Using a HackRF to Investigate Why WiFi on the Raspberry Pi 4 Doesn’t work when Running HDMI at 1440p
The Raspberry Pi 4 launched with it's fair share of problems, but a new problem seems to have been recently discovered and documented. It turns out that the Pi 4's WiFi stops working when running at a screen resolution of specifically 1440p.
Suspecting interference generated by the HDMI clock, Mike Walters (@assortedhackery) used a HackRF and a near field probe antenna to investigate. By placing the near field probe on the Raspberry Pi 4's PCB and running a screen at 1440p resolution he discovered a large power spike showing up at 2.415 GHz. This interferes directly with 2.4 GHz WiFi Channel 1.
There's an interesting story doing the rounds about the Raspberry Pi 4 WiFi not working at higher HDMI resolutions. I had a quick look with a HackRF & near-field probe and there's definitely a big spike that stamps right on channel 1 pic.twitter.com/FXRebYYJxw— Mike Walters (@assortedhackery) November 28, 2019
An article by ExtremeTech article notes:
There’s a giant spike that could easily interfere with Channel 1 of a Wi-Fi adapter. So why is this happening? Because a 2560×[email protected] has a pixel clock of 241.5MHz and has a TMDS (transition-minimized differential signaling) clock of 2.415GHz, according to Hector Martin (@Marcan42). And what frequency does the RBP4 use for Wi-Fi? 2.4GHz. Which means… outputting on HDMI over 1440p can cause interference in a Wi-Fi channel.
The ExtremeTech article also notes that this problem is not unique to the Raspberry Pi 4 only. It turns out that USB 3.0 hardware is to blame, and this problem has occurred before with USB3.0 hard driver and on some MacBooks.
While the interference appears to be localized to the near field around the Pi4 PCB, we suspect that you could use TempestSDR to remotely eavesdrop on the Pi 4's video output if the interfering signal was boosted.
I wonder if the problem reduced a smidgen by using the micro HDMI connector that is physically closer to the USB ports and further away from the 2.4/5GHz WiFi/Bluetooth 5.0 module. Or by using 5GHz WiFi, or by using a better quality HDMI cable.
Looking at images of the RPi4, have they removed the ceramic antenna that was on some of the older boards (which I think was only 2.4GHz – https://i.stack.imgur.com/koc0a.jpg ) and just left a (tuned) floating track on the PCB to act as the antenna.