Coaxial Cable Transmission Line

             Coaxial cable may be the most common transmission line for shortwave and VHF/UHF. It consists of concentric two-layer conductors, called "coaxial" because the two-layer conductors share the same axis (Fig. 1). The inner layer conductor is a solid standard conductor, and the outer conductor forms a shielding layer. The coaxial conductor used for the receiver, the shielding layer is usually woven from the wire, but sometimes the multi-line is also seen. The coaxial conductor impedance of the TV antenna system is 75ω, and the outer conductor is made of metal foil. Cables consisting of this shielding layer have a low signal loss in a large frequency range, but are not ideal for most applications outside of television. The problem is that the metal foil used for the outer conductor is aluminum and the aluminum is not welded. So these antennas are usually connected with F-joints, and the signal loss rate for other applications of this connection is too high.Coaxial Cable

            The inner insulating layer isolates the two-layer conductor as the dielectric, and its material has many choices. Commonly used are polyethylene, foam and Teflon (the latter mainly used in high UHF and microwave frequency bands).Coaxial Cable

            The large diameter type is slightly smaller than the small diameter signal loss, but its main advantage is power control ability. While this is important for radio amateurs, it is not important for receivers. Unless in a very long line (far greater than 100ft, 1ft=3.048x10-1m), the cumulative loss becomes very large, and other times a smaller diameter (RG 58/u or rg-59/u) cable is used as the receiving antenna because they are easier to control.Coaxial Cable

            The micro-diameter RG 174 is sometimes used on the receiving antenna, but its primary use is as a connection between the devices (e.g. receivers and preselection or ATU), in balancing transformers and coaxial phase converters and instrumentation applications.Coaxial Cable