Re: NEC-LIST: Current Source - Correct NT Card

I came up with a method some years ago which has been incorporated
into Nittany Scientific's software (NEC-Win Pro, GNEC, etc.) and also
EZNEC.

It basically uses a Network in NEC (NT) and needs a short dummy
segment to make it work for each current source.

You can make the dummy segment some really small wire that is located
far away from the antenna structure. It is best to put it after any
scaling inputs (GS) and make it very short.

Ex. GW 901 1 999 999 999 999 999 999.001 .0001

I usually make it have some really large tag like 901 or 9001 and make
it only 1 segment. Make it only .001 meters long and the radius 1/10
of the length.

Then you add a NT with the following parameters:

NT 901 1 TAG_OF_WIRE SEGMENT_OF_WIRE 0 0 0 1 0 0

The network connects between the dummy segment wire and the actual
wire tag and segment that you want the current source in.

The two-port Admittance (Y) parameters are: Y11 = Y22 = 0 + J0
Y12 = 0 + J1

This will ensure that the current at port 2 will be equal to the
voltage at port 1 with just a 90 deg phase shift. If you have more
than one current source, you will need a separate dummy segment and
network for each source. Since all are shifted by a constant 90
degree phases shift plus the phase of the voltage sources relative to
each other, the relative phases are correct.

You need to add the EX voltage source as follows:

EX 0 901 1 0 REAL_PART_OF_CURRENT IMAGINARY_PART_OF_CURRENT

The only problem is that the Antenna Input Impedance printout will not
be correct for the current source at port 2. It will show a very
small value which is the impedance of the short dummy segment.

You need to find the impedance in the Network Excitation Table
printout in NEC which gives the impedance at each port of the network.
The impedance at Port 2 is the correct value.

This has worked great for feeding phased arrays, etc.

I hope this helps and feel free to get back to me if you have any
further questions.

I won't be around the next 4 days, however, since I am going to the
largest Ham Radio convention in the world called the Dayton
Hamvention.

My ham radio call is WA3FET and I hope to see some of you there from
this reflector.

Good luck with this.

Jim Breakall

Prof. Jim Breakall
Department of Electrical Engineering
Penn State University
202 EE East
University Park, PA 16802

Tel: 814-865-2228 (Office)
      814-865-6339 (225 EE East Lab)
      814-692-7779 (Antenna Test Range)
      814-863-8457 (FAX)
      814-692-4261 (Home)
Received on Tue May 23 2000 - 06:27:09 EDT