Transmission Lines as Circuit Elements PDF

Transmission Lines as Circuit Elements Transmission Media and Antenna System (ECE108) Prepared by: Engr. Rochelle M. Sabarillo Resonant Circuits and Reactive Components The standing wave conditions resulting f...

Transmission Lines as Circuit Elements Transmission Media and Antenna System (ECE108) Prepared by: Engr. Rochelle M. Sabarillo Resonant Circuits and Reactive Components The standing wave conditions resulting from open- and short-circuited loads must usually be avoided in working with transmission lines. However, with one-quarter and one-half wavelength transmissions, these open- and short-circuited loads can be used as resonant or reactive circuits. A shorted one-quarter wavelength line A shorted one-half wavelength line Acts as a parallel resonant circuit. acts as a series resonant circuit. If the line length is less than one-quarter wavelength at the operating frequency, the shorted line looks like an inductor to the generator. If the shorted line is between one-quarter and one- half wavelength, it looks like a capacitor to the generator. Department of Electrical Engineering & Technology 2 Resonant Circuits and Reactive Components Similar results are obtained with an open line. To the generator, a one-quarter wavelength line looks like a series resonant circuit and a one-half wavelength line looks like a parallel resonant circuit, just the opposite of a shorted line. If the line is less than one-quarter wavelength, the generator sees a capacitance. If the line is between one-quarter and one-half wavelength, the generator sees an inductance. One-quarter and one-half wavelength open lines look like resonant circuits to a generator. Department of Electrical Engineering & Technology 3 Resonant Circuits and Reactive Components Shown is the summary of the conditions represented by open and shorted lines of lengths up to one wavelength. The horizontal axis is length, in wavelengths, and the vertical axis is the reactance of the line, in ohms, expressed in terms of the line’s characteristic impedance. The solid curves are shorted lines, and the dashed curves are open circuit lines. If the line acts as a series resonant circuit, its impedance is zero. If the line is of such a length that it acts as a parallel resonant circuit, its impedance is near infinity. If the line is some intermediate length, it is reactive. Department of Electrical Engineering & Technology 4 Stripline and Microstrip At low frequencies (below about 300MHz) the lines are just too long to be used as reactive components or as filters and tuned circuits. However, at UHF (300 to 3000 MHz) and microwave (1 GHz and greater) frequencies the length of one-half wavelength is less than 1 ft; the values of inductance and capacitance become so small that it is difficult to realize them physically with standard coils and capacitors. Special transmission lines constructed with copper patterns on a printed circuit board (PCB), called microstrip or stripline, can be used as tuned circuits, filters, phase shifters, reactive components, and impedance-matching circuits at these high frequencies. A PCB is a flat insulating base made of fiber glass or some other insulating base material to which is bonded copper on one or both sides and sometimes in several layers. Teflon or ceramic is used as the base for some PCBs in microwave applications. In microwave ICs, the base is often alumina or even sapphire. The copper is etched away in patterns to form the interconnections for transistors, ICs, resistors, and other components. Thus, point-to-point connections with wire are eliminated. Diodes, transistors, and other components are mounted right on the PCB and connected directly to the formed microstrip or stripline. Department of Electrical Engineering & Technology 5 Microstrip Microstrip is a flat conductor separated by an insulating dielectric from a large conducting ground plane. The microstrip is usually one-quarter or one-half wavelength long. The ground plane is the circuit common. This type of microstrip is equivalent to an unbalanced line. Shorted lines are usually preferred over open lines. Microstrip can also be made in a two-line balanced version. The characteristic impedance of microstrip, as with any transmission line, is dependent on its physical characteristics. It can be calculated by using the formula: Department of Electrical Engineering & Technology 6 Examples of Microstrip Department of Electrical Engineering & Technology 7 Stripline Stripline is a flat conductor sandwiched between two ground planes. It is more difficult to make than microstrip; however, it does not radiate as microstrip does. Radiation produces losses. The length is one-quarter or one-half wavelength at the desired operating frequency, and shorted lines are more commonly used than open lines. The characteristic impedance of stripline is given by the formula: Even tinier microstrip and striplines can be made by using monolithic, thin-film, and hybrid IC techniques. When these are combined with diodes, transistors, and other components, microwave integrated circuits (MICs) are formed. Department of Electrical Engineering & Technology 8 Seatwork: 1. A microstrip transmission line is to be used as a capacitor of 6 pF at 1000 MHz. The PCB dielectric is 3.9. The microstrip dimensions are h = 0.0628 in, w = 0.15 in, and t = 0.002 in. What are (a) the characteristic impedance of the line and (b) the reactance of the capacitor? 2. What is the length of the transmission line in Problem No. 1? Department of Electrical Engineering & Technology 9 Reminder Prepare for oral recitation. Department of Electrical Engineering & Technology 1 0 Thank You for Listening :) Do you have any questions? Email me at [email protected] or you may post a message in our Google Classroom.