Kraus comes up with Z=73 + j42.5. He then goes on to say that an actual
dipole is made a few % shorter, which yields 65 + j0. When I did an EZNEC
calculation on a 1/2 wave dipole at 3MHz, I did not quite get that. For a
#30 wire in free space I got 76.81 + j43.89 at 3 MHz, and 72.88 + j0.3465
at 2.94 MHz.

I let EZNEC tell me what the wavelength was, and used 1/2 of that for the
length of the dipole.

Tam/WB2TT
"W5DXP" wrote in message
...
Dr. Slick wrote:
Do you know of anyone who has mathematically derived the 73 Ohms
of a dipole in free space?

_Fields_and_Waves_in_Communication_Electronics_, Ramo, Whinnery,
& Van Duzer. Pages 647, 648, sections 12.05, 12.06.

The feedpoint current of a dipole is caused by the in-phase superposition
of the forward current and reflected current at the balanced feedpoint
of a standing wave antenna. A 1/2WL dipole and an open 1/4WL stub have
similarities. The deviation of the feedpoint impedance from zero ohms
gives an indication of the losses due to dissipation and radiation.
--
73, Cecil http://www.qsl.net/w5dxp