[TowerTalk] Ferrite Cores in Trapped Dipoles

Jim Lux jimlux at earthlink.net
Thu Feb 8 00:01:16 EST 2007

At 09:31 AM 2/7/2007, Bill Turner wrote:
>On Tue, 6 Feb 2007 19:10:58 -0500, "Dan Zimmerman N3OX"
>  <n3ox at n3ox.net> wrote:
> >Yet many of us are using autotuners (which all have powdered iron
> >inductors) on pathological matching cases, probably with high
> >circulating currents.
>------------ REPLY FOLLOWS ------------
>Yes, it can be done but doing it right requires some careful
>engineering. It's not something you want to slap together in a couple
>of minutes and put up in the air.

You got that right....

>I still maintain that the best traps (i.e. lowest loss and most
>reliable) have cores made of nothing but air.

That *might* not be true...  But, it's a pretty complicated analysis 
to find out whether it is.  You'd have to look at core losses (which 
will be pretty low), which is fairly straightforward.  The tricky 
part would be looking at the skin effect and proximity effect losses 
from adjacent turns, etc.

>  Also, the only
>capacitance present should be the self-capacitance present between
>turns of the coil. A trap like this is a bit harder to make because it
>requires considerable cut and try trimming to achieve resonance, but
>once made, will work without problems for years on end.

No question there.. the air wound coil is a LOT easier to "cut and 
try" than a cored inductor.  But it is an intriguing idea... you 
could do something like a mobile whip with a big "bugcatcher" loading 
coil that isn't big and doesn't catch bugs.

Jim, W6RMK 

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