How calculate antenna impedance?

[Myron A. Calhoun]

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?

--
-- Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member & Certified Instructor for Rifle, Pistol, & Home Firearm Safety
Also Certified Instructor for the Kansas Concealed-Carry Handgun (CCH) license

[Helmut Wabnig]

On Sat, 19 May 2012 02:55:38 +0000 (UTC),
mcal...@sdf.lNoOnSePsAtMar.org (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 procedure:
http://en.wikipedia.org/wiki/Finite_element_method

In the antenna software this is called modelling.
http://www.k5sld.com/presentations/may-2009/EZNEC-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 beware: the formula for the field of a dipole is about
half a page long and is still imprecise.

w.

[W5DXP]

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.
--
73, Cecil, w5dxp.com

[Sal]

"Myron A. Calhoun" <mcal...@sdf.lNoOnSePsAtMar.org> wrote in message
news:jp723a$r8d$1...@odin.sdf-eu.org...

> I've been playing with several programs (tunehalf.exe, endfeed.exe,
> and EZNEC).

< snip >

> 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?
>
> --
> -- Myron A. Calhoun.

Isaac Newton is credited with saying, "If I have seen further it is by
standing on the shoulders of giants."

Duplicating the work of those who have gone before may be only an
entertaining exercise. (I got 1 credit in college for a Physics lab, in
which I confirmed the results of experiments first performed centuries ago.
Frankly, I felt a little silly doing it.)

If you're after an entertaining exercise, more power to you. (I've
"created" computer programs that I knew existed.)

"Sal"
KD6VKW

[Jeff Liebermann]

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

>I've been playing with several programs (tunehalf.exe, endfeed.exe,
>and EZNEC). All ask these questions:

(...)

>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?

If you want to see how a program works, it's often best to go to the
source code. Most of the major programs are user friendly front ends
to the Fortran or C NEC "engine" that actually grinds the numbers.
However, there is a C version of the code available:
<http://5b4az.chronos.org.uk/pages/nec2.html>
<http://www.si-list.net/NEC_Archives/nec2c.tar.gz>
There are other source code versions that might help:
<http://www.si-list.net/swindex.html>
<http://www.qsl.net/m5aiq/nec.html>
etc...


--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558