NEC-LIST: LPDA simulation

Hi All,

I am attempting to simulate a narrow band LPDA in NEC. I know that the
general approach is very easy for LPDA design, and I have also simulated
numerous wideband LPDA's before. But with the current scenario I have
come upon an interesting phenomena, in that I cannot get good
correlation between the narrow band LPDA and the measurements I make.
The normal simulation is a number of dipoles (straight) with a source in
the center segment, and a transmission line with Zo in the order of 70 -
80 Ohm, phase reversal between elements. The actual source is in the
center of the shortest element, and the transmission line then feeds the
other dipoles. According to me this is the most basic, and generally
well understood model. What I now have is a 5% bandwidth LPDA (a design
for improved F/B ratio and proper gain (10dBi)) and the measured antenna
seems to work 10-20% higher in operating frequency. I am still fiddling
around with the model, but I would like to know if there is perhaps some
sub-logical explanation for what I am doing wrong. Two additional
remarks from my side are the following:
1) I now have a simulation where I physically have the two sides of the
dipoles off-center, exactly like the actual prototype. This seems to
result in a remarkable upward frequency shift, although this does not
seem logical to me. I expected downward shift if any. A small linking
element between the two sides of the dipole ensure that they are excited
very much identical to the way normal (ideal) dipoles are.
2) The second remark is simply that LPDA's are normally extremely
wideband, therefore I reckon frequency shift is normally not that much
an issue. For this narrow band application the requirements are very
different, and I would like to predict the correct frequency in the
simulation model.

Best Regards,
David

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