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Optimize antennas using NEC and a genetic algorithm

Project description

This project can optimize anntennas using genetic algorithms. It uses my pgapy Python wrapper for PGApack, the parallel genetic algorithm library, originally by David Levine at Argonne National Laboratory and currently maintained by me. It also uses PyNEC, the Python wrapper for NEC2++, the C++ version of the Numerical Electromagnetics Code.

The project currently lacks an installer and I’m experimenting in the working directory. It is still very much work in progress.

Originally this started out with a low-gain two-element antenna where the driven element is a folded dipole. One of the requirements for that antenna was that it should have 50 Ω impedance and at least some forward gain. Optimizing this antenna by hand soon turned out to be tedious and I started experimenting with optimization by a genetic algorithm.

You can find the original experiments in folded.nec and folded2.nec. These are input files to the command-line NEC programs. You can either use the nec2c command-line program which produces an output file that can be viewed with xnecview or you can use graphical xnec2c program. All run under Linux and are included in the Debian Linux distribution. The .nec files may also be usable with other NEC versions on other operating systems but I have not tried.

For optimizing the two-element Yagi-Uda you can use folded.py. Later a 3-element antenna with the same principles was added in folded_3ele.py.

The antenna_model.py factors out the common parts of the various antennas.

The hb9cv.py models the well-know HB9CV antenna but only very crudely. NEC2 isn’t really suited for modelling the phasing stubs of that antenna because it doesn’t like parallel wires that are too close. I mainly did this for comparing some of the antennas resulting from optimization with the well-known performance of the HB9CV.

In the file logper.py you can find a 9-element log-periodic antenna. It can currently not be optimized, the performance of the real antenna is better than the results obtained with NEC, so I didn’t implement an optimizer for it yet.

All the antenna programs take an action as mandatory argument. The action is typically either optimize for running the optimizer or necout for creating a .nec file which can then be fed to one of the nec programs mentioned above. When running the optimizer it makes sense to experiment with different random seeds, each random seed will usually produce a different antenna. In addition there are some experimental actions, frgain prints the forward and backward gains (in dBi) for the lowest, the middle, and the highest frequencies and the VSWR for those. The gain action visualizes the 3D antenna gain pattern and the swr action visualizes the VSWR over the given frequency range. Note that both, the gain and the swr action compute the antenna data over the whole frequency range using NEC and may take some time.

The output of the optimizer is text (usually redirected to a file) that prints the evaluation, the VSWR, maximum gain, and forward/backward ratio of the best antenna for every 10th generation of the genetic algorithm. In addition the command-line options to create that antenna are printed. When the genetic algorithm doesn’t make any more progress, the search terminates and the data of the best evaluation are given. An example of the last lines of such a text is as follows. The data is from one of the best 2-element antennas was obtained with the random seed 26 of an earlier version of the program:

The Best Evaluation: 2.886437e+02.
The Best String:
-r 0.0364 -d 0.0444 -l 0.1704 -4 0.1075
VSWR: [1.7901433511443068, 1.1495780609982815, 1.7995760521232753]
GMAX: 6.69913175227, RMAX: -3.03663376703
Cache hits: 5670/9243 61.34%
Eval: 288.64
[ 101011001100101001111000000010011 ]

This tells us the evaluation (which is meaningful only to the genetic algorithm), the genetic algorithm maximizes this value. The command-line options after the line The Best String: can be used to create a .nec file for that antenna. The antenna in the example has a voltage standing wave ratio of < 1.8 at the band ends and around 1.15 in the middle of the band (the 70cm band from 430 to 440 MHz in that case). The forward gain (in the middle of the band) is 6.7 dBi. The RMAX value is the (maximum) backward gain (in a 30 degree area in the back). So the F/B ratio of that antenna is:

6.7 dB - -3.0 dB = 9.7 dB

The last line of the text output contains the genetic representation of that antenna. The .nec file for the antenna above can be created with the command:

./folded.py -r 0.0364 -d 0.0444 -l 0.1704 -4 0.1075 necout > folded-opt.nec

The command-line options specify the radius of the folded dipole, the distance of the reflector from the folded dipole, the (half) length of the reflector, and the (half) length of the straight part of the folded dipole, respectively.

According to NEC it has a standing wave ratio (VSWR) of < 1.8 from 430-440 MHz, a forward gain of > 6.5 dBi over the whole frequency range and a Forward/Back Ratio of 8-11 dB.

If you want to implement an optimizer for your own antenna, look at the file folded.py: You need to implement a class that defines the geometry of the new antenna and an optimizer class that initializes the gene ranges and implements a compute_antenna method that returns an instance of your antenna class with the parameters obtained from the given gene. All lengths in the models are metric (in meters) as is the default in NEC.

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