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
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.
Over on YouTube Drone and Model Aircraft enthusiast channel Paweł Spychalski has uploaded a video showing how he determined that cheap HD cameras that are commonly used on hobbyist drones can cause locking issues with the on board GPS. He writes:
You might believe it or not (today I will prove it, however) that HD cameras, especially cheap ones, can be responsible for GPS problems on your drones and model airplanes. The majority of HD cameras (RunCam Split, Runcam Split Mini, Foxeer Mix, Caddx Tarsier) generate RF noise on different frequencies. Some of them on 433MHz, some on 900MHz, but most of them also at around 1GHz. Just where one of the frequencies used by GPS signal sits. As a result, many GPS modules are reported to have problems getting a fix when the HD camera is running.
In the video he uses an RTL-SDR and SDR# to demonstrate the interference that shows up when a cheap HD camera is turned on. He shows how the interference is present at almost all frequencies from the ISM band frequencies commonly used for control and telemetry to the 1.5 GHz GPS frequencies.
Reddit user [SDR_LumberJack] writes how he was recently featured in his local newspaper [Part2] in Ontario, Canada thanks to his efforts in helping to hunt down the cause of an RF deadspot with an SDR. He began his journey by reading a story in his local newspaper called the [Windsor Star]. The story was about locals having found a ‘dead-spot’ for car key-fobs. In the dead-spot key-less cars wouldn’t start, key-fobs wouldn’t unlock cars, and alarms would go off.
Being intrigued by the story [SDR_LumberJack] investigated by driving around with an RTL-SDR, HackRF and a laptop running SDR#. Eventually he found that there was what appeared to be a WBFM Broadcast radio station interfering at 315 MHz. This frequency happens to fall into the ISM radio band that used by car remotes and key-fobs. The exact source of the interference hasn’t been nailed down just yet though.
While it’s possible a broadcast station is at fault it is also possible that his SDR was just overloading, causing broadcast FM imaging. Perhaps a WBFM filter could be used to prevent signal imaging that could interfere with the investigation.
Hopefully [SDR_LumberJack] will continue his investigation and we’ll get an update on this story.
If you’re interested, back in 2016 we posted a very similar story about the exact same thing happening at a car park in Brisbane, Australia. The conclusion to that story was that the dead-spot only occurred in particular locations in the car park, and this was due to the shape of surrounding building causing the RF signals to reflect off the walls and distort the signal.
Over on YouTube user ALI6359 has uploaded a video showing what severe interference from a neighbors poor quality solar power inverter looks like on his RTL-SDR dongle. An inverter converts the DC power produced by solar panels into AC power which is used by common household equipment. Inverters typically use switching techniques to convert the power, and this can cause RF noise if the inverter is poorly designed and not shielded.
In the video ALI6359 shows strong interference all across the VHF spectrum. He also writes in the video description that the interference also occurs all over the entire HF band. He writes:
This is what happens if you or your neighbours install a dodgy quality solar power system. i am using a uhf phased array antenna facing away from the source of interferance but i am picking up very strong interferance. just touching the antenna connector of the rtlsdr is enough for the interferance to show up. i once had a HF upconverter (stopped working now) it used to show very strong interferance through the enitre HF band. the solar inverter certainly fails the part 15 FCC requirements.
In a previous post we also showed how interference from Ethernet over powerline adapters can destroy the entire HF band as well.
Solar power inverter interfernace RTL-SDR sdrsharp 30mhz to 120mhz
The differences in generated noise probably come from the fact that the iMac is probably much better shielded with an aluminum case and that they have high build quality standards for their monitors. The author suggests that an alternative to using an iMac could be to build your own PC, ensuring that dual chamber metal enclosures are used, which ensures that the power supply is isolated in its own separate steel compartment.
I walked out to my car from Bunnings, and there was a new HSW Maloo parked in front of me with the owner staring at his key fob and shaking his head.
I said “let me guess, car won’t open?” and he said yeah, and he’d been trying for about 5 minutes. I said that I’d had the same thing happen to me a few months back in the same spot, and then went to open my car.
Nothing. No beep, door stayed locked. Looked around and there was another couple trying to get into their car as well (late model C Class).
It took about 5 minutes of me trying the door every 20 seconds or so before it opened. HSV owner was still there when I left. The only thing he and I could think of causing it was the mobile phone tower in front of Aldi.
After reading the post, user u/riumplus decided to go out to the same spot with his Funcube dongle SDR and see if there was any interference that might explain the issues. But he found no such interference. However, when he pressed the wireless entry on his own keyfob he noticed reflections from the main transmission that were coming from the buildings walls. He wrote:
In his latest tests he tried a metal outlet box as the case and saw improved results over the aluminium case. His conclusions seem to indicate that the aluminium box is not a good EMI shield. We’re not sure why he found these results, but one theory might be that because the aluminium case is anodized, it has a non-conductive surface, which might cause poor grounding.
Over on the Reddit thread discussing his work, there are some concerns about excessive crystal drift due to there being no ventilation holes. However, it seems that the general consensus is that lack of ventilation will not significantly affect crystal drift and may actually help to stabilise the crystal over time by keeping the internal temperature more constant.