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Coax Cables

Basics of RF Coaxial Cable

by Peter McNeil | Mar 06, 2023

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Coaxial Cable Assembly

RF Coaxial Cables are one of the most common transmission line types in use today. Part of the reason for this is that there is a wide range of design freedom and material use that allow for RF coax cables to be designed for a wide range of applications. Moreover, RF Coaxial Cables are predominantly used in the transverse electromagnetic (TEM) mode, with a frequency range that is dedicated by the physical geometry of the inner and outer conductor of the RF cable. The TEM mode is relatively efficient and exhibits predictable performance, as the RF conductor losses and dielectric losses are relatively straightforward to model and measure.

Coax cables are used as part of RF coax cable assemblies when terminated with RF coax connectors, but also on their own when used as jumpers in between transmission lines or RF ports that are crimp or solder attached. Most coax RF cables are either 50 Ohm coaxial cables or 75 Ohm coaxial cables, where the majority of applications use 50 Ohm coax, except for broadcast and other specialized applications using 75 Ohm coax.

RF cables are either flexible coaxial cable, formable coaxial cable, corrugated coaxial cable, or semi-rigid coaxial cable, or rigid coaxial cable. Flexible coaxial cables are one of the most common types, with braided, banded, or taped jackets/outer conductors that allow a high degree of flexibility and flex cycles. Corrugated coax cables can also be flexed to some degree but are generally stiffer and heavier than flexible coax and made for longer runs in conditions where it may be likely that a coax cable could be crushed. Formable coax cables are made with a stiff conductors/jackets that can be hand formed into a given position, but usually only have a limited number of cycles and should only be formed a limited number of times. This cable type is very high performance and can be relied upon to hold the formed shape once bent, which can be advantageous for applications that require consistent performance.

Semi-rigid and formable coaxial RF cables are often confused with each other. The main difference is that semi-rigid coax use is made with a solid conductive outer conductor/jacket and should only be formed once with the appropriate tools. Whereas a formable coaxial cable is typically formed or reformed by hand.

RF coaxial cables are made of various sizes and materials to accommodate different frequency ranges, power handling capabilities, and environmental requirements. Many RF cables are designated with a “RG” number, which is a designation method that rates back to early military specifications for coax. There have been many innovations with coaxial cables, and specialized coaxial cable designs that fill the needs in virtually every application.

An RF cable works by transmitting high-frequency electrical signals from one component to another while minimizing signal loss and interference. The signal travels through the center conductor, while the surrounding dielectric maintains a consistent spacing that controls impedance. The outer shielding confines the electromagnetic field within the cable and blocks external interference, ensuring stable and reliable signal transmission. This controlled structure allows RF cables to carry signals over a wide frequency range with minimal reflection, attenuation, and noise, critical for RF, microwave, and high-speed communication systems.

Common Types of RF Cables

RF cables are available in several types, each designed to meet specific performance and installation requirements. Flexible RF coaxial cables are widely used due to their ease of routing and versatility in general RF applications. Semi-rigid RF cables provide superior phase stability and low loss, making them ideal for high-frequency and precision systems. Low-loss RF cables are engineered with advanced dielectric materials to reduce attenuation over longer distances. High-frequency and microwave cables are optimized for gigahertz-level performance, supporting applications such as radar, satellite communications, and test and measurement environments.

Key Applications of RF Cables

RF cables are essential components across numerous industries where high-frequency signal integrity is critical. They are commonly used to connect antennas, transmitters, receivers, and amplifiers in wireless communication systems. In test and measurement setups, RF cables ensure accurate signal delivery between instruments and devices under test. RF cables also play a vital role in aerospace, defense, broadcasting, medical equipment, and industrial automation, where reliable and interference-free signal transmission is required under demanding operating conditions.

There are several commonly used methods of carrying electromagnetic energy along conductive pathways. The most common of which for RF applications are RF cable assemblies, such as coaxial cable or twisted pair. The better shielding and lower loss capability of coaxial cable means that for frequencies over a couple hundred-megahertz, coaxial cable assemblies are the most commonly used for high frequency applications from hundreds of megahertz to over 110 GHz. These cables are used in virtually every RF application, from home fixed wireless access to space communications. There are RF cable assemblies made for every environment RF equipment is used, including automotive, train/rail, marine/naval, aerospace, military/defense, and commercial.

