We also ordered the ESP32-C5 with a u.FL connector, so we can use it with an external antenna:

The schematics of the board can be viewed here:

KiCanvas: Display the circuit diagrams of ITS-G5-Receiver (live from Codeberg.org)

On our antenna, made by Ubiquiti, horizontal and vertical polarization can be connected separately:

We use a Ubiquiti airMAX 16 dBi here. There are certainly antennas that are better suited, though.

It turned out that the antenna performs significantly better with vertical polarization.

We have a test setup with the integrated antenna of the ESP32-C5 (top) and two receivers for vertical and horizontal polarization.

On the heatmap we can see that

1. the vertical antenna performs best
2. the PCB antenna of the ESP32-C5 is actually also very good
3. the horizontal antenna performs worst