Dipole, Off-Center, Balun, No Balun, 50 or 75 Ohm Coax?

Just started playing with the home-brew antennas for my scanner family. I have 2 I am using now, plus 4 purchased antennas...



I've been reading, and reading, and looking at pictures, studying designs...



... and there's a few things that seem unclear to me:



1) for dipoles, real dipoles, center-fed, do you use 50 ohm or 75 ohm coax?



2) For center-fed dipoles do you use a balun?



3) For center-fed dipoles, do you used a balun ONLY if you used 75 ohm coax?



Again, this is for scanner use.



Also, just for listening, is the length of a center-fed dipole critical?



For instance, if a 36" dipole (2x 18") works well for 154 MHz and 440-470MHz, if the measurements were increased to 10' (2x 5'), will it bring in the signals better because there is more metal in the air?



prcguy mentioned in another thread that 1/2 wave is bad, but that was "end-fed". I believe "odd multiples" of a 1/4 wave were good, so would 2x 54" legs be better since they are 3x 18"?



I've got results from several antennas already, but I don't quite understand the results, or at least I don't want to draw conclusions from assumptions. I'll leave this for a later post till these basic questions are answered.



Thanks in advance :)