Hans Nusbaum, DJ1UGA, has published in [1] a simple noise source for VHF & up. But he used a manhattan style PCB. This didn't work above 1GHz. I used a commerical 50 Ohm striplineboard.
The unit covers the frequency range up to 3.6GHz within +-1db, the ENR is about 32dB.

PCB noise source Spectrum noise source

[1] HF-Messungen für den Funkamateur, VTH-Verlag, Germany