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This is the answer. That mode puts the second harmonic going out of the HDMI cable smack in the 2.4GHz WiFi band (the fundamental would be at 1/2 the bit rate).

Now the question is who screwed up. Is it a leaky cable? Is it bad PCB design? Is it a problem internal to the SoC? Is it a power delivery issue? Knowing the RPi foundation and Broadcom, I bet one of them screwed this up for all RPis and it isn't just a bad cable.

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I'm trying to understand this so I don't know a lot of what's going on here, but why is the fundamental half the bit rate instead of the bit rate?

I'm not 100% sure, but I'm assuming because of Double Data Rate transfers: https://en.wikipedia.org/wiki/Double_data_rate

Imagine a signal that alternates every second (e.g. 10101010). This has a frequency of 1/2 Hz, not 1 Hz, because it repeats every 2 seconds.

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