Lecture notes on Antenna Theory and Design

basic antenna theory and concepts and antenna theory analysis and design pdf free download
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ICTP-ITU-URSI School on Wireless Networking for Development The Abdus Salam International Centre for Theoretical Physics ICTP, Trieste (Italy), 5 to 24 February 2007 Basic Antenna Theory Ryszard Struzak Note: These are preliminary notes, intended only for distribution among the participants. Beware of misprints Purpose • to refresh basic physical concepts needed to understand better the operation and design of microwave antennas R Struzak 2Outline • Introduction • Review of basic antenna types • Radiation pattern, gain, polarization • Equivalent circuit & radiation efficiency • Smart antennas • Some theory • Summary R Struzak 3Quiz Transmitting antennas used to radiate RF energy, whereas receiving antennas designed to capture RF energy Somebody told that receiving antennas, radiate radio waves during the reception Is it a true fact or a slip of the tongue? R Struzak 4• It is true... R Struzak 5Intended & unintended radiators • Intended antennas – To produce/ receive specified EM waves: • Radiocommunication antennas; • Measuring antennas; • EM sensors, probes; • EM applicators (Industrial, Medical, Scientific) R Struzak 6• Unintended antennas - active – EM waves radiated as an unintended side- effect: • Any conductor/ installation with varying electrical current (e.g. electrical installation of vehicles) • Any slot/ opening in the screen of a device/ cable carrying RF current R Struzak 7• Unintended antennas - passive • Any discontinuity in transmission medium (e.g. conducting structures/ installations) irradiated by EM waves – Stationary (e.g. antenna masts or power line wires); – Time-varying (e.g. windmill or helicopter propellers); – Transient (e.g. aeroplanes, missiles) R Struzak 8Antenna function • Transformation of a guided EM wave (in waveguide/ transmission Space wave line ) into an EM wave freely propagating in space (or vice versa) – Transformation from time-function into RF wave (= vectorial field dependent on time and 3 space-dimensions) – The specific form and direction of the wave is defined by the antenna structure and the environment Guided wave R Struzak 9• Transmission line – Power transport medium – the transition ideally without power reflections (matching devices) • Radiator – Must radiate efficiently – must be of a size comparable with the half-wavelength • Resonator – Unavoidable - for broadband applications resonances must be attenuated R Struzak 10Monopole (dipole over plane) Sharp Thin radiator transition Thick radiator region Smooth transition region High-Q Uniform wave Narrowband traveling Low-Q along the line Broadband • If there is an inhomogeneity (obstacle, or sharp transition), reflections, higher field- modes and standing wave appear. • With standing wave, the energy is stored in, and oscillates from electric energy to magnetic one and back. This can be modeled as a resonating LC circuit with Q = (energy stored per cycle) / (energy lost per cycle) • Kraus p.2 R Struzak 11Outline • Introduction • Review of basic antenna types • Radiation pattern, gain, polarization • Equivalent circuit & radiation efficiency • Smart antennas • Some theory • Summary R Struzak 12Dipole antenna • Java apllet on thin linear dipole antenna (length effects): http://www.amanogawa.com/archive/Dipol eAnt/DipoleAnt-2.html • Java applet on detailed analysis of dipole antennas: http://www.amanogawa.com/archive/Anten na1/Antenna1-2.html R Struzak 13Dipole, Slot & INF antennas • Slot antenna: a slot is cut from a large (relative to the slot length) metal plate. • The center conductor of the feeding coaxial cable is connected to one side of the slot, and the outside conductor of the cable - to the other side of the slot. – The slot length is some (λ/2) for the slot antenna and (λ/4) long for the INF antenna. • The INF and the slot antennas behave similarly. • The slot antenna can be considered as a loaded version of the INF antenna. The load is a quarter-wavelength stub, i.e. a narrowband device. • When the feed point is moved to the short-circuited end of the slot (or INF) antenna, the impedance decreases. When it is moved to the slot center (or open end of the INF antenna), the impedance increases R Struzak 14Antennas for laptop applications Source: D. Liu et al.: Developing integrated antenna subsystems for laptop computers; IBM J. RES. & DEV. VOL. 47 NO. 2/3 MARCH/MAY 2003 p. 355-367 R Struzak 15• Patch and slot antennas derived from printed-circuit and micro-strip technologies • Ceramic chip antennas are typically helical or inverted-F (INF) antennas, or variations of these two types with high dielectric loading to reduce the antenna size Source: D. Liu et al.: Developing integrated antenna subsystems for laptop computers; IBM J. RES. & DEV. VOL. 47 NO. 2/3 MARCH/MAY 2003 p. 355-367 R Struzak 16• Patch and slot antennas are – Cheap and easy to fabricate and to mount – Suited for integration – Light and mechanically robust – Have low cross-polarization – Low-profile - widely used in antenna arrays – spacecrafts, satellites, missiles, cars and other mobile applications R Struzak 17Aperture-antenna • Aperture antennas derived from EM wave waveguide technology (circular, rectangular) Power density: 2 PFD w/m Power • Can transfer high absorbed: P watt power (magnetrons, A = APFD Effective klystrons) 2 aperture: Am • Above few GHz • Will be explored in practice during the school • Note: The aperture concept is applicable also to wired antennas. For instance, the max effective aperture of 2 linear λ/2 wavelength dipole antenna is λ /8 R Struzak 18Leaky-wave antennas • Derived from millimeter- wave guides (dielectric guides, microstrip lines, coplanar and slot lines). • For frequencies 30 GHz, including infrared • Subject of intensive study. – Note: Periodical discontinuities near the end of the guide lead to substantial radiation leakage (radiation from the dielectric surface). Source: adapted from N Gregorieva R Struzak 19Reflector antennas • Reflectors are used to concentrate flux of EM energy radiated/ received, or to change its direction • Usually, they are parabolic (paraboloidal). – The first parabolic (cylinder) reflector antenna was used by Heinrich Hertz in 1888. • Large reflectors have high gain and directivity – Are not easy to fabricate – Are not mechanically robust – Typical applications: radio telescopes, satellite telecommunications. Source: adapted from N Gregorieva R Struzak 20

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