Antenna simulation vs measurement

When simulating the radiation pattern of an antenna using NEC, we nomally get slight different patterns over a certain bandwidth (e.g. 108-118MHz). However, the measured radiation pattern could be quite different cross tha band. How to explain this phenomena?

Thanks.

There are several sources of error with NEC. One is the number of elements. More is not necessairly better. Another is the interaction between parallel elements which is not simulated accurately.

A good cheap book on learning the problems with NEC is http://www.nittany-scientific.com/ and click the book link on the left of the page.

The radiation patterns of closely spaced frequencies (e.g. 108 MHz, 113 Mhz and 118MHz) are almost identical. Is this because of the error or NEC? I don't think so.

The radiation pattern characteristics depend very much on the frequency and antenna size. What kind of an antenna are you designing? Traveling-wave type antennas are usually very broadband and can have quite an uniform pattern for a narrow frequency range (like in this case seems to be), so tell us more about the problem.

Hobbie

I feel it is not due to error of NEC and agree with hobbymat. The frequencies you mentioned are very close to each other. Similar pattern is possible.

So it could be due to something else. I learned that it could happen for certain frequencies, but forget the explanation. Measured patterns are similar for the upper band but not for 108MHz. This is a small loop antenna (0.03wl).