Topband: Beverage feedline isolation

n0tt1@juno.com n0tt1@juno.com
Fri, 18 Oct 2002 02:55:59 +0100


Hi Gents,

I was considering several methods of building a common
mode choke for my Beverage coax line.  I wanted something
that was small and could fit inside an existing box.  Small
coax wound on a core was an option, but then I didn't have 
a large enough ferrite toroid to use.

Then it dawned on me that a simple twisted-pair transmission
line might work.  I made a few tests with some rather crude
instruments with some tightly twisted #26 enamel wire
and one #73 binocular core (the same kind made "famous"
by Tom, W8JI.

I wound 6-1/2 turns of the line through the core exiting 
at opposite ends.  Here's what I observed (again, crude instuments 
were used)....The twisted pair transmission line impedance 
measured about 60 ohms, and the common mode choke 
impedance measured about 1950 ohms reactance at 1.8 Mhz.

If the relatively low breakdown voltage of the enamel wire insulation
could be tolerated, I see no reason why a choke like this couldn't 
be used, that is if the above measurements are reasonably correct.

Could someone with better instruments and the cores make 
some measurements on the described choke and make a quick report?

73,
Charlie, N0TT

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