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Coaxial RF Cables Classification Guide

Coaxial RF Cables Classification Guide

Contents

1. Introduction
2. Classification of cables based on characteristic impedance
    2.1 Co-axial cable - 50 ohms
    2.2 Co-axial cable - 60 ohms
    2.3 Co-axial cable - 75 ohms
    2.4 Co-axial cable - 93 ohms
3. Classification of cables based on unique design feature
    3.1 Hard line cable
    3.2 Radiating Co-axial cable
    3.3 Tri-axial cable
    3.4 Twin-axial cable
    3.5 Biaxial cable
    3.6 Ladder line
4. Annexure

 

1. Introduction

This article aims to classify a coaxial cable into two parts, namely

  • Based on the characteristic impedance
  • Based on a unique design feature

2. Classification of cables based on characteristic impedance

Most commonly used coaxial cables have characteristic impedance Z0 of either 50 ohms or 75 ohms, regarded as the standard impedances. However, for some applications we do require coaxial cable with different characteristic impedance.

The following section discusses the origin of 50 ohms and 75 ohms characteristic impedance and some of the non-standard impedances in use today. Readers may be aware, that 50 ohms cable is used in radio transmitter antenna connections, test and measurement equipment and in data communications such as Ethernet. 75 ohms coaxial cable is used for video signal transmission, TV antenna signals and digital audio signals.

How did we arrive at 50 ohms and 75 ohms characteristic impedances?

The answer to this lies in the fact that a cable designed for different characteristic impedance offer different set of characteristic properties. A coaxial cable when constructed for a specific geometry and material exhibits characteristic electrical properties. A cable designed for given characteristic impedance exhibits benefits and gains in certain areas of its electrical properties. For example, a coaxial cable exhibits maximum power handling capability when designed for optimum characteristic impedance between 30 and 44 ohms. Dielectric filled coaxial cable designed for 77 ohms characteristic impedance exhibit lowest attenuation and a 93-ohm coaxial cable exhibit low distributed capacitance per foot of the cable.

2.1. Coaxial Cable - 50 Ohms

In telecommunication system (such as radio transmitter and a receiver) and test and measurement equipment (such as Network analyzer or a power meter), the most commonly used source and load impedances are 50 ohms. A 50 ohms coaxial cable, when connected between a source and a load, is used to provide excellent impedance match between the two, for example, between a radio transmitter and an antenna. Therefore, microwave and RF subsystems are built with their specifications around 50 ohms, for example power amplifiers are designed for maximum power output and power transfer using impedance matching technique. Apart from this, the test and measurement equipment used for high frequency measurements such as high frequency probe in an oscilloscope use 50 ohms. High frequency digital signals such as ECL and PECL logic signals are transmitted over 50 ohms cable. Coaxial Ethernet network are built with 50 ohms cable. Commonly used 50 ohms cables are RG8 and RG58 cables.

2.2. Coaxial cable - 60 Ohms

Introduced in Europe around 1950s, as a standard 60 ohms cable for radio applications, this has been phased out and replaced either with 50 ohms or 75 ohms, depending on the applications. 60 ohms cable were used in radio transmitter and antenna connection applications. During the same time, some of the radio transmitter equipment was designed for 30 ohms, wherein the output was transmitted over a 30 ohm coaxial cable, mainly for high power application.

2.3. Coaxial cable - 75 ohms

75 ohms-co-axial cable graph 75 ohms characteristic impedance has been made a standard after it was found that dielectric filled coaxial cable exhibit low attenuation figures somewhere around 77 ohms.

The telecommunication industry thus adopted 75 ohms characteristic impedance as a standard since then. 75 ohms coaxial cable is a standard characteristic impedance cable used for long distance network connectivity, video and audio transmission and telecommunications systems (in some cases, baseband and IF stages are designed at 75 ohms characteristic impedance). Generally, all baseband video applications (analog and digital) use 75 ohms cable.

