The Fair-Rite data sheets showing recommended frequency range are for a
single turn for the core. The most detailed data sheet for each
suppression part include curves for 1, 2, and 3 turns, and provide FAR
less choking impedance than needed to kill receive noise and to prevent
feedline pickup from filling nulls in the pattern of a Yagi.
My designs are for CHOKES, not "baluns," and are based on suppression of
receive noise and pattern control, and provide guidance for power
handling. Depending on the design of the antenna, they MAY work as a
"balun," but may defeat coupling action with some multiband designs. For
example, at least one of N6BT's excellent designs is happy with a 2-wire
line choke, but not a RG400 choke. My recommendation is to use whatever
matching device is provided by the mfr, and add the choke between that
and the feedline.
My designs are intended for use at the feedpoint of antennas that are
reasonably well matched to their feedline. Secondary uses are as "egg
insulators" on feedlines to antennas like high dipoles, to prevent their
interaction with nearby verticals.
Chokes at feedpoints also minimize interference between stations with
closely spaced antennas, like Field Day, DXpeditions, and and
multi-transmitter contesting stations.
73, Jim K9YC
On 6/25/2025 12:58 PM, Jim Lux wrote:
I use 31 mix up to 30 MHz (and higher) to good effect.
The real part of the impedance keeps going up to 1GHz (that's the lossy
component)
The imaginary part peaks at about 30 MHz, is decent to 100 MHz, and then falls
off. (that's the pure inductance)
https://fair-rite.com/31-material-data-sheet/
don't forget that Z is mu*frequency, so even though both real and imaginary
parts of my start to roll off at 2MHz, the*impedance*, which is what you want
for a choke, keeps going up.
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