Mark wrote:
The best way to ask my question is to use a thought experiment.

Imagine I have a bucket of ferrite rings. Assume that the permeability
and other material properties of each ring is such that I get about
100 Ohms of resistive loss for one turn through a ring at 144 MHz.

Now assume I have 19" whip on top of a car connected to a 144 MHz
transmitter. If I slip one ring over the whip and let it down to the
bottom feedpoint, it will attenuate the signal by adding 100 Ohms in
series with the feed of the antenna which otherwise would look like
about 30 Ohms. That seems clear and seems to agree with common sense
and observations.


Now if I stack up 20 ferrite rings on the antenna, if we analyze each
ring as a lumped element we would have 2000 Ohms in series and
essentially an open circuit and little current or power will flow.

However this does not pass the common sense test. Common sense says
the ferrite tube will absorb the energy from the antenna and
potentially will get hot and that the feedpoint Z of the antenna will
NOT raise indefinitely with more and more ferrites but rather it seems
it would level out to some value. We need to use distributed rather
than lumped analysis.

Yes. The effect of a ferrite ring part way up the antenna won't be the
same as one at the base. A 100 ohm resistor in the center of the
vertical will add 50 ohms at the feedpoint. One 3/4 of the way to the
top will add about 17.5 ohms.

Regarding the ferrite absorbing energy from the antenna -- the amount
absorbed will be maximum when the ferrite's impedance is the complex
conjugate of the antenna's. For example, if the vertical is resonant and
grounded with no feed system, you'll get maximum ferrite heating when
the ferrite's impedance is around 36 + j0 ohms. If you add more ferrite,
the amount of power absorbed from a passing wave and delivered to the
ferrite will decrease, approaching zero as the ferrite impedance
increases to a large value.

Bottom line question... How to determine the feedpoint Z of a wire
that is inside a ferrite tube?

You could probably do a pretty decent job with an antenna modeling
program, placing a number of lumped loads along the wire.

Roy Lewallen, W7EL