direct conversion and heterodyne receiver

hi all, from UBX daughter board from ettus, I see that for HF & UHF (<500MHz ), they use heterodyne receiver architecture, and for microwave, they use direct conversion receiver architecture. why that ? is there any advantages of heterodyne arc for low frequency and in reverse ? tks in advanced.

Probably at microwave the circuit DC offset and flicker noise is a smaller problem than filter out the image frequency and mixing to an intermediate frequency or frequencies. Just a guess.

Image frequency is a more serious problem at lower frequencies.For better filtering, heterodyne receivers are more convenient.
But direct conversion is also possible.For instance Cable TV Tuners use-nowadays- Direct Conversion technique because CATV spectrum is pretty clean compare to terrestrial broadcasting.

why is Image freq more serious problem at lower freq ? I heard much about it but no one can give the reason.

Because the spectrum is quite dirty up to 3GHz .( Radios,TVs,Comm.s,VHF,UHF Military,Air Traffic etc etc..,GSM,LTE.. )