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Re: [TenTec] 450 ohm Feed Line

To: <tentec@contesting.com>
Subject: Re: [TenTec] 450 ohm Feed Line
From: "JAMES HANLON" <knjhanlon@msn.com>
Reply-to: tentec@contesting.com
Date: Wed, 14 Apr 2004 23:11:07 -0600
List-post: <mailto:tentec@contesting.com>
Just remember when you are using open-wire feedline that you are not 
matching the feedline to the feed-point impedance of the antenna and 
therefore you are running a high swr on the feedline.  That's no problem for 
the open-wire line since it has quite low loss compared to coax.  But if you 
use a balun of any ratio and switch to coax in the middle of the feedline, 
you are also likely to run a high swr on the coax as well, and that could 
present a problem, at least in higher loss and perhaps also in high voltage 
arcing over the coax if you are running high power.  It would be better to 
run the open-wire line all the way to the tuner in your shack if at all 
possible.

I've been using open-wire fed antennas since 1952 with excellent results. 
There are no particular lengths of feedline that are magic.  I use whatever 
length is necessary to fit between the antenna and the shack.  The impedance 
presented at the transmitter end of the line will be a function of the 
termination impedance at the antenna and the electrical length of the line 
of course.  A Smith Chart would be a good aid in figuring out what that 
impedance will be if you are concerned about it.  So long as the impedance 
at the shack is within the matching capabilities of your tuner there, you 
probably won't care exactly what it turns out to be.  If your tuner can't 
match it, one thing you can do is to add an additional length of open wire 
feedline between the tuner and the line coming into the shack.  A quarter 
wavelength would be a good first try, although something considerably 
shorter might work.  That will change the impedance and likely put it into 
the range of your tuner.

Jim, W8KGI 
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