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1.1 Microwave Spectrum and Bands, Historical Background

Loosely speaking, the felds o microwave and RF engineering together encompass thedesign and implementation o electronic systems utilizing requencies in theelectromagnetic spectrum rom approximately 1 !"z to over 1 #"z$

!icrowaves are a type o electromagnetic radiation, as are radio waves, ultravioletradiation, %&rays and gamma&rays$ !icrowaves have a range o applications, includingcommunications, radar, cooking$ 'lectromagnetic radiation is transmitted in waves atdi(erent wavelengths and requencies$ )his *road range o wavelengths is known asthe electromagnetic spectrum +'! spectrum $

RF requencies range rom very high requency +-"F +. /. !"z to ultra highrequency +0"F +. /. !"z $ )he term microwave is typically used or requencies

*etween . #"z and . #"z, with a corresponding electrical wavelength *etween 2c3 21 cm and 21 mm, respectively$ )he *oundary *etween 4RF5 and 4microwave5 issomewhat indistinct$

6n RF and microwave engineering, must o ten work with !axwell7s equations and theirsolutions$

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A Short History of Microwave Engineering

!icrowave engineering is o ten considered a airly mature discipline *ecause the

undamental concepts were developed more than 9 years ago, and pro*a*ly *ecauseradar, the frst ma:or application o microwave technology, was intensively developed asar *ack as ;orld ;ar 66$ "owever, recent years have *rought su*stantial and continuing

developments in high& requency solid&state devices, microwave integrated circuits, andcomputer&aided design techniques, and the ever&widening applications o RF andmicrowave technology$

)he oundations o modern electromagnetic theory were ormulated in 1ames?lerk !axwell 1, who hypothesized, solely rom mathematical considerations,electromagnetic wave propagation and the idea that light was a orm o electromagneticenergy$

!axwell7s ormulation was cast in its modern orm *y @liver "eaviside A during the periodrom 1

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6t was not until the 1B s and the advent o radar development during ;orld ;ar 66 thatmicrowave theory and technology received su*stantial interest$ )hat was very intensiveperiod o development in the microwave feld$ 6t included the theoretical and experimentaltreatment o waveguide components, microwave antennas, small&aperture couplingtheory, and the *eginnings o microwave net&work theory$

?ommunications systems using microwave technology *egan to *e developed soon a terthe *irth o radar$ )he advantages o(ered *y microwave systems, including wide*andwidths and line&o &sight propagation, have proved to *e critical or *oth terrestrialand satellite communications systems and have thus provided an impetus or thecontinuing development o low&cost miniaturized microwave components$

)hese days use o microwave technology include wide range o applications, such as,microwave oven or cooking, mo*ile phones, wif, radar, *luetooth, wimax, satellitecommunication, #8G, #L@ CGG, radio telescope etc$

1.! 'imitations of conventional circuit theory, concepts at microwave

fre#uencies )he most undamental characteristic that distinguishes conventional circuit analysis rommicrowave engineering is directly related to the requency +and thus the wavelength, o the electronic signals *eing processed$ 6n ree space, 2c3 , where is the requency o the signal and c is the speed o light$

For low& requency circuits +with a ew special exceptions such as antennae , the signalwavelength is much larger than the size o the electronic system and circuit components*eing examined$ 6n contrast, or a microwave system the sizes o typical electroniccomponents are o ten compara*le to +i$e$, within approximately 1 order o magnitude o

the signal wavelength$

Cs illustrated in a*ove fgure, or components much smaller than the wavelength +i$e$,

lM 31 , the inite velocity o the electromagnetic signal as it propagates through thecomponent leads to a modest di(erence in phase at opposite ends o the component$ Forcomponents compara*le to or larger than the wavelength, however, this end&to&endphase di(erence *ecomes increasingly signifcant$

