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

I appreciate everyone's replies. I'm a software guy, not an antenna
guy...but I'm reading like crazy. I realize that the simulator is a
tool much easier misused than used correctly.

My funny looking antenna was generated by software I wrote using genetic
programming. My software uses the same principles as a genetic
algorithm, but instead of encoding the antenna as a string of 1's and
0's that get reproduced and recombined, my antennas are represented via
LOGO instructions (move, rotate left, rotate right, etc).

I'm particularly interested in small antennas for 802.11 wireless LANs
and for GSM mobile phones. Right now I'm working with NEC because it is
so accessible, and I can build small wire antennas by hand. If things
work out well I'd like to use a simulator like Agilent ADS/Momentum for
PCB antennas.

The antenna I attached isn't the best result, but it's small, relatively
uncomplicated size makes it easier to deal with. When built it will
have an MMCX connector at its base which will be plugged into a MMCX to
U.FL mini coax cable attached to an 802.11a WLAN card with a microstrip
feed and ground plan on the PCB.

-Rian Sanderson

-----Original Message-----
From: Best Steven R Civ AFRL/SNHA [mailto:Steven.Best_at_hanscom.af.mil]
Sent: Thursday, December 30, 2004 4:43 AM
To: nec-list_at_robomod.net
Subject: 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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