Topband: Common Mode Noise: PERSONAL EXPERIENCE

[David Sinclair]

Ed Swynar wrote:

> 
> I have two K9AY loops at my QTH --- each fed with its own run of RG-6 coax
> cable --- with the ability of four point switching from inside the shack.
> Anyway, I was always curious as to any "extraneous" signal pick-up by way of
> the feedlines alone to each loop (the runs are 100' long, and simply laid
> atop my lawn). For want of something to do with a couple of ferrite toroid
> cores that I salvaged from the picture tube deflector yoke/coil assemblies
> out of some TVs here, I wound an eleven turn winding of RG-6 on each toroid,
> & put appropriate connectors at the ends of each winding. I then
> disconnected the loop from one of the feeders, and tuned around the AM BC
> band frequencies of some powerhouse stations...
> 
> Interestingly enough --- and as expected, I guess --- I could detect the
> stronger stations from before, at about the S-1 level on my Icom 751A's
> meter. I then inserted one of the homebrewed "filters" ahead of the rig, &
> the signals disappeared.
> 

RG/6 construction varies a good bit from one manufacturer to another.
I believe they all feature foam center dielectric, however. I would
advise caution so far as wrapping this type of coax too tightly around
ferrite cores on a small radius- especially with those which have a
copper-plated steel center conductor. Foam dielectric is infamous for
deforming to allow the center conductor to migrate and eventually
short to the outer conductor, if it is bent on too small a radius.
It would probably be best to make filter assemblies using regular,
small diameter, polyethylene dielectric coax such as RG-58, RG-174,
etc. Connectorize them and insert in line with your straight runs
of RG/6, or just pot/encapsulate them inline.

A great idea about using the TV flyback cores. I'm going to remember
to harvest some of them myself, whenever I get a chance to go over
dead receivers before they go to recycling.

73, David K3KY