Coaxial cable consists of a hollow outer cylindrical conductor and a single inner conductor surrounded by it. The cylinder is separated from the conductor by insulating material, and its frequency characteristics are better than the twisted pair, which can perform higher-speed transmission. Due to its good shielding performance and strong anti-interference ability, it is usually used for baseband transmission. Coaxial cables can be divided into two basic types: baseband coaxial cable (thick coaxial cable) and broadband coaxial cable (thin coaxial cable). The shield of a thick coaxial cable is made of a copper mesh, with a characteristic impedance of 50Ω, such as RG-8 and RG-58; the shield of a thin coaxial cable is usually made of stamped aluminum, with a characteristic impedance of 75Ω, such as RG-59.

I. Physical Structure of Coaxial Cable

Coaxial cable consists of a central conductor, an insulating material layer, a shielding layer composed of mesh fabric, and an outer insulation material layer, as shown in the figure below.

Structure of coaxial cable

The coaxial cable is flexible enough to support a bending radius of 254 mm (10 in). The center conductor is a solid copper wire with a diameter of 2.17 mm ± 0.013 mm. The insulation material is required to be an insulating material that satisfies the electrical parameters of the coaxial cable. The shielding layer is composed of metal tapes or sheets that satisfy the transmission impedance and ECM specifications, with an inner diameter of 6.15 mm and an outer diameter of 8.28 mm. The outer insulation material is generally polyvinyl chloride (such as PVC) or similar materials.

II. Main Electrical Parameters of 50Ω Coaxial Cable

The characteristic impedance of coaxial cable: The average characteristic impedance of coaxial cable is 50Ω ± 2Ω. The periodic variation of impedance along a single coaxial cable can reach a sinusoidal average value of ± 3Ω, and its length is less than 2 m.

Attenuation of coaxial cable: When measured with a 10 MHz sine wave, the attenuation value of a 500 m long cable section does not exceed 8.5 dB (17 dB/km), and when measured with a 5 MHz sine wave, it does not exceed 6.0 dB (12 dB/km).

The propagation speed of coaxial cable: The minimum propagation speed is 0.77c (c is the speed of light).

DC loop resistance of coaxial cable: The sum of the resistance of the center conductor and the resistance of the shielding layer does not exceed 10 mΩ/m (measured at 20℃).

III. Physical Parameters of 50Ω Coaxial Cable

The coaxial cable is flexible enough;

Can support a bending radius of 254 mm (10 in);

The center conductor is a solid copper wire with a diameter of 2.17 mm ± 0.013 mm. The insulation material is required to be an insulating material that satisfies the electrical parameters of the coaxial cable;

The shielding layer is composed of metal tapes or sheets that satisfy the transmission impedance and ECM specifications, with an inner diameter of 6.15 mm and an outer diameter of 8.28 mm. The outer insulation material is generally polyvinyl chloride (such as PVC) or similar materials.

IV. Thin Coaxial Cable

Thin coaxial cables cannot be twisted together, and the sections are connected by low-loss 75Ω connectors. The connectors are physically matched to the cable. Intermediate joints and couplers are wrapped with tubes to prevent accidental grounding. If the cable is to be buried in a place where it will not be exposed to light, it is best to bury the cable in a layer of earth below the freezing point. If you do not want to bury the cable underground, it is best to use a utility pole to string it up. The coaxial cable is marked every 100 meters for easy maintenance. If necessary, the cable should be supported every 20 meters. When installing inside a building, consider the convenience of maintenance and expansion, and also provide pipelines to protect the cable where necessary.

V. Cables Used in Communication and Cable TV

Cables commonly used in communication and cable TV include: series physical foamed cable TV cable, series access network physical foamed coaxial cable, series 50Ω physical foamed coaxial cable, series physical foamed corrugated copper tube coaxial cable, series solid polyethylene insulated radio frequency coaxial cable, and series leaky coaxial cable.