What is the longest cable length I can use?

For passive antennas, 20 meters is about as far as you can practically go (you can go farther, but the size of the cable, rigidity, and cost make it impractical). From 20 meters to 40 meters, consider an Active (powered) antenna solution. Beyond this, our Comcenter Outdoor (also used for our Citadel Solution) is the only option we know of currently. We see alot of these installed onto skyscrapers and some of the world’s largest ships!