WAVE PROPAGATION IN ANTENNAS AND WAVE PROPAGATION
- 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