Also, analog RF modulation of video signal such as in CATV and CCTV system use 75 ohms coaxial cable for their distribution networks (feeder, branch and drop cables). Feeder cables are low loss, thicker 75 ohms cable for running long lengths between a headend to a node or a branching site. Drop cables are used to run cable connection between a pole and a subscriber residence or providing in house connections.

Digital audio signals such as S/PDIF and coaxial AES/EBU uses 75 ohms coaxial cable. Satellite radio receivers installed in automobile and residence use 75 ohm coaxial cable In telecom applications, for example some of the primary 2 Mbps E1 links use 75 ohms coaxial cable. Some of the common 75 ohms cables in the market are RG6, RG11 and RG59 cables.

2.4. Coaxial cable - 93 Ohms

93 ohms coaxial cable exhibit low capacitance per foot, an electrical property, which lead to its usage in applications such as connecting the computer monitor to its CPU. Because of the low distributive capacitance, the coaxial cable did not load the electrical circuit and was used in long cable runs in applications such as digital communication network like the IBM 3270 terminal networks and the LAN systems. With technological advancement in cable manufacturing techniques and capabilities, the 93 ohms cable has been phased out from the industry.

3. Classification of cables based on unique design feature

Till now we have discussed about coaxial cables, which are flexible or semi-rigid. As we will see, there are few other types of coaxial cables used in the industry. Radiating cable is another type of coaxial cable and was briefly discussed in the preceding sections. Let us examine these different types of coaxial cable.

3.1. Hard line cable

16 mm Hard line cable
16mm hard line cable Hard line, as the name suggests, is a type of coaxial cable, which has special design features, and are used for specific applications. A hard line consists of a solid outer conductor, in the form of a tube, and constructed out of metal such as copper, silver or gold. This forms the shield for the cable, and in some specific case, may be slipped inside an outer jacket made of PVC. In some cases, the outer shield is made of aluminium, which is a lower quality version of the hard line. Because of the oxidation of aluminium metal as compared to copper, silver or gold, the effective conductivity of the outer conductor is lowered at the cost of the electrical performance. Hence, extra effort is put by the cable manufacturer to make the connection air and watertight.
   

The center conductor is made of solid copper or copper plated aluminium. Copper plating of the center conductor addresses the skin effect phenomenon at microwave frequencies, by offering extra surface area for an effective center conductor. Hard line cables are generally chassis mounted in an outdoor environment where it is exposed to climatic variations involving temperature, humidity, wind and rain. In such cases, hard lines are covered in PVC jacket. Hard lines are ideally suited to handle high power microwave signal, offering low attenuation, such as when connecting a ground based radio transmitter with an antenna mounted high on a tower. Hard lines are thick cables with outer diameters, 0.5 inch onwards. In earlier section, we have discussed features of a hard line cable, Heliax, from Commscope/Andrew in more detail. Cablewave (RFS/Cablewave) in another such hard line cable used in the industry.

In larger hard line the center conductor is constructed from either rigid or corrugated copper tube and dielectric material chosen from polyethylene foam or pressurized gas such as nitrogen or dry air. In gas filled hard line, nylon spacers are used to maintain spacing between the inner and outer conductor. The purpose of gas filling is to reduce moisture and environmental contamination inside the dielectric space, which leads to a stable dielectric constant. The gas also reduces the risk of internal arcing. Gas filled Hard lines are used in high power radio transmitters such as earth station satellite transmitters, TV broadcasting and military transmitters. However, in higher microwave region, rectangular waveguide is used to connect a radio transceiver to an antenna because of its electrical (power handling, impedance control, single mode etc) and precision mechanical properties. The shielding material used in hard line cable construction varies from solid rigid tube to corrugated tube (for flexibility in routing and avoidance of discontinuities in the form of a kink when bend). In indoor applications, hard line cable is used in high frequency applications such as microwave equipment where shielding is required between different stages or various sub systems.

