"Walter Maxwell" wrote in message
...
On Thu, 1 Sep 2005 07:35:35 +0000 (UTC), "Reg Edwards"
wrote:


"Walter Maxwell" wrote
Will you please describe the details of your experiment where you
claim 90% of the power is radiated from the feedline? How did you
make
this measurement?

==================================

Walt,

If you erect a 10 meter 1/2-wave dipole and center-feed it with a
50
feet length of coax, and then transmit on 1.9 MHz, 99.9 percent of
available power will be radiated from the feedline and only 0.1
percent from the antenna itself.

SWR on the line will be about 1400 and the amount of power
available
will not be very great.
----
Reg.

Sorry to disagree, Reg, But if a balun at the feedline-antenna
junction is a perfect balun at 1.9 MHz, NO power will be radiated
from
the feedline with any amount of SWR, even 1400. It is true that only
0.1% of the power will be radiated, but the 99.9% will be reflected
back to the input of the line, not radiated,, making a 50-ohm
transmitter see a 1400:1 mismatch. In the steady state the
transmitter
will only deliver 0.1% of its available power into the line due to
the
mismatch.

The 1400 SWR inside the coax will not cause it to radiate, unless
the
outer conductor is full of holes.

============================
Walt,

Back along the thread you asked for details of the experiment which
demonstrated that a feedline could radiate as much as 90 percent (of
the available power). But you received no reply.

So I provided an example in which nearly all the power is radiated by
the feedline simply because the short antenna is unable to accept very
much and is inefficient. The power available at the antenna input HAS
to go somewhere if it cannot be accepted by the antenna.

Walt, my statement is correct. There is NO choke balun involved. The
unbalanced coax goes straight into the balanced antenna which
inevitable injects power into the outside of the coax. The choke is
omitted just to acheive that purpose.

I made the statement because you seemed to doubt it was possible for
90 percent of the available power ever to be radiated from an
unbalanced feedline. I may possibly have mis-interpretted you.

Admitted, there may not be very much power available at the antenna
due to high SWR and loss within the coax. But nevertheless most of the
power will be radiated by the outside of the feedline. The feedline
makes an excellent, efficient antenna wire compared with the dipole.

An approximate analysis can possibly be done by using Eznec and
placing the transmitter directly at the center of the dipole.

The outer conductor of the coax can be represented by a thick wire
down to ground, the transmission loss inside the coax being ignored
without having the slightest effect on the experiment.

Incidentally, the length of the feedline in wavelengths and the
resistance of the bottom end ground connection will have a
considerable effect on results.

I mention Eznec because of the number of people who swear by it.

For Roy's benefit, the experiment will not tell him how many watts are
radiated from the feedline because it is impossible to separate the
three radiation resistances involved and there will be only one very
peculiar radiation pattern from their combination.

I entered this extremely long-standing discussion only to say that, in
general, the amount of "Power Radiated from a Feedline" is not worth
the effort which has been expended in waffling about it.

I use the words "Power Radiated from a Feedline" only because, to make
myself understood, I am obliged to use the same confusing language as
used by old-wives for the last 50 years. I try to be helpful.
----
Reg, G4FGQ