Specifications of RF Cables and Applications
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Radio Frequency cables are significant components of every RF systems range from RF transceiver systems to aviation industry. What are the major specifications of an RF test cables and applications?
RF test cables are used to make efficient signal (power) transfer between two equipments, devices or to a transmitting antenna. Choosing a proper cable is important step to make sure the system works efficiently. There are many types of RF cable for specific uses; it manly varies in its specifications.
Specification of RF Cables
1. Impedance Specification
Characteristic impedance is one the most important specifications of RF cables. Impedance of a line is determined by the ratio of voltages and current of a wave transmitting (propagating) along that line. The factors affecting characteristic impedance are the conducting material, dielectric and geometry of the material.
SI unit of characteristic impedance is Ohm. There are mainly two categories of cables in terms of the characteristic impedance.
50 Ohm cables: used in high precision RF test equipments and high frequency applications.
75 Ohm cables: used in domestic video applications, digital audio, TV distribution networks.
Impedance of a standard RF cable is independent of its physical length. However, cable losses are higher with increase in length.
2. Frequency Specification
Frequency range is another important specification of RF cables. RF cables are often designed to operate at specific frequency range due to limitation of material used for construction. It doesn’t offer required performance outside specified frequency range (impedance, loss, phase stability etc…).
Most of the RF cable performance is frequency dependant. Professional RF test applications require higher accuracy for measurement results. High end cables are usually preferred in RF test and measurement applications.
3. Phase Stability
In RF testing, phase stability is also a significant parameter which determines accuracy of the test system. Standard RF cable should have good phase stability; position of the cable after calibration should not affect phase difference.
In transmission systems, phase difference cause efficiency of the overall system.
4. Attenuation Specification
Attenuation of the cable is the amount of signal loss due to cable losses / attenuation which is also a qualifying factor of a standard RF cable. Cables with higher loss always affect the entire system efficiency.
RF test labs will use low loss standard RF cables for device measurements.
5. Power Specification
Standard RF cable developed to operate within specific power range to ensure safety and efficiency. Wide ranges of RF cables are available to fit applications range from low power transmission, medium power RF test applications, high power cables for transmission and testing purposes.
High power broadcasting stations and mobile transmitters uses high end cables which can handle power up to few kilo watts.
In low power applications maximum power rating is also specified as maximum voltage rating.
6. Temperature Stability
Temperature stability becomes an important parameter for high temperature applications. Engineers have to ensure accuracy of the measurement setup and make sure cable characteristic has no contributions to the test results.
Most of the co-axial cable has different characteristic under high temperature and low temperature. Unusual behavior of cable might be interpreted wrongly as performance of device under test (DUT).
Cables designed for high temperature applications uses materials (dielectric, inner conductor, outer layer and connector assembly) which handles higher temperature. Deformation of cables is common issue during high temperature test.
7. Physical dimensions
Dimension of RF cable is yet another specification which determines the connector type, size, shape, physical diameter and length.
Connector type: many types of RF connectors used in industries depend on frequency range and type of applications.
BNC: used in low frequency low power applications
SMB: used in mid frequency low power applications
SMA: used in high frequency high power applications
N-type: used in high frequency high power applications
Flexibility: cable flexibility requirements vary according to applications. Some application requires rigid cables which are permanently fixed after implementation. RF test equipment requires cable with flexibility and stable performance for many cycles.
Application of RF cables
Radio communication systems
Mobile communication networks
Domestic applications, digital audio and TV channel distribution network
Professional RF test and measurement systems, labs and development units
Aerospace industry for accurate communication equipments
Wired telephone and communication networks
Military and space application
Ships and navy communication systems
Satellite communications
RF equipment in medicine and healthcare
How to choose a proper RF cable
RF cables are designed to perform application specific. Some applications requires higher accuracy and other requires higher power handling capability. Selection of RF cable is based on the main specifications which a particular applications demands (cable specifications can be found from manufacturer data sheet). Engineers have to analyze the real performance of RF cables for each applications.