STEP2: NEC2 simulations. Antenna measurements Nordic VHF Meeting, Íland 970606

(Feb 16 1998)

Why all these details ??

Since the method of measuring antenna performance by comparision of far field amplitudes and phases is not commonly used and since it leads to a considerable amount of data processing it is described in great detail here.

All data and software needed is presented in full detail in such a way that the evaluation process can be repeated by anyone interested. It is possible to investigate how sensitive different parameters in the computer model are to changes. For example it is possible to move the center of rotation along the boom or perpendicular to see if the change that occurs can be compensated by changing other parameters.

This site serves as a back up for software and general information about this method for later use at better sites than the one used at the Nordic VHF Meeting on Íland 1997.

Detailed description for NEC2 simulations

First follow the instructions for STEP1. DownloadSTEP2.ZIP containing the source code and .EXE file for STEP2 into the main directory and unpack. DOS4GW is still needed.

If you have a FORTRAN compiler, you may recompile the source code that is supplied with the .EXE file. Note that some of the .FOR and .INC files contained in STEP1.ZIP are also needed.

Once you have STEP2.EXE in the main directory, start it by typing DOS4GW STEP2.

The STEP2 program uses the file ANTLIST.TXT which is one of the files in STEP2.ZIP. The ANTLIST.TXT file has to contain the names of the subdirectories where the input files for NEC2 are stored.

The parameter ELCORR in the ANTPAR.TXT file is used as a "boom correction" which means that all element lengths are modified by the addition of this number (length in mm) before the NEC2 simulation is performed.

When STEP2.EXE is run, the files TEOPAT1.TXT and TEOPAT2.TXT are produced in the subdirectories specified by ANTLIST.TXT.

TEOPAT1.TXT contains the NEC2 radiation pattern in 360 points calculated with the boom correction specified by the ELCORR parameter. TEOPAT2.TXT contains the NEC2 pattern calculated with a slightly larger boom correction.

STEP2.EXE uses the files CMPLXAMP.TXT in the respective subdirectories to determine the frequencies for which patterns are needed. CMPLXAMP.TXT are the output files from STEP1.EXE

When STEP2 uses the NEC2 code to calculate the radiation patterns at the required frequencies, the corresponding gain and impedance values are displayed on the screen. These NEC2 output data are saved in the files GAIN1.TXT and GAIN2.TXT in the respective subdirectories.

The data from the GAIN1.TXT file is an accurate estimate of the real performance for those antennas that fit very closely in the third evaluation step.

Note that the gain is calculated in two different ways. The first value, within parenthesis, is the normal NEC2 gain calculated from the absolute value of the far field. When simulating folded dipoles this value is seriously incorrect. The second gain figure is calculated from the NEC2 radiation pattern using the ohmic losses of the NEC2 model. This radiation pattern is the very pattern that fits very well (except for the BSZ26 antenna) with the measurements. The gain is therefore calculated indirectly from the measured pattern and the errors are presumably very small. The error in ohmic losses caused by the poor modeling of the folded dipole impedance is extremely small due to the very small current in folded dipoles.