Microwave engineering is a specialized field within electrical and electronics engineering that focuses on the study, design, and application of devices and systems utilizing electromagnetic waves at microwave frequencies

Microwave engineering is a specialized field within electrical and electronics engineering that focuses on the study, design, and application of devices and systems utilizing electromagnetic waves at microwave frequencies. It involves the analysis and design of circuits, components, and systems, finding applications in telecommunications, radar, navigation (like GPS), medical imaging (MRI), remote sensing, and consumer electronics

1. CHAPTER 10: STRIPLINE CIRCUITS

2. 10.1 OVERVIEW A stripline is a planar type transmission line which is well suited for microwave integrated circuitry and photolithographic fabrication. It is usually constructed by etching the center conductor of width w, on a substrate of thickness b/2 and then covering with another grounded substrate of the same thickness. With the voltage applied between the center strip and the pair of ground planes, current flows down the center strip and returns by means of the two ground planes.

3. Although the structure is open at the sides, it is basically a non-radiating TL. The fields are found to decrease quite rapidly away from the center conductor. In practice, However , any unbalance in the line causes energy to be radiated out the sides. To prevent this, The ground planes are shorted to each other with screws.

4. Basic Structure of Stripline Schematic diagrams of Strip Lines.

5. Prior to 1965 nearly all microwave equipment utilized coaxial, waveguide, or parallel strip-line circuits. In recent years-with the introduction of monolithic microwave integrated circuits (MMICs)- microstrip lines and coplanar strip lines have been used extensively, Because they provide one free & accessible surface on which solid-state devices can be placed.

6. 10.2 Substrate Materials Microstrip transmission lines consist of a conductive strip of width "W" and thickness "t" and a wider ground plane, Separated by a dielectric layer (a.k.a. the "substrate") of thickness "H" as shown in the figure below. Variants of microstrip include embedded microstrip and coated microstrip , both of which add some dielectric above the microstrip conductor

7. 10.3 STRIPLINE, COUPLED STRIPLINE, MICROSTRIP, COUPLED MICROSTRIP, COPLANAR STRIPLINE The stripline belongs to the category of planar circuits that can be fabricated with printed circuit technology . It consists of a metal strip embedded in a dielectric substrate symmetrically sandwiched b/n two metal ground planes . Geometry of the stripline (a) and electric field lines of the TEM mode (b). The stripline can be considered as a printed version of the coaxial cable.

8. Stripline requires three layers of conductors where the internal conductor is called the “hot conductor,” While the other two, always connected at signal ground, are called “cold” or “ground” conductors. The hot conductor is embedded in a homogeneous and isotropic dielectric, of dielectric constant “ ε r ”. So , unlike the case of Microstrip , the word “substrate” is not appropriate since the dielectric completely surrounds the hot conductor .

9. COUPLED STRIPLINE Coupled Striplines Are Used Extensively As: The Basic Building Blocks For Directional Couplers & Filters In Low Temperature Co-fired Ceramic.

10. MICROSTRIP Microstrip TLs Consist Of A Conductive Strip Of Width "W" & Thickness “T" & A Wider Ground Plane, Separated By A Dielectric Layer (A.K.A. The "Substrate") Of Thickness “H“ Microstrip Is By Far The Most Popular Microwave TL, Especially For Microwave Integrated Circuits &  MMICs .

11. The Major Advantage Of Microstrip Over Stripline Is That: All Active Components Can Be Mounted On Top Of The Board. The Disadvantages Are That: When High Isolation Is Required Such As In A Filter Or Switch, Some External Shielding May Have To Be Considered. Given The Chance, Microstrip Circuits Can Radiate, Causing Unintended Circuit Response.

12. A Minor Issue With Microstrip Is That: It Is Dispersive, Meaning That, Signals Of Different Frequencies Travel At Slightly Different Speeds. Microstrip Does Not Support A TEM Mode, Because Of Its Filling Factor. For  Coupled Lines, The  Even & Odd Modes Will Not Have The Same Phase Velocity.

13. The microstrip line, or simply microstrip , is the most common transmission line employed in microwave & RF engineering. This is because of a number of advantages such as its simplicity of manufacturing , low cost , low weight & excellent integrability with active devices. It Is A Planar TL Like The Stripline , But, In Contrast, It Is An Open Structure; It Is Not Fully Embedded In A Dielectric, But Printed On Top Of A Dielectric Substrate Metallized On The Opposite Side.

14. Characteristic impedance equation for a Microstrip line   Microstrip Lines Are Used Extensively To Interconnect High-speed Logic Circuits In Digital Computers Because They Can Be Fabricated By Automated Techniques And They Provide The Required Uniform Signal Paths. The Characteristic Impedance Of A Microstrip Line Is A Function Of: The Strip-line Width (W), The Strip-line Thickness (B), The Distance B/n The Line & The Ground Plane (d), & The Homogeneous Dielectric Constant Of The Board Material Er . The Well-known Equation Of The Characteristic Impe - dance Of A Wire-over- ground TL Is Given By:

15. Using The Dielectric Constant Equation & An Equivalent Diameter Of A Microstrip , The Equation Of Characteristic Impedance For A Narrow Microstrip Line Yields:

16. The Characteristic Impedance For A Wide Microstrip Line Was Derived By Assadourian & Others & Is Expressed By: Losses in Microstrip Lines The Attenuation Constant Of The Dominant Microstrip Mode Depends On Geometric Factors, Electrical Properties Of The Substrate & Conductors, & On The Freq.

