College of Computer & Information Science
College of Computer & Information Science
Northeastern University
Wireless Networks
Lecture 2
Antennas and Propagation
The notes in this document are based almost entirely on Chapter 5 of the textbook [Sta05]. Rappaport’s text is also a good reference for wireless signal propagation [Rap95].
1
Antennas
An antenna is an electrical conductor or a system of conductors that radiates/collects (transmits or receives) electromagnetic energy into/from space. An idealized isotropic antenna radiates equally in all directions. The directivity of an antenna is captured by its beam width; it is the angle within which power radiated is at least half of that in the most preferred direction (that is, half of the maximum power).
Antenna gain is the power output, in a particular direction, compared to that produced in any direction by an idealized omnidirectional antenna. If f is the carrier frequency and Ae is the effective area of the antenna, then the antenna gain is given by
4πAe f 2
,
c2 where c is the speed of light. The effective area of an antenna depends on the size and shape of the antenna. The effective area of an idealized isotropic antenna is 1, by definition.
G=
2
Propagation modes
Wireless transmissions propagate in three modes: ground-wave, sky-wave, and line-of-sight. Ground wave propagation follows the contour of the earth, while sky wave propagation uses reflection by both earth and ionosphere. Finally line of sight propagation requires the transmitting and receiving antennas to be within line of sight of each other. Which of these propagation modes dominates depends on the frequency of the underlying signal.
Examples of ground wave and sky wave communication are AM radio and international broadcasts such as BBC. Above 30 MHz, neither ground wave nor sky wave propagation operates and the communication is through line of sight.
If h is the height of a transmitting (resp., receiving) antenna in meters, then the distance to
Northeastern University
Wireless Networks
Lecture 2
Antennas and Propagation
The notes in this document are based almost entirely on Chapter 5 of the textbook [Sta05]. Rappaport’s text is also a good reference for wireless signal propagation [Rap95].
1
Antennas
An antenna is an electrical conductor or a system of conductors that radiates/collects (transmits or receives) electromagnetic energy into/from space. An idealized isotropic antenna radiates equally in all directions. The directivity of an antenna is captured by its beam width; it is the angle within which power radiated is at least half of that in the most preferred direction (that is, half of the maximum power).
Antenna gain is the power output, in a particular direction, compared to that produced in any direction by an idealized omnidirectional antenna. If f is the carrier frequency and Ae is the effective area of the antenna, then the antenna gain is given by
4πAe f 2
,
c2 where c is the speed of light. The effective area of an antenna depends on the size and shape of the antenna. The effective area of an idealized isotropic antenna is 1, by definition.
G=
2
Propagation modes
Wireless transmissions propagate in three modes: ground-wave, sky-wave, and line-of-sight. Ground wave propagation follows the contour of the earth, while sky wave propagation uses reflection by both earth and ionosphere. Finally line of sight propagation requires the transmitting and receiving antennas to be within line of sight of each other. Which of these propagation modes dominates depends on the frequency of the underlying signal.
Examples of ground wave and sky wave communication are AM radio and international broadcasts such as BBC. Above 30 MHz, neither ground wave nor sky wave propagation operates and the communication is through line of sight.
If h is the height of a transmitting (resp., receiving) antenna in meters, then the distance to
References: [Rap95] Theodore S. Rappaport. Wireless Communications: Principles and Practice. PrenticeHall, Inc., Upper Saddle River, NJ, USA, 1995. [Sta05] William Stallings. Wireless Communications and Networks. Prentice Hall Professional Technical Reference, 2005. 2nd edition. 4