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Co-axial Cables - An Introduction

Coaxial cables - An introduction


  1. Brief description of RF cable

    a. What is an RF cable
    b. Characteristics of an RF cable

  2. Coaxial cable construction

  3. Types of coaxial cables

    a. Braided Flexible cable
    b. Semi-rigid
    c. Flat ribbon

  4. Applications of RF coaxial cable

   1. Brief description of RF cable

Anyone associated with electronics or as an user of electronic gadgets and appliances will be aware with different forms of cable, one comes across in our day-to-day life: from a simple wire carrying electrical power (such as a mobile phone charger), to the sophisticated ones found in submarines and satellite, to the ones seen connecting the antenna on a cellular base station tower (a familiar sight in the metros these days!) with the indoor equipment. However, the aim of this article is to cover different aspects of, a special category of cable, called the RF cable, used in Microwave and Millimeter wave engineering applications. We begin with defining a simple cable, and then proceed on, to discuss in detail about microwave/RF cable.

a. What is an RF Cable?

In simple term, it is notional, to think about the cable as an entity, which acts as a conduit for transporting energy from point A to point B. The energy being transmitted could be electrical in nature, such as, a DC power being supplied to different sections of an electronic circuit or an alternating one, such as AC power supplied from mains to an electronic system. The energy transported through a cable, could be viewed as a signal propagating through it, such as an audio cable, a video cable or a LAN data cable. In each of the above types, the stress is on the kind of signal it is transporting: data, audio or video. If this cable transports a signal, which is a very high oscillation-alternating signal, we enter the realm of microwave and RF, and we need special cable (as we will see later) to transport such a signal. The cables used for transporting such signals are broadly known as rf cables.

b. Characteristics of an RF Cable

Microwaves range from about 1 GHz to 1000 GHz corresponding to wavelengths from 30 cm down to 0.3 mm. It is characterized by the short wavelengths involved, which in turn mean that the propagation time for electrical effects from one point in a circuit to another point is comparable with the period of the oscillating currents and charges in the system. As a result, conventional low frequency circuit analysis such as Kirchhoff's laws and voltage-current concepts do not describe the electrical phenomena adequately. Instead, an analysis is done on the basis of description of the electric and magnetic fields associated with electromagnetic propagation of the signal.

One of the essential requirements of a cable is the ability to transfer signal power from one point to another without radiation loss. This requires the transport of EM energy in the form of a propagating wave, by a guiding structure referred to as the transmission line. There are various transmission lines, such as the open two-conductor line, coaxial line, and shielded strip line. At higher microwave frequencies (wavelengths < 10 cm), hollow-pipe waveguide is used because of better electrical and mechanical properties. Before we go ahead with coaxial cable properties, we explain different modes that exist in the propagation of electromagmetic waves.

Modes of Propagation of EM waves

Propagation of EM waves may be classified into different types:

  • TE modes (Transverse Electric) have no electric field in the direction of propagation.
  • TM modes (Transverse Magnetic) have no magnetic field in the direction of propagation.
  • TEM modes (Transverse ElectroMagnetic) have neither electric nor magnetic field in the direction of propagation.
  • Hybrid modes have nonzero electric and magnetic fields in the direction of propagation.

Tubular waveguides such as rectangular/circular waveguide support TE, TM, and Hybrid modes, whereas coaxial cables support TEM mode of propagation of signals.

Two main characteristics in a transmission line or a waveguide are single-mode propagation over a wide band of frequencies and small attenuation. Most of the transmission lines fall into one of the following three categories:
1. Transmission lines on which the dominant mode of propagation is transverse electromagnetic wave (TEM). An example of this type transmission line is coaxial cable.
2. Closed cylindrical conducting pipe, an example for this type of transmission line is a rectangular or circular wave-guide. The propagation is primarily using TE or TM, but not TEM.
3. Open-boundary structures that support surface wave mode of propagation. An example of this propagation is our FM radio signals that travel through the open air.
In this article we will restrict ourselves to coaxial and rectangular waveguide structures. Transmission lines consist of two or more parallel conductors and will guide a TEM wave. The commonest form of a transmission line, used at rf frequencies, is the coaxial transmission line or coaxial cable.

  2. Coaxial cable construction

Conceptually, a coaxial cable consists of circular inner and outer conductor separated by an insulator (a dielectric medium, can be air or plastic material like PTFE) and sharing the same geometrical axis. A cross sectional view of a coaxial transmission line is shown below.

Co-axial Transmission Line Model

In reality, the coaxial cable is an electrical cable with an inner metallic conductor surrounded by a flexible and tubular insulating layer, which in turn is surrounded by the outer metallic conductor shield. Oliver Heaviside invented coaxial cable in 1880. The coaxial structure is ideally suited to transport microwave signal because of the confinement of the propagating electric and magnetic fields (associated with the microwave energy) within the enclosed space between the inner conductor and the outer shield. Unlike other transmission line structure, the coaxial structure does not allow any electromagnetic energy to escape or radiate outside and also serves the purpose of protecting the transmission line from outside environment and interference from other sources. The outer conductor or the shield is held at an electrical ground potential to ensure that it does not radiate any microwave energy.

Co-axial Cable Components

A relevant question to ask at this point is: can an ordinary wire be used to transport microwave energy? The answer is no, since an ordinary wire acts as a radiating element like an antenna leading to power loss.

The propagation of microwave signal through a coaxial cable assumes that the dimensions of the inner conductor, the outer conductor and the space in between are held uniform along its length. If this is not so, any abrupt change in its dimension causes a discontinuity at that point, which alters the characteristic impedance of the transmission line leading to signal reflections towards the signal source and lossy propagation of microwave energy.

