Presentation on theme: "ENE 428 Microwave Engineering"— Presentation transcript:
1 ENE 428 Microwave Engineering Lecture 6 Transmission lines problems and microstrip linesRS
2 Review Input impedance for finite length line Quarter wavelength line Half wavelength lineSmith chartA graphical tool to solve transmission line problemsUse for measuring reflection coefficient, VSWR, input impedance, load impedance, the locations of Vmax and Vmin
3 Ex1 ZL = 25+j50 , given Z0 = 50 and the line length is 60 cm, the wavelength is 2 m, find Zin. Ex2 A 0.334 long TL with Z0 = 50 is terminated in a load ZL = 100-j100 . Use the Smith chart to find a) L b) VSWR c) Zin d) the distance from load to the first voltage minimum
4 Ex3 ZL = 80-j100 is located at z = 0 on a lossless 50 line, given the signal wavelength = 2 m, find a) If the line is 0.8 m in length, find Zin.b) VSWRc) What is the distance from load to the nearest voltagemaximumd) what is the distance from the input to the nearest point atwhich the remainder of the line could be replaced by a pureresistance?
5 Ex4 A 0.269- long lossless line with Z0 = 50 is terminated in a load ZL = 60+j40 . Use the Smith chart to finda) Lb) VSWRc) Zind) the distance from the load to the first voltage maximum
6 Impedance matching To minimize power reflection from load Zin = Z0 Matching techniques1. Quarter - wave transformers for real load2. single - stub tuners3. lumped – element tunersThe capability of tuning is desired by having variable reactive elements or stub length.
7 Simple matching by adding reactive elements (1) EX5, a load 10-j25 is terminated in a 50 line. In order for 100% of power to reach a load, ZLoad must match with Z0, that means ZLoad = Z0 = 50 .Distance d WTG = ( ) = to point 1+ j2.3. Therefore cut TL and insert a reactive element that has a normalized reactance of -j2.3.The normalized input impedance becomes1+ j2.3 - j2.3 = 1which corresponds to the center or the Smith chart.
8 Simple matching by adding reactive elements (2) The value of capacitance can be evaluated by known frequency, for example, 1 GHz is given.
9 Single stub tunersWorking with admittance (Y) since it is more convenientto add shunt elements than series elementsStub tuning is the method to add purely reactive elementsWhere is the location of y on Smith chart?We can easily find the admittance on the Smith chart bymoving 180 from the location of z.Ex6 let z = 2+j2, what is the admittance?
10 Stub tuners on Y-chart (Admittance chart) (1) There are two types of stub tunersShorted end, y = (the rightmost of the Y chart)opened end, y = 0 (the leftmost of the Y chart)Short-circuited shunt stubOpen-circuited shunt stub
11 Stub tuners on Y-chart (Admittance chart) (2) ProcedureLocate zL and then yL. From yL, move clockwise to1 jb circle, at which point the admittance yd = 1 jb.On the WTG scale, this represents length d.2. For a short-circuited shunt stub, locate the short end at0.250 then move to jb, the length of stub is thenl and then yl = jb.3. For an open-circuit shunt stub, locate the open end at 0,then move to jb.4. Total normalized admittance ytot = yd+yl = 1.
13 Microstrip (1)The most popular transmission line since it can be fabricated using printed circuit techniques and it is convenient to connect lumped elements and transistor devices.By definition, it is a transmission line that consists of a strip conductor and a grounded plane separated by a dielectric medium
14 Microstrip (2)The EM field is not contained entirely in dielectric so it is not pure TEM mode but a quasi-TEM mode that is valid at lower microwave frequency.The effective relative dielectric constant of the microstrip is related to the relative dielectric constant r of the dielectric and also takes into account the effect of the external EM field.Typical electric field linesField lines where the air anddielectric have been replaced bya medium of effective relativepermittivity, eff
16 Evaluation of the microstrip configuration (1) Consider t/h < and assume no dependence of frequency, the ratio of w/h and r are known, we can calculate Z0 as
17 Evaluation of the microstrip configuration (2) Assume t is negligible, if Z0 and r are known, the ratio w/h can be calculated asThe value of r and the dielectric thickness (h) determines thewidth (w) of the microstrip for a given Z0.
18 Characteristic impedance of the microstrip line versus w/h
19 Normalized wavelength of the microstrip line versus w/h
20 Ex8 A microstrip material with r = 10 and h = 1 Ex8 A microstrip material with r = 10 and h = mm is used to build a TL. Determine the width for the microstrip TL to have a Z0 = 50 . Also determine the wavelength and the effective relative dielectric constant of the microstrip line.
21 Wavelength in the microstrip line Assume t/h 0.005,
22 Attenuation conductor loss dielectric loss radiation loss where c = conductor attenuation (Np/m)d = dielectric attenuation (Np/m
23 Conductor attenuation If the conductor is thin, then the more accurate skin resistance can be shown as