3.2. Radiating coaxial cable

The objective of radiating or leaky cable is to intentionally radiate microwave signal in a controlled manner in areas where antenna installation or provision is not feasible; for example in underground operations such as mining, exploration; elevator shafts, and transportation tunnels. In construction, a radiating cable, is similar to a hard line coaxial cable except that it has tuned slots cut inside the shield, which act as radiating antenna elements. These slots are designed for a specific to band of RF frequency and the dimension of the slots are determined by tuning it to RF signal wavelength. The slots on the cable offer a bi-directional desired propagation or leakage of RF signal to a RF transceiver. As mentioned in an earlier section, Radiax, manufactured by Commscope/Andrew is an example of a radiating cable.

3.3. Triaxial cable or Triax

A Triaxial cable, as the name suggests, is a three-conductor cable. Apart from an inner and an outer conductor, a third conductor is added as a protective shield. Since, all the three conductors share a common axis, it is named as triaxial.

Tri-axial Braid Shield

Similar to a coaxial cable, the triaxial cable has center pin followed by dielectric insulation separating the outer shield. The third shield is added as a sheath and is connected to the electrical ground. The third sheath protects the inner shield from EMI. The triax shield configuration allows for separate termination of the external braid and this technique provides a higher degree of isolation between signal carrying ground plain and external noise grounding. It provides greater bandwidth and rejection of interference than coax. In TV industry, triax cable is used to connect a camera to its controlling stations. The outer sheath is used as an electrical ground. When in operation, the center conductor acts as a communication channel between the camera and its control unit, by carrying frequency multiplexed bi-directional audio and video signals, DC power for the camera and the control information signals for the camera. The inner shield provides a DC return path. It is ideally suited to connections between an out door TV broadcast van and the camera at venues such as a sporting arena. Another application is in precision low-current measurements.

Comparison of Triaxial cable with Coaxial cable - note the extra shield in Triax cable Coaxial Cable Triaxial Cable

Coaxial Cable Triaxial Cable
Co-axial cable Tri-axial cable

3.4. Twin-axial cable or Twinax

Twin axial cable is a form of balanced transmission line wherein a balanced and twisted pair of coaxial line is placed inside a cylindrical outer shield. Many applications, such as audio signal transmission use a pair of balanced coaxial line, which allows differential signal propagation in a shielded environment. Advantages of using twin axial cable are EMI, crosstalk and noise pick up reduction because of shielding and differential signalling due to balanced configuration. It is used in short-range high-speed differential signalling applications.

Twinax Key features:  
  • A balanced line configuration 
  • Cylindrical outer shield 
  • EMI, crosstalk shielded 
  • 78 Ω as per MIL-STD-1553 
  • Used in high-speed Ethernet applications. 
A type of cable similar to coax, but with two inner conductors instead of one. It was used in IBM midrange (AS/400, System/3x) communications environments.

Twinax cable has been used in legacy computer systems such as IBM5250 for connecting host to the terminals and printers. It uses a half duplex method of data transmission at 1 Mbps on a single shielded 110-ohm twisted pair of twinax cable. Another such legacy system NEC Astra uses twinax for computer network. Twinax cable designed and manufactured as per MIL-STD-1553 specification has a characteristic impedance of 78 ohms and is an industry standard. As per the standard, the twinax cable has a characteristic impedance of 78 ohms at 1 MHz and is used to connect bus and stub devices. Other applications include 10 Gigabit Ethernet implementation (such as Cisco System) and DisplayPort cable using twinax cable assembly. 

3.5. Biaxial cable or Biax

Biaxial cable or biax is commonly referred to as Twin-Lead configuration of two 50 ohms coaxial cables and used in computer networks. In the past, 75 ohms biax was used in CATV application. A twin-lead or biax is a two-conductor ribbon cable transmission line for RF signals. 

