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U2 (AP1117) tab is internally connected to Vout. The symbol used is a three-terminal device, however being in an SMT SOT-223 package it has a 4th pad for the tab. This got connected to the top layer groundplane. Workaround is to cut the 2 traces and insure a significant gap before populating the board.

The PCB trace inverted F antenna KiCad library has no solder mask on top of the antenna. Per the TI DN007 appnote describing the design, the thickness should be 1oz copper. Without soldermask the antenna is covered in plating, increasing the thickness beyond the 1oz copper layer. Additionally, the mask protects from direct contact with other metal objects. The appnote does not specifically mention putting mask over the antenna, but the example photos show green mask over the antenna.

The antenna performance is decent on the v0.1 protos, which may be due to a consistent ENIG finish used by the board house. But if the board were to be built using HASL the thickness and consistency may change enough to detune the antenna.

Fix is to clone the symbol to the project library and add a mask.

A few problems with the path from the ESP LNA pin to the antenna feed:

1. Strip line width is all wrong for this PCB thickness. 0.2mm gap and .5mm trace as used in v0.1 give ~70 ohms. For OSH Park with dielectric constant of 4.6 and board thickness of 1.6mm, a gap of 0.2mm and 1.22mm gives an impedance of 49.98 ohms (see http://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm)

2. Via stitching around the path should be increased but should be careful not to intrude into the gap.

3. The 90 degree bend should be properly mitered, or preferably done with a smooth curve.

4. The guide ground plane should be continuous around both parts and the signal trace. There are some spots where vias for other signals are interrupting the path.

5. The ground plane under the path should be uninterrupted. In particular, the 3.3V distribution that jumps the waveguide on the bottom layer is not OK.

The Pi HAT standard has the mounting holes in the corners at 2.75mm diameter, while the holes on v0.1 are 3.5mm. Refer to this for Zero dimensions: https://www.raspberrypi.org/forums/download/file.php?id=12279