At 50 feet or less, it's not really a critical factor. Sure, every bit helps, but unless you are listening to pretty weak or distant signals to begin with, the loss usually isn't enough to notice, even with really cheap coax. However, if you are indeed trying to receive really weak or distant signals, especially anything in the 800mhz range, or if you are going over 50 feet in length, then it pays to get quality.
The chart that scannerpro gave you is helpful in putting some quantitative info in your choice. Unfortunately, that chart doesn't cover the LMR series of coax, which is arguably some of the best choices, and more widely available than some of the obscure RGs. Here is another chart that covers the LMRs:
LMR-400 is widely considered to be the best compromise for scannists. It is much lower loss than the RG series coaxes, but not so thick and/or stiff that it is too difficult to work with. And it's pretty commonly available, even at ScannerMaster.com. You can get better coax, but at fifty feet, there's really no reason to. However, if you are going to go overboard on anything in your system, then coax is indeed a good thing to do that on, lol.
Thanks for the input.
I don't know if it actually make any differance between 50 Ohms and 75 Ohms cables.
What I want to do is to connect a "ROYAL DISCONE 2000" antenna and a normal DiPole antella to a BNC
"T" connector which in turn feeds into an "M100 PRE AMPLIFIER" which ends serialy in a Realistic rpo 2006 scanner, an AOR1000 and an AE30H.
I don't know how efficient this setup is, but I think the cable is important as well.
I don't think the ohms are going to make much of a difference at all. I use LMR400 to my Yagi 800 and it has N-Type connectors running at 50 Ohm. That runs into a N to BNC adapter and on to my 396T. Works very well. I bought a 75ft roll of the LMR400 with the connectors already attached. I really only needed about 40ft though. I have a bunch spooled up in my closet but don't really want to cut and resplice the connectors.. Never dealt with N-Type other than prebuilt.
Don't try to mix multiple multi-band signals into one feed. You'll waste a lot of time and money and end up with notably worse performance than what you get off of either antenna individually. More is not better.
Why are you trying for so much gain? Are there distant signals you need to hear that you are not currently hearing? At least half the time you add a preamp, all you get is more noise, yet no improvement in reception from the systems you want to hear. It's not as easy as the "Tool Time" school of radio communications. More tools and more power does not always equal more performance. It's about quality, not quantity.