ITNW 1351 Fundamentals of Wireless LANs

1. ITNW 1351Fundamentals of Wireless LANs Chapter 2 Antennas

2. Antennas… • Antennas are most often used to increase the range of a wireless LAN system • Proper antenna selection can also enhance security of a wireless LAN • …are most sensitive to RF signals whose wavelength is an even multiple of the antenna’s length (including fractional multiples – such as ½ or ¼)

3. Basic Antenna Principles • Antennae convert electrical energy to RF waves in the case of transmitting antennae OR …RF waves into electrical energy in the case of receiving antennae • The physical dimensions (especially length) of an antenna are directly related to the frequency at which the antenna can propagate or receive waves

4. Basic Antenna Principles • The physical structure of an antenna is directly related to the shape of the area in which it concentrates most of its radiated RF energy

5. Generic Categories of RF antennas… • Omni directional • Semi-directional • Highly-directional • Each category has multiple types of antennas, each having different RF characteristics and appropriate uses

6. Omni directional (Dipole) Antennas… • Most common wireless LAN antenna is a dipole • Standard equipment on most access points • Radiates energy equally in all directions around its axis

7. Omni directional (Dipole) Antennas… • Radiates in a 360-dgree horizontal beam • Sphere = isotropic radiator – sun = example – only theoretical • Radiates in all directions around axis, but does not radiate along the length of the wire itself • Looks like a donut • The higher the gain, the more the donut is squeezed until it looks like a pancake

8. Omni directional (Dipole) Antennas… Dipole Side-View Coverage Area – Top View

9. Omni directional (Dipole) Antennas… Coverage Area – High-gainSide View Coverage Area – Top View

10. Omni directional (Dipole) Antennas… • If placed in center of a single floor of a multistory building, most energy will be radiated along the length of that floor, with some sent to the floors above and below • High-gain omnis offer more horizontal coverage area, but vertical coverage is reduced • Important consideration when mounting on high indoor ceiling

11. Omni directional (Dipole) Antennas… • …used when coverage in all directions around the horizontal is required • most effective where large coverage areas are needed around a central point • commonly used for point-to-multipoint designs with star topology • outdoors – should be placed on top of structure in the middle of the coverage area

12. Passive Gain… • Antennas use passive gain • Total amount of energy emitted by antenna doesn’t increase – only the distribution of energy around the antenna • Antenna is designed to focus more energy in a specific direction • Passive gain is always a function of the antenna (i.e. independent of the components leading up to the antenna

13. Passive Gain… • Advantage… • Does not require external power • Disadvantage… • As the gain increases, its coverage becomes more focused • Highest-gain antennas can’t be used for mobile users because of their tight beam • Active gain involves an amplifier

14. Omni directional Antenna Usage… • Used when coverage is required in all directions around the horizontal axis • Most effective when large coverage areas are needed around a central point • Commonly used for point-to-multipoint designs

15. Omni directional Antenna Usage… • …2 to 5 dBi • treat as an isotropic radiator • Signal above and below center line will be weaker • …5 to 8 dBi • Appropriate for mounting above users • Increase in gain means decreased vertical tolerance • …8 to 10 dBi • Very flat

16. Semi-directional Antennas… • Direct energy from the transmitter significantly more in one particular direction • Often radiate in a hemispherical or cylindrical coverage pattern • Have back and side lobes that, if used effectively, may further reduce the need for additional access points

17. Semi-directional Antennas… • Frequently used types: • Patch & Panel– flat, designed for wall mounting • Focus coverage in horizontal arc of 180° or less • Yagi – elongated, ribbed, and usually housed in an enclosure for moisture protection • Common vertical & horizontal beamwidths of 90° or less • 30° or less average

18. Semi-directional Antennas… Directional Yagi Antenna Directional Patch Antenna

19. -3 dB -3 dB Beamwidth… • Calculated by measuring the number of degrees off-axis where beam drops to ½ (3 dB) of strength at the 0° position… Beamwidth (degrees)

20. VerticalBeamwidth HorizontalBeamwidth Beamwidth… • Two vectors must be considered when discussing an antenna’s beamwidths… • Horizontal = parallel to Earth • Vertical = perpendicular to Earth

21. Beamwidth…

22. Azimuth & Elevation Charts… • Provide a more accurate picture of antenna’s beamwidth • Standard way of representing coverage pattern • Azimuth = top-down view • Elevation = side-view See Text…

23. Semi-directional Antenna Usage… • Ideally suited for short and medium-range bridging • In some cases, semidirectional antennas provide such long-range coverage that they eliminate the need for multiple access points in a building • Patch or panel typically used on short range building-to-building & in-building directional links • Yagis most often used on short to medium length building-to-building bridging up to 2 miles (3.3 km)

24. Highly-Directional Antennas… • High-gain antennas that emit the most narrow signal beam of any antenna type • Greatest gain of any of the types • Typically concave, disk-shaped devices (similar to satellite TV antenna) • Parabolic dishes • Some are grids (grid antennas) – provides good resistance to wind loading • Ideal for long distance, point-to-point wireless links

25. Highly-Directional Antenna Radiation Pattern… Highly-directional antennas are never appropriate for mobile users

26. Highly-Directional Antennas… • These are not for clients usage… • Used for point-to-point communication links • Have a very narrow beamwidth and must be accurately aimed at each other • May be aimed directly at each other within a building in order to “blast” through an RF signal absorbing obstruction • Can transmit at distances up to 58 km (about 35 miles)

27. Basic Antenna Principles… • Line of Sight (LOS) = the apparently straight line from the transmitter to the receiver • Why “apparently” straight? • Remember refraction, diffraction, & reflection? • Can be affected by blockage of the Fresnel Zone

28. Basic Antenna Principles… • Fresnel* Zone = an area centered on the visible LOS between the transmitting and receiving antenna • It defines an area around the LOS that can introduce RF signal interference if blocked • As an obstacle obstructs the zone, energy is absorbed and prevented from getting to the receiver *frā-něl'

29. Basic Antenna Principles… • Another way of defining the Fresnel Zone is a series of concentric ellipsoid-shaped areas around the LOS path… TX RX Fresnel Zone LOS

30. Fresnel Zone… • 20 to 40 % blockage introduces little to no interference into the link • Best to allow no more that 20 % blockage • >40 % means link will be unreliable • Usually not encroached indoors unless signal is partially or fully blocked • Constantly changes in mobile environment • Users dismiss it to simply bad coverage

31. Fresnel Zone… Formula to calculate the 60 % unobstructed (minimum clear) radius around the visual LOS… r = 43.3* X√d/4f r = radius in feet d = link distance in miles f = frequency in GHz • The radius is 60 % smaller than the radius of the entire zone. • *Substitute 72.2 to calculate the radius of the Fresnel Zone itself • The beamwidth is NOT a factor in calculating

32. Basic Antenna Principles… • Antenna Gain = results from focusing the RF radiation into a tighter beam which creates a seemingly “brighter” beam • Example: radiating at 30 degrees rather that 360 degrees at the same power – the beam will radiate farther

33. Basic Antenna Principles… • Intentional Radiator (IR) = an RF device specifically designed to generate and radiate RF signals • Includes: • RF device • All cabling • All connectors up to but not including the antenna • Any reference to power output of the IR refers to power at the last connector before the antenna