The Raspberry Pi 4 is truly an amazing design. It has received widespread acclaim since its launch. However, a test report from Enrico Zini shows that under certain conditions, the HDMI display output of the Raspberry Pi 4 can interfere with its own WiFi.

On the Raspberry Pi 4, some users have noticed that its onboard WiFi can become unresponsive and difficult to connect in certain situations. This includes Enrico Zini, who uses the Raspberry Pi 4 as a display controller for a giant LED screen, therefore inevitably using high-resolution HDMI output. Similarly, during this process, they discovered the relationship between the two. To replicate this issue, they tried using different HDMI cables, different Raspberry Pi 4 units, and power supplies, ultimately isolating the problem to the display resolution.

HDMI is the port for display output, which includes clock and signal outputs. The transmission rate of these signals is related to the set resolution. If you want to use a higher resolution, then at the same refresh rate, the amount of data through the interface will increase, causing the signal and clock lines to switch levels faster. As we know, any wire can act as an “antenna” under certain conditions, radiating electromagnetic waves into the air.

To determine the problem, they used HackRF SDR as a receiver to analyze the electromagnetic waves around the Raspberry Pi 4 and noted that when the HDMI output of the Raspberry Pi 4 was set to 2560×1440, the leaked electromagnetic wave frequency from the HDMI clock or signal line coincided with WiFi channel 1. Since these lines are very close to the WiFi section (in fact, they are all on the same circuit board), the WiFi section of the Raspberry Pi 4 is actually blocked by the leakage from its own HDMI output.

This phenomenon requires many “coincidences” to replicate successfully, but it is not a strange occurrence in itself. For example, in 1982, an incident occurred on the amateur radio satellite UO-9, when a software error in the onboard computer caused both the VHF and UHF beacons to turn on simultaneously, blocking its own receiver and preventing the satellite from receiving commands sent from the ground. In the end, researchers at Stanford University used a 45m parabolic antenna to transmit a signal with an EIRP of 15MW, “forcing” their way through the blockage from the satellite itself, allowing it to resume normal operation.

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