Antennas – Part 1 Antenna Characteristics and Line of Sight Paths

1. Antennas – Part 1Antenna Characteristics and Line of Sight Paths Cisco Fundamentals of Wireless LANs version 1.1 Rick Graziani Cabrillo College

2. Acknowledgements • Thanks Jack Unger and his book Deploying License-Free Wireless Wide-Area Networks • Published by Cisco Press • ISBN: 1587050692 • Published: Feb 26, 2003 Rick Graziani graziani@cabrillo.edu

3. Antenna Directivity • Antennas radiate wireless power • Accept wireless signal energy from the transmission line connected to a transmitter • Launch that wireless energy into free-space Rick Graziani graziani@cabrillo.edu

4. Antenna Directivity • Antennas focus wireless energy like a flashlight reflector (focusing element) focuses light from a flashlight bulb. • Without the focusing element, the bulb radiates light energy in all direction. • No direction receives more light than any other direction. Rick Graziani graziani@cabrillo.edu

5. Antenna Directivity • Light energy from an unfocused flashlight bulb is similar to the wireless energy radiated from a theoretical isotropic antenna. • Like a light bulb, an isotropic antenna radiates wireless energy equally in all directions and does not focus the energy in any single direction. Theoretical Isotropic Antenna Rick Graziani graziani@cabrillo.edu

6. Antenna Directivity • A flashlight focuses the light into a beam that comes out the front of the flashlight. • The flashlight (reflector) does not amplify the power or total amount of light from the bulb. • The flashlight simply focuses the light so all of it travels in the same direction. Rick Graziani graziani@cabrillo.edu

7. Antenna Directivity • By focusing the light, the flashlight provides more directivity (beam focusing power). • An antenna provides directivity for the wireless energy that it focuses. • Depending upon the design of the antenna, antennas focus and radiate their energy more strongly in on favored direction. • When receiving, antennas focus and gather energy from their favored direction and ignore most of the energy arriving from all other directions. Rick Graziani graziani@cabrillo.edu

8. Antenna Radiated Patterns • Antennas exhibit directivity by radiating most of their power in one direction. • Major or Main Lobe – Main direction of the power from the antenna • Minor or Side Lobes – Small amount of power in other directions • Nulls – Where no power is radiated Top View Main Lobe Front Null Back Side Lobes Rick Graziani graziani@cabrillo.edu

9. Antenna Radiated Patterns • Antennas provide the same directivity for transmitting and receiving. • Antennas radiate transmitter power in the favored direction(s) when transmitting. • Antennas gather signals coming in from the favored directions(s) when receiving. Top View Main Lobe Front Null Back Side Lobes Rick Graziani graziani@cabrillo.edu

10. Antenna Radiated Patterns • When selecting antennas, remember: • When receiving, antenna directivity not only gathers incoming signals from the favored direction, but also reduces noise, interference, and unwanted signals coming in from other directions. Patch Antenna (Directional Antenna) Rick Graziani graziani@cabrillo.edu

11. Antenna Radiated Patterns • An omnidirectional antenna radiates equally well in all horizontal directions around the main lobe, surrounding the antenna like a donut. • More later… Top View (H) Side View (V) Dipole Antenna (Omnidirectional Antenna) Rick Graziani graziani@cabrillo.edu

12. Antenna Gain • Antenna gain – Measurement of the power in the main lobe of an antenna and comparing that power to the power in the main lobe of a reference antenna. • Gain - This refers to the amount of increase in energy that an antenna appears to add to an RF signal. • Measure in dBi or dBd • dBd – “d” is the gain measured relative to the gain of a dipole reference antenna. • dBi – “i” is the gain measure relative to the gain of a theoretical isotropic antenna. • More later… Like a flashlight, there is always a tradeoff between gain, which is comparable to brightness in a particular direction, and beamwidth, which is comparable to the narrowness of the beam. (coming) Rick Graziani graziani@cabrillo.edu

13. Antenna Gain • The dBi is a unit measuring how much better the antenna is compared to an isotropic radiator. • An isotropic radiator is an antenna which sends signals equally in all directions (including up and down). • An antenna which does this has an 0dBigain. • The higher the decibel figure the higher the gain. • For instance, a 6dBi gain antenna will receive a signal better than a 3dBi antenna. +21 dBi or about 100 times the signal strength when comparing it to an isotropic antenna Top View Rick Graziani graziani@cabrillo.edu

14. Antenna Gain Dipole antenna • A dBd unit is a measurement of how much better an antenna performs against a dipole antenna. • As a result a dipole antenna has a 0dBd gain. • Note: Wireless power never stops exactly on a sharp line like the lobe drawings show, but tapers off. • More later… Rick Graziani graziani@cabrillo.edu

