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Use of a single stub to provide impedance matching. Generally short circuited stubs are preferred comparable to open circuited stub as open circuited stub radiates some energy at high frequencies. It consists of an open or short circuited section of transmission line of length l t,connected in parallel with the main line at distance l s from the load Z R.Stub has the same characteristic impedance as the main line. (a) Connection of stub in parallel with transmission line (b) Adjustable susceptance can be added in shunt with the transmission line. (a) The length and characteristic impedance of the line remains unchanged. It is possible to connect sections of open or short circuited line called stub in shunt with the main line at some point or ponts to effect impedance matching. When this condition is achieved,it is referred to as impedance matching, and the methods employed to achieve this are termed as impedance matching devices. That is if the source is inductive ,the load should be capacitance and vice-versa. This means that if maximum power transfer has to take place between the source and the load,the resistance of the load should be equal to that of the source and the reactance of the load should be equal to that of the source but opposite in sign. So in communication networks, the elements of network should be designed such that maximum power transfer takes place between the source (or transmitter)and load (or antenna). Therefore ,the maximum power transfer is possible. X=1/3 x=1 x=2.5 x=-1/3 x=-1 x=-2.Thus, there is no reflected wave so the incident power is fully absorbed by the load. all still have the same properties, but the results can be generalized to any line with the same normalized impedances The Smith Chart is a clever tool for analyzing transmission lines The outside of the chart shows location on the line in wavelengths The combination of intersecting circles inside the chart allow us to locate the normalized impedance and then to find the impedance anywhere on the line
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all repeat every half wavelength The magnitude of the reflection coefficient, the standing wave ratio (SWR) do not change, so they characterize the voltage & current patterns on the line If the load impedance is normalized by the characteristic impedance of the line, the voltages, currents, impedances, etc. Smith Chart Impedances, voltages, currents, etc. Assume for a certain terminated line, we have measured the SWR on the line and lmin, the distance from the load to the first voltage minimum on the line.Ģ8 Impedances, voltages, currents, etc. ZL is complex 2 distinct quantities must be measured.
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With this device the SWR and the distance of the first voltage minimum from the load can be measured, from this data ZL can be determined. (b) Standing wave for unknown load.ġ1 The Slotted Line A transmission line allowing the sampling of E field amplitude of a standing wave on a terminated line. Short-circuited line Figure (a) Voltage, (b) current, and (c) impedance (Rin = 0 or ) variation along a short-circuited transmission line.ĩ Open-circuited line Figure (a) Voltage, (b) current, and (c) impedance (Rin = 0 or ) variation along an open-circuited transmission line.ġ0 Figure Voltage standing wave patterns (a) Standing wave for short-circuit load. Parallel (Shunt reactance )Ħ Types of Stubs : Single Stub Double stub Triple stubħ Figure Reflection and transmission at the junction of two transmission lines with different characteristic impedances. It can be connected in two ways (OC & SC) 1. Stub Matching : For high microwave frequencies, a section of a transmission line can be used as matching network at suitable distances. Impedance matching using short circuit stubs. Impedance matching using half wave lines. Impedance matching using quarter wave transformer. Impedance matching using L- C of a transmission line. Impedance matching using L or C of a transmission line. Different types of impedance matching : Reflection less match Conjugate match Zo matchĤ Reflection less match Conjugate match Zo match In order to avoid the problems we need a Matching Network. Signal get reduced due to reduction of signal-to-noise ratio. Frequency stability problem in generator.
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All the above will be affected due to mismatch problems.ģ Effects of Mismatch : Power loss in feeder line. Presentation on theme: "IMPEDANCE MATCHING & SMITH CHART"- Presentation transcript:Ģ Impedance Matching of RF Transmission Lines :īasic requirements needed for a better transmission line High transmission efficiency Very small VSWR Can able to operate over a range of frequencies.