The formula for the number of coil turns I'm using is:  N = the square root of [ (3 a + 9 b) x L) /  (0.2 x a x a) ]  where N is the number of turns, a is the coil diameter in inches, b is the coil length in inches, and L is the coil inductance in microhenries.  You can plug my numbers into the formula to check that you are making the calculation correctly, and then you can try other numbers for diameter and length to see what size coil would best suit your needs.  
 
All considered, some series inductance would probably be a lot easier to put together than a substantial amount of additional feedline, so that's what I would suggest you try first.
 
Good luck.  Let us know how it comes out.  73,
 
Jim Hanlon, W8KGI