Listening to Atmospheric noise
If you just want a source of true random numbers, you might as well use the RF noise at 400 MHz, which is just as random as the noise at 7 MHz. It's true that most of the atmospheric noise power is in lower frequencies, but you don't need a lot of power, you just need a lot of entropy, and entropy is everywhere.

For that matter, if you need true random numbers, there are plenty of ways to do that, which might be simpler than a radio, and perhaps also less vulnerable to attack, given that anyone with a radio transmitter can compromise your RNG.