Hi Bill,
> I know I've suggested this before, but why not use a mix of bead
> materials? Have the beads closest to the feedpoint be 43 material, and
> the ones away from the feedpoint be 73 material.
I don't understand what any particular sequence of beads would do, since
they are all in series without shunting components to "sap off" current. RF
is just like low frequency circuits so far as series and parallel
components are concerned. Whatever current flows in one end of the beads,
provided the string is not heavily coupled to something via a strong
electric field, flows right out the other end.
For example consider a mobile base loading coil. Current at each terminal
is almost exactly the same, except voltage becomes out of phase with
current as you move up through the coil. Losses in the coil are evenly
distributed, since they are predominately I^2R losses. If current from the
coax is one ampere, you can be sure current in the base of the whip is very
very close to one ampere unless the coil is jammed up against sheet metal
that provides shunt C.
(As a matter of fact, that IS why some traps have poor performance even if
measured in circuit Q seems good. That's why I never use coaxial stubs or
traps or put loading coils in tight metal shields or make them with large
metal end caps or put large capacitance hats very near loading coils.)
I've never been able to measure an advantage by stacking beads in any
particular order, the end result is the same no matter what order is used.
If you measure current at each end or anywhere in the string, it is almost
exactly the same throughout. The ONLY time I've seen a change is when the
beads are series resonant at or near the operating frequency, and that
situation requires high Q beads in a really long stack or a lot of shunt C
from the middle of the stack to each end!
There is a small taper in current, do to the small reduction of current
through a "displacement current" through the weak electric field.
> This has the advantage of requiring fewer overall beads. (Which is why
> you'd use 73 material in the first place) But still has the power
> handling of 43 material.
I don't understand why that would be true. The result is simply a mix of
the two. This is a series circuit, not a low pass filter with shunt and
series components.
> If you had an infinite variety of bead materials, seems the optimal way
> to approach the bead balun would be to start with a low-permability
> material at the feedpoint and increase the permability as you move away
> from the feed. Done right, this would evenly distribute the loss (and
> thus heating) across all the beads. None of the beads would get "hotter"
> than the rest, and fewer beads would be needed overall.
I don't understand why that would be true. It is a simple series circuit,
and power dissipation is I^2 R.
Since current is equal throughout the string, heat would be proportional to
the ESR (R) of each bead. It always works that way, unless Kirchoff and Ohm
were wrong and their laws are just suggestions.
73 Tom
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