)his gives rise to a reasona*le working defnition o the two design areas *ased on theunderlying approximations used in design$ Gince in conventional circuit, the circuitcomponents and interconnections are generally small compared to a wavelength, theycan *e modeled as lumped elements or which Nircho(7s voltage and current laws apply atevery instant in time$ 6t means the description or analysis o such circuits may *eadequately carried out in terms o loop currents and node voltages without considerationo propagation e(ects assuming that the current +and voltage at any given instant hasthe same value at every point in the element$ )he time delay *etween cause and e(ect atdi(erent points in these circuits is so small compared with the period o the applied signal

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as to *e negligi*le$ 6n practice, a rule o thum* or the applica*ility o a lumped&elementequivalent circuit is that the component size should *e less than 31 at the requency o operation$

Cs the requency is raised to a point where the wavelength is no longer large comparedwith the circuit dimensions, propagation e(ects can no longer *e ignored$ Go ormicrowave requencies or which component size exceeds approximately 31 , the initepropagation velocity o electromagnetic waves can no longer *e as easily a*sor*ed intosimple lumped&element equivalent circuits$ For these requencies, the time delayassociated with signal propagation rom one end o a component to the other is anapprecia*le raction o the signal period, and thus lumped&element descriptions are nolonger adequate to descri*e the electrical *ehavior$ 6n this case phase di(erences withinelement *ecome signifcantO at a given instant the current at one point in the elementmay *e passing through its maximum value, while at another point it is zero$ Cdistri*uted&element model is required to accurately capture the electrical *ehavior$ Thetime delay associated with fnite wave propagation velocity that gives rise to thedistributed circuit e ects is a distinguishing eature o the mindset o microwaveengineering $

)he extension o conventional circuit theory to microwave system is urther complicated*y the use o circuit elements such as waveguides, in which voltages and currents are notuniquely defned$ )he analysis o these elements must *e approached rom the point o view that they serve to guide electromagnetic wavesO attention is centered on electric andmagnetic felds rather than on voltage and current$

6n addition to material properties, some physical e(ects are signifcant at microwaverequencies that are typically negligi*le at lower requencies$ For example, radiation

losses *ecome increasingly important as the signal wavelengths approach the componentand interconnect dimensions$ For conductors and other components o compara*le size tothe signal wavelengths, standing waves caused *y rePection o the electromagneticwaves rom the *oundaries o the component can greatly enhance the radiation o electromagnetic energy$

1.( Applications of Microwaves

>ust as the high requencies and short wavelengths o microwave energy make ordiQculties in the analysis and design o microwave devices and systems, these sameaspects provide unique opportunities or the application o microwave systems$

• Cntenna gain is proportional to the electrical size o the antenna$ Ct higherrequencies, more antenna gain can *e o*tained or a given physical antenna size,

and this has important consequences when implementing microwave systems$

Antenna gain ,G ∝ A / λ2

, C & 8hysical aperture area o antenna, & wavelength• !ore *andwidth +directly related to data rate can *e realized at higher requencies$

'g$ )- *andwidth 2 K !"zCt K !"z +-"F , 1 E; or 1 ?hannelCt K #"z +0&Eand 1 E; or 1 channels

C 1 *andwidth at K !"z is K !"z, which +with *inary phase shi t keyingmodulation can provide a data rate o a*out K !*ps +mega*its per second , whileat K #"z a 1 *andwidth is K !"z, allowing a K !*ps data rate$

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• !icrowave signals travel *y line o sight and are not *ent *y the ionosphere as arelower requency signals$ Gatellite and terrestrial communication links with very highcapacities are there ore possi*le, with requency reuse at minimally distantlocations$

• )he e(ective rePection area +radar cross section o a radar target is usuallyproportional to the target7s electrical size$ )his act, coupled with the requency

characteristics o antenna gain, generally makes microwave requencies pre erredor radar systems$

• -arious molecular, atomic, and nuclear resonances occur at microwave requencies,creating a variety o unique applications in the areas o *asic science, remotesensing, medical diagnostics and treatment, and heating methods$

RF and microwave communications systems are pervasive, especially today when wirelessconnectivity promises to provide voice and data access to 4anyone, anywhere, at anytime$5

!odern wireless telephony is *ased on the concept o cellular requency reuse$

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