| -----------------------------------------------------------------
| Subject...: 73 Ohms, How do you get it?
| Sent......: Monday, September 19, 2005 5:27 PM
| Newsgroups: rec.radio.amateur.antenna
| From......: "Cecil Moore"
| -----------------------------------------------------------------
| [...]
| As the length of a dipole is increased, for the same
| power input, more energy is radiated during the first
| transcient cycle and less is available for reflection
| from the ends of the dipole. Reflected energy is what
| is causing the feedpoint impedance to change. As the
| length of the dipole is incrementally increased, the
| magnitude of the reflected energy is incrementally
| decreased. I believe Balanis alludes to this characteristic
| of standing-wave antennas.
|
| The feedpoint impedance is Zfp = (Vfor+Vref)/(Ifor+Iref)
| using phasor addition.
|
| The limit of that equation as Vref and Iref go to zero
| is Vfor/Ifor. That's what happens for an infinitely
| long dipole. That's also what happens during the transient
| phase of a finite dipole. Thus, Vfor/Ifor can be thought
| of as the characteristic impedance of the dipole. Seems
| to me, Vfor/Ifor could actually be measured during the
| transient phase of a long finite dipole. Will a TDR
| report the ratio of V/I for an RF pulse?
| --
| 73, Cecil http://www.qsl.net/w5dxp

| -----------------------------------------------------------------

Do you mean that
since the length is infinite
there is no reflected wave?
But then here it is,
once again,
one of the most controversial issues...

Well,
I think we are in the front of a case in which
the limit depends on the way we approach it.
Every logical way to approach a limit is permissible.
And these ways are infinite in number of course.
But the convergence is too demanding:
"She" wants all these limits to be equal.
If just two of them are unequal
the convergence simply does not exist.
Mathematically this is not a rare case,
they say.

Sincerely,

pezSV7BAXdag