17. For A Nonmagnetic Dielectric Substrate, Two Types Of Losses Occur In The Dominant Microstrip Mode: (1) Dielectric Loss In The Substrate (2) Ohmic Skin Loss In The Strip Conductor & The Ground Plane. The Sum Of These Two Losses May Be Expressed As Losses Per Unit Length In Terms Of An Attenuation Factor α . From Ordinary TL Theory, The Power Carried By A Wave Traveling In The Positive Z Direction Is Given By:

18. Dielectric Losses. When The Conductivity Of A Dielectric Cannot Be Neglected, The Electric & Magnetic Fields In The Dielectric Are No Longer In Time Phase. In That Case The Dielectric Attenuation Constant Is Expressed As: Ohmic Losses   In A Microstrip Line Over A Low-loss Dielectric Substrate, The Predominant Sources Of Losses At Microwave Freqs Are The Non-perfect Conductors. These Lossess Are Referred To Omic Losses The Microstrip Conductor Contributes The Major Part Of The Ohmic Loss.

19. The Radiation Loss For Several Discontinuities Can Be Calculated Using The Following Approximations, Suggested By Lewin : 1 . TEM transmission 2 . Uniform dielectric in the neighborhood of the strip, equal in magnitude to an effective value 3 . Neglect of radiation from the transverse electric (TE) field component parallel to the strip 4 . Substrate thickness much less than the free-space wavelength

20. Radiation losses.   In addition to the conductor and dielectric losses, microstrip line also has radiation losses . The radiation loss depends on the sub- strate's thickness and dielectric cons- tant , as well as its geometry . Theoretical Conductor Attenuation Factor Of Microstrip As A Function

21. Lewin's results show that the ratio of radiated power to total dissipated power for an open-circuited microstrip line is : where F (Ere) is a radiation factor given by: in which Ere is the effective dielectric constant and λo = c / f is the free-space wavelength.

22. Quality Factor Q of Microstrip Lines   Many Microwave Integrated Circuits Require Very High Quality Resonant Circuits. The Quality Factor Q Of A Microstrip Line Is Very High, But It Is Limited By The Radiation Losses Of The Substrates With Low Dielectric Constant.

23. Coupled Microstrip   So Far, Each Microwave Line Is Being Considered In Isolation. In Practice, Two Or More TLs Are Located, Intentionally Or Not, Close To Each Other So That, The Field Guided By Each Of Them Interacts With The Field Of The Other. The Field Is Simultaneously Guided By The Whole Set Of Coupled Lines, So That One Must Refer To The Modes Of The Entire Set Rather Than To The Modes Of Each TL.

24. The Coupling B/n Different Lines Is Used In The Realization Of Directional Couplers & Coupled line Filters. The Characterization Of Coupled Lines Is Required, Not Only For The Design Of Such Devices, But Also To Model Cross-coupling & Interference Phenomena Occurring Within The Same Circuit.

25. Cross-sections of some coupled lines: (a) stripline ; (b) microstrip line; (c) coaxial cable; (d) square coaxial cable .

26. Coplanar Stripline   It Is A Type Of TL Fabricated Using PCB & MMIC Technologies, Deployed To Convey Microwave-frequency Signals. Coplanar Trip Line Consists Of Two Conducting Strips On One Substrate Surface With One Strip Grounded. Conventional Coplanar Consists Of A Single Conducting Track Printed Onto A Dielectric Substrate, Together With A Pair Of Return Conductors, One To Either Side Of The Track. Schematic Diagram Of A Coplanar Strip Line.

27. The Coplanar Strip Line Has Advantages Over The Conventional Parallel Strip Line Because Its Two Strips Are On The Same Substrate Surface For Convenient Connections. In MICs, The Wire Bonds Have Always Presented Reliability & Reproducibility Problems. The Coplanar Strip Lines Eliminate The Difficulties Involved In Connecting The Shunt Elements B/N The Hot And Ground Strips. As A Result, Reliability Is Increased, Reproducibility Is Enhanced, Production Cost Is Decreased.

28. The characteristic impedance of a coplanar strip line is: Where Io Is The Total Peak Current In One Strip & P avg Is The Average Power Flowing In The Positive Z Direction. The Average Flowing Power Can Be Expressed As:

29. PARALLEL STRIP LINES A parallel strip line consists of two perfectly parallel strips separated by a perfect dielectric slab of uniform thickness. The plate width is w , the separation distance is d, and the relative dielectric constant of the slab is Ed. Schematic diagram of a parallel strip line.

30. SHIELDED STRIP LINES A Partially Shielded Strip Line Has Its Strip Conductor Embedded In A Dielectric Medium Its Top & Bottom Ground Planes Have No Connection Shielded strip line.

31. The characteristic impedance for a wide strip ( w/d >> 0.35) is:

32. 10.4 Stripline Terminations, Attenuators, Couplers, Power Dividers, Circulators, Isolators & Resonators Splitline Attenuators ,

33. Splitline Couplers ,

34. Splitline Power Dividers,

35. Splitline Circulators ,

36. Splitline Isolators

37. Splitline Resonators

38. 10.5 Lowpass Filters, Bandstop Filters, Bandpass Filters, Wideband Filters . Please The Materials Here Are The Same As Those Studied In The Undergraduate Programme, Thus, Kindly Refer To, & Review Those Literature.

39. THE END OF LECTURE No. 10