RG-59/U is a specific type of coaxial cable, often used for low-power video and RF signal connections. The cable has a characteristic impedance of 75 ohms, and a capacitance of around 20pF/ft (60pF/m). The 75 ohm impedance matches a dipole antenna in free space.
RG59 is a single shielded coaxial cable about 6.1 mm in diameter with a smaller centre conductor than your standard RG6 cable. Although RG59 can be used for cable TV applications (CATV), there is a significant amount of signal loss over higher frequencies when run at longer distances.