Ch. 7 - Antennas

1. Ch. 7 - Antennas Cisco Fundamentals of Wireless LANs version 1.1 Rick Graziani Cabrillo College

2. Overview • Everything about antenna choice involves a tradeoff. • If maximum range is desired, coverage must be traded. • With a directional antenna, the same amount of power reaches the antenna, but the antenna design can reflect and direct the RF energy in tighter and stronger waves, or wider and less intense waves, just as with a flashlight. Rick Graziani graziani@cabrillo.edu

3. Antennas

4. Introduction • Antennas generally fall into two categories: • Directional • Omnidirectional Radiate RF energy equally in all horizontal directions. Radiate RF energy predominantly in one direction. Rick Graziani graziani@cabrillo.edu

5. Introduction • The antennas used for WLANs have two functions: • Receive: • This is the sink or terminator of a signal on a transmission medium. • In communications, it is a device that receives Information, control, or other signals from a source. • Transmit: • This is the source or generator of a signal on a transmission medium. Rick Graziani graziani@cabrillo.edu

6. Introduction • Two way radio communications can take place with: • FDD (Frequency Division Duplex) • Full duplex • A different frequency is used in each direction • Must allocate two spectrum in two bands, one for each direction. • TDD (Time Division Duplex) • Half duplex • Uses same channel or frequency, but with alternating periods of transmitting and listening. Rick Graziani graziani@cabrillo.edu

7. Variables • Antenna maximum distances are usually expressed in kilometers or meters. • The maximum link distance is not easy to solve and is governed by all of the following: • Maximum available transmit power • Receiver sensitivity • Availability of an unobstructed path for the radio signal • Maximum available gain, for the antenna(s) • System losses (such as loss through coax cable runs, connectors, and so on) • Desired reliability level (availability) of the link Rick Graziani graziani@cabrillo.edu

8. Variables More later… Rick Graziani graziani@cabrillo.edu

9. Ranges • Vendor ranges are usually optimized for best conditions. • A link distance can exceed standard distances, if consistently higher error rates are acceptable. Rick Graziani graziani@cabrillo.edu

10. Antenna Bandwidth: Frequency Range • (There are various definitions of antenna bandwidth.) • The bandwidth of an antenna is the band of frequencies, over which it is considered to perform acceptably. • The wider the range of frequencies a band encompasses, the wider the bandwidth of the antenna. • Antennas are ordered pre-tuned by the manufacturer, for use in a specified band segment. • The trade-off in designing an antenna for a wider bandwidth is that it would generally not have as good of performance in comparison to a similar antenna that is optimized for a narrower bandwidth. Rick Graziani graziani@cabrillo.edu

11. 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

12. Gain – It’s all relative! theoretical isotropic antenna • The gain of any antenna is essentially a measurement of how well that antenna focuses radiated RF energy, in a particular direction. • There are different methods for measuring this. • Cisco is standardizing on dBi to specify gain measurements. • This method of measuring gain uses a theoretical isotropic antenna as a reference point. • Some antennas are rated in dBd, which uses a half -wave dipole type antenna. • To convert any number from dBd to dBi, simply add 2.14 to the dBd number. dBi = dBd + 2.14 Half-wave dipole antenna Rick Graziani graziani@cabrillo.edu

13. Decibel references (Review) • Example: • 1 mW = .001 Watts • Using 1 mW as our reference we start at: 0 dB • Using the dB formula, doubling the milliwatts to 2 mW or .002 Watts we get +3 dBm • +10 dBm is 10 times the original 1 mW value or 10 mW • +20 dBm is 100 times the original 1 mW value or 100 mW Rick Graziani graziani@cabrillo.edu

14. Path-loss (Review) • Every time the distance from the transmitter to the receiver is doubled, the signal level is lowered (or increased) by 6 dB 1/4th or 4 times). • 6 dBm = 4 times or ¼ • 3 dB + 3dB = 2 times + 2 times = 4 times • -3dB + -3dB = ½ + ½ = ¼ • This is also know as the inverse square law. • “Signal strength does not fade in a linear manner, but inversely as the square of the distance. This means that if you are a particular distance from an access point and you move measure the signal level, and then move twice a far away, the signal level will decrease by a factor of four. You move 2x and the signal decreases by 1/4x; hence the inverse square law. (Move 4x, signal decreases by 1/16x.) In any case, the fact that exponential measurements are involved in signal strength measurement is one reason why the use of logarithmic scale of measurement was developed as an alternative way of representing RF power.” WildPackets White Paper Rick Graziani graziani@cabrillo.edu

15. Half-wave dipole antenna (FYI) • Half-wave dipole • The length from end to end is equal to half the wavelenth at that frequency. • “At any angle, the distance of the "surface" from the origin indicates the intensity of radiated power in that particular direction. The surface is shaped something like a "bagel", such that zero power is transmitted along the line of the axis, and the maximum power is radiated along the "equator" (i.e. the plane orthogonal to the axis). In the "equatorial plane", however, the antenna is omnidirectional - that is to say it radiates energy uniformly in all directions.” http://www.kingston.ac.uk/~ku12881/bcomms/lect3.htm Half-wave dipole antenna Rick Graziani graziani@cabrillo.edu

