I think that 31 material is probably a good choice for cleaning RF from
wiring that isn't handling RF power and where there isn't much in the way
of
shunt impedance for the choke to work against -like a line from an
electricity meter carrying a much as 480 VAC (530 VAC at 480 high-line).`
That line has shunt impedances. Almost everything does. A main area for
shunting impedance is the entrance ground.
The individual wires couple together fairly tight, so even the hot wires
have shunt impedance.
This is one of the reasons antenna systems, with system design flaws causing
common mode issues, have better reception (less noise) with improved
grounding on the mains. The shunt impedance is as important as any series
bead or choke string. The better the ground, the less important choke
impedance is.
Another common point is downstream at the station. If the feeders have
common mode from poor feed arrangements, reducing shunt impedance on the
path from antenna feeders to power mains reduces noise.
Common mode chokes are part of a system.
Because I have a good entrance layout and cable bonding, just a single bead
makes a large difference. If I had a sloppy arrangement with high shunt
impedances, I could have fifty beads and not change much.
This is the problem with small antennas, or antennas with high common mode
impedance. The antenna itself doesn't present much shunt impedance. The
series part of the pi filter or attenuator formed by the shunt impedances
and series beads or chokes is in a high impedance path, and has very little
attenuation even with astronomical series impedances.
It is a system with huge variables if the variables are not controlled, not
something that generally the same where one rule fits every system.
That's also an issue on high-speed digital circuits that can't tolerate
much
in the way of shunt capacitance.
Not on individual lines, but when the lines are properly grouped they can
have low shunt impedance for common mode and high shunt impedance for
desired signal modes. Although we might not always think about it, we do
that all the time in systems.
Not so sure about it as a "balun" on coax carrying much RF power!
Every different system has to be looked at as a unique system, but the best
way to avoid troubles is to design feedpoints and equipment entrance points
properly. Proper basics are far better than randomly peppering a mess of a
system with beads.
The nice thing is what we do for lightning also works for RF, and vice
versa.
An air coil choke is just as good as beads when the system is planned. As a
matter of fact, we can build a nearly perfect single band balun with no
beads and no solenoid chokes at all. An 80 meter dipole with a feedline in
air and a ground ~~40-80 feet from the antenna has virtually no common mode
without any balun at all. As a matter of fact, a balun could make it worse.
We need to think about the system more, and guess less.
73 Tom
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
|