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[TowerTalk] Baluns - Primer and Questions

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Subject: [TowerTalk] Baluns - Primer and Questions
From: W8JI@contesting.com (Tom Rauch)
Date: Thu, 9 Aug 2001 12:36:42 -0400
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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