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