[SECC] Switcher Noise

[Hal Kennedy]

While working VK9HR this morning I heard what sounded like switching
power supply noise drifting around on 80 including right through their
QRG.  I pass this along because it's not obvious - at least to me, and
most of us have this potential problem.  I should know better as this is
the second time in 6 years this has happened at N4GG.
 
I keep switchers OUT of this station.  They are noise sources and I do a
lot of digging on the low bands at sunrise.  But there are two in the
shack that I have not replaced with linear supplies.  One runs a Dell
laptop and sits on the floor, and the other runs my LCD computer screen
and it sits on the floor.
 
Six years ago I traced noise originating from the Dell supply.  It was
laying on top of a coax run on the floor.  Moving it a few inches cured
the problem.  This morning, the LCD P/S was laying on the K9AY coax on
the floor.  Moving it a few inches cured the problem.  I kick both
around with my feet so they move a little now and then.  I will get that
fixed today by fastening those two switching supplies up off the floor.
 
Coax does not provide a magnetic shield, nor does it act like coax for
electro-magnetic fields below the cut-on frequency.  We like to think
coax is a shielded cable, but at 120 Hz zip cord has better RFI
properties than coax does, for magnetic fields.  Zip cord is balanced
line WRT a magnetic field, coax is unbalanced.  The transformer in a
switching supply will induce a differential voltage between the shield
and the center conductor of coax if the two are tightly coupled (laying
on top of).  
 
If you have any little supplies - STEPPIRs use them, LCD screens use
them, notebook computers use them, keep those supplies 3 or 4 inches
away from any coax your receive signals are going through.  The best
alternative of course is to go all linear, but for things like notebook
computers and the like that's painful.
 
73,
Hal N4GG
 
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