Hi Brian,
> The "design" consists of about 6 turns of about
> pencil diameter coax wound on a toroid.
>
> I'm more used to designs with multiple windings or
> bead baulns.
>
> I'm interested in using this to feed my 402CD 40
> meter beam (50 ohms at
> the antenna feed)
>
> Questions:
>
> Is this a valid "bauln" design?
Depends on core material. You'd have to measure it to tell how good it is.
There are two basic rules at work....
1.) When you wind multiple turns on one closed core, the
impedance goes up about (not exactly as often claimed) by the
SQUARE of the turns.
2.) The impedance of a GIVEN core material is closely related to
the length of the core exposed to the wire inside the core. For
example, #61 ferrite has about 8 ohms per linear inch of core
thickness that parallels the wire on 2 MHz, and about four times
that at four times the frequency (about 32 ohms on 8 MHZ).
Let's assume you have a one inch tall stack of 61 material cores.
With one pass through the core the impedance would be ~32 ohms
on 8 MHz. With six turns it would be six squared times 32, or 1150
ohms. That would be plenty of impedance for 80 meters on up, and
OK even on 160.
Let's say you use the same material in beads. It would require a
stack of beads 32 inches long to get the same impedance!
So for a given material, multiple turns always produce MUCH more
choking impedance for a given core length.
> Does it really transform unbalanced to balanced?
Almost certainly it does, unless they used powdered iron cores
which are almost useless in baluns of any type.
> I'm convinced from past experience that best beam
> performance occurs when the feed is truly
> balanced.
Me too.
> Is it just a shrinkage (because of the toriod
> permitivity) the 12 turn coax design which guys
> seem to have both success and failure with?
Success and failure depends on the core material selected, the
construction, and the operating parameters. I almost never use
bead baluns, unless the application is low power, low common
mode voltage and current, or very high frequency.
> Has anybody used similar designs on their 402CD?
Wouldn't matter unless you all get together and tell what core you
used. Myself, in something like a yagi where the antenna can hold
all kinds of weight, I just use an air wound choke of RG8 on a 4
inch PVC pipe spaced up off the boom. About 12 turns makes a
VERY good choke for 40 meters, with an impedance in the kilo
ohms, and only costs a few dollars.
In my 160 meter phasing networks where space is a problem, I use
a stack of eight 61 material cores (giving me about 30 ohms base
reactance with one turn on 160) with 6 turns of teflon coax for a Z of
about 1100 ohms.
73, Tom W8JI
w8ji@contesting.com

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