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Re: [TowerTalk] Running Coax in the plane of a VHF antenna

To: "" <>
Subject: Re: [TowerTalk] Running Coax in the plane of a VHF antenna
From: "Jim Brown" <>
Date: Thu, 06 Nov 2008 13:25:14 -0800
List-post: <">>
>So I'm considering loading the coax with Type 61 Ferrite beads, every 
>6" or so, the idea being to make the coax "transparent" to the antenna

This is quite solid engineering, except that #61 is not a good material 
for 146 MHz. #43 would work much better. Study Fair-Rite data for the 
beads you want to use, selecting beads that show an impedance peak near 
your operating frequency and using enough in series so that their 
impedance adds to at least 1,000 ohms, and more is better. I would  
spread them out by about 0.1 wavelength along the part of the coax 
that's near the antenna, assuming a velocity factor of 0.97 (that is, 
adjusting only for the diameter of the coax and the presence of an outer 

Note that my comment is NOT meant to address the relative usefullness of 
other methods of decoupling the coax, merely to state how THIS method 
can be made to work well. This method works by reducing the current in 
the coax due to the antenna current to a very small value by adding a 
large resistance in series with the coax. 


Jim Brown K9YC


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