Re: NEC-LIST: ex card near open wire end

Rian,

An EX card near the open end of a wire antenna is not inherently
a bad idea as long as you understand what you are modeling relative
to what you would construct. End-fed wire antennas are commonly used
in commercial antenna designs.

The simplest example to consider is an end fed dipole. If you feed
 a 1/2 wavelength dipole at the bottom end the current distribution
is much like that of a center fed dipole. The peak or maximum current
is at the center of the dipole. The current at the bottom of the
dipole is different because of the different feed location.
The radiation pattern is similar to that of a dipole with a
slight uptilt of the peak directivity.

To end-feed this antenna in practice you would need a good balun
and impedance matching circuit. In practice, a microstrip feed is
used where the balun and matching are integrated into the microstrip
circuit. Many times ferrite beads are used on the coax to aid in
"choking" the common mode currents on the feed coax.

I looked at your antenna and I am wondering how you intend to
actually feed it. Practical issues to consider are whether the
antenna is meant to be a center conductor extension of a coax
cable for microstrip feed line (in which case a balun and
matching circuit will be needed) or whether a ground plane
(circuit board) will be present.

Steve Best

-----Original Message-----
From: nec-list-bounces+steven.best=hanscom.af.mil_at_robomod.net
To: nec-list_at_robomod.net
Sent: 12/25/2004 11:39 AM
Subject: NEC-LIST: ex card near open wire end

Can anyone point me towards information why an ex card near an open wire
end is a bad idea?

I've recently upgraded to using 4nec2 version 5.4.1 and it gave me a
warning on this model. The waring:

Error: Wire 1, seg 1 (tag 1), EX-src: Not allowed near open wire-end.

Perhaps there's a more appropriate way to excite my antenna at the same
spot?

Thanks in advance,

Rian Sanderson

The model:

CM GP Generated nec input
CM 802.11a antenna
CM
CM Using screen coords. Excite at first wire
CM dimensions in mm, scaled to meters with GS
CM
CM Rian Sanderson
CE
GW 1 1 0 0 0 1 1 0 .120
GW 2 1 1 1 0 2 2 0 .120
GW 3 1 2 2 0 3 3 0 .120
GW 4 1 3 3 0 3 2 0 .120
GW 5 1 3 2 0 2 1 0 .120
GW 6 1 2 1 0 3 1 0 .120
GW 7 1 3 1 0 2 0 0 .120
GW 8 1 2 0 0 2 1 0 .120
GS 0 0 0.004
GE
EX 0 1 1 0 1 0
FR 0 1 0 0 5220
RP 0 73 73 1000 -180.0 0.0 5.0 5.0
EN

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