ADVANCES IN MICROWAVE ENGINEERING

What is the range of MicrowaveFrequency ?

Microwaves (f≈1-300GHz or λ≈ 30Cms- 1mm)

What is a “Microwave?”(Part of the RF spectrum

1 - 300 GHz)

Uses of Microwaves

• Cooking

• Communication– Radios– Satellites– RADAR

• Medicine• Astronomy

• In the microwave frequency region, power is considered to be in electric and magnetic fields that are guided from place to place by some physical structure. Any physical structure that will guide an electromagnetic wave from place to place is called a RF Transmission Line.

Transmission Line Theory• Introduction:• In an electronic

system, the delivery of power requires the connection of two wires between the source and the load. At low frequencies, power is considered to be delivered to the load through the wire.

• This is called a low frequency Transmission Line.

Types of Transmission Lines

1. Two wire line2. Coaxial cable3. Waveguide

Rectangular Circular

4. Planar Transmission Lines Strip line Microstrip line Slot line Fin line Coplanar Waveguide Coplanar slot line

• At low frequencies, the circuit elements are lumped since voltage and current waves affect the entire circuit at the same time.

• At microwave frequencies, such treatment of circuit elements is not possible since voltage and current waves do not affect the entire transmission line circuit at the same time.

Analysis of differences between Low and High Frequency

Analysis of differences between Low and High Frequency

• The entire transmission line circuit must be broken down into unit sections within which the circuit elements are considered to be lumped.

• This is because the dimensions of the transmission line circuit are comparable to the wavelength of the waves transmitted on the line according to the formula:

l = c/fwhere,c = velocity of light f = frequency of voltage/current

Microwave Transmission Lines

• Waveguide and other transmission lines for the low-loss transmission of microwave power.

• Early microwave systems relied on waveguide and coaxial lines for transmission line media.

• Waveguide: high power-handling capability, low loss, but bulky and expensive

• Coaxial line: high bandwidth, convenient for test applications, difficult medium in which to fabricate complex microwave components.

• Planar transmission lines: stripline, microstrip, slotline, coplanar waveguide compact, low cost, easily integrated with active devices.

Microwave Transmission Lines

Transmission LinesTransmission Lines

Cross-sectional view of typical transmission lines (a) coaxial line, (b) two-wire line, (c) planar line, (d) wire above conducting plane, (e) microstrip line.

Transmission LinesTransmission Lines

• (a) Coaxial line connecting the generator to the load; (b) E and H fields on the coaxial line

Electric and magnetic fields around single-phase transmission line

E and H Field lines oncross-sectioned Coaxial lines

Stray field

Triplate line

Planar transmission Lines

• MICROSTRIP:

• The great majority of planar circuits are realized in microstrip. Microstrip is a practical medium for a wide variety of components and is a natural choice for large, integrated systems. Microstrip, like most planar circuits, is a "quasi- TEM" transmission line. This means that it is usually treated as a TEM line at frequencies low enough for dispersion to be negligible. At higher frequencies, dispersion corrections are usually necessary. Again, a number of methods exist. One of the most popular and most accurate is that of Kirschning and Jansen. Another good one is by Wells and Pramanick. A simple approximate expression for the cutoff frequency of the lowest non- TEM mode is 75/h(k-1)^0.5. where this is got in GHz and h is in mm. k is dielectric constant of medium.

Planar transmission Lines

Planar transmission Lines

• CPW:

• For many purposes CPW is a good alternative to microstrip. In CPW the ground surfaces are alongside the strip conductor instead of underneath it. This configuration causes many characteristics to differ from those of microstrip. First, the fields are not as fully contained in the dielectric and extend farther into the air above the substrate. This causes dispersion and radiation to be worse in CPW than in microstrip. Second, the currents are more strongly concentrated in the edges of the conductors. Because the edges are likely to be much rougher than the surfaces, losses are higher.