Cross-Sectional view of co-axial cable

Fig. Construction detail

The manufacturer of the cable ensures that the dimension remains uniform by embedding the circular inner conductor in a flexible yet semi-rigid dielectric material like Polytetrafluroethylene (PTFE), which can be easily bend and handled during its use. This avoids abrupt kinks or discontinuities, which are the primary cause of reflections of microwave energy. In fact, a good quality cable is measured in terms of its construction, uniformity of its dimension, materials used in construction and the electrical parameters like return loss, insertion loss and radiation leakages.

As mentioned earlier, a single TEM mode of propagation is a desirable property of a coaxial cable system. What happens is that above the cutoff frequency or the highest frequency of operation that the coaxial cable supports, other modes such as TE and TM tend to be generated (as in the case of wave guide). These additional modes in turn interfere with the single dominant mode (TEM) leading to undesirable results, as far the microwave signal transmission is concerned. It is to be noted that the highest frequency or the cutoff frequency is determined by the dimension of the outer conductor; smaller the outer diameter, higher the cutoff and vice versa. Compared to an air dielectric coaxial system, the presence of a solid insulator dielectric material, like the PTFE, lowers the cutoff frequency and adds to the insertion loss. For most practical purposes, a coaxial cable is constructed using a solid dielectric material between the inner and the outer conductor, trading-off insertion loss and cutoff frequency with the stability and strength it offers to the anchoring of the outer conductor on the inner conductor and maintaining uniform dimension along the length of the cable. The outer conductor is shielded from EMI (electromagnetic interference) by a sheath of outer cover called the jacket. The jacket is usually made of Polyvinyl chloride (PVC) material and comes in different colours such as black, gray, white and tan.

The entire effort when selecting a coaxial cable assembly goes in ensuring that the cable exhibits a single TEM mode of propagation below the cutoff frequency (specified by the outer conductor dimension), maintaining flexibility along the cable length, low insertion loss, VSWR close to one, maintaining uniform inner conductor dimension (complying to one of the industry standard dimension. When it comes to designing and manufacturing coax for military and space related applications, there are other stringent specifications, such harsh environmental compliance, easy installation and quick field deployability, thermal management etc.

  • Popular dielectric materials used for cable fabrication are:

    FPE is foamed polyethylene                                                PEAS polyethylene air spaced
    PF is polyethylene foam                                                       FEP Fluorinated ethylene propylene
    PEF polyethylene foam                                                         LDPE low density polyethylene
    PTFE is Polytetrafluroethylene                                            PE is propylene
    ASP is air space polyethylene                                             PESS polyethylene semi solid

  3. Types of RF Coaxial Cables

Typical coaxial cable constructions are classified as braided, semi-rigid or flat ribbon and described briefly below.

a. Braided Flexible cable

Flexible (Braided) Coaxial Cable is the most common form of coax cable mainly used for its flexibility. It consists of entwined or 'braided' layer of fine wire acting as the outer conductor; hence the name "braided coaxial cable".

Braided cable


Fig Braided Cable

The braided outer conductor acts a good shield to the center conductor; its effectiveness being measured by the number of strands weaved in the braid and the number of braid layers used in the construction. The shielding offered by the braid is enhanced if an extra aluminum foil is added over the braid, particularly for short wavelength and high frequency applications, where high degree of radiation shielding is of prime importance. Replacing braided cable with a semi-rigid cable in such applications where a high degree of shielding is required mitigates this problem.

b. Semi-rigid

Semi rigid Coaxial Cable uses a solid tubular outer conductor usually made of copper, so that all the RF energy is contained within the cable. It is also a preferred cabling option for fixed connections such as inter-subsystem connections inside a microwave system.

Semi-rigid Cable

The semi-rigid cable connections are expected to remain fixed during the normal operation of the microwave system and not disturbed frequently. Special bending tools are available for bending or giving shape to the semi-rigid cable. Conformable semi-rigid cable is in between flexible and semi-rigid cable in terms of their rigidity and conformability. They are a popular choice of hand formable cable used widely in testing laboratories and systems.

c. Flat ribbon coaxial cable

Flat ribbon coaxial cable is special variety of ribbon cable, where multiple coaxial cable runs are grouped side by side and placed together inside an insulating jacket. Each coaxial cable consists of a center conductor separated by dielectric material from the outer conductor or the foil. A drain wire connected to the foil runs along the length of the ribbon coaxial cable and offers an electrical ground connection. Ribbon Coaxial Cable combines the advantages of both ribbon cable and coaxial cable. It is advantageous to use this type of cable when multiple high frequency RF signals are required to be transported to another point in the system, such as a panel mounted termination box. It offers a quick and easy installation option for applications requiring simultaneous transportation of microwave signal, radiation shielding and cross talk shielding from each other. The ribbon coaxial cable can be mass terminated with the insulation displacement technique.

  4. Applications of RF coaxial cable

Coaxial cable is widely used in applications such as the following:

  • Connecting the radio transmitters and receivers to the antenna in a telecommunication system,
  • Distribution of CATV, HDTV, and CCTV signals, 
  • Coaxial Ethernet networking (Computer networking), 
  • Remote monitoring, live broadcast of a sport event from the stadium to a studio,
  • Laboratories to test different microwave systems using test and measurement equipment,
  • Aerospace applications, where the cables are used on-board the aircraft, and
  • On-board satellite (where space grade, radiation hardened cables are used), and several others.

Key Words : RF cable ,  co-axial cable construction ,  Braided cable ,  Semi-rigid cable ,  Flat ribbon co-axial cable ,  RF coaxial cable

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