This is a hot topic, judging from the e-mails. I'l eventually post
something up at www.w8ji.com...but it might be a while.
In the mean time, there are many good articles around. Now, keep
in mind I'm not saying EVERY bead balun is no good, just that you
have to be careful what you do and many people obviously are NOT
careful.
Just a couple more comments.
> >Type C and D baluns, however, can expect little or no feedpoint current
> >to excite the core. In either case, common-mode currents flowing on the
> >sheild are inhibited by the inductive reactance of the core. Differential
> > currents flowing on the INSIDE of the sheild should not excite the core
> >at all. This would mean that the core only needs to be sized according to
> > the expected magnitude of the common-mode currents.
>
> W8JI pointed out to me in a private e-mail that this isn't true.
No Bill, that is true. The differential mode currents do NOT excite
the core *directly*. What really happens, is the terminals of the
antenna must have a voltage difference with respect to the feedline
reaching the antenna. The voltage from the shield connection side
of the antenna to the shield of the coax below the balun is across
the beads or whatever choke you use.
So there is always indirect excitation of the beads unless the
antenna is perfectly *UN-balanced* with the shield side of the
antenna at zero volts with respect to the area around the feedline.
In the case of perfect balance and a zero voltage feedline shield
(perfect feedline routing and grounding), as you say below, exactly
half the differential feedline voltage appears from end-to-end and
excites some current in the balun.
I think what messes people up is they might not realize how little
power the beads can dissipate (especially if potted in a thermal
insulating media) over a period of time. Even a very small amount of
power can cause the beads to heat their way towards curie
temperature...which is the point where the material stops behaving
like a soft-iron. Some beads actually will never recover from that!
> In the BEST case (all impedances matched and no reactive impedance or
> common coax currents), these baluns will see HALF of the antenna terminal
> voltage. This is because the choking impedance is shunted across one
> antenna terminal and the coax sheild ground.
Yep. That is the case for a perfect balanced antenna and perfect
feedline. It would be close to what we might expect with a yagi
antenna that is well-balanced, or a dipole hanging in the open with
a vertical feedline dropped straight away for some distance and
grounded exactly 1/2 wl away.
> In the worst case of complex feedpoint impedances, mismatched impedances,
> and complex currents flowing on the coax sheild -- the actual voltage
> across the balun could be MANY TIMES the best case.
Let me give a working example. A vertical fed at the center, with
the feedline routed down parallel to the vertical (and maybe a few
feet away). Or a dipole with the feedline pulled off to the side in a
similar fashion.
In that case, 200 beads are often not enough even for QRP
because there can be thousands of volts required to balance
antenna currents...even if the feedpoint impedance is 50 ohms!
> >3) The typical rule of thumb for a Type C or D balun is the inductor
> >should have at least 10x the reactance of the antenna impedance. (eg 500
> >ohms reactance for a 50 ohm feedpoint impedance)
Not true at all, when we look at the real world and run some power.
Let me roughly approximate this from memory, so I don't have to
dig out papers. As I recall, a 73 bead is about 120 ohm per linear
inch of core material at 14 MHz. There is some reactance, but it is
small. So most of the choking is in the form of a dissipative
resistance.
If we use 4 linear inches of beads, we will have about 500 ohms. If
the system is perfect, with 1500 watts we will have about 140 volts
across the beads. Dissipation in the beads will E^2 over R, or
about 40 watts.
Now put this into perspective of something we all know, a light
bulb. The beads have considerably LESS surface area than a 60
watt light bulb, yet they have to get rid of 2/3 of the heat the light
bulb does. How long do you think they will last? How hot will they
get?
That has satisfied the requirement of balancing the antenna within
a few percent in the ideal case, but NOT the power dissipated in
the beads! In a few seconds, the beads would be so hot they would
reach curie temperature. ESPECIALLY if you close them off in a
tube, then much lower power could do it over time because there
would be no air circulation.
Double the number of beads, and things get four times better. You
now have 1000 ohms so 140 volts squared over 1000 is 20 watts
over ten inches. You have halved the dissipated power while
doubling the area. This only works in free air, with the beads
horizontal, so the lower beads do not heat the upper beads by
convection.
You likely need about a foot or more linear area of beads in free-air
or a thermal conductive media with exposure to cooling air to
handle 1500 watts CCS in a perfect situation.
I forgot the exact details, but this is very plainly what is wrong with
the Unadilla Big Signal balun I tested (W2DU says he wants no
part of his call being associated with that balun). The balun is too
short to have enough beads inside, it is a sealed PVC tube
preventing cool air from reaching the beads, so there is simply no
way it will work with power....and it does not work in tests. It would
be fine at maybe 100 watts RTTY or 400 watts PEP SSB, but
certainly not at a kW despite having enough impedance for a
perfect load.
I'd bet a lot of baluns fail to look at airflow and dissipation, or worse
case conditions, when they pick the design.
This is all the time I have for now, and I rushed this. But hopefully it
will give some insight into the power problem!
Keep in mind the feedpoint situation can be much worse than the
ideal case. A 50-ohm feedpoint can have ANY value of common
mode problem!
73, Tom W8JI
W8JI@contesting.com
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