In article ,
Roy Lewallen wrote:

Big Endian wrote:

Here is something strange I discovered this weekend. While using a F.S.
meter as a RF sniffer, I was probing along some lamp cords that are
plugged into one of those extention outlet strips. I have a split block
ferrite core which measures one inch square and a 1/2 inch ID hole. So
while monitoring the FS meter I placed the core on the 115 to the house
outlet thinking that I will see a decrease in FS reading, much to my
surprise just the opposite happened, the FS meter pegged out. Why would
adding the core cause the FS reading to increase?

There are at least three possible explanations.

The first is one I've come across many times in doing EMI work, and is
probably the most likely. What happens is that you've got two or more
radiating sources whose fields cancel or partially cancel at the field
strength meter. When you reduce the radiation from one of those sources,
the field at the meter increases.

The second can be a bit subtle. Suppose you have a wire near an antenna
and that wire is, say, 3/4 wavelength long. Very little current will be
induced in this wire because it's far from self-resonance. Now put a
choke in the wire 1/4 wavelength from one end. Presto, a lot of induced
current in the now-isolated 1/2 wavelength portion. This phenomenon can
cause common mode current to increase when you add a common mode choke
on the feedline, if the current is being induced in the feedline (as
opposed to conducted) and the wire length and choke position are
favorable for this to happen.

The third is that the core you're using is a high frequency ferrite. If
it is, it will act as a loading coil, which could make a previously
non-resonant system resonant. I don't think this is likely, though,
because most clamp-on cores are made from ferrites suitable for EMI
suppression. A common type of material for this purpose is type 43
ferrite, which has a Q of 1 at a few MHz. So this type of ferrite won't
cause resonant effects like a high frequency ferrite (e.g., type 61) would.

Roy Lewallen, W7EL

I suppose the right thing to do is to place ferrite cores on all the
conductors in the area. Could get expensive. I have so many wires all
over the place the job seems a bit hopeless.

Thanks for the information Roy.