"Cecil Moore" bravely wrote to "All" (19 Sep 05 09:27:25)
--- on the heady topic of " 73 Ohms, How do you get it?"

CM Subject: 73 Ohms, How do you get it?
CM Xref: core-easynews rec.radio.amateur.antenna:216868



CM pezSV7BAXdag wrote:
The limit for Z does not exist
or is (in general) the complex infinity.

CM As the length of a dipole is increased, for the same
CM power input, more energy is radiated during the first
CM transcient cycle and less is available for reflection
CM from the ends of the dipole. Reflected energy is what
CM is causing the feedpoint impedance to change. As the
CM length of the dipole is incrementally increased, the
CM magnitude of the reflected energy is incrementally
CM decreased. I believe Balanis alludes to this characteristic
CM of standing-wave antennas.

CM The feedpoint impedance is Zfp =3d (Vfor+Vref)/(Ifor+Iref)
CM using phasor addition.

CM The limit of that equation as Vref and Iref go to zero
CM is Vfor/Ifor. That's what happens for an infinitely
CM long dipole. That's also what happens during the transient
CM phase of a finite dipole. Thus, Vfor/Ifor can be thought
CM of as the characteristic impedance of the dipole. Seems
CM to me, Vfor/Ifor could actually be measured during the
CM transient phase of a long finite dipole. Will a TDR
CM report the ratio of V/I for an RF pulse?


Cecil, an infinitely long antenna is simply an impedance transformation
between different mediums. i.e. wire to free space.

A*s*i*m*o*v