[Top] [All Lists]

Re: [TenTec] RF Ground

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] RF Ground
From: Jim Brown <k9yc@audiosystemsgroup.com>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sat, 04 Apr 2009 22:12:43 -0700
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Bob McGraw - K4TAX wrote:
> Why would one feed a dipole, generally considered a balanced antenna, with 
> an unbalanced coax?  Should you do so, then you do need a RF choke at the 
> feed point to reduce current on the outside of the shield.  
Actually, coax is the BEST feedline for a dipole, BECAUSE you can kill 
common mode current on it VERY effectively with a ferrite choke, You 
cannot do that with twinlead -- way too much leakage flux. 

Virtually ALL ham antennas are unbalanced by their surroundings, even 
before you put a feedline on them. Trees, buildings, ground slope, 
varying conductivity under the antenna, slight asymmetry in the antenna, 
etc. Put a parallel wire feedline on a dipole and there will still be 
common mode current on the feedline due to those imbalances.  That 
unbalance creates common mode current on the feedline, the common mode 
component of the current radiates, and by reciprocity, couples RX noise 
onto the antenna (and fills in the nulls of your beam).


Jim Brown K9YC
TenTec mailing list

<Prev in Thread] Current Thread [Next in Thread>