Jim,
Your suggestion to add 33 uH in series with each leg of the 600-ohm open-
wire feedline for 160M operation should do the job.
Mike,
I'd be interested to know how you end up resolving this. Wishing you a
speedy recovery from surgery.
73,
Marsh, KA5M
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of JAMES
HANLON
Sent: Tuesday, October 15, 2013 5:24 PM
To: prosolar@sssnet.com; marsh@ka5m.net; tentec
Subject: [TenTec] 160 meter antenna matching
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
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|