lecture 2.ppt
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This document discusses various topics related to antennas and propagation. It describes what antennas are, their characteristics, and different types of antennas like dipole, parabolic, and arrays. It also covers radiation patterns, antenna gain, and different propagation modes like ground wave, sky wave, and line-of-sight. Key factors affecting line-of-sight transmission are discussed, including attenuation, free space loss, noise from thermal, intermodulation, crosstalk and impulse sources, and atmospheric absorption and multipath effects. Common antenna types and their uses as well as concepts like radiation patterns, antenna gain, and propagation modes are summarized.
lecture 2.ppt
- 1. Antenna and Propagation By Waheed ur Rehman wahrehman@gmail.com
- 2. Antenna • Electrical conductor for conduction electromagnetic energy • Electric energy < -- > electromagnetic energy • Antenna characteristics are essentially the same whether an antenna is sending or receiving electromagnetic energy.
- 3. Radiation pattern • A common way to characterize the performance of an antenna • Graphical representation of the radiation properties of an antenna • The simplest pattern is produced by an idealized antenna known as isotropic antenna (sphere with an antenna in center)
- 5. Isotropic Antenna • Isotropic Antenna is a point in space that radiate power in all directions equally • Its not a practical antenna just used as a reference.
- 6. Antenna Types • Isotropic radiator • Monopole • Loop • dipole • Rhombic • Dielectric Rod • Yagi • Horn • Parabolic dish • Patch • Dielectric lens • arrays
- 7. Dipole Antenna • Lamda / 2 half wave dipole • Lamda / 4 Quarter-wave dipole
- 8. Parabolic Antenna • Used for terrestrial and satellite communication • Signals reflected back to focus
- 10. Antenna Types
- 11. Antenna types
- 12. Antenna Gain • Measure of directivity of an antenna • Defined as power output in a particular direction, compared to that produced in any direction by a perfect radiator (isotropic) • Its only possible on the expense of radiation in other directions • If an antenna has a gain of 3dB that means that antenna improves upon isotropic antenna in that direction by 3dB.
- 13. Antenna Gain • Antenna gain is not related to more output power but with directionality • Effective area of an antenna is related to that of the physical size of the antenna and its shape G = 4 х pi х Ae/lamda ^2 Ae is different from antenna to antenna
- 14. Propagation Modes • A radiated signal from antenna travels along one of three routes – Ground wave – Sky wave – LOS • We will only be concerned with LOS
- 15. Ground Wave Propagation • More or less follow the earth curvature • Propagate considerable distance well over the visual horizon • Frequency upto 2MHz • One factor is that electromagnetic wave induces a current in the earth’s surface (causes a bend towards the earth) • Another factor is diffraction • Electromagnetic waves in this frequency range are scattered in such a way that they don’t penetrate the upper atmosphere. • AM radio
- 18. Sky Wave Propagation • Signal from the earth-based antenna is reflected from the ionized layer of the upper atmosphere back down to the earth • Seems like reflection but actually refraction. • 2 – 30 MHz • BBC , VoA
- 21. LOS Propagation • Above 30MHz • Not reflected by ionosphere (satellite comm) • For ground-based LOS communication both Tx and Rx antennas must be within effective LOS of each others • Optical Vs Radio LOS
- 22. LOS Propagation • Optical LOS – d = 3.57 √h – d= distance b/w antenna and horizon – h= height of antenna in meters • Effective / Radio LOS – d=3.57√Kh – K = adjustment factor to account for refraction, typically K=4/3 • Max distance between two antennas – 3.57 (√Kh1 + √Kh2) – h1, h2 are height of antennas
- 23. LOS Transmission • Signal received is not similar to signal transmitted • Significant impairments are – Attenuation and attenuation distortion – Free space loss – Noise – Atmospheric Absorption – Multipath – Refraction
- 24. Attenuation • Strength of the signal falls with the distance • Expressed in decibels dB • For unguided medium attenuation is a complex function of distance and makeup of the atmosphere
- 25. Attenuation • Attenuation involves these factors – Receive signal must be sufficiently strong to be detected and interpreted – Signal level must be sufficiently higher than noise – Attenuation is greater at higher frequencies, causing distortion. Amplifiers Repeaters can be used Amplifiers that amplify higher frequency more Than lower frequency
- 26. Free Space Loss • Signal disperses with distance • Signal spreads larger over distances • This type of attenuation is called free space loss • In ideal free space propagation – Pr = Pt Gt Gr (lamda/4 х pi х d) 2 • For microwave systems • Ls = 32.45 +20log d(km) + 20 log f (MHz)
- 27. Noise • Unwanted signal created from the source other than the transmitter • Four categories – Thermal noise – Intermodulation noise – Cross talk – Impulsive noise
- 28. Noise Thermal Noise • Due to thermal agitation of electrons • Always present and cannot be eliminated • Uniformly distributed across the frequency spectrum hence referred to as white noise • Independent of frequency • Thermal noise in watts present in a bandwidth of B Hertz can be expressed as N = kTB where k = boltzmann’s constt 1.38 х 10-23 J/K T is Temp, in Kelvin • Or in decibels-watt – N = 10 log k + 10log T + 10 log B
- 29. Noise Intermodulation Noise • When signal with different frequencies share the same medium, results in I.N. • It produces signal at frequency that is the sum, difference or multiple of two other frequencies. • E.g. f1 , f2 would result in f1+f2
- 30. Noise Crosstalk • Unwanted coupling between signal paths. • The effect of one wire over the other in twisted pair • Can also occur when unwanted signals are picked up by microwave antenna
- 31. Noise Impulse Noise • Irregular , continuous pulses • Unpredictable therefore not possible to engineer a transmission system to cope with it • Generated from external electromagnetic disturbance like lightning and faults and flaws in the communication system • A sharp spike of energy of 0.01 s duration can destroy 560 bits of data being transmitted at 56kbps