In article ,
"Robert11" wrote:

Hi,

Thanks for suggestions on Ferrite Bead usage from older post, and for
all the past help. Sure is a lot to learn.

Have one question on the ferrite beads:

For my receiving only antenna.

If I put some ferrite beads over the coax run in from the antenna to
the house, wouldn't the beads (also) degrade the incoming received
signal level?

I guess what's bothering me is that I don't see how they can absorb
noise "selectively" from the actual signal (30 MHz on down) ?

What's some of the very basic theory behind how these things actually
work, then ?

Ferrite can be used to either absorb or block RF energy. People in this
news group will mostly only consider the later action. The ferrite as
used conventionally at HF frequencies is used to increase the
inductance of a path. An increase in inductance in a path results in an
increase in inductive reactance at any frequency. Here the idea is to
prevent a common mode RF current on the outside of the coax shield from
traveling the length of the coax. The differential mode RF current on
the inside of the coax is not affected by the common mode choking
action of the ferrite.

So the basic concept is the ferrite increases the impedance of the path
to a common mode noise on the outside of the coax where the RF current
is traveling in one direction. The RF electrical current is converted
into magnetic field energy. Some of this energy is lost (dissipated) in
the ferrite during the conversion from electric to magnetic and back
again. The ferrite is not a perfectly conducting element for a magnetic
field and has resistive loss to the field so it dissipates some of the
energy.

The differential current on the inside is going both ways and the
magnetic field of the current traveling in one direction on the inside
of the shield is canceled by the current on the center conductor going
in the other direction. The differential current fields cancel so the
conversion, magnetic field path loss does not occur. The net result is
no change in the impedance of the path for a differential signal.

--
Telamon
Ventura, California