Topband: Common mode filters...help

[n0tt1@juno.com]

Hi Steve,

You got it right....it's nothing more than a good RF choke on the shield
to help prevent the "snake" antenna effect.  You can indeed
use a "large" hi-u ferrite core in place of the beads.  
As you mention you can also use a large coil of coax to make a choke.
Whatever material you use the choke should have roughly
1500 ohms reactance at 1.8 MHz....this is not especially critical.
These chokes must be used with a good ground tap on the coax shield,
otherwise they will not be effective.

In some "tough dog" situations one would use even another choke and
ground combination in the line some distance away from the
antenna end to provide even more unwanted signal attenuation.

Without going into this further, I would suggest to anyone who is
interested in this subject to obtain and **actually read** a copy of
ON4UN's latest book on low-band DX-ing....he covers this subject
in detail.

BTW, 100 of those beads from the "Wireman" costs about
$15 USD.

73,
Charlie, N0TT



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