[TowerTalk] Ferrite Cores in Trapped Dipoles

[Bill Turner]

ORIGINAL MESSAGE:

On Mon, 05 Feb 2007 14:18:34 -0700, "Brent Baum" <bbaum at utah.gov>
 wrote:

>I searched through the archives and found plenty of references to using ferrites/toroids as baluns, but nothing that led me to using ferrites/toroids as part of LC traps for dipoles.
> 

------------ REPLY FOLLOWS ------------

Baluns are not resonant circuits, but traps are. Baluns do not have
the high circulating currents that traps do and that is one of the
important differences.

A trap, especially at high power, is not a good place to use any kind
of ferrite or powdered iron core. Ferrite is notorious for saturating
on current peaks and thereby causing distortion and harmonics. If you
must use something, powdered iron is preferred, but air is best of
all. The voltages around a resonant trap are amazingly high and if you
use a conductive core, you must insulate it extremely well.

IMO, the best trap of all is a coil which is self-resonant at the
desired frequency and requires no additional capacitors at all. This
is used in the classic W9INN trap dipoles and easily handles full
legal power. I have had a W9INN dipole up for seven years running at
full RTTY power with no trouble at all. If you make your own you will
need a dip meter and lots of cut and try, but you will end up with a
trap superior to any other type. Again, that is my opinion and others
may disagree.

In other words, there are good reasons why nobody uses ferrite or iron
cores. :-)

Bill, W6WRT