I agree that strings of ferrite beads don't present a lot of inductive
reactance on the low HF bands, but you lost me with your comment about
the reactance canceling the capacitive reactance of the cable. I've
not thought about this deeply, but it seems to me the cable
capacitance is differential, between the center conductor and the coax
braid. The ferrite beads present a common mode reactance, and so I
don't see how the cable capacitance enters into the picture. I'd think
any common mode inductive reactance would always inhibit common mode
currents, at least far below the series resonant frequency of the
beads.
Jim,
For an unbalanced line, the issue is common mode impedances at the
particular point where the "choking impedance" is inserted. It really is all
about what the line looks like at the choke insertion point, if the line was
simply cut, conductors paralleled, and viewed as a single wire line
transmission line at that point in both directions. Any changes caused by a
CM choke or balun are a problem of series impedances and the
characteristics of whatever "source" happens to be driving the common mode.
Ignoring resistance (to keep it simple), if we series a reactance of one
sign, and a second impedance of an opposing sign, we decrease system
impedance unless the choking reactance is more than twice the initial
reactance. In other words, if the line common mode is -j150 0 and we series
a choke of j150 0, we have j0. So we made a short. We'd have to add j300 to
break even and have +j150, and above that CM starts to decrease. Often
overlooked, if the choke is j150 100, it still decreases common mode. It
would be more effective if the j150 was a higher value or not there at all,
but it still decreases common mode.
The question is always, "how much do we actually need in a particular case?"
This answer is never "this is needed everywhere".
Of course the series R factors in, so it is NOT necessary to have resonance
in the choke. As a matter of fact that almost never happens, and probably is
not desirable in most cases. In general, and this varies with application
and power or CM voltages driving the line, we could want something entirely
different. But in cases of modest voltages pure resistances work.
The amount of resistance necessary depends entire upon the particular
system. The most effective thing that can be done, and the smartest thing,
is to use strategically placed grounds, and planned distances to grounds, to
control the common mode impedance and not make the system require
astronomical choking impedances.
Everyone wants an answer like, "do X and it works for all systems", but
that never is the case. It's a big world, and the problems run a wide range.
This is why some people without baluns or chokes are happy as can be, and
why some people have problems no matter what they do.
I would say for a cable laid on the ground, and to isolate a vertical with a
reasonable number of radials, you need almost no choking impedance at all
and it probably doesn't matter if it is inductive or pure resistance for
modest cable lengths. You won't have that much of a problem to start with,
often unnoticeable, and the cable common mode probably looks inductive and
fairly low impedance to start with.
This is why, in my shack, just a few beads over a wire works wonders. This
is why, decoupling an open wire line with high SWR at the voltage peak,
almost nothing works.
73 Tom
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Topband Reflector
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