On 04/03/2011 15:21, Olin Lathrop wrote:
> Now that we've beat to death what a balun is, I agree with Russell that
> these coils are unlikely to be deliberate baluns.
>
> My guess is the reason for the extra wire is not electrical.  They may be
> service loops to make future changes easier.  It's hard to tell from a
> single picture at a single point in time, but this could be a temporary
> installation.  Maybe the phone pole is scheduled to be moved soon.
> Sometimes things are done for short term expedience or don't make sense
> without context within the bigger picture.

It's the occam explanation I think.

I'd agree.

But a coil of coax never hurts. one at TV aerial (as tight as won't=20
damage coax dielectric or screen, about 3 or 4 turns) and at TV socket,=20
can reduce interference from CB radio, or even reduce peak voltage=20
static discharge if  storm (nothing helps a direct strike, that removes=20
the Chimney usually!).

If it's a cheap "contract" yagi with no built in balun, then it acts=20
actually as a balun.

A 1:1 RF trans former wired with the two windings in series breaking the=20
two wires, rather than wired as DC isolation has advantage that it's=20
wider band for desired differential signal, passes DC power supply and=20
audio signalling. The DC won't saturate it as the DC currents cancel if=20
a single bifilar wind is done to get the two 1:1 windings. it's not=20
uncommon to see RG174 miniature coax or twisted pair of particular=20
impedance wound on a ferrite.

If a separate "balun" is wound like this to feed DC to a pair of FETs=20
(the fields  DC bias cancels, but separate RF chokes would saturate),=20
they can be driven push-pull. They can have about 25 Ohm output, so a=20
2nd "balun" like this connects to 50 output via two capacitors for DC=20
isolation.

A third balun used to drive gates. You can achieve 100MHz to 2000MHz =20
with suitable ferrite, which is impossible using the baluns like=20
conventional 1:1 isolation transformers.

--=20
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