On Oct 16, 8:19 pm, Roy Lewallen wrote:
Thanks for the comments. Once again, I scanned a posting too hastily,
and somehow missed the "transmitter" part. My comments were appropriate
for a receiving antenna, not transmitting. So let me try to answer both
Tom's and Mark's postings more appropriately.

None of us has a constant current source with which to drive an antenna,
but generally a source with a fixed amount of power and a finite source
impedance. If we did have a constant current source, then yes, adding
more and more ferrite cores would result in more and more power being
delivered by the source, a larger and larger fraction of which would be
dissipated in the ferrite. A 1 megohm resistor would get a bundle of
power from our source, and a 10 megohm resistor would get 10 times as much.

If we have a tuner, we can adjust our source impedance over some range.
Provided that the feedpoint impedance is within that range with the
ferrites in place, we can deliver all our power to the ferrite-antenna
combination. I believe that the fraction of the power applied to the
antenna which ends up in the ferrites monotonically increases as we add
ferrites (assuming we don't move the previously added ones). If the
ferrites were all at the base, the equivalent load circuit would be just
two impedances in series -- the ferrite impedance and the antenna
feedpoint impedance, and it would behave as Tom said. But putting the
ferrite cores anywhere but the base changes the antenna current
distribution, which has a potentially complex effect on the feedpoint
impedance other than just adding the transformed impedance of the core.
This means that not only does Tom's Z3 increase as we add ferrites, but
Z1 changes also.

Roy Lewallen, W7EL



thanks for the replies...
so for talking purposes:

Z1 is the antenna feedpoint Z and we will define "antenna" as the
exposed wire after the end of the ferrite tube.

Z2 is the Z of the wire passing though the ferrite

Z3 is the source Z which I will stipulate is 50 Ohms

OK as we add ferrite to the antenna, Z1 changes because the antenna is
getting shorter as the ferrite is getting longer. i.e. if there is 7"
of ferrite, then there is only 12" of exposed antenna and it is
elevated over the ground plane so Z1 is going up. In the end case,
when the ferrite is 19" there is no antenna Z1 becomes infinity.

Then looking into the base (thinking as lumped elements), we have Z2 +
Z1. Since Z1 is infinity, the base must look like infinity, but this
does not pass common sense.


In other words, what is the Z looking into a 19" wire that is inside
19" of ferrite. Thinking in lumped element terms, it would be very
high and little power will flow. Thinking in distributed terms there
will be some relatively low Z looking into the base, power will flow
and the ferrite will dissipate heat. The base Z would be related to
some property of the ferrite like the property of free space has a Z
of 377.

What is that propery and what would a typical Z be?

Mark