Topband: 160m noise

[Jeff Maass]

> -----Original Message-----
> From: topband-bounces at contesting.com
> [mailto:topband-bounces at contesting.com]On Behalf Of Tom Rauch
> Sent: Wednesday, November 10, 2004 1:48 PM
> To: Larry Molitor; topband at 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