ESP32C3 Chip-Antenna Matching

[Fredric]

Hi there,

i've designed a little PCB with an ESP32C3 to transmit some sensor data over wifi. However, I'm having a few questions regarding the matching of the antenna.
Now I only have a nanoVNA to do the matching with, so I don't expect a perfect match and it's just a one off board. This is also the first board I'm trying this with, so I really don't have a lot experience with RF and could very well be wrong on some things.

So, to my question:
According to posts from the Espressif forum, the ESP32C3 has an RF output impedance of 35+j10 Ohm. So I understand that I can't just connect the VNA in place of the ESP32C3 on a blank board with just the RF components on it and match the antenna, since the VNA has 50 Ohms. Because when I then put the ESP on, the match will be off, because of the difference in output impedance, correct?

If you look at the attached image, I have a pi-network to match the 35+j10 Ohm to 50 Ohm and then another matching network for the antenna.
In order to match the antenna, I would just remove the components from the pi-network and attach the vna right after it (image 2).
But then how would I figure out what components I need for the pi network to match the 35+j10 Ohm to 50 Ohm and vice versa?
I mean I assume I can't just calculate them, since the real world will be very different from ideal calculated values.

All the resources I've looked at always assume 50 Ohms output impedance of the RF transceiver, but I haven't found anything that would help me here. Would be great if someone could give me some hints or tell me if I'm completely wrong on my assumptions... 

[RFDx]

So, to my question:
According to posts from the Espressif forum, the ESP32C3 has an RF output impedance of 35+j10 Ohm. So I understand that I can't just connect the VNA in place of the ESP32C3 on a blank board with just the RF components on it and match the antenna, since the VNA has 50 Ohms. Because when I then put the ESP on, the match will be off, because of the difference in output impedance, correct?

Yes, you can connect the VNA instead of the chip. Technically you have two separate matching networks. The first matching network transforms a 50 Ohm load to the complex conjugate value of the chip, namely 35-j10 Ohm. This is the impedance your VNA would show you. As an example see the attached file.

The second network transforms the unknown impedance of the antenna to 50 Ohm. You need to measure the unknown antenna first, devise a suitable matching network and then measure again at the input of the second matching network to confirm you now got 50 Ohms.