I'm trying to design a low-power passive communication system. It receives its power via wireless transmission from a nearby transmitter and to power a sensor which digitally modulated a signal via backscattering. The power calculated to the antenna @ 915 MHz with Tx and Rx gain around 9dBi was to be roughly 1.5 Watts or 1.76 dB. FCC regulations limit the directed power to 4 W maximum in the ISM Band. I was considering using multiple receive antennas and using a Wilkinson Power splitter as a combiner to increase the power received, however, I know the theoretical power loss is 3 dB, which is more than what I receive. My question is can I use the Wilkinson Power combiner in low power environments such as my example? Is there an alternate way to increase the power received without altering the antenna design?


1 Answer 1


You are confusing absolute power (dBW) with gain (dB). Your received power is +1.75 dBW. A Wilkinson has 3 dB insertion loss so your signal would drop to -1.25 dBW (1.75 dBW - 3 dB).

When using a Wilkinson as a combiner, if both your signals are coherent (same magnitude and phase) you will actually get a 3 dB boost in received power. If you're receiving two +1.75 dBW signals, you'd see +4.75 dBW at the combined output of the Wilkinson.

However, this only applies if the signals are in phase. If they are completely out of phase, they would cancel and you'd receive 0 W (-inf dBW).

  • \$\begingroup\$ Thank You. That also solves some confusion I had around logarithms and units. \$\endgroup\$
    – Aaron
    Commented Mar 11, 2021 at 1:25
  • \$\begingroup\$ @Aaron yes, logarithms are always relative something. dBW implies "relative to 1 W" and dBm means "relative to 1 mW". So when working with absolute power, it's extremely important to include the proper unit (by the way, in many industries, dBm is more common than dBW). \$\endgroup\$
    – Jason
    Commented Mar 11, 2021 at 3:29

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