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
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
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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