15. Antenna Beamwidth • Beamwidth – The width of the main beam (main lobe) of an antenna. • Measures the directivity of an antenna • The smaller the beamwidth in degrees, the more the antenna focuses power into its main lobe. • The more power of the main lobe, the further the antenna can communicate. Rick Graziani graziani@cabrillo.edu

16. Antenna Beamwidth • Beamwidth is a measurement used to describe directional antennas. • Beamwidth is sometimes calledhalf-power beamwidth. • Half-power beamwidth is the total width in degrees of the main radiation lobe, at the angle where the radiated power has fallen below that on the centerline of the lobe, by -3 dB (half-power). 15 dBi -3 dBi 12 dBi 15 dBi Rick Graziani graziani@cabrillo.edu

17. Remember, wireless power does not stop and start exactly along a straight line, but declines gradually with distance. • The smooth outlines of the main lobes show the approximate intensity of the wireless power at various distances away from the antenna. • The dotted lines pass through the half-power points – the points on each side of the center of the main lobe where the wireless power is one-half as strong as it is at the center of the lobe. Rick Graziani graziani@cabrillo.edu

18. Line-of-Sight (LOS)

19. Line of Sight • When a wireless signal encounters an obstruction, the signal is always attenuated and often reflected or diffracted. • It is important to try and obtain a wireless line-of-sight whenever possible, especially in a wireless WAN environment (outdoor connections between building or different parts of a campus). • A wireless LOS typically requires visual LOS plus additional path clearance to account for the spreading of the wireless signal (Fresnel Zone – coming). Attenuated Signal Diffracted Signal Rick Graziani graziani@cabrillo.edu

20. Visual LOS “I see you!” • There is a difference between visual LOS and wireless LOS. • This is because of the difference in wavelengths. • The wavelength of visual light is very small. • For example, the wavelength of a green light is only about 1/50,000th of an inch • Remember, the wavelength of a 2.4 GHz WLAN signal is about 4.8 inches. “And, I see you!” 1 Mile 1 Mile Rick Graziani graziani@cabrillo.edu

21. LOS • A lightwave and a wireless wave are similar. • Both are forms of electromagnetic radiation. • Both must obey the same laws of physics as they propagate. • Wireless signals are like lightwaves that you cannot see. 1 Mile 1 Mile Rick Graziani graziani@cabrillo.edu

22. LOS • The shorter the wavelength of an electromagnetic wave, the less clearance it needs form objects that it passes as it travels between two points. • The less clearance it needs, the closer it can pass to an obstruction without experience additional loss of signal strength. • The clearance distance is known as the Fresnel Zone. 1 Mile 1 Mile Rick Graziani graziani@cabrillo.edu

23. LOS • The green light has a shorter wavelength so only needs a fraction of an inch to avoid additional attenuation. • A 2.4 GHz (802.11b/g) wireless signal has a larger Fresnel zone and needs to clear the building by quite a few feet (about 10 feet in this example). 1 Mile 1 Mile Rick Graziani graziani@cabrillo.edu

24. Fresnel Zone • Fresnel zone (pronounced “frA-nel”; the “s” is silent). • Provides a method for calculating the amount of clearance that a wireless wave (or light wave) needs from an obstacle to avoid additional attenuation of the signal. Rick Graziani graziani@cabrillo.edu

25. Fresnel Zone • Fresnel Zone = 72.1 * SqrRoot (dist1Mi * dist2Mi / FreqGHz * DistanceMi) • At least 60% of the calculated Fresnel Zone must clear to avoid significant signal attenuation. Rick Graziani graziani@cabrillo.edu

26. 19.7 feet Example: Diameter = 72.1 * [ SquareRoot (D1 * D2) / FreqGhZ * (D1 + D2) ] = 72.1 * [ SquareRoot (1 * 1) / 2.4 * (1 + 1) ] = 72.1 * [ SquareRoot 1 / 2.4 * (2) ] = 72.1 * [ SquareRoot 1 / 4.8 ] = 72.1 * [ SquareRoot .208 ] = 72.1 * .456 = 32.9 feet 60% of FZ = 0.6 (32.9) ft. = 19.7 feet 1 Mile 1 Mile Rick Graziani graziani@cabrillo.edu

27. 60% of FZ = 0.6 (32.9) ft. = 19.7 feet So the wireless wave must clear the building by one-half of the 19.7 ft. diameter or or 9.85 feet 9.85 feet Rick Graziani graziani@cabrillo.edu

28. Fresnel Zone Calculators • http://www.wisp-router.com/calculators/fresnel.php • http://www.tuanistechnology.com/education/calculators/fzc.htm • http://www.firstmilewireless.com/calc_fresnel.html Rick Graziani graziani@cabrillo.edu

29. Antennas – Part 1Antenna Characteristics and Line of Sight Paths Cisco Fundamentals of Wireless LANs version 1.1 Rick Graziani Cabrillo College