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Re: Topband: 160M Balun

To: topband@contesting.com
Subject: Re: Topband: 160M Balun
From: donovanf@starpower.net
Date: Fri, 30 Mar 2018 13:29:37 -0400 (EDT)
List-post: <mailto:topband@contesting.com>
Hi Peter, 


What you need in a common mode choke is adequate resistive 
component to absorb common mode signals as heat. The choke 
doesn't need to be at resonance to achieve adequate choking 
resistance, but it doesn't hurt either. It takes some thought 
and technical understanding to determine how much common 
suppression is enough. And c ommon mode chokes aren't the 
only way to adequately suppress common mode signals. 



A low impedance radial system provides good common mode 
suppression. Grounding the coax feed line to a center-fed dipole 
1/4 wavelength or 3/4 wavelengths from the dipole feed point also 
provides good common mode suppression, certainly more than 
you ever need for a dipole. This assumes that the feed line runs 
at approximately 90 degrees from the dipole. Pulling the feed line 
significantly away from 90 degrees significantly increases common 
mode. An off-center-fed dipole is much worse because its very 
difficult to build a choke with adequate power handling capability 
and good choking performance. 


You can't understand your needed common mode signal suppression 
without understanding your signal environment and the directive 
performance you want to achieve from your antenna. 


For example: 


A multi-transmitter or SO2R station needs much more attention to 
common mode suppression than a single operator low power station 


A highly directive multi-element Yagi needs much more common 
mode suppression than a dipole or a vertical. 


Coax feed lines properly installed and bonded to a tower has less 
common mode signals than a coax feed line suspended well above 
the ground. 


Perhaps the worst case need for common mode suppression is 
directive receiving antennas with very low signal levels such a small 
terminated loop (K9AY, Flag, etc) or a small "magnetic" loop. Both 
require extreme attention to common mode signal suppression, 
including a strong preference for buried feed lines, control lines 
and power cables. 


73 
Frank 
W3LPL 




----- Original Message -----

From: "Peter Voelpel" <dj7ww@t-online.de> 
To: "Jim Thomson" <jim.thom@telus.net>, topband@contesting.com 
Sent: Friday, March 30, 2018 4:47:44 PM 
Subject: Re: Topband: 160M Balun 

Why do you want it resonant? 

73 
Peter 

-----Original Message----- 
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Jim 
Thomson 

The 160m CM choke needs to resonate lower in freq. It requires more cores 
to shift the res freq down. 

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