Topband: Grounds

[Tom W8JI]

Hi Mike,

I hope we can all collectively rethink RFI suppression a bit.

It is electrically impossible for a device to transfer *significant* common 
mode on 160 meters or other low frequencies without some other low impedance 
terminal to push against. If it does have common mode, it will be very hard 
to suppress it with a bead or choke because the source impedance will be 
very high.

Imagine a florescent light fixture with a two or three wire cord entering, 
insulated, with no other path. Let's exclude direct radiation, since a 4 
foot fixture makes a poor antenna and it is a long distance from the 
receiving antenna.

There are two ways it can excite the wiring:

1.) Differential mode between the wires that enter via one cord.

2.) Common mode on the cord that the fixture pushes in differential to the 
lamp housing. Since a floating housing is an extremely high impedance on 
160, the housing moves around a lot more in voltage than any wiring and 
becomes a fairly weak E-field dominant source.

When you throw beads at that system because everyone else throws beads at 
things, it solves neither problem. For common mode, the beads would have to 
add an impedance the same sign (capacitive) or extremely high resistance 
compared to the reactance of the fixture to space around the fixture. The 
last thing you would probably want is inductive reactance, because it would 
increase common mode current.

If it is differential mode excitation of the wires entering the fixture, you 
can add all the beads you want over the cord wire group and you do nothing 
at all. The beads change the common mode, but the excitation is differential 
mode. The differential mode is largely unaffected by the beads.

This is the result of poor analysis and poor planning, and is why people 
have to sometimes resort to unnecessarily high choking impedances for baluns 
and RFI suppression. I can site many dozens of examples where a single 
jumper wire or bypass capacitor has hundreds of times more effect than 
beads.

> Thanks for your thoughts. I looked at things here just now, and found that
> (1) unplugging my amplifier's power cord and (2) disconnecting an unused
> 50' run of RG-213 from my main TX antenna switch drastically drops that
> noise. :-)

That doesn't surprise me in the least. It simply means there is some 
differential excitation between the TX system grounding and the mains being 
excited by the lamp(s).

I had a problem with a switching supply in a battery charger in a neighbor's 
house differentially exciting the mains. I could connect and disconnect 
things and change the levels. To cure it, since I didn't have access to the 
source, I put a bypass capacitor in a plug and moved it around my house to 
different outlets. I found an outlet that made the noise virtually go away.

If I had access to the charger, the cure would have been a bypass at the 
battery charger.

The only way a choke would have any effect would be if separate independent 
chokes were on each wire leaving the charger.

73 Tom