Topband: Noise in the Shack - A new noise!

[Jim Brown]

On 6/26/2012 5:18 AM, Tom W8JI wrote:
> I'm not a big fan of throwing beads at noise issues, or wrapping things.
> Usually that isn't a good solution. Running a series of tests to locate the
> point of leakage is usually faster and better.

Most RF noise escapes from equipment as current flowing on wiring, 
either inside the box due to inadequate shielding and/or poor circuit 
layout, and on external wiring due to improper termination of cable 
shields (the Pin One Problem).  The part of that current on external 
cables, and thus the noise radiated by that current, can be greatly 
reduced by ferrite chokes TUNED to the frequency(ies) of the noise.  The 
resonance of a ferrite choke is quite broad, with Q typically on the 
order of 0.4 - 1, and a single turn through most ferrite cores typically 
resonates around 150 MHz.

Like ANY inductor, we lower the resonant frequency by winding more turns 
-- L increases as the square of the number of turns, C increases 
proportionally with the number of turns, and R, which is coupled 
inductively from the core, also increases as the square of the number of 
turns. This R (in the parallel equivalent circuit) is what suppresses 
the current.

While we would all love to fix the root cause of the problem, that isn't 
always easy or practical. Most modern equipment is built with external 
wiring terminated in a manner that puts RF on it -- which shields either 
not bonded to the chassis, or balanced wiring not bypasses to the 
chassis. Yes, it's POSSIBLE to terminate cables properly without a 
chassis or chassis connection, but that requires a ground layer on a 
board and proper layout of that board. Most equipment is NOT built that 
way -- it costs more, and many circuit layout designers don't understand 
the causes of noise (nor do they know about noise or care about it).

  There are graphs of measured data for the choking impedance of various 
numbers of turns on a selected group of cores in my RFI tutorial.  
http://audiosystemsgroup.com/RFI-Ham.pdf  Other core sizes and shapes 
using the same material will behave similarly -- the same number of 
turns through a longer core will simply resonate at a somewhat lower 
frequency than through a shorter one. This is not a large effect -- 
inductance increases approximately linearly with the length of the core, 
and resonance moves with the square root of inductance.

ANY of the cables connected to noisy equipment will act as TX antennas 
if the equipment puts noise current on them, so there is potential 
benefit for choking all of them.  BUT -- I firmly agree with Tom that 
simply throwing beads on cables is next to useless for HF noise -- you 
MUST use multiple turns to move the resonance down to the HF spectrum.

And I also agree that if it's a shielding issue (that is, noise is 
radiated by internal wiring/circuit traces), wrapping the noise source 
is a flaky enterprise at best.  Such equipment requires the bucket 
treatment.  Fill a bucket with water, put it in twice, take it out once.

The good news for topband is that, thanks to wavelength, most noise we 
hear is noise flowing on external cables, because the internal cables 
are too short to be very effective antennas.

73, Jim Brown K9YC