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S11 and VSWR optimization for Compact Printed LPDA

Erique

Hello,

I have a design of printed LPDA (100MHz - 2GHz) designed from first principles and the results are okay. The design is implemented on FR4, wih top and bottom feedlines, therefore no groundplane. However, I need to make it compact to be able to fabricate, as a result, I have reduced the spacing factor and the s11 results as shown:

I am seeking assistance on how I can make the S11 go below -10dB for the whole band, I would highly appreciate anyone in the community who can assist with the techniques of achieving the same. Ready to share the file.

@Torben Voigt

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Torben Voigt

Hi Erique,

Could you please attach the model (.cfx) and also name the variables that could be used for the optimisation?

Best regards,
Torben

Erique

Erique_pclpda_25el_100MHz-2GHz_0.06_spacing-factor.cfx

Hi Torben,

Here is the file. Also kindly let me know if the file can be fabricated (what is the size limit for fabrication? Is it 1m by 1m).

Erique

Erique_pclpda_25el_100MHz-2GHz_0.06_spacing-factor.cfx

Hi Torben,

Here is the file. Kindly advice if the file can be fabricated as is or there is need for further size reduction (what is the size limit for fabrication?)

Torben Voigt

Hi Erique,

Unfortunately I have no idea what the limit for fabrication might be. Let me see, what can be optimised at all. Please be patient.

Best regards,
Torben

Erique

Hi Torben,

Take your time.

Regards.

Michellemiller

Reducing the spacing factor is a good way to make a printed LPDA more compact, but on FR4 it usually increases coupling between adjacent elements and disturbs the current distribution, which is why the S11/VSWR starts degrading at the lower and mid bands. I would try a gradual taper instead of a uniform reduction, slightly optimize the boom/feedline width and maintain the log-periodic scaling ratio as closely as possible, since even small deviations can shift the impedance behavior across 100 MHz–2 GHz.

Torben Voigt

I would agree with @Michellemiller on that. @Erique , please give it a try. You may want to use an infinite substrate layer while optimising the shape of the antenna, see attched model. This will reduce runtime dramatically. Note, results might be slightly different from the finite substrate. So if you have an optimized model later on, you should simulate it using a finite substrate, since the proximity to the edge of the substrate will have an effect.

test_pGF.cfx

Best regards,
Torben

Erique

Hi Torben,

The file you have shared with me is incompartible with the Feko 2026 version I have.

Was there a recent update?

Regards.

Erique

Hi @Michellemiller

Thank you. The major challenge I have here is to try to low frequency performance with the fabrication size limits. The antenna from first principles is about 1.8m and may need techniques to reduce the size by more than 50% without adversely affecting broadband performance.

Erique

Hi @Torben Voigt

Please guide on how to implement an infinite ground plane, I seem to be struggling with it.

Torben Voigt

Hi Erique, please update to Feko 2026.1

Torben Voigt

Delete your substrate and define an infinite one like shown here:

Erique

Thank you, let me give it a try.

Erique

Hi @Torben Voigt

I have managed to use the gradual taper to make the design compact (more than 50% boom reduction) without degrading s11, see below.

I have been trying to bend the first three dipoles to fit into the board (The board to be used for my fabrication measures 580 mm by 960mm). May I request your expertise in bending the 3 dipoles, as my meshing takes too long to process and I am also running into tolerance and geometry errors.

Taper_1D-5D_0.45_6D-10D_0.55_11D-15D_0.70_16D-20D_0.85_pclpda_25el_100MHz-2GHz_MoM.cfx

Torben Voigt

Hi @Erique ,

I don't get any warnings/errors when running your model. Could you please attach screenshots that show the problem?

Erique

Hi @Torben Voigt

The model attached has no errors, I said I was getting errors upon trying to meander the dipoles to fit in a board of 580mm by 960mm. Here is the model, kindly assist meander the first three dipoles.

Taper_Bent_1D-5D_0.45_6D-10D_0.55_11D-15D_16D-20D_0.85_pclpda_25el_100MHz-2GHz_MoM.cfx