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Coaxial cables - An introduction
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.
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.
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:
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
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.
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