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How much trouble can a via cause at 2.4GHz

gervasi

I have a 2.4GHz chip antenna working fine with the transceiver on the same side of the PCB as at the chip antenna.

 

I laid out my board with the same antenna on the other side of the board, using a 16-mil VIA.  I put the via on the antenna side of the matching network.  The signal is always referenced to ground. 


image

 

The chip antenna was a royal pain to match.  It started out looking like a short at 2.4GHz.  (It was not DC shorted or shorted at lower frequencies.) 

image

 

When I finanally got it matched, it was only matched (1.4:1) in the center of the 2.4GHz band.  It was around 4:1 at the edges. 


image

 

The performacne was 20dB worse than the board with the antenna on the same side.  How much trouble can a via cause at 2.4GHz?  20dB?

  • Hi Gervasi,

     

    You might well have created a small parasitic inductor path.  The via could set up a cross magnetic field that could then cause an oscillation between the board layers as sort of an active filter.

     

    My math skills are no longer up to the task of proving this, but my experience tells me that I have seen a lot of strange EM effects at those frequencies.

     

    I would redo the board and make sure that I had a good ground plane under your antenna so that you can radiate inside a EM quiet zone.

     

    RF can be really strange stuff.

     

    Just a thought,

    DAB