Richard Clark wrote:
On Tue, 28 Mar 2006 22:33:18 -0500, John Popelish
wrote:
(snip)
Make that, "impedance (mostly inductive, if the ferrite is well suited
to the frequency)" and I agree.


Hi John,

No, the principle of the impedance is overwhelmingly resistive, not
reactive. One core (#75) that snuggly fits over RG58, at about 10MHz,
exhibits 25 Ohms resistance to common mode current.

Yep. Type 75 operated at 10MHz is very lossy.

My BalUn uses 50
such whose combination presents 1250 Ohms resistance. This value
varies over frequency, but easily finds a 1000:50 ratio over the
entire MF/HF Ham spectrum.

Thank you for some specific facts. I had no idea that you were using
a ferrite whose permeability falls to half the low frequency value
(about 5000 at 200kHz) at only 700kHz and is not recommended for flux
coupling applications higher than about 750kHz. It also has a very
low bulk resistivity of 300 ohm cm. At 3 MHz, its permeability is
only about 150 (and plunging).

Type 43 material has a permeability of about 500 at 3 MHz and falling
much more slowly.

I also have two engineering samples boxes full of various material
types and geometries and have measured them all. Some exhibit
reactance, but are vastly overwhelmed by their intrinsic resistance.

(snip)
I agree with this, except that the purpose of the ferrite is to
increase the common mode inductance of the section of coax passing
through it, not add resistance. Some resistance is inevitable,
because no ferrite is lossless, but the intention is for inductance.

Quite the contrary. Inductance, or more properly to the application,
reactance, may come as a consequence of these cores, but that is
really just a minor component to the vastly greater resistance.

If the cores don't have relative hi permeability (causing mutual
inductance in the two conductors passing through) they can't produce
any impedance in series with the common mode current. That is also
transformed over to the two conductors by mutual inductance (as a
loaded secondary reflected to those conductors as a primary).

Try transmitting through such a resistance and you are going to lose a
lot of your power.

That power, actually current, was forced into a common mode because of
one of several problems:
1. Massive imbalance in the radiator design,
2. Massive catastrophe with the radiator.

If you are destined to "lose a lot of power" into such resistance, you
have far greater problems you are ignoring. However, let's return to
this "loss of a lot of power." You have an antenna with a nominal 50
Ohm load resistance (presumably balanced to the common). You have a
common mode choke with 1000 Ohms resistance in parallel (to the common
of the common mode). Now, why would the power choose the path of 10
to 40 times more resistance to invest its calories in?

I will be generous and pour 1.5KW into a balanced 50 Ohm load to
develop 275V that is also across the 1000 Ohms of the BalUn. 76W into
50 beads. Now this verges on serious power for this load I will
admit, but it also presumes key-down conditions which would only be
found for RTTY or SSTV. By my bench measurements, I figure that the
core's can tolerate as much power as the bulk equivalent carbon
composition resistor - or least this is my rule of thumb. This means
about 1 or 2W per bead to offer enough heat to cause pain, but perhaps
not enough to blister. This is another benchmark from my days in the
Navy.

If the radiator is perfectly balanced, the impedance of the ferrite
(transformed to the primary side) sees half of the voltage that is
between the center conductor and the shield. So if the coax carries
275 volts, half of that or about 1378 volts appears across the bead
bead common mode impedance. So a 1000 ohm common mode choke limits
the common mode current to 138/1000 = 138 mA. And if that impedance
is dominated by resistance, the beads will absorb 138*.138=19 watts.
50 beads will stand that indefinitely.

I don't think the O.P.'s balun has 50 beads, though.

You are missing one path. The two from the source in the form of the
inner shield of the coax, and the center conductor, and the one from
the load in the form of the outer shield of the coax (same shield, but
isolated circuits).

I can't parse this. There are two metal conductors entering the
choke, and two exiting it. All currents pass through those 4 conductors.


Your count is four terminals (not conductors), there are six. You
need to come to terms with this shortfall in your count as it explains
the utility and design of BalUns.

If the other two are the ends of the loss resistance, they can be
transformed back to the two conductors.
(snip)

The common mode current causes flux in the core, and the conductors
passing through that flux produce a voltage proportional to the rate
of change of that flux, just as the conductor passing through any
inductor would.


This "rate of change" is spurious, call it frequency dependant, but
then frequency dependency is neither here nor there at this moment.
More to the matter trying to turn this BalUn into a power transformer
is doomed in this analysis as it has absolutely no impact to

It may not be the approach you are familiar with, but I think it is
valid. The transformer just has a lot of core loss, if you use a
ferrite optimized for a much lower frequency. And that loss shows up
as if it were a resistor connected across the ends of the two
windings. If a low loss ferrite (at the operating frequency) is used,
then the impedance across the windings is dominated by inductance, as
one normally expects with a transformer. But the analysis handles the
whole range of cases.