Solenoid coil baluns are effective so long as you avoid their stray
resonances, such as where the outside is a halfwave long (including the
outer covering which may be considerably lossier than the polyethylene
between the braid and the center conductor. My friend K0DOK built one of
the transposed coax UHF collinears once and when he put it up it stank.
Didn't get as good a signal as a quarter wave ground plane. He took it
to work (McDonald Aircraft) and ran it on the antenna range. Its pattern
was good, but its efficiency was rotten. He stripped off the outer
jacket and then it worked with good efficiency and vertical directivity.
This loss effect can affect a balun too depending on the plastic of the
outer jacket.). At the frequencies where the inductance and the stray
capacitance, or the overall length of the wire makes it series resonant
the isolation of the balun is gone. Using bucking layers or a ferrite
core can change the characteristics. Specifically the ferrite core
allows there to be fewer turns for a given impedance of the isolation
inductance. An RF broad band transformer criteria is that that impedance
should be about 5 times greater than the circuit Z to reduce the effects
of the isolation inductance to a reasonable value. That sets the LF end
of the passband. The mid band is generally where the wire of the coil is
about a quarter wave long, where the coil is self resonant, but parallel
resonant. The high end is either where the coil wire is a 1/2 wave long,
or where the strays and the coil become series resonance. All coils do
that. Winding chokes in pis and with variations in spacing are
techniques applied to try to keep the impedance from getting too low in
a desired band. At Collins making a PA plate choke to handle a few KW
(or couple hundred KW) from 3 to 30 MHz was not a trivial pursuit. It
often took several separate chokes of different construction to keep the
choke impedance from getting too low and dissipating too much RF power.
One design that I saw, used blackened wire so it would radiate heat
better than shiny wire to keep it from getting too hot.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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