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Re: [TowerTalk] Dipole Center Connector

To: "Tower Talk List" <>
Subject: Re: [TowerTalk] Dipole Center Connector
From: "Jim Brown" <>
Date: Mon, 17 Nov 2008 10:36:49 -0800
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On Mon, 17 Nov 2008 10:08:58 -0800, Bob Moore wrote:

>Jim-  This discussion sparked this question-  My set up here is 100' of 7/8"
>hardline (used because I got it for free and seemed like a good thing to do)
>running from near the rig in the house to the base of the tower where it
>goes into a remote switch with RG-213 making a run to each antenna.  The
>question is what will I do to the efficiencies if I run 100' of RG-59 to a
>new wire antenna I am putting up for 80m.  

Hi Bob,

That smaller cable will likely add a dB or so of loss when the antenna is well 
matched, and an additional dB or so far off resonance. The transition from 75 
ohms to 50 ohms will do a bit of impedance transformation, but a decent antenna 
tuner should handle it. The other effect may be counter-intuitive. Using higher 
loss coax will result in a lower VSWR at the transmitter for the same VSWR at 
antenna, because reflections are attenuated more by the coax. That will make 
antenna easier to match to the rig, but you'll be burning more power in the 

BTW -- the ARRL Antenna Book comes with a great computer program that computes 
loss in various types of coax for any frequency, length, and antenna impedance. 
It was written by Antenna Book editor Dean Straw, N6BV, who also wrote the very 
useful HFTA software that models how a horizontal antenna interacts with nearby 
terrain, thus allowing us to find the optimum height. A bunch of the serious 
operators in our club have used it and vouch for it (including me and my 
neighbor, K6XX). HFTA is also on the CD that comes with the Antenna Book. So is 
beginning version of W7EL's EZNEC. 

I happen to have about 300' of
>the stuff and am looking for a good use for it.  The only reason I would
>consider it is the smaller diameter and lighter weight and I don't want to
>buy more RG-8x unless I have to.

I found a 1,000 ft spool of Belden RG11 at HSC last year that was about 80% 
and bought it at a good price. I used it on both of my high 80/40 fan dipoles, 
and I'm very glad I did. It takes more tension on the guy wires to hold it up, 
but I figure it's good for a dB or so averaged over the bands, and for several 
on 30M and 17M where the mismatch is greater and coax loss is greater. I use 
on these antennas because they're high (100 ft), and thus a closer match to 75 
ohms than to 50 ohms. 

Bottom line -- if you can hold bigger coax up in the air and can get it at a 
price, it's well worth doing.  


Jim K9YC


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