RF (Radio Frequency) cables are distinct from coaxial video cables. They are internally filled with 100% polyethylene, a design that significantly reduces signal energy loss. Capable of transmitting relatively large frequency amplitudes, RF cables are generally easier to install and feature a simpler overall structure. Given their diverse functions and applications, RF cables are classified using various standards. This article provides a detailed breakdown of RF cables based on several common classification systems.
I. Classification by Insulation Type
The insulating dielectric is a critical component affecting the high-frequency performance of the cable.
Air Insulated Cable
In this type, the majority of the volume within the cable's insulation layer is air, with only a portion occupied by solid material that supports the inner and outer conductors. Its defining structural feature is that electromagnetic waves can travel between conductors without passing through a solid dielectric layer. Air insulated cables offer extremely low signal attenuation and are a commonly used structural type for ultra-high frequency (UHF) and higher applications.
Semi-Air Insulated Cable
This structure serves as an intermediate type between solid and air insulation. The insulation is composed of a mix of air and solid medium, but unlike purely air-insulated cables, electromagnetic energy must pass through some solid dielectric layer when traveling from one conductor to another.
Solid Insulated Cable
This is the most prevalent type for flexible coaxial cables. The space between the inner and outer conductors is completely filled with a solid high-frequency dielectric material.
II. Classification by Cable Structure
The physical structure defines the electromagnetic field distribution and primary application scenarios.
Symmetric RF Cable
The electromagnetic field in a symmetric RF cable circuit is an open structure. At high frequencies, the radiation of electromagnetic energy leads to increased attenuation and poorer shielding performance. Additionally, its performance can be influenced by atmospheric conditions. Therefore, symmetric cables are less commonly used and are primarily found in low-frequency RF or symmetric feeding applications.
Spiral RF Cable
In this specialized design, the conductor in coaxial or symmetric cables is wound into a spiral coil shape to increase the cable's inductance. This allows for:
High Impedance Cable: Increased inductance raises the cable's wave impedance.
Delay Cable: Designed to deliberately delay the transmission time of electromagnetic energy.
Cables with varying coil density along their length can be manufactured to create variable impedance cables.
Coaxial RF Cable
In this structure, the inner and outer conductors are positioned concentrically. This confines the electromagnetic energy within the dielectric medium between them, resulting in significant advantages: low attenuation, high shielding performance, wide frequency bandwidth, and stable performance. They are typically used to transmit RF energy in the range of 500 kHz to 18 GHz. Two common characteristic impedances are:
50 Ω RF Coaxial Cable: Widely used in wireless communications, radio equipment, and test instrumentation.
75 Ω RF Coaxial Cable: Commonly employed in CATV (Cable Television) networks, hence it is often referred to as "CATV cable." Its transmission bandwidth can reach up to 1 GHz, with 750 MHz being standard for currently prevalent cables.
III. Other Common Classification Criteria
RF cables can also be categorized based on other properties to meet specific application needs.
By Insulation Material:
Plastic Insulated Cables
Rubber Insulated Cables
Inorganic Mineral Insulated Cables
By Flexibility:
Flexible Cables
Flat Flexible Cables
Rigid Cables
By Transmission Power:
Low-Power Cables (below 0.5 kilowatts)
Medium-Power Cables (0.5 - 5 kilowatts)
High-Power Cables (above 5 kilowatts)
By Special Application Features:
Low Attenuation Cables
Low Noise Cables
Miniature Cables
High Stability Phase Cables
