I am new to RF field and trying to use an Archimedean Spiral Antenna design from this paper to capture 915MHz RF for the P2110B power harvester chip. The estimated antenna impedance is 188 Ohm (see page 5), whereas the chip's unbalanced RF input expects a 50 Ohm antenna (see page 4).

How can I match the impedance of the antenna to the RF input and how the antenna should be connected to the chip since it has no ground plane? Should one end of the antenna be connected to the GND pins on both sides of the RF pin?

There will be a dedicated directional RF power transmitter (not just ambient RF) emitting at 915MHz center frequency. The plan is to transfer energy across some surface (up to 2 inch thick), so, please, share your thoughts if you think there is a better approach.

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  • $\begingroup$ Looking at the link, I see it uses a Cockroft-Walton voltage multiplier. How do you plan on implementing that at these frequencies? $\endgroup$ – Mike Waters May 30 at 18:09
  • $\begingroup$ @MikeWaters I assume the IC chip will handle that. I am currently using two "stick" antennas on both TX and RX side, and I was hoping that the spiral antenna would be more efficient. $\endgroup$ – Nazar May 30 at 18:12
  • $\begingroup$ Ah, I think that I see what you're trying to do. Sorry. $\endgroup$ – Mike Waters May 30 at 18:24

The P2110B datasheet quotes the maximum receiver input power as +23dBm, which is 200mW. You might be able to use an off-the-shelf surface-mount 4:1 balun such as the Mini-Circuits TC4-25+, which is specified to handle 250mW with less than 0.4dB of insertion loss at 900MHz.

  • $\begingroup$ So, this would be like an impedance matching circuit? How should it be connected with the antenna and the chip? $\endgroup$ – Nazar May 30 at 20:32
  • $\begingroup$ You would connect the radio to PRI and the antenna to the upper and lower SEC terminals of the balun. One of the PRI terminals would be grounded at the radio. The center tap of the SEC would be unconnected. $\endgroup$ – Brian K1LI May 31 at 1:53
  • $\begingroup$ Could you, please, confirm that at the very center of the antenna, the two arms are not connected, but instead separated by 1.2mm according to the zoomed-in image of Figure 3? Also, do you know what are the two white circles at the tips? I tried to design such antenna using MatLab and the design shows that the two tips are touching. $\endgroup$ – Nazar May 31 at 20:39
  • $\begingroup$ @Nazar The two white circles are the antenna's feed points. See this description of the Spiral Antenna. The connection between the two arms at this point in your MatLab model is opened by insertion of a current source. $\endgroup$ – Brian K1LI Jun 1 at 10:24
  • $\begingroup$ Oh, so in my case, those are the inputs to my power harvester, right? $\endgroup$ – Nazar Jun 3 at 17:16

188 Ohm is theoretical result. In practice think like 200 Ohm, and such can be connected via 2:1 balun (2:1 in voltage, 4:1 in impedance)

  • $\begingroup$ Could you, please, explain your answer in more detail? $\endgroup$ – Nazar May 30 at 20:33

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