Jim wrote: "...what is a good source of electronic broadband noise
from low HF through high UHF -- say, 5 to 500 MHz.?"

A linear feedback shift register. Small, repeatable. 500MHz should be
no particular problem these days. (There's an idea for some IC
manufacturer...32 bits clocked at 1G/sec repeats every 4 seconds, which
would be OK, but I'd prefer 40 or more bits. Should fit nicely into a
5 pin SOT-23: power, gnd, reset, out, ...)