In practical reality, feedline length is irrelevant to anything, if both Source and Load are at the same Impedance, AND of course the Line Loss of the Feedline itself, is relevant.... Where you get into problems is when that is NOT the case... If your Antenna is NOT the same Impedance as the Source, (i.e. Frequency is NOT at Resonance point of the Antenna) then things can get a little wonky... The worse the Impedance mismatch the more wonky things get... This is why Autotuners, at the Source, work well... They help match the source Impedance to whatever the Load Impedance turns out to be at the frequency of the RF... At LF/MF/HF these difference can get out of hand very fast as the Antenna Impedances change rapidly as a percentage of Resonance Mismatch.... At VHF/UHF you have to change the Frequency a LOT to get a significant difference in Antenna Impedance... (i.e. USUALLY YOU CAN GET A WHOLE BAND TO BE GOOD, FOR A GIVEN ANTENNA, WHERE AS AT LOWER FREQUENCIES, SOMETIMES YOU ARE LUCKY TO GET EVEN 1/3 OF THEN BAND TO WORK WELL)