Choosing the right power supply for your Raspberry Pi with attached Moitessier HAT is a very important task, as it influences the reception performance of the Moitessier HAT’s AIS receiver.
The HAT is connected via the 40-pin IO header to the 3.3V power rail of the Raspberry Pi. The voltage input of the HAT does feature an input filter to minimize conducted noise on the supply produced by the step-down converter integrated on the Raspberry Pi. In addition, the HAT provides a screening shield on the AIS receiver to be less sensitive to radiated interference.
However, excessive noise might still have a negative impact on the performance. To ensure noise does not exceed the limits, use a high-quality and low-noise converter. In general, do not use the cheapest converter you can get. Rule of thumb: The cheaper the converter, the higher the noise. Most of the low-cost converter designs do not really care about noise, ringing, oscillation, etc. These converters might work satisfactorily to load your smartphone battery but are not recommended to power RF device, like the Moitessier HAT.
See the video below, that shows a comparison of two different power supplies (step-down regulator designs) and how they affect the quality of the AIS receiver: