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Off center impedance on half wave dipole

T

Tak

Jan 1, 1970
0
Need a graph of impedance (both R and X) relative to off-center
distance on a half wave dipole.

Anybody have ot handy? Searched on web and books but could not find.

Thanks
Tak
 
J

Joe McElvenney

Jan 1, 1970
0
Hi,
Need a graph of impedance (both R and X) relative to off-center
distance on a half wave dipole.

Can't help with a graph but I think that the easiest way, and one
that could be used for any future odd-ball antennas, is to employ an
antenna modelling program such as MMANA by Makoto Mori. It is freeware
and downloadable from -

http://mmhamsoft.ham-radio.ch/

Import a half-wave dipole from the 'MMANA/ANT/HF SIMPLE' folder and
then edit the element lengths to see what happens.


Cheers - Joe
 
R

Reg Edwards

Jan 1, 1970
0
"Tak"wrote
Need a graph of impedance (both R and X) relative to off-center
distance on a half wave dipole.

Anybody have ot handy? Searched on web and books but could not find.
===============================

You're looking in the wrong places. Even Terman and probably Kraus are
silent.

The input impedance of an off-centre fed 1/2-wave dipole is always purely
resistive (because the antenna is resonant), with approximately -

R = 73 divided by Square( Cosine( Theta ) ) ohms.

Where Theta is the angle 180 times D divided by L degrees,

D is the distance from centre and L is the overall length in any units you
like.

But, be warned, if not at the centre, as soon as you connect your feedline
you will make a mess of it depending on height and on length and impedance
of the line. The feedline becomes a part of a far more complicated antenna.
On the other hand, the approximation is good enough for other calculations
provided the connection is, say, not more than 1/2 of the way to the end of
the wire.

You will have to plot your own graph. ;o)
 
T

Tam/WB2TT

Jan 1, 1970
0
Tak said:
Need a graph of impedance (both R and X) relative to off-center
distance on a half wave dipole.

Anybody have ot handy? Searched on web and books but could not find.

Thanks
Tak

Go to www.eznec.com and download the demo version for free. You will be able
to see the feedpoint impedance and radiation pattern for any offset and any
height that you put in.

Tam
 
T

Tak

Jan 1, 1970
0
Thanks for good advice.
Reg's formula gave me the picture.

Will also visit sites suggested by Joe and Tam, for simulators.

Thanks again. Tak
 
T

Tak

Jan 1, 1970
0
You're looking in the wrong places. Even Terman and probably
Kraus are silent.

I actually have books of Terman and Kraus. You are right. :)
Tak
 
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