Choosing a Feedline Choke

How to choose the best feed line chokes, line isolators, baluns, or ununs for coax fed dipoles, verticals, hex beams, slopers, loops, windom, OCF, G5RV, ladder line, and yagi antennas

 

Download a copy of this article in PDF : Choosing a Feedline Choke – RFI Tip Sheet RC-1 by AK6R

Radio Communication 

Choosing a Coax Feed Line Choke

By Bob Brehm, AK6R

RFI Tip Sheet #RC-1

 

How to choose feed line chokes, line isolators, baluns, or ununs for coax fed dipoles, verticals, hex beams, slopers, loops, windom, OCF, G5RV, ladder line, and yagi antennas

Overall Objective:  Minimize common mode current (Radio Frequency Interference or RFI) causing feed line radiation on transmission, and rfi generated broadband “noise” on reception. 

This objective can be met when:

EVERY coax fed antenna has a common mode choke at the antenna feed point! 

EVERY rotor control, remote antenna selector also has a common mode choke at each end of the cable!

 

EVERY coax fed antenna has a high impedance common mode choke (a.k.a. Common Mode Noise Filter) at the receiver end of the coax.

Antenna Feed Line Choke Definitions

Feed line choke: 1:1 (50 to 50) have the same input and output impedance.  

 

Impedance transformer: <1:1 or > 1:1 have different input and output impedances.  

 

This document is concerned only with feed line chokes and their effective use to meet the objective.  A separate document is available discussing impedance transformers and their use for specific antenna types.  Some antenna systems will need both a feed line choke and an impedance transformer.

 

Coax feedline chokes (and impedance transformers) all have an unbalanced input and are made with several output options dependent on whether the output is balanced or unbalanced.  The output option determines whether it is an UNUN or BALUN.  There are multiples types of ununs and baluns.  Here are a few examples applicable for feed line chokes (1:1) and impedance transformers (<1:1, >1:1):

 

 

Unun Type 1 (+/- output, coax input) – for unbalanced output like end fed, verticals, etc.