On Wed, 2008-05-07 at 04:25 -0700, Jim WA9YSD wrote:
> Hi all
>
> Another confusion.
>
> An isolator or to obtain isolation or high inductance on the out side of the
> coax feed line is to wrap 6 to 8 turns around a plastic 5 lbs coffee can.
> Some people call this an balun or current balun. Why? This use of the term
> balun to me seams to be a miss application. What should the term really be?
>
Its one of the ways to increase the impedance of the outside of the
coax. It allows the impedance to ground to vary on both the center and
outer (inside) conductors so the antenna sets the balance. It doesn't
have the bandwidth of the ferrite core current balun which is often a
ferrite sleeve around the coax as a 1 turn coil ferrite cored coil.
The plain coil has a lower frequency where the inductive reactance is
enough to decouple adequately, then at some higher frequency, the
capacitance between turns makes it parallel resonant where it functions
best, then it has a high frequency range where its capacitive and when
the reactance of that distributed capacitance gets down, it doesn't
decouple well enough. Typically such a coil works from 14 to 30 MHz.
There is also a region where the coax in the coil is a quarter wave long
and its highly effective there. Sometimes that may not be
distinguishable from the parallel resonance.
When a ferrite core is added, it takes less conductor length to get the
minimum impedance at the low frequency side, yet the high frequency side
is still determined by that quarter wave length and the distributed C
and the ferrite isn't so much effect which allows for a broader
bandwidth of useful impedance. Whether the ferrite is a pot of toroids
over the coax or the ferrite has multiple turns of coax or wire pairs
through it.
The effect is the same, so the name can be generally the same, just the
ferrite cored balun will work from 80 or 160 through 10 and the air core
balun will only work over a 2:1 range like 14 to 30 MHz.
> Keep The Faith, Jim K9TF/WA9YSD
>
>
73, Jerry, K0CQ
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