Co-Axial feedline, or simply referred to as COAX, is the most commonly used and is perhaps the safest and easiest way to get your RF to the antenna. With all feedlines there are several determining factors that should be considered before you go out and buy your coax. First, and most important, is to look at your transmitters manual and see what IMPEDANCE LOAD is recommended. Some transmitters can "load" into a variety of impedances and some transmitters will only work correctly with certain impedances. The most common impedance is 50 Ohms. Right now, you should not be concerned with the How-To's and Why's of antenna feedline impedances. The second concern is the RF power rating of the transmitter. Feedlines are rated in both IMPEDANCE and POWER HANDLING, so it's important that you know what you need. For 99% of any type of transmitter/antenna combination you will probably encounter the most generic and all around used coax is RG/8U or the same spec, but smaller RG/8X. Some prefer the RG/8X because it's smaller. RG/8U,X impedance is 50 Ohms and can handle up to 1000 watts. Buy quality cable and not the cheap Radio Shack stuff!
There is no critical length to use with feedlines, just keep the length of coax between the transmitter and antenna as short as possible to avoid losses. The more feedline, the more power you will lose before it ever gets to your antenna. Usually, losses are not apparent until the length of the feedline gets to be 100 feet or longer.
The Outside end of your coax that connects to the antenna, you should take great care to seal the end to prevent moisture entering the coax! Use lots of electrical tape, silicon sealer, or COAX SEAL tape. The capillary action on the outer braid of your coax cable will actually draw water up through the entire cable. I have seen water come out of coax cable that looked like it was a garden hose! Once your coax becomes water logged it's pretty much useless, but can be used for ground connections in your station! You will also need a medium duty (Several 100 watt) soldering iron for soldering COAX connectors. PL-259 types are the most commonly used and you should ask for silver plated with teflon insulators to get the best connectors. Again, if you buy the Radio Shack cheapies, they could turn out to be more trouble than they are worth!