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Understanding Antennas: Types, Radiation Patterns, and Propagation Models in Wireless Communication

This chapter presents a comprehensive overview of antennas and their critical roles in wireless communications. It defines antennas as conductors used for transmitting and receiving electromagnetic energy. Various types of antennas are discussed, including isotropic, dipole, and parabolic antennas, along with their radiation patterns. The chapter also explores propagation models like ground wave, sky wave, and line-of-sight, detailing how different frequencies and environmental factors affect communication. Understanding these concepts is essential for optimizing wireless signal transmission and reception.

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Understanding Antennas: Types, Radiation Patterns, and Propagation Models in Wireless Communication

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  1. Antennas and Propagation(William Stallings, “Wireless Communications and Networks” 2nd Ed, Prentice-Hall, 2005, Chapter 5) by Ya Bao http://eent3.sbu.ac.uk/staff/baoyb/acs

  2. Introduction • An antenna is an electrical conductor or system of conductors • Transmission - radiates electromagnetic energy into space • Reception - collects electromagnetic energy from space • In two-way communication, the same antenna can be used for transmission and reception

  3. Types of Antennas • Isotropic antenna (idealized) • Radiates power equally in all directions • Dipole antennas • Half-wave dipole antenna (or Hertz antenna) • Quarter-wave vertical antenna (or Marconi antenna) • Parabolic Reflective Antenna

  4. Radiation Patterns • Radiation pattern • Graphical representation of radiation properties of an antenna • Depicted as two-dimensional cross section • Beam width (or half-power beam width) • Measure of directivity of antenna

  5. Radiation patterns

  6. Three-dimensional antenna radiation patterns. The top shows the directive pattern of a horn antenna, the bottom shows the omnidirectional pattern of a dipole antenna.

  7. or as separate graphs in the vertical plane (E or V plane) and horizontal plane (H plane). This is often known as a polar diagram

  8. outdoor enclosure featuring a wide band 2.5GHz panel antenna

  9. Other antennas Helical Antenna Patch (microstrip) antenna Multiband antenna: for GSM 900+GSM 1800+GSM 1900+Bluetooth; or GSM and 3G

  10. Antenna Gain • Antenna gain • Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) • Effective area • Related to physical size and shape of antenna

  11. Antenna Gain • Relationship between antenna gain and effective area • G = antenna gain • Ae= effective area • f = carrier frequency • c = speed of light ( 3  108 m/s) •  = carrier wavelength

  12. Propagation Models • Ground Wave (GW) Propagation: < 3MHz • Sky Wave (SW) Propagation: 3MHz to 30MHz • Effective Line-of-Sight (LOS) Propagation: > 30MHz

  13. Ground Wave Propagation • Follows contour of the earth. • Can propagate considerable distances. • Frequency bands: ELF, VF, VLF, LF, MF. • Spectrum range: 30Hz ~ 3MHz, e.g. AM radio.

  14. Sky Wave Propagation • Signal reflected from ionized layer of upper atmosphere back down to earth, which can travel a number of hops, back and forth between ionosphere and earth’s surface. • HF band with intermediate frequency range: 3MHz ~ 30MHz. • e.g: International broadcast.

  15. Line-of-Sight Propagation • Tx. and Rx. antennas are in the effective ‘line of sight’ range. • Includes both LOS and non-LOS (NLOS) case • For satellite communication, signal above 30 MHz not reflected by ionosphere. • For ground communication, antennas within effective LOS due to refraction. • Frequency bands: VHF, UHF, SHF, EHF, Infrared, optical light • Spectrum range : 30MHz ~ 900THz.