Planar transmission Lines• Nevertheless, CPW has significant advantages over microstrip

for monolithic circuits. The most important is that ground connections can be made on the surface of the substrate; there is no need for "via" holes, which are used to make ground connections in microstrip circuits. CPW grounds usually have much less inductance than microstrip, an important consideration for many types of high-frequency circuits. Another important advantage is size. CPW conductors can be very narrow, even with low characteristic impedances. Low-impedance microstrip lines often are impractically wide. Finally, CPW is much less sensitive to substrate thickness than microstrip, so the thinning of the monolithic substrate is much less critical. CPW monolithic circuits often are not thinned at all.

Planar transmission Lines• STRIPLINE:

• Strip line is one of the oldest types of planar transmission media, developed in the late 1950s and originally called triplate. Of the lines listed in Table 1.1,stripline is the only true TEM transmission line. As such, it is non-dispersive, but it is not immune to moding, especially if the strip conductor is not centered evenly between the ground planes. Strip line components invariably use composite substrates. One technique is to create a sandwich of two substrates, one having a ground plane and a strip conductor, the other having only the ground plane. These two substrates are clamped firmly together to prevent the formation of an air gap, which would create variations in the dielectric constant of the medium between the ground planes.

Planar transmission Lines

• Stripline is a great medium for directional couplers.

• This is virtually impossible in microstrip or CPW, which can use only edge coupling. The homogeneous dielectric of stripline makes its even-mode and odd-mode phase velocities equal, resulting in high directivity. Broadside coupling is also possible in suspended-substrate stripline. Stripline is not a favored transmission medium these days, probably because it is not really suitable for components that include chip diodes, transistors, or other discrete circuit elements, and it does not integrate well with the media.

Microstrip: basic microwave interconnection structure

Summary of key material requirements at RF:

Conductors:- low bulk resistivity• - good surface finish (low surface

roughness)• - high line/space resolution• - good temperature stability

Dielectrics: - low loss tangent (<10-2)• - good surface finish• - precisely defined r (stable with

frequency)• - isotropic r

• - consistent substrate thickness

• - low Tf (< 50 ppm/oC)

•

Metamaterials

• Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior.

• The primary research in metamaterials investigates materials with negative refractive index. Negative refractive index materials appear to permit the creation of superlenses which can have a spatial resolution below that of the wavelength. In other work, a form of 'invisibility' has been demonstrated at least over a narrow wave band with gradient-index materials. Although the first metamaterials were electromagnetic, acoustic and seismic metamaterials are also areas of active research.

Metamaterials

• Metamaterials consist of periodic structures. An electromagnetic metamaterial affects electromagnetic waves by having structural features smaller than the wavelength of light. In addition, if a metamaterial is to behave as a homogeneous material accurately described by an effective refractive index, its features must be much smaller than the wavelength. To date, subwavelength structures have shown only a few questionable results at visible wavelengths

Electromagnetic Metamaterials

• For microwave radiation, the structures need only be on the order of few centimeters. Microwave frequency metamaterials are usually synthetic, constructed as arrays of electrically conductive elements (such as loops of wire) which have suitable inductive and capacitive characteristics. These are known as split-ring resonators

Electromagnetic Metamaterials

APPLICATIONS OF MICROWAVE ENGINEERING

• Various molecular, atomic, and nuclear resonances occur at microwave frequencies, creating a variety of unique applications in the areas of basic science, remote sensing, medical diagnostics and treatment, and heating methods.

• Today, the majority of applications of microwaves are related to radar and communication systems. Radar systems are used for detecting and locating targets and for air traffic control systems, missile tracking radars, automobile collision avoidance systems, weather prediction, motion detectors, and a wide variety of remote sensing systems.

APPLICATIONS OF MICROWAVE ENGINEERING

• 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 broadcast satellite (DBS) television, personal communication systems (PCSs), wireless local area networks (WLANS), cellular video (CV) systems, and global positioning satellite (GPS) systems rely heavily on microwave technology

APPLICATIONS OF MICROWAVE ENGINEERING

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