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
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