  16. LOS calculations dr • What is the relationship between h and d ? do optical horizon radio horizon earth h • For optical LOS: • where • h = antenna height (m) • d = distance between • antenna and horizon (km) • K = adjustment factor for • refraction, K = 4/3 • For effective or radio LOS:

  17. Line-of-Sight Equations Effective, or radio, line of sight • d = distance between antenna and horizon (km) • h = antenna height (m) • K = adjustment factor to account for refraction, rule of thumb K = 4/3 • Maximum distance between two antennas for LOS propagation:

  18. LOS Wireless Transmission Impairments • Attenuation and attenuation distortion • Free space loss • Noise • Atmospheric absorption • Multipath • Refraction • Thermal noise

  19. Attenuation • Strength of signal falls off with distance over transmission medium • Attenuation factors for unguided media: • Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal • Signal must maintain a level sufficiently higher than noise to be received without error • Attenuation is greater at higher frequencies, causing distortion

  20. Free Space Loss • Free space loss, ideal isotropic antenna • Pt = signal power at transmitting antenna • Pr = signal power at receiving antenna •  = carrier wavelength • d = propagation distance between antennas • c = speed of light ( 3  108 m/s) where d and  are in the same units (e.g., meters)

  21. Free Space Loss • Free space loss equation can be recast:

  22. Free Space Loss • Free space loss accounting for gain of other antennas can be recast as

  23. Categories of Noise • Thermal Noise • Intermodulation noise • Crosstalk • Impulse Noise

  24. Noise (1) • Thermal noise due to thermal agitation of electrons. • Present in all electronic devices and transmission media. • As a function of temperature. • Uniformly distributed across the frequency spectrum, hence often referred as white noise. • Cannot be eliminated – places an upper bound on the communication system performance. • Can cause erroneous to the transmitted digital data bits.

  25. Noise (2): Noise on digital data Error in bits

  26. Thermal Noise • The noise power density (amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor) is: • N0 = noise power density in watts per 1 Hz of bandwidth • k = Boltzmann's constant = 1.3803  10-23 J/K • T = temperature, in kelvins (absolute temperature) • 0oC = 273 Kelvin

  27. Thermal Noise • Noise is assumed to be independent of frequency • Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts (dBW),

  28. Noise Terminology • Intermodulation noise – occurs if signals with different frequencies share the same medium • Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies • Crosstalk – unwanted coupling between signal paths • Impulse noise – irregular pulses or noise spikes • Short duration and of relatively high amplitude • Caused by external electromagnetic disturbances, or faults and flaws in the communications system

  29. Signal to Noise Ratio – SNR (1) • Ratio of the power in a signal to the power contained in the noise present at a particular point in the transmission. • Normally measured at the receiver with the attempt to eliminate/suppressed the unwanted noise. • In decibel unit, where PS = Signal Power, PN = Noise Power • Higher SNR means better quality of signal.

  30. Signal to Noise Ratio – SNR (2) • SNR is vital in digital transmission because it can be used to sets the upper bound on the achievable data rate. • Shannon’s formula states the maximum channel capacity (error-free capacity) as: • Given the knowledge of the receiver’s SNR and the signal bandwidth, B. C is expressed in bits/sec. • In practice, however, lower data rate are achieved. • For a fixed level of noise, data rate can be increased by increasing the signal strength or bandwidth.

  31. Expression of Eb/N0 (1) • Another parameter that related to SNR for determine data rates and error rates is the ratio of signal energy per bit, Eb to noise power density per Hertz, N0; →Eb/N0. • The energy per bit in a signal is given by: • PS = signal power & Tb = time required to send one bit which can be related to the transmission bit rate, R, as Tb = 1/ R. • Thus, • In decibels: – 228.6 dBW

  32. Expression of Eb/N0 (2) BER versus Eb/N0 plot • As the bit rate R increases, the signal power PS relative to the noise must also be increased to maintain the required Eb/N0. • The bit error rate (BER) for the data sent is a function of Eb/N0 (see the BER versus Eb/N0 plot). • Eb/N0 is related to SNR as: Higher Eb/N0, lower BER where B = Bandwidth, R = Bit rate

  33. Wireless Propagation Mechanisms • Basic types of propagation mechanisms • Free space propagation • LOS wave travels large distance with obstacle-free • Reflection • Wave impinges on an object which is large compared to the wave-length  reflection Lamp post diffraction • Diffraction • Occurs when wave hits the sharp edge of the obstacles and bent around to propagate further in the ‘shadowed’ regions – Fresnel zones. • Scattering • Wave hits the objects smaller than  itself. e.g. street signs and lamp posts. scattering

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