[Lowfer] Can too much be too much ??

[C. Turner]

I've had to combat noise in my shack several times over the years, and 
here are a few random observations  As usual, your mileage may vary:

- I've never had much problem with a properly-functioning computer 
causing much QRM on the LF bands.  What I've seen, however, is that the 
screws holding the board down inside the power supply are often not very 
tight or work their way loose, "un-grounding" the power supply board.  
Simply tightening them down made the problem go away.  If that wasn't 
enough, adding a second bifilar choke (and some caps) to the line input 
beefed up the decoupling.

- One one computer, I found that the rear-panel jacks weren't making 
good connection with the case allowing noise (on HF, mostly) to be 
conducted out through the keyboard and mouse connectors.  Typically, 
they have these spring-things that are supposed to make this connection, 
but they are rarely springy or stiff enough to be reliable.  In that 
case, I simply used a very hot iron and soldered the rear-panel 
motherboard jack panel to the case in a few strategic places (doing what 
the springs were supposed to have done) - it only took one or two to 
ground it adequately on LF/HF.

- In the past, it wasn't the computer's video card that cause the 
problem, but the monitor attached to it.  I've found that LCD monitors 
are far less offensive than CRTs - probably having something to do with 
not having a huge, high-power, high-voltage oscillator buzzing around in 
a plastic box.  The correlation with the video card came about because 
harmonic of a refresh rate happened to land near the frequency of 
interest:  Simply changing video mode/refresh rate was usually enough to 
QSY the QRM.

- My LF antennas come in on RG-58 type coax that have been passed 
through some pretty hefty inductance - although I'd have used RG-174 
instead, had I any on hand at the time I wound it.  My favorite thing 
for this is the core of a dead flyback transformer as you can usually 
get a dozen or more turns on it and achieve 100's of uH of inductance, 
although the complete ferrite yoke (winding, pretending that it was a 
large, odd-shaped toroid) will work, too - but is a lot uglier and takes 
more wire.  This - along with a good ground - can go a long way to 
decouple the coax from the LF antenna from the "Shack Grunge".  On my 
LF-400B, I have found that I was able to achieve effective decoupling 
down below 10 kHz with this method.  Likewise, a balanced line with good 
longitudinal isolation (such as might be found on those that use CAT-5) 
can work, too, but a similar decoupling choke can further-enforce that.

- I've had surprisingly little QRM from compact fluorescent lights.  
These generally seem to operate in the 30-35 kHz area (although that 
varies) and while they'll kill a WWVB clock, they've been fairly quiet 
for me above 150 kHz or so:  It's the light dimmers that are the *real* 
problem!  Nevertheless, I'd keep an eye out for them and be ready to 
swap out to a different brand if there's a QRM issue.  (No, I don't know 
which brand is better or not - and if I did, it probably wouldn't 
matter, as they seem to change suppliers all of the time, even for the 
same brand...)

- *Other* switching supplies seem to cause problems.  I have a "Z-Box" 
(Z3801 GPS unit) that utterly demolished my LF listening a few years ago 
- until I added more filtering to it:  Now it's completely quiet.  I 
recently had to beef up the filtering in a Samlex SEC-1223 switching 
supply to keep it out of the LF band (the first clue was that none of 
the WWVB clocks in the room would auto-set anymore) and that was done by 
adding a couple capacitors and a bifilar choke swiped from an old 
switching supply - and now its completely quiet on HF, too!  (See - 
switching supplies *can* be good for LF listening!)

- For a while, a neighbor had a Plasma TV that was tearing everything 
up, but the problem fixed itself.  A guy who repairs things like that 
told me that there are two types of Plasma TVs:  Those that have already 
blown up, and those that will!  The QRM-ing TV apparently moved itself 
into the first category and hasn't been heard from since!  (That's 
another reason why I'd never own one!)

Now, if these things come from outside your house, then you have an 
entirely different set of challenges - but appropriately-wielded loops 
can help with that, too...

73,

Clint
KA7OEI