Harry wrote:
I am a newbi in antennas.

Here's my question:

I know that a half-wave dipole in free space has
a feed-point impedance of approximately 73 ohms.

Can anyone tell me **exactly** how this number is calculated.

(or tutorial webpages)

I mean I like to see a general formula (a function of
antenna height above earth....) and all the detailed steps
that will get the impedance number.

Thanks!

-- Harry

That calculation comes about as the culmination of a two-semester
junior-level college course in Electrodynamics. Because of their
mathematical intensity most people suffer through such courses with grim
determination rather than greeting them with joy.

The 73 ohm number assumes an antenna in free space with a magic
zero-size current source at it's center and no wires going to the
antenna. Then some handwaving simplifications are made, a current
gradient is assumed, a voltage gradient is calculated, and the impedance
(and antenna pattern) is calculated. If you want to avoid the
handwaving simplifications you take _another_ year of antenna-specific
E&M and/or you write a program like NEC to do the calculation numerically.

This is why the antenna chapters in the Handbook start with very basic
theory then take a very long jump to a compendium of results, without
trying to fill in all the space in between.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com