Roy Lewallen wrote:

But I assume you
are talking of something like NEC which breaks antennas into segments.

Yes. You can find a good description of the method of moments in the
second and later editions of Kraus. The fundamental equation can only be
solved numerically, and the method of moments, used by NEC and MININEC,
is an efficient way to do it.

Since you clearly know more about this stuff than me, do you know of the
best freely available software for this which works under Unix? (I use
Sun's Solaris for 99% of the things I do, including sending this
message. I use Solaris on my laptop too, rather than Windows).

Hence I'm almost certainly looking for source code in either C, C++ or
Fortran. Anything that works under Linux would almost certainly be able
to be compiled for Solaris without too much effort.
I found this page:

http://www.si-list.net/swindex.html

which has some source. I downloaded one

http://www.si-list.net/NEC_Archives/necpp-1.1.1.tar.gz

It would not compile immediately on my Sun. gcc 4.3.1 complained about
some ambiguous code. gcc 3.4.1 did not, so I got past that bit.

It then tries to link with the 'blas', 'atlas' and 'lapack_atlas'
libraries, none of which my Sun has.

I then swapped to the Sun C/C++ and Fortran compilers, removed
references to 'blas', 'atlas' and 'lapack_atlas' , and replaced them
with 'sublibperf' which is the optimised library on Solaris. That worked
ok, and I had an executable:

$ ./nec2++
usage: nec2++ [-iinput-file-name] [-ooutput-file-name]
-g: print maximum gain to stdout.
-b: Perform NEC++ Benchmark.
-h: print this usage information and exit.
-v: print nec2++ version number and exit.


I've not done any more than that at this point, but proved it will
compile on Solaris with little effort.

Anyway, if you have any recommendations for the best freely available
Unix/Linux code, I would be interested.


Hallen's integral equation is exact, but it's not a formula, since you
can't plug numbers into one side and get a result on the other. Nor can
it be solved in closed form at all. That's why so much work was done on
approximate solutions and on developing numerical solution methods. Feel
free to write your own program to solve it, but such programs have
existed for decades and have been verified countless times as well as
being highly optimized.

OK, I understand that.

Getting the resistance is pretty straightforward once you assume the
shape of the current distribution. Assume some arbitrary current at the
feedpoint which, along with the assumed current distribution, gives you
the field strength in any direction. With the impedance of free space,
this directly gives the power density. Integrate the power density over
all space to get the total radiated power. Then you know how much power
is radiated per ampere of current at the feedpoint, from which you can
calculate the feedpoint resistance.


This calculation is done in all editions of Kraus, I'm sure; I have only
the first and second, but I can't imagine it was deleted in later ones.
Be careful when reading Kraus, however. Unlike many authors, he uses a
uniform, rather than triangular, current distribution for his short
elemental dipole examples. This is equivalent to a very short dipole
with huge end hats, not just a plain short dipole. The half wavelength
and other dipoles in his text are conventional.

I think I found what I was looking for in either Kraus or Balanis last
night. The book is beside the bed, and as my wife is still asleep I'm
not going to look for it.