I have a confusion about the impedance matching between the antenna and the RF-DC rectifier. The antenna(915MHz) was bought from the market and the datasheet says its impedance is 50ohm. However, I use a mini-Vector network analyzer to measure the impedance of the antenna, which is (42.4+22.5j)ohm. To reach maximum power transfer, the impedance of the source and the load should be the conjugate with each other (R+j=R-j).

My confusion is which impedance should I choose for matching (42.4+22.5j or 50ohm)?

Tuning the impedance matching is also difficult. First, I use ADS2017 to calculate the value of the L matching network. Then, I modify the capacitor and inductance around the calculated value. However, it hard to reach perfect matching. For example, the impedance of the antenna is 42.4+22.5j so I think the impedance of the rectifier should be tunned to 42.4-22.5j. If add a serial capacitor, the point should move counter-clockwise on the Z circular of the smith chart but in practice, the point just jumps to another position where it shouldn't be. Maybe it is because of the parasitic effect?

If the matching is well, the point of Zs and ZL should be as shown in the picture, right? enter image description here

How do you usually matching the impedance according to the smith chart? Is there any mistake in my operation?

www.ebyte.com/en/pdf-down.aspx?id=362 antenna datasheet

Many thanks

  • \$\begingroup\$ is it an embedded antenna or external antenna with connector? Embedded antennas can change easily with the environment. Could you share the antenna datasheet? \$\endgroup\$
    – Marcos
    Nov 4, 2019 at 15:43
  • \$\begingroup\$ @Marcos Thank you. It an external antenna. The datasheet is in the Chinese language. I found the English description at : cdebyte.en.alibaba.com/product/62312628623-803998762/… \$\endgroup\$
    – penli
    Nov 4, 2019 at 15:56

1 Answer 1


The impedance magnitude of 42.4 + j22.5 is 48.0 ohms and I expect that someone in the marketing department for that antenna felt it wouldn't hurt rounding that figure up to 50 ohms.

I would assume the correct impedance is the complex value stated providing: -

  1. The manufacturer is reputable
  2. The dealer is reputable
  3. The data sheet contains very few anomalies like the 50 ohm one

If not a reputable manufacturer/dealer then I wouldn't be keen to trust the data sheet. If the data sheet is of poor quality (in terms of errors/anomalies) then I wouldn't trust the product period.

  • \$\begingroup\$ Thank you. I think the manufacturer is reputable. Maybe the precision and repetitive of the mini VNA is not so good? I had done the measurement serval times, sometimes it will very close to 50ohm. \$\endgroup\$
    – penli
    Nov 4, 2019 at 15:59
  • \$\begingroup\$ Well, I don't regard alibaba as a reputable dealer and I don't see a proper pdf or downloadable data sheet. \$\endgroup\$
    – Andy aka
    Nov 4, 2019 at 16:28
  • \$\begingroup\$ ebyte.com/en/pdf-down.aspx?id=362 I find their official website. This is the pdf datasheet of the antenna. \$\endgroup\$
    – penli
    Nov 4, 2019 at 16:31
  • \$\begingroup\$ Double check that the series capacitance you added has 22.5 ohms impedance at 915 MHz. My iPad isn’t allowing me access to the data sheet for some strange reason. \$\endgroup\$
    – Andy aka
    Nov 4, 2019 at 17:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.