[TowerTalk] Coax, conduit and toroids

K8RI K8RI-on-TowerTalk at tm.net
Wed Jun 27 11:40:59 PDT 2012

On 6/27/2012 12:59 PM, Steve Hunt wrote:
> Jim's comment should not be misunderstood. A 1:1 choke balun feeding a
> _perfectly_ balanced load does not have zero common-mode voltage across
> it; the choke CM voltage will be exactly half the voltage across the load.

In the case of a perfectly matched antenna the voltage on the inside of 
the shield is half the "differential mode" voltage, not common mode.
Common mode voltage on the outside of the coax indicates the coax is 
also serving as an antenna.  As Jim says, the antenna "system" can be 
very complicated particularly when unbalanced.  It would be extremely 
rare to have an antenna that is perfectly balanced, so the choke at the 
feed point of the antenna to eliminate, "or reduce" the common mode 
voltage is a much better idea than the commonly used 1:1 balun.  This 
can also reduce received noise and in some cases present a much quieter 

Also there are some problems with the air core coil at the base of the 
tower.  Air core coils are relatively narrow band with the optimum 
number of turns varying with the band in use.  Even when resonant they 
do not present the resistance/isolation of a ferrite core choke to 
common mode voltages.

I know they are expensive for good ones, but when you spend all that 
money on the rig, (amp?) coax, tower(s), and antennas, the extra for the 
chokes is a very small percent of the totals. I consider the ones at the 
antennas essential. The others are easily installed later if proven 


Roger (K8RI)

> If we gradually unbalance the load in one direction the choke CM voltage
> reduces and becomes zero when the load is completely unbalanced. If we
> unbalance the load in the other direction, the choke CM voltages
> increases and in the limit will equal the load voltage.
> 73,
> Steve G3TXQ
> On 27/06/2012 17:08, Jim Brown wrote:
>> My bifilar choke designs (which are simply a short length of parallel
>> wire line wound around a single #31 toroid) will handle full legal ham
>> power levels if the common mode voltage is reasonably low -- that is,
>> if the system is reasonably close to balance -- and they are excellent
>> chokes.
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