RF coaxial cable assemblies are composed of three separate components and the method of attachment between the components. Each coaxial cable assembly has a coaxial cable and two coaxial connectors. The coaxial connectors need to be compatible with the coaxial cable, but not necessarily the same coaxial connector type. A coaxial cable assembly needs to be attached, or otherwise manufactured, in such a way that the coaxiality of the transmission line is maintained throughout the assembly. Coaxiality refers to the alignment of the inner conductor of the cable or connector and the outer shielding of the cable or connector. In some cases, a RF cable assembly may have a connector that is not a coaxial connector, but instead a banana connector or even exposed center conductor for specific use cases.

Coaxial Cables typically have a center conductor, dielectric spacer, an outer conductor, and an outer cable jacket. There may be a variety of additional layers to enhance environmental protection, physical ruggedness, or to enhance electrical performance. Though the most common type of coaxial cable found as cable assemblies is flexible cable, or flex cable, there is also rigid and semi-rigid coaxial cable. These cables tend to exhibit better electrical performance than flexible coaxial cables at the cost of less physical flexibility and the flexibility to be used in a variety of applications. RF Cables may have one of a variety of different connectors, the most common of which are threaded coaxial connector types, such as SMA, N-type, 4.3-10 DIN, 3.5mm, etc. There are also several common types of coaxial connectors that are push-connect, such as SMP, MCX, MMCX, QMA, and others. Bonnet style fittings are also common but are limited in frequency to 2 GHz. Different connectors can be used for each end of the coaxial cable assembly, but it is important to note that the maximum frequency of the cable, and many other of the electrical performance parameters, will be limited by the lowest rated of the coaxial cable assembly components

Recommendations

For Engineers & Designers

Select RF cables based on frequency range, impedance (50Ω or 75Ω), and loss requirements.

Minimize adapters and transitions to reduce signal reflections.

Consider environmental conditions such as temperature, vibration, and moisture.

For Installers & Maintenance Teams

Avoid tight bends beyond the cable’s specified bend radius.

Ensure connectors are properly torqued to maintain signal integrity.

Inspect cables regularly for jacket damage or connector wear.

Peter McNeil | Dec 27, 2023 | RF Cable Assemblies

In large facilities and buildings, it often comes up that there may be a need for fire rated coaxial cable. The term “plenum” cable gets tossed around as well. Many are confused on the terms and the rating systems and how this pertains to the United States National Electrical Code (NEC) and what regulations the National Fire Protection Association (NFPA) set out for cables in the NEC. Though not all states or regions adopt the NEC in the US, it is the most widely used electrical code, and is a good standard to base safe designs on. Of the 15 cable ratings in the NEC, there is only one that is rated for Plenum spaces.

A Plenum space is a cavity above a ceiling, below a floor, or beyond a way that is used for air circulation. This includes ductwork and any spaces that allows for the free transfer of air between regions of a building. Though plenum spaces are necessary for circulating air throughout a building for temperature control and ventilation purposes, these spaces also pose a danger during a fire. As plenum spaces naturally cannot contain fire breaks, any fire that enters a plenum space has a free and fresh-air-flowing passage to other parts of the building and can generate fumes and smoke that can be circulated throughout an entire building. As this is an especially life-threatening scenario, plenum spaces have a cable rating that is special to the space, called Communications Multipurpose Plenum (CMP). This is not to be confused with Communications Multipurpose (CM) or Communications Multipurpose Riser (CMR), which have their own ratings and limitations, and are not rated for plenum spaces.

CMP cables are often just referred to as “plenum” cables. These cables are made in such a way that if they are exposed to fire, they burn slowly, and exude a low enough amount of toxic fumes, gasses, and smoke. This requires very careful construction and testing of materials to ensure they meet safety regulations. Generally, there are well known materials that meet Plenum ratings, such as PTFE/Teflon jackets and certain types of PVC jackets. In the case of a coaxial cable, this means the jacket material and inner dielectric must be constructed of such materials. To further enhance the ratings of some materials, some plenum rated cables are further coated with flame retardants that help to ensure the cable doesn’t combust in the first place.

Special construction of any sort results in higher cable costs, which is understandable. This is also why plenum cables don’t tend to be used for all applications, even though they are also often designed to a high quality, low-loss, and are subjected to relatively thorough quality control screening.

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Basics of RF Coaxial Cable

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