Re: NEC-LIST: LPDA simulation

Try the boom correction factor as applicable to yagis with grounded
elements: 66% of the boom diameter to be added to the overall length of
all grounded elements.
If this is not done the antenna ends up high-frequency due to the RF
shortening effect of the metallic boom.
In wideband LPs (gain 6-8 dBi, b/f ratio ~25 dB) this does not show up,
but presumably the effect becomes important in narrow-band LPs as well.

SWR (within reason) does nor seem to effect the gain or pattern of most
antennas. In LPs the SWR is set by spacing the two booms (assuming they
are vertically separated) the required amount, a la standard transmission
line theory.

Please let us know what you find out.

Ian ZS6BTE

> 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.

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