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Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
From: Jim Brown <>
Date: Tue, 24 Apr 2012 11:26:24 -0700
List-post: <">>
On 4/24/2012 3:03 AM, Ian White GM3SEK wrote:
> I agree with Jim, but calling this a "brute force" approach is being
> unkind to ourselves. Real-life RFI problems provide a deeper technical
> rationale.

Not unkind at all -- I consider it a very solid engineering approach.

Here' the basic problem. We have equipment manufactured by others, often 
poorly documented, mostly with Pin One Problems that are impractical to 
fix by virtue of their construction, often difficult to take apart. Even 
if we had doc could open them up, and could modify them, any engineer 
who has ever built anything halfway complex realizes that changing 
things, especially "grounds," in a device that is stable opens up the 
possibility that it may no longer be stable. Another component of the 
problem is that often the victim equipment (or the noisy equipment 
emitting trash that we'd like to keep out of our RX) is often owned by a 
neighbor. Yet another part of the problem is that RFI problems are often 
broadband in nature, so any solution limited to a single frequency is 
likely to be problematic.

All of this DEMANDS that the solution be as non-invasive as possible.  
We've got to choke cables, but the last thing we should be doing is 
going inside the box.  Period.

My recommendations of the common mode choke as a solution is based both 
on considerable research in the lab shoving RF into victim equipment, 
and upon considerable anecdotal observations of problems and solutions.  
The overwhelming preponderance of RFI of all sorts is excited by common 
mode current, so suppressing that common mode current is the obvious 

Brute force is the solution here because once you realize the problem 
there's no need to get elegant with the analysis by studying resonant 
cable lengths, or figuring out more complex networks that work at each 
frequency of interest.  If the choking impedance has a sufficiently high 
series resistive component at every frequency of interest, that's 
enough! Not only that, more is better.  And better yet, suitable ferrite 
parts are pretty inexpensive as compared to going into a lab and taking 
it apart.

Brute force is also elegant because when working in a neighbor's living 
room, you often have only one shot at it, so you'd better get it right 
the first time, and you certainly don't want to break anything.  :)

And brute force is elegant on a DX trip or on Field Day when you have 
limited time for setup, have luggage weight restrictions, yet want to be 
prepared for the worst.

73, Jim Brown K9YC


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