Coax loss can be a big issue.
Given the low power levels you will suffer less loss on a long run using 75 ohm coax cable which is designed for low loss, even assuming you do nothing about the mismatch at the ends of the run. All that is good up to about 1ghz where more specialized coax is needed for minimum loss.
The antenna is a significant part of the equation and you are well advised to use an antenna suited for the frequency of interest. Broadband antennas like discones have very low gain, and they pull in other strong signals you don't want.