I agree with Sean, and would just emphasize two points: Get the best shielded cable you can, and the installation will go much more smoothly if you can go with a cable that displays a "limp" pliability, instead of resisting you at every turn. The combination of these two factors will usually mean you'll have to spend a little bit more to get a quality cable that employs a finely braided internal shield construction along with a flexible jacket material. Also, gold plated terminations are desirable if you're placing the antenna outside for corrosion resistance. I was able to find these features at a fairly reasonable price in a Monster Cable product (sorry I don't remember the exact model), but mine is only 20' - I don't know if they make a 100' version.
