[TowerTalk] Balun question

[Jim Brown]

On 10/9/2021 12:14 PM, CAROL RICHARDS wrote:
> I built several 1:1 Current Baluns from the April 2021 issue of QST and they work fine.

Hi Carol,

This "balun" is really a common mode choke, and while it adds inductance 
to the common mode circuit, that circuit includes the feedline, and in 
the common mode circuit, it becomes part of the antenna. By virtue of 
its electrical length, an antenna (the feedline) can be inductive, 
capacitive, or resistive (at resonance). At frequencies where the 
feedline is capacitive, it cancels the some or all of the inductive 
reactance of choke (balun) you've added, reducing or defeating its 
effectiveness.

The most important function of a common mode choke at the feedpoint is 
to prevent noise picked up on the feedline from coupling to the antenna. 
For this to be effective, a very large resistive impedance is required. 
And because they're resistive, they dissipate power; if the resistance 
is not large enough, and if the choke is not exposed to air, it can fry.

Bottom line -- while that design may not fail due to overheating on most 
bands (it can on higher bands where the choke resonates but the 
resistance at resonance is too low), it's also unlikely to be 
universally effective at reducing noise on receive.

I've been studying all of this since 2003, and published what I believe 
are far superior designs. http://k9yc.com/2018Cookbook.pdf

This is a tutorial on how chokes work, and a lot more.
k9yc.com/RFI-ham.pdf

73, Jim K9YC