There is a lot of good information in this thread but I think there is some confusion between the concepts of a receiver selectivity and its sensitivity. Selectivity is the receiver ability to distinguish your signal from all the others around you. Sensitivity is its ability to detect a weak signal. Single and dual conversions circuits are means to increase a receiver selectivity. Better selectivity does not mean greater sensitivity.

My first receiver had no conversion. It had no IF's. It was a super regenerative circuit. It had as good of range as any present day dual conversion receiver. But it had no selectivity and it would pick up everything in the band. At the time that wasn't a problem since we had only one frequency, 27.255MHz.

Allan