On Friday, May 18, 2012 9:55:38 PM UTC-5, Myron A. Calhoun wrote:
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?

The Telegrapher's Equation can be used to give an *estimate* of antenna feedpoint impedance for a center-fed dipole if one knows the loss factor which includes radiation. The feedpoint impedance of a center-fed dipole is:

(Vfor+Vref)/(Ifor+Iref)

where all math is phasor math. The Z0 of a center-fed dipole can be *estimated* from the single-wire transmission line formula. For instance, The Z0 for a #14 wire horizontal at 30' above ground is very close to 600 ohms and the SWR on a 1/2WL dipole (including is ~20:1 in order to obtain a 50 ohm feedpoint impedance (which includes end effects). For the length of time it takes the key-down signal to travel the length of the dipole and back to the feedpoint from the open ends, the antenna exhibits a feedpoint impedance of approximately 600 ohms - similar to a traveling wave antenna. The loss factor, including radiation "loss", can be simulated using resistance wire.
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73, Cecil, w5dxp.com