> -----Original Message-----
> From: topband-bounces@contesting.com
> [mailto:topband-bounces@contesting.com]On Behalf Of Tom Rauch
> Sent: Wednesday, November 10, 2004 1:48 PM
> To: Larry Molitor; topband@contesting.com
> Subject: Re: Topband: 160m noise
>
>
> > Much has been said about ferrite solutions, but they have
> been a big disapointment for me. I have NEVER had a snap on
> type of core make any difference on any frequency. One piece
> cores sometimes help, but it takes a dozen or more to raise
> the impedance enough to be effective on 160, so that's
> usually not practical.
>
> They are a BIG disappointment for most people Larry.
>
> All a hunk of ferrite does is increase the common mode
> impedance of the cable. If you have a cable ten feet long
> between two solid grounds, the bead can make a many-fold
> increase in impedance and common mode rejection.
>
> If you have a poor ground at either end of the cable or a
> long cable, a ferrite bead as a general rule does little or
> nothing to the system.
>
> Think of the bead as the series "L" in a pi network. If
> shunt Z and series Z is very low, adding a bead choke of
> modest impedance will have a large effect. If the shunt Z is
> high *or* the series Z is high, adding the bead will do
> nothing.
>
> If the bead is primarily inductive, adding the bead can
> actually increase problems if the cable length is such that
> the common mode impedance is capacitive.
>
> The bottom line is just slapping a bead on a cable without
> planning, even though that is the first thing we tend to do
> nowadays, is most often a wasted effort. Odds are about 1
> out of 4 it will help.
>
> It's an impedance ratio problem. One 6 ft ground rod in the
> right spot would do more than ten beads.
>
> 73 Tom
>
This has been a very interesting, and somewhat frustrating,
discussion!
My applications are:
1) For keeping RF out of audio lines in a high RF multi-
transmitter station. I think I understand this one,
although I'm interested in seeing K9YC's article to
learn to better select ferrite materials.
2) Reducing the effect of the feedline on the effectiveness
of a Beverage or pennant/flag antenna. From Tom's
description above, ferrite is pretty much useless in
the case of my pennant/flag (long feedline, poor ground
at both ends), but we are told in the various pennant/flag
articles that a choke at the feedpoint is pretty much
essential to proper operation.
So, who has *practical* advice of how to apply a choke
for flag/pennant antennas? I've been winding 15-20 turns
of RG8X in an FT240-77 core.
Next week I'll be flying down to Curacao for a few weeks, and
I'll have some time to try out some more receiving antennas.
73,
Jeff Maass K8ND
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