RF cables are used in applications that require ultra-high-speed data and information to be transferred, specifically covering electromagnetic radio frequencies from 3kHz to 300GHz. These frequencies may consist of satellite communication, Wi-Fi, and cellular signals as well as AM/FM radio and analog/digital television.
Coaxial cables are the industry standard for manufacturing RF cables because they allow these RF frequencies to be transmitted with minimal loss and performance impacts.
RF cables are used everywhere – in the household, in the workplace, in vehicles, and widely used in military and aerospace applications. Before streaming services became prevalent replacing cable TV, F-type CATV connectors were among the most recognizable RF cables and connectors around. Every kid growing up from the 1980s to 2010s has tinkered with one of these finicky connectors only to learn the hard way how delicate the center pin is on one of these coaxial cables. With the emergence of Wi-Fi and high-speed internet, the prominence of these all-too-familiar cable TV connections has waned in recent years. But still, RF cables are needed for countless devices and applications as data transfer speeds increase and connectivity becomes even more widespread.
F-type CATV connectors.
Coaxial cables consist of two electrical conductors made in concentric layers that are protected with a dielectric insulator between each conductor. Most RF cables are flexible, but the subset of RF cables are semi-rigid and made of extruded metal tubing that gives them special properties. Semi-rigid cables are used in microwave frequency applications and perform at the ultra-highest frequency ranges. The cross-section of coaxial cables is unique compared to traditional wiring, and this physical construction allows them to operate where a conventional single conductor hook-up wire simply will not function.
While coaxial cables can be used to transmit either analog or digital signals, they are primarily used in RF applications. Much simpler than the name implies, coaxial cables are just high-performance two-conductor wires with the center conductor acting as the first leg, and the outer shield acting as the second leg of the wire. In between the center conductor and outer shield is where the magic happens- literally. The dielectric material sandwiched between the two legs is where the RF signal lives.
Special Characteristics of RF Cables
When specifying the requirements of an RF cable, there are many initial considerations to review before starting the process. First, the operating frequency of the signal needs to be confirmed to determine what type of coaxial cable and connector are required. If the frequency is below 3kHZ, an RF cable is unnecessary, and a lower-cost standard communication cable or hook-up wiring harness could be used instead.
Next, several performance characteristics need to be confirmed. These cable characteristics include the impedance, which is a property of the bulk coaxial cable and typically 50Ω or 75Ω.
Also known as insertion loss, the cable's attenuation is the dB reduction across the length of the cable. Higher-performing cables tend to have a lower loss and thus improved signal integrity.
Power in an RF cable is described as watts at a given frequency. Usually referred to as Watts at MHz, the maximum input power rating of an RF cable is important because too much power can compromise the signal or damage the system. High-power cables can be susceptible to overheating, arcing, or otherwise damaging to components.
Coaxial cable with push-on connector.
Some say this next cable property phonetically- here in New England people pronounce it as vizwaa. But the VSWR (Voltage Standing Wave Ratio) is another important characteristic of an RF cable that helps define how stable the cable’s impedance is along its length.
Primary Components of a Coaxial Cable
Coaxial cables have 4 primary components: the center conductor, the dielectric, the braid/foil, and the outer jacket.
- Center Conductor: A single conductive copper wire that is either solid or stranded residing in the center of the cable. It is this item for which the coaxial cable is named.
- Dielectric: A layer of material with special electrical properties that helps the RF signal propagate. Common materials include FEP (Fluorethylenepropylene), PTFE (Polytetrafluorethylene), and ETFE (Ethylene tetrafluoroethylene). This layer also provides mechanical stability.
- Braid & Foil Wraps: Braided copper wire or aluminum/copper foil is used as shielding from radio frequency interference (RFI). Foil shields can offer 100% coverage but have some reliability concerns over time. Braided copper shields offer strength and flexibility but with a lower coverage area. Specialty coaxial cables may contain both braided and foil shields.
- Outer Jacket: The jacket is made with UV-stable or flame-retardant materials to encapsulate the conductors, dielectric, and braid/foil to prevent ingress of water, dust, and other contaminants. It protects the inner components of the cable.
- Wraps: Some outer jackets are made from an extruded thermoplastic where others are formed from several layers of wrapped tapes. These wrapped jackets can be just as rugged as an extruded thermoplastic, but without the complexities of building with pelletized raw materials.
Common Connector Types for RF Connectors
Standard Threaded Connectors
Examples of standard threaded connectors include SMA, SMC, TNC, Type N, and UHF.
Threaded connectors are among the most common RF connector types and provide a robust means of attachment. These types of connectors offer stability, especially when used in environments that have high vibrations. The main disadvantage of threaded connectors is the possibility of over-torquing the connector and causing damage. They are also slow and can take lengthy times for installation and removal by requiring each connector to be turned several times.
SMA connectors are among the most common.
Bayonet Style Connectors
Examples of bayonet-style connectors include BNC, BNT, SHV, MHV, and Type C. Bayonet-style connectors offer quick attachment and removal due to the circular locking rings. This type doesn't require any tools to connect, and the bayonet feature provides a secure mate with only one-third of a turn. This connector style has pins resembling bayonets that interface with corresponding ramp-shaped features on the opposite connector. Disadvantages of this connector include poor performance in high-vibration environments and reduced transmission performance due to the susceptibility to rocking and intermittent connections.
BNC connector with a custom overmolded strain relief.
Spring-Loaded High-Density Connectors
Examples of spring-loaded high-density connectors include 1.0/2.3 and 1.6/5.6 connectors. Spring-loaded high-density connectors have an internal spring that offers self-locking capabilities. These connector types represent a quick installation option for RF coaxial cables especially where space is limited between equipment, walls, and other structures. These connectors can be expensive with limited options and availability.
Micro Snap-On Connectors
Examples of micro snap-on connectors include MMCX, MCX, and SMB. Micro Snap-On Connectors have a straight push/pull feature without mated ends. They come in high-density variations and are ideal for small spaces that require frequent access as some are rated for a high number of mating cycles. The connectors offer a stable and quick disconnect method. However, the snap connection requires a large amount of force to mate and un-mate.
Highly specialized micro snap-on connectors include MMCX, MCX, and SMB types.
What is an RG Coaxial Wire?
Radio Guide (RG) coaxial wires are among the most common wire types. RG coaxial cables were originally introduced to comply with military specifications dating back to World War II. RG cables have numbers to designate the cable's diameter and are usually a two- or three-digit coding. Higher RG numbers will have smaller central conductors. The RG numbers correspond to an industry standard for the jacket type, shielding type, and dielectric material.
RG coaxial wire is a common wire type used for industrial, commercial, and residential applications. These cables are widely used for satellite and cable transmissions, television, and video communications, and even everyday electronic devices.
Many RG cables are military-grade and can be used indoors or outdoors. They can handle a range of differing temperatures and come with single or double shielding. The most common RG coaxial types include RG188, RG55, RG141, RG174, RG178, RG214, and RG 316.
Summary
Coaxial cables are effectively used in countless applications, with cable lengths varying from millimeters to hundreds of meters. Coaxial cable pricing is volatile and varies considerably with simple cables costing dollars and high-performing cables costing thousands of dollars. Much of the cost for an RF cable resides with the connector which leaves few options if a specialty component is needed.
The key to being successful for a new RF cable design is to identify the critical performance characteristics needed in a cable and make this information abundantly clear to potential vendors. Working with a full-service manufacturer like Epec can help address these concerns early in the project.