300 ohm Twin-lead 300-to-75-ohm balun, showing twin-lead on the right hand side
300 ohm twin lead 300-to-75 ohm balun

Twin-lead is consists of two multistranded copper wires (or copper clad steel), separated by a plastic (usually polyethylene) ribbon. The precise spacing between the two is held uniform, to function as a parallel transmission line. If the spacing between them is not held constant, then any abrupt change in it results in RF power reflection towards the source. The plastic cover also acts as a cover and insulates the wires. In a 300-ohm twin lead cable, the wire strand is either 20 or 22 AWG and spacing dimension is 7.5 mm (0.3 in). Parallel transmission line such as the twin-lead is used to connect the radio transmitter and receiver to the antenna. It exhibits less attenuation loss compared to a coaxial cable by an order of magnitude. Its main drawback is that, it is prone to external interference owing to lesser shielding compared to coaxial cable. It is generally shielded from metal objects, poles and antenna masts when installed in outdoor applications, using standoff insulators. An expression for characteristic impedance of a twin-lead is given in Annexure A. Twin-lead cables are available in different sizes and characteristic impedance options such as 75, 300, 450 and 600 ohms. In the past, 300-ohm twin-lead were used in connecting the TV sets and FM radios to their receiving antennas. Since then, they have been replaced with 75-ohm coaxial cables. Twin-lead cable is also used in amateur radio stations for balance transmission of RF signals. Some of the typical electrical properties of a twin-lead cable is given in Annexure B.

3.6. Ladder Line

450-Ohm Ladder line

  450-Ohm Ladder line installation

450 ohm ladder line 450 ohm ladder line installation

Ladder line or window line is a type of transmission line similar to twin-lead for balanced connection to antennas. Ladder line has a different design and is constructed using a support mechanism consisting of plastic webbing separating a pair of wire evenly and holding them apart. The structure also has cutouts or windows next to the webbing spacer, which results in reduction of attenuation in the cable. The alternate occurrence of a window and webbing spacer gives it an appearance of a ladder and hence the name.

Transmission line impedance matching technique is used to match a twin-lead cable. For example when connecting a 300-ohm twin-lead cable to a 75-ohm coaxial source or a load (such as an antenna port), a 2:1 impedance transformer called a balun is used. The impedance transformation from 300-ohms to 75-ohms by the balun also results in the transition from balanced symmetric (twin-lead side) to an unbalanced asymmetric transmission line (coaxial side). When used as a feed line, the ladder line offers better electrical properties compared to a coaxial cable, such as, a better impedance mismatch handling capability and lower insertion loss property.

4. Annexure

Annexure A

The characteristic impedance of a parallel-wire transmission line like the twin lead or ladder line is expressed in terms of its dimensions; the diameter of the wires d and their separation D as shown below

Where Z0 is the free space impedance (approximately 377 Ω), εr is the effective dielectric constant (for air it is 1.00054). If the separation D is much greater than the wire diameter d then the expression is approximated as:

 

An expression of the separation needed to achieve given characteristic impedance is derived as:

Annexure B 

Electrical Characteristics Characteristic Impedance
300 Ω 75 Ω
Capacitance (pF/m) 11.8 20
Propagation speed (% of light) 80 % 71 %
Loss (dB/100m) 100 MHz 3.6 3.6
300 MHz 7.2 7.2
500 MHz 10.2 10.2

A Twin-lead can be connected directly to a suitably designed antenna such as: 

  • A dipole (designed for resonance impedance 73 ohms in free space) 
  • A folded dipole (designed for characteristic impedance of 300-ohms in free space) 
  • A Yagi Uda antenna or similar balanced antenna

Key Words : Co-axial RF cables ,  Co-axial cable-50 ohms ,  Co-axial cable-60 ohms Co-axial cable-75 ohms ,  Co-axial cable-93 ohms ,  hard line cable ,  radiating co-axial cable ,  tri-axial cable ,  twin-axial cable ,  biaxial cable ,  ladder line

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