16. Other decibel references besides mW • dB dipole (dBd) - This refers to the gain an antenna has, as compared to a dipole antenna at the same frequency. • A dipole antenna is the smallest, least gain practical antenna that can be made. • dB isotropic (dBi) - This refers to the gain a given antenna has, as compared to a theoretical isotropic, or point source, antenna. • Unfortunately, an isotropic antenna cannot exist in the real world, but it is useful for calculating theoretical coverage and fade areas. • A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. • For example, a simple dipole antenna has a gain of 2.14 dBi or 0 dBd. From Ch. 3 Rick Graziani graziani@cabrillo.edu

17. EIRP - Effective Isotropic Radiated Power • EIRP – Effective Isotropic Radiated Power • The actual power transmitted by a radio connected to an antenna. • EIRP takes the gain of an antenna in units of dBi, relative to an isotropic antenna, and multiples it by the net power offered by the transmitter to the antenna. • Measured in dBi • ERP (Effective Radiated Power) • Same as EIRP but with gain expressed relative to a dipole antenna. • Measured in dBd Rick Graziani graziani@cabrillo.edu

18. Other decibel references besides mW • Effective Isotropic Radiated Power (EIRP) - EIRP is defined as the effective power found in the main lobe of a transmitter antenna. • It is equal to the sum of the antenna gain, in dBi, plus the power level, in dBm, into that antenna. • Gain - This refers to the amount of increase in energy that an antenna appears to add to an RF signal. • There are different methods for measuring this, depending on the chosen reference point. • Cisco Aironet wireless is standardized on dBi to specify gain measurements. • Some antennas are rated in dBd. • To convert any number from dBd to dBi, simply add 2.14 to the dBd number. From Ch. 3 Rick Graziani graziani@cabrillo.edu

19. Gain • 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. Rick Graziani graziani@cabrillo.edu

20. Gain • Antennas have gain in particular directions • Direction other than the main intended radiation pattern, are typically related to the main lobe gain Rick Graziani graziani@cabrillo.edu

21. Cisco Aironet 802.11b Antennas • FCC requires that ALL antennas sold by spread spectrum vendor be certified with the radio they are to be sold with • All Cisco Aironet 802.11b supplied cables, RF devices and antennas have reverse polarity TNC (RP-TNC) connectors • Cisco Aironet supplied antennas meet all FCC rules Wide variety of 802.11b antennas for most applications Rick Graziani graziani@cabrillo.edu

22. Cisco Aironet 802.11a Antennas • FCC requires that all radios utilizing the UNII-1 Band (5.15 GHz – 5.25 GHz) must have non-removable or integrated antennas • FCC allows radios utilizing the UNII-2 Band (5.25 GHz – 5.35 GHz) to have external or removable antennas • The Cisco Aironet 802.11aradios utilize both UNII-1 and UNII-2 bands, therefore cannothave external or removable antennas • Cisco 802.11a antennas are integrated into the radio module • Cisco 1400 radios utilize UNII-3 bands, therefore have external or removable antennas Rick Graziani graziani@cabrillo.edu

23. Polarization • Polarization is the physical orientation of the element on the antenna that actually emits the RF energy. • An omnidirectional antenna is usually a vertically polarized antenna. • AllCisco antennas are set for vertical polarization. Rick Graziani graziani@cabrillo.edu

24. Using different Antennas • The antennas for both ends of a link do not need to be the same size or type. • In some cases, the antenna mounting arrangements at one end of a link may only be able to physically support a relatively small antenna. • The link may require a larger antenna at the other end to provide the needed antenna gain for the path length. • On the other hand, a high-gain, narrow-pattern antenna may be needed at one end in order to avert an interference problem, which may not be a concern at the other end. • If two antennas have different gains, it does not matter which antenna is at which end, except in consideration of mounting or interference issues. • Remember that even though the two antennas for a link may look very different from each other, they must have the same polarization for the link to work properly. Rick Graziani graziani@cabrillo.edu

25. Radiation patterns • Imagine pressing in the top and bottom of a balloon. • This causes the balloon to expand in an outward direction, which covers more area in the horizontal pattern. • It also reduces the coverage area above and below the balloon. • This yields a higher gain, as the balloon, which represents the antenna, appears to extend to a larger horizontal coverage area. Rick Graziani graziani@cabrillo.edu

26. Space Diversity • With space diversity, the receiver of a microwave radio accepts signals from two or more antennas that are spaced apart by many wavelengths. • The signal from each antenna is received and then simultaneously connected to a diversity combiner. • Depending upon the design, the function of the combiner is either to select the best signal from its inputs or to add the signals together. Rick Graziani graziani@cabrillo.edu

27. Frequency Diversity • With frequency diversity, the information signal is simultaneously transmitted by two transmitters operating at two different frequencies • If the separation in frequencies of the two transmitters is large, the frequency selective fading will have low probability of affecting both paths to the same extent. • This will improve the system performance • Access points can have two antennas attached to them. • These two antennas are for diversity in signal reception, not to increase coverage. Rick Graziani graziani@cabrillo.edu

28. Omni-directional Antennas

29. Omni-directional Antennas • An omni-directional antenna is designed to provide a 360 degree radiation pattern. • This type of antenna is used when coverage in all directions from the antenna is required. • Omni-directional antennas come in many different styles and shapes. • Most operated in the 2.4 GHz ranges, whereas a few operate in the 5 GHz range. • Omni-directional antennasinclude dipoