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September 28th 04, 03:28 AM

In article ,
(bpnjensen) wrote:

Friends -

I have a few queries regarding people's experience with Ferrite, esp.
beads and so forth.

Can these things actually reduce noise? I see them suggested for
encircling coax cable, electrical wires on appliances and
electronics, and many other devices. If one were to put them around
a coax cable, how many would one use in a row?

Yes they can help. Clamped around a cable they are a 1 turn choke coil.
How many you need depend upon how much attenuation you need. The more
noise you have the larger a choke you will need.

The idea here is that you have noise traveling the outside of cables or
coax and you are attempting to stop it by inserting inductive reactance
in the path.

I have also seen suggestions that ferrite cores with the offending
wires wound through them are useful as well - how do these compare to
beads?

I'm not sure exactly what you are referring to as a "core" and "beads."
I'll guess that you mean the two piece snap together as the "core" and
a toroid as the "bead." They work the same way. The one piece core is
more reliable in that you must make sure that the two piece units fit
snugly together with no gap between them or they become non-effective.

If one wound a wire around a ferrite rod, would this also have a
similar effect?

Yes. The closed cores will work better from the standpoint that their
magnetic path is closed. No stray fields with the closed core type.

What ever type of core used make sure to keep the ends away from each
other or you will make the choke non-effective.

I presently use an A-D DX-Ultra, have used random wires, and intend
to put up some more wire including a new random wire and probably a
loop encircling most of my property along the fence and roof eaves.
I expect to feed these wires using coax with matching tranformers.
The radio and accessories are grounded, which clearly increases both
the signal and external noise.

I appreciate any and all answers. The noise in my neighborhood,
mostly the result of artificial external problems, is really
discouraging, and if these things would help even a few dB, it would
be a blessing.

Generally what you are trying to accomplish with the chokes on the coax
to the antenna is to stop noise coming from the radio end (which is
connected to the AC noisy mains) from getting to the antenna. Noise
traveling from the radio to the antenna couples to the antenna and
mixes with any received signal back down the center conductor.

If you don't have a noisy AC mains supply, radio or the coax shield over
its run is not picking up noise then the chokes will not do much for you.

Often these chokes are used on the source of noise generating appliances
to stop them from radiating.

--
Telamon
Ventura, California

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