CHAPTER-I MICROWAVE TRANSMISSION LINES The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation emitted or absolved. The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom as shown in Fig 1.1. The limit for long wavelengths is beyond one‟s imagination. One theory existing depicts that the short wavelength limit is in the vicinity of the Planck length. A few scientists do believe that the spectrum is infinite and continuous. Microwaves region forms a small part of the entire electromagnetic spectrum as shown in Fig 1.1. Microwaves are electromagnetic waves generally in the frequency range of 1 G Hz to 300 GHz. However with the advent of technology usage of higher end of the frequencies became possible and now the range is extended almost to 1000 G Hz. Fig:1.1: Electromagnetic Spectrum 1 Brief history of Microwaves • Modern electromagnetic theory was formulated in 1873 by James Clerk Maxwell, a German scientist solely from mathematical considerations. • Maxwell‟s formulation was cast in its modern form by Oliver Heaviside, during the period 1885 to 1887. • Heinrich Hertz, a German professor of physics carried out a set of experiments during 1887-1891 that completely validated Maxwell‟s theory of electromagnetic waves. • It was only in the 1940‟s (World War II) that microwave theory received substantial interest that led to radar development. • Communication systems using microwave technology began to develop soon after the birth of radar. Advantages and disadvantages of Microwaves compared to VHF Advantages Disadvantages 1. Large bandwidth: The bandwidth 1. Higher Radiating losses in available is proportional to operating transmission lines and connecting frequency. ∆푓 = 푄. 푓 wires Q = Quality factor 2. The dimensions of the antenna gets 2. Transit time effects make conventional minimized to a great extent for a given devices unusable at microwave directive gain. frequencies. 3. Satellite communications was possible 3. Lumped elements such as Resistors, due to usage of microwaves as Capacitors, and Inductors cannot be antenna size became practicable. used. 4. Fading effect is less compared to lower 4. Inter electrode capacitances, lead frequencies. inductors cause severe problems in circuit design 5. As the wavelength is smaller, the attenuation during adverse weather conditions is higher. Table 1.1: Advantages and disadvantages of Microwaves 2 MICROWAVE FREQUENCY BANDS Microwave Band Frequency range L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Table 1.2: Microwave bands Applications of Microwaves Microwaves have a broad range of applications in modern technology. They are mostly used in long distance communication systems, radar, radio astronomy, navigation, medical applications etc. 1. Tele Communications (a) Satellite communications (b) Mobile communications (c) Wireless Communications (d) Telemetry links 2. Radars (a) Surveillance Radars (b) Tracking radars (c) Weather radars (d) Terrain mapping radars (e) ATC radars (f) Police radars (g) Sports radars (h) Motion detectors 3. Commercial and Industrial applications 3 (a) Microwave oven (b) Drying machines(Textile, food, paper, etc.,) (c) Rubber industry, plastics, chemicals etc., (d) Non-destructive Testing (e) Sterilization of instruments (f) Collision avoidance systems (g) Proximity sensors 4. Medical applications (a) Physio-therapy (b) Diagnostics 5. Microwave communication systems handle a large fraction of the world‟s international and other long haul telephone, data and television transmissions. Most of the currently developing wireless telecommunications systems, such as Direct To Home(DTH) television, Personal communication systems (PCSs), Wireless local area networks (WLANS), Cellular video (CV) systems, Global positioning satellite (GPS) systems rely heavily on microwave technology. Transmission lines at Microwave Frequencies There are generally three types of transmission lines used at microwave frequencies. 1. Coaxial Cables 2. Wave guides 3. Strip lines and micro strip lines Coaxial Cables Coax cable, coaxial feeder is normally seen as a thick electrical cable. The cable is made from a number of different elements that when together enable the coaxial cable to carry the radio frequency signals with a low level of loss from one location to another. The main elements within a coaxial cable are: 1. Centre conductor 2. Insulating dielectric 3. Outer conductor 4. Outer protecting jacket or sheath The overall construction of the coaxial cable can be seen in the Fig 2.1 below and from this it can be seen that it is made up of a number of concentric layers. Although there are many varieties of coaxial cable, the basic overall construction remains the same. 4 Fig 1.2: Cross section though coaxial cable 1. Centre conductor The centre conductor of the coax is universally made of copper. Sometimes it may be a single conductor whilst in other RF cables it may consist of several strands. 2. Insulating dielectric Between the two conductors of the coaxial cable there is an insulating dielectric. This holds the two conductors apart and in an ideal world would not introduce any loss, although it is one of the chief causes of loss in reality. This coax cable dielectric may be solid or as in the case of many low loss cables it may be semi-air spaced because it is the dielectric that introduces most of the loss. This may be in the form of long "tubes" in the dielectric, or a "foam" construction where air forms a major part of the material. 3. Outer conductor The outer conductor of the RF cable is normally made from a copper braid. This enables the coax cable to be flexible which would not be the case if the outer conductor was solid, although in some varieties made for particular applications it is. To improve the screening double or even triple screened coax cables are sometimes used. Normally this is accomplished by placing one braid directly over another although in some instances a copper foil or tape outer may be used. By using additional layers of screening, the levels of stray pick-up and radiation are considerably reduced. The loss is marginally lower. 4. Outer protecting jacket or sheath Finally there is a final cover or outer sheath to the coaxial cable. This serves no electrical function, but can prevent earth loops forming. It also gives a vital protection needed to prevent dirt and moisture attacking the cable, and prevent the coax cable from being damaged by other mechanical means. How RF coax cable works A coaxial cable carries current in both the inner and the outer conductors. These current are equal and opposite and as a result all the fields are confined within the cable and it neither radiates nor picks up signals. This means that the cable operates by propagating an electromagnetic wave inside the cable. As there are no fields outside the coax cable it is not affected by nearby objects. Coaxial cable attenuation The power loss caused by a coax cable is referred to as attenuation. It is defined in terms of decibels per unit length, and at a given frequency. Obviously the longer the 5 coaxial cable, the greater is the loss, but it is also found that the loss is frequency dependent, broadly increases with frequency. For virtually all applications the attenuation or loss is to be minimized. The losses in coaxial cables can be classified into: (a) Resistive loss (b) Dielectric loss © Radiated loss Of all these forms of loss, the radiated loss is generally the least important as only a very small amount of power is generally radiated from the cable. Accordingly most of the focus on reducing loss is placed onto the conductive and dielectric losses. Resistive loss: Resistive losses within the coax cable arise from the resistance of the conductors and the current flowing in the conductors results in heat being dissipated. The actual area through which the current flows in the conductor is limited by the skin effect, which becomes progressively more apparent as the frequency rises. To help overcome this multi-stranded conductors are often used. To reduce the level of loss due in the coax cable, the conductive area must be increased and this results in low loss coax cables being made larger. However it is found that the resistive losses increase as the square root of the frequency. Dielectric loss: The dielectric loss represents another of the major losses arising in most coax cables. Again the power lost as dielectric loss is dissipated as heat. It is found that the dielectric loss is independent of the size of the RF cable, but it does increase linearly with frequency. This means that resistive losses normally dominate at lower frequencies. However as resistive losses increase as the square root of frequency, and dielectric losses increase linearly, the dielectric losses dominate at higher frequencies. Radiated loss: The radiated loss of a coax cable is normally much less than the resistive and dielectric losses. However some very cheap coax cables may have a very poor outer braid and in these cases it may represent a noticeable element of the loss. Power radiated, or picked up by a coax cable is more of a problem in terms of interference. Signal radiated by the coax cable may result in high signal levels being present where they are not wanted.