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> From: Michael Lamb <n7ml@imt.net>
> To: 'towertalk@contesting.com'
> Subject: FW: [TowerTalk] Weird choke "balun" failure
To: <towertalk@contesting.com>
> Date: Monday, September 28, 1998 2:43 PM
Mike Wrote:
> Apparently, there is some controversy between Walt Maxwell and Jerry
Sevick over this one and those are two titans I would not want to get into
the middle of a technical argument with. But after thinking about things a
little, it appeared to me that maybe those beads do actually dissipate some
power when they stop spillover currents from coming down the coax. Based
on that theory, I replaced my all my bead baluns with Amidon Guanella
baluns on the triband stack.
>
With the "choke balun":
This is without a doubt the very BEST type of balun for antenna systems. It
is far superior to the Guanella Balun for power handling, balance, and
common mode isolation when proper materials are used in the choke balun.
The problem is in the choice of materials, not in the theory.
Even a few watts of dissipation will overheat the beads. The 73 material
beads I tested for this application can only dissipate about 1/2 watt for
one minute before curie temperature is reached. It is a power vs time
problem, not a saturation problem, and the power levels it takes in each
bead to cause problems are very low. If you burn the flesh off your fingers
touching a string of beads one foot long after one minute it can be from 50
or so watts! Think of a 100 watt light bulb, it's large surface area, and
how much power it takes to heat it up in a few minutes and how hot the
surface gets. Now consider a group of beads in a closed space with poor
thermal conductivity! 100 watts of heat shatters a six inch string of 1/2
inch diameter beads in about one minute.
The energy wasted is nothing, since several times that amount would have
been wasted as common mode radiation anyway!
The unfortunate thing that happens is when the beads get hot the balun
quits acting like a balun. If the beads get hot enough, the beads will
NEVER act like a balun again. SK beads. SK balun.
With the Guanella balun, the transformer core material suggested is
correct for high power applications. The core presents mainly a reactance,
so even if the common mode power becomes high the balun won't get hot. It
also won't do much to prevent the unwanted common mode current!
There are two errors in the Guanella balun analysis:
1.) The theory is based on the flawed assumption the antenna requires
balanced voltages. The real goal is to provide balanced currents with a
high common mode impedance through the balun. Guanella baluns have almost
zero common mode isolation, and work best in a test lab or when driving
push-pull transistors or FET's.
2.) It is also stated, quite incorrectly, magnetizing forces are minimal in
Guanella baluns. That isn't true, since they have a winding system that
forces the core to be magnetized (that winding connects directly across the
differential mode voltages of the feedline).
I would rather correct the incorrect materials or choke styles used in the
much better choke balun system, than use a system with correct materials
that doesn't correct balance or common mode problems as well as the one
with wrong materials would.
73 Tom
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