Structure and transmission characteristics of RF coaxial cable

structure

RF coaxial cable is composed of 4 parts: inner conductor, insulator, outer conductor and sheath. The insulator insulates the inner and outer conductor and keeps the axis coincident, which is the coaxial cable. The inner and outer conductors are separated by a dielectric (insulating material), which determines the transmission speed and loss characteristics of the coaxial cable to a large extent. The commonly used insulating material is a mixture of dry air, polyethylene, polypropylene, polyvinyl chloride and other materials. The physical foam cable is optimized for its low loss, good frequency characteristic and difficult water intake.

Transmission characteristic

(1) Electromagnetic field distribution in coaxial cable

The electric field strength is distributed sinusoidal, the waves transmitted in the coaxial cable will not leak outside the cable, in the application, the outer conductor is usually grounded, so it has a good shielding effect, the transmitted TV signal is not interfered with external clutter, and the signal inside will not radiate out.

Coaxial cable performance

(1) Characteristic impedance

The characteristic impedance Zc is defined as the ratio of voltage to current at any point on the coaxial cable with a matching terminal. The characteristic impedance of a coaxial cable is determined by the diameter of the conductor and the medium between the conductors, and has nothing to do with the cable length. In the CATV system, the characteristic impedance of the coaxial cable is 75Ω.