From the un-un/bal-un traffic, it seems there is some confusion and a
couple of "alternative facts" regarding this subject. Fortunately, it
is much more straightforward that it might seem.
A bal-un is a transformer. Technically, it has two windings on a
ferromagnetic core material which for RF is typically some flavor of
ferrite, usually in the form of a toroid. Practically, the transformer
is often configured as an auto-transformer but it's still a
transformer. In the classic case, a balanced load [e.g. center of a
half-wave wire] becomes unbalanced [coax, shield grounded] by the bal-un.
An un-un is exactly the same thing except both primary and secondary are
unbalanced [i.e. one side is grounded ... and often connected together].
Transformers [bal-un or un-un] transform one complex impedance into
another. The transformation ratio is equal to the square of the turns
ratio between primary and secondary. The little transformer that used
to go between the 300 ohm TV twin lead and the coax to the TV set had a
2:1 turns ratio, transforming 300 ohms to 75 ohms. A 4:1 turns ratio
gives a 16:1 impedance transformation, 50 ohms becomes 750 ohms -- a 9:1
turns ratio gives an 81:1 impedance ratio and 50 ohms looks like 4
Kohms, roughly the practical impedance at the end of a half-wave wire.
So Eric, the terms "advantages" and "disadvantages" are somewhat
misleading. The goal is to provide your transmitter with a load of
50+j0 ohms. To work, the turns ratio of the bal-un must accomplish that
and that in turn depends on your frequency and length of your wire. The
"advantage" of the Elecraft balun is that it is switchable so one device
can be used in multiple situations.
Note that the impedance at the feedpoint is complex. It has both
resistive and reactive [inductive/capacitive] components. Only at a
resonant frequency will the reactance be zero. The bal-un/un-un
transforms both components by the square of the turns ratio. It doesn't
eliminate the reactance.
So-called "current baluns" are really chokes that present a very high
impedance to the current that might be flowing on the outside surface of
the coax shield [common mode current]. They do not affect the equal and
opposite currents flowing on the center conductor and inside surface of
the shield.
A dipole is anything that has two "poles." A length of wire has two
ends [poles]. It doesn't matter how long it is. As Ron has pointed out
before, the classic ham usage of "dipole" is a half-wavelength wire fed
in the center, but technically, length is irrelevant. A water molecule
is an electrical dipole built like a dumbell, one end is positive, the
other negative. The field in the microwave makes them spin and their
friction heats up your coffee.
Hope this helps.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 1/30/2017 10:59 PM, gliderboy1955 via Elecraft wrote:
What are the advantages/disadvantages of using a 9:1 balun v. using the
switchable Elecraft balun at 1:1 or 4:1 or no balun at all when using a random
wire portable?
Why 9:1?
Thanks
73 Eric WD6DBM
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