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[SOLVED] Ground plane near antenna affects performance?

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Apr 17, 2011
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Below is the layout of a 2.4GHz RF system.

Will the RF ground plane on the left of the antenna affect the antenna's performance? Is it better for the antenna to stay where it is, or should it moved to the right? I am new to RF design, so I would like to avoid 100 Ohm differential matching for the time being, that's why I brought the antenna very close to the low pass filter after tranceiver's output.


Yes, metal objects (and even dielectric objects) close to an antenna will screw up the radiation pattern.

Odd looking antenna shape!

How much the metal effects the radiation pattern depends on how you are feeding that antenna. Is it being differentially fed? If so, then the metal nearby will have a much smaller effect.

If it is not being differentially fed, and the metal ground plane is part of the antenna, it will have a much bigger effect.
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Thank you biff for your answer.

The antenna is differentially fed, but as you said it is good for the antenna to be moved from the ground plane. So I used a free application (thank you Mazz for the link), that came up to the following slotline characteristics:

So according to this software, to achieve 100 Ohm differential impedance I need 22.9mm trace length and 1.07mm gap in between them.

But how about Electrical Length, Phase Constant and Effective Diel. Const.? Are these values shown OK for a 2.4Ghz system?

Thanks in advance.

Your differential line is not a slotline. The line impedance is independent of the length. Your calculator only tells you that this length would give 90 degree phase shift at 2.4GHz.

If you want to upload the Gerber RS274X file, or send it to me by PM, I can give it a quick run in the Sonnet EM simulator.

volker_muehlhaus said:
Your differential line is not a slotline.
Then what line is it? Which category do I have to simulate it to?

volker_muehlhaus said:
If you want to upload the Gerber RS274X file, or send it to me by PM, I can give it a quick run in the Sonnet EM simulator.
Thank you very much volker_muehlhaus, very kind of you. Below are the gerber files for top and bottom layer.

View attachment gerbers.rar

Thanks again.

Your line does not fit any of the types available in TXline.

I have now EM simulated your lowpass and antenna in Sonnet. The good news is that it seems to work well, at least with the ideal components that I have used in simulation. Antenna matching is ok (slightly offset in frequency) and the antenna gain is ok as well.


Thank you volker_muehlhaus.

One more question please.
Do you have any suggestions to improve the design? Or is it OK to move forward to the construction of the board with no changes?

Thanks for once more.

I would manufacture a prototype now, and see how it performs.

The low pass is ok. I was not sure about the extra capacitance of the pads on the shunt capacitor, but that's ok.
The only thing we could do now: simulate the SMD with measured data/vendor models to include the parasitics, if this data is available.

For the antenna, there will be some influence from tolerances of the FR4 material. I simulated with your value of 4.6, but that value might change depending on the supplier. I think the design is good enough to build a protoype now, with this layout.

Thank you volker_muehlhaus, this is what I will do.

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