Quang Nguyen <q.nguyen_at_ee.latrobe.edu.au> wrote:
> I would like to know : Is it possible to model an array of 16
> Log-periodic antennas (using either linear array (broadside) or
> circular array structure).
Yes. This should be very straightforward, providing that you have
enough RAM in your computer. You will also need to compile the
FORTRAN source code after setting certain array dimensions
sufficiently high. The dimensions I'm mainly thinking of pertain to
the number of transmission lines in the model. An LPA includes
several transmission-line segments, and you'll have 16 LPA's and
possibly additional transmission lines interconnecting these. I
believe that in the already-compiled executable versions of NEC that
are widely distributed, the number of transmission lines is limited to
a modest value such as 30 or 50. However, it is very easy to change
this limit in the source code. Just one number in an INCLUDE file
needs to be changed.
> I am trying to find out how NEC does it; what NEC input file must
> contain; how do I feed each antenna of the array; and can NEC
> simulate the radiation pattern of the array at different scan angle
> off the boresight (this can be done by applying phase shift in array
> but in NEC, I do not know how).
"How NEC does it" is much too big a question to answer in an e-mail.
I refer you to the ample documentation, which is available at modest
cost from the U. S. Department of Commerce, National Technical
Information Service (NTIS), 5285 Port Royal Road, Springfield, VA
22161, telephone 703-487-4650. IIRC, the NTIS has a website.
BTW, NEC-4 is export-restricted, but NEC-2 should do the job for you.
In NEC you will model each LPA with a number of straight "wires"
representing the elements of the LPA. Each wire will be composed of a
number of thin right-circular-cylindrical "segments". You will
connect the centers of these wires with transmission-line segments.
You will define a source to feed the LPA, possibly through an
additional transmission line. You will specify the complex amplitude
(mag. & phase) of the source. You may use a different source for each
LPA in your array of 16, or you may feed all 16 from a single source
via transmission lines. You may set the complex amplitudes of
different sources arbitrarily, and/or set transmission lengths as you
prefer, to phase your array. NEC will form and solve n simultaneous
equations in n unknowns representing the complex currents in all of
the wire segments. The electromagnetic couplings between all of the
segments, and the constraints imposed by all of the transmission
lines, will be accounted for in these equations. From its solution
NEC can calculate the radiation in any direction. NEC can also
calculate near fields, currents, impedances, etc. NEC can also model
lossy ground, BTW.
A NEC solution is done for a single specified radio frequency, but in
an input command you can specify that it be repeated for a series or
progression of frequencies.
There are websites devoted to NEC that you should visit. My info my
be outdated, but try <http://www.dec.tis.net/~richesop/nec/> and
<http://www.qsl.net/wb6tpu>. Do a web search. If you wish to
purchase NEC-2, contact Jerry Burke <burke2_at_llnl.gov>.
Received on Fri Sep 10 1999 - 13:27:38 EDT
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