I have got this pcb with a Bluetooth antenna and the electrical engineer who did the job gave me this chart : this antenna graph

My questions :

  • what does decibel loss means in reality ? My guess : the antenna is supposed to emit at power 100% but there are some losses in the pcb and the actual signal strengh is of 100-x%, hence a loss of x%. Correct ?

  • At the Bluetooth frequency range, my antenna is -13db. Is that good or not ?


Parameter S11 (aka return loss) is a measure of how accurately the antenna "looks like" 50 ohms. A perfect return loss scenario is -infinite dB but anything lower than -10dB is likely to be acceptable in most circumstances.

If you want a more detailed explanation note the following: -

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See this wiki page where I stole the picture formula. It's also related to VSWR: -

enter image description here

Image stolen from this wiki page.

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  • \$\begingroup\$ stupid follow up question : if I want to know how much my body absorbs the antenna signal, is the formula Power emission = Power reflected + Power absorbed + Power emit_beyond_my_body ? or is it Power absorbed = Power reflected ? \$\endgroup\$ – Thomas Sep 12 '15 at 21:59
  • \$\begingroup\$ If you are up close to the antenna you are in the near field and in the NF a proper EM wave has not formed so you can't base any calculations on power levels. If you are beyond the near field, your body receives a tiny fraction of the transmit power with nothing measurable at the transmit antenna. As for how much the body absorbs versus how much travels thru and beyond I have no idea but it will certainly be freq dependent with absorption being max at 2.45 GHz. \$\endgroup\$ – Andy aka Sep 13 '15 at 8:12

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