On Sat, 19 May 2012 02:55:38 +0000 (UTC),
(Myron A. Calhoun) wrote:

I've been playing with several programs (tunehalf.exe, endfeed.exe,
and EZNEC). All ask these questions:
* Height of vertical portion of wire
* Length of horizontal portion of wire
* Wire diameter
* Frequency

Endfeed.exe also asks these questions:
* Number, length, & diameter of buried radial wires
* Length of strap from radio to radial connection

Tunehalf also asks these questions:
* Transmitter output impedance
* Length * diameter of coil
* Parallel and serial capacitance
* Estimated ground loss resistance

Among other things, both calculate the antenna impedance.

HOW DO THEY DO THIS? I'd like to write some programs of my own.
What is the formula or equation for estimating/calculating antenna
impedance given the above inputs?

You are far off, there is no single formula or equation.
All the advanced software programs split the antenna structure
into small parts, calculate each part separately and then
put the result together. The current in each small element
generates a field and all those split up parts interact
with the rest of the structure.
There is a name for that procedu
http://en.wikipedia.org/wiki/Finite_element_method

In the antenna software this is called modelling.
http://www.k5sld.com/presentations/m...NEC-basics.pdf
The antennas are put together as small pieces of wires.

Older softwares as e.g. YAGICAD use complete formulas
which give only an approximation value of the real results.
But bewa the formula for the field of a dipole is about
half a page long and is still imprecise.

w.