Decerning antenna impedance

Decerning antenna impedance

Post by Bria » Mon, 01 Mar 2004 00:28:03

Anyone here know of a formula or other manner in which to determine the
impedance of a wire antenna, knowing it's length and height above ground?

Brian

 
 
 

Decerning antenna impedance

Post by Bria » Mon, 01 Mar 2004 00:30:11


Sorry, can't spell today. Should've been discerning I believe.


Quote:> Anyone here know of a formula or other manner in which to determine the
> impedance of a wire antenna, knowing it's length and height above ground?

> Brian

 
 
 

Decerning antenna impedance

Post by Telamo » Mon, 01 Mar 2004 06:48:31



> Anyone here know of a formula or other manner in which to determine the
> impedance of a wire antenna, knowing it's length and height above ground?

Z = 138log(4H/D)

Z is in ohms
H is the height of the wire above ground in feet.
D is the diameter of the wire in inches

The length of the wire does not matter.

This is Z is the result of electric field effects measured relative from
earth ground.

You can see by formula that the higher the wire is the higher the
impedance and the larger the wire size the lower the impedance will be
for the wire.

A typical wire will represent hundreds of ohms. An 18 guage wire would
be ~ 500 to ~ 600 ohms 5 to 15 feet above ground.

--
Telamon
Ventura, California

 
 
 

Decerning antenna impedance

Post by Frank Dresse » Tue, 02 Mar 2004 05:34:22


Quote:> Anyone here know of a formula or other manner in which to determine
the
> impedance of a wire antenna, knowing it's length and height above
ground?

> Brian

A good source for these formulas is the ARRL antenna book.  The ARRL
radio amateur's handbook also has some antenna formulas, and is often
found in public libraries.

Here's a couple of radio amateur sites:

http://www.cebik.com/radio.html

Look for the "Practical Antenna Notes"

Reg Edwards has written some computer programs which greatly simplify
using some of the more complicated formulas.  There are a few of them
for antennas:

http://www.btinternet.com/~g4fgq.regp/page3.html#S301"

But the effects of the antenna's impedance are far more important when
transmitting.  As a practical matter, receivers work fine over a broad
range of impedances.

Frank Dresser