Question on antenna matching

aschweiz
Posts: 22
Joined: Wed Nov 30, 2016 1:57 pm

Question on antenna matching

Postby aschweiz » Fri Jan 27, 2017 5:52 pm

Hi,

I'd like to connect an external antenna to the ESP32 but am not sure how to implement the impedance matching. Basically, I'd like to know the impedance of the chip's RF pin.

The hardware design guidelines document specifies in section 3.2.4.1, "RF", that the "...characteristic RF impedance must be 50 Ohm", and it also describes a matching network.

Is my understanding correct that the characteristic impedance of the chip is different from 50 Ohms and the matching network is required to match the chip's impedance to 50 Ohms of the antenna? Or is it required to match the impedance of the reference design's PCB antenna to the 50 Ohms input impedance of the chip?

One additional point: I was unable to figure out how the impedance matching network works. To my knowledge, there exist two variations of the PI network, one with the inductor in series and the capacitors as legs, and another one with a capacitor in series and the inductors as legs. But the proposed circuit has an inductor in one leg and a capacitor in the other leg??

Thank you and greetings
Andreas

aschweiz
Posts: 22
Joined: Wed Nov 30, 2016 1:57 pm

Re: Question on antenna matching

Postby aschweiz » Mon Jan 30, 2017 7:47 pm

It seems the matching network is a "tapped capacitor" circuit. According to the formula on this web page, the circuit transforms the input impedance (in parallel to the inductor) down by a factor of about 2.6: http://analog.intgckts.com/impedance-ma ... -matching/

So, if the target output impedance is 50 Ohms, the impedance that the ESP32 should see is about 130 Ohms.

Here is an LTspice simulation of the circuit. The source has an internal series resistance of 130 Ohms:
ac3.png
ac3.png (388.33 KiB) Viewed 4249 times
The red curve shows the ratio of the output power to the input power which is close to 0dB at 2.4 to 2.5 GHz.

aschweiz
Posts: 22
Joined: Wed Nov 30, 2016 1:57 pm

Re: Question on antenna matching

Postby aschweiz » Mon Jan 30, 2017 7:59 pm

And some additional information on the antenna in the reference design:

This seems to be called an "inverted F antenna", see for example here: http://www.ti.com/lit/an/swra117d/swra117d.pdf

And the wikipedia article: https://en.wikipedia.org/wiki/Inverted-F_antenna

WiFive
Posts: 2952
Joined: Tue Dec 01, 2015 7:35 am

Re: Question on antenna matching

Postby WiFive » Mon Jan 30, 2017 9:57 pm

Interesting thanks for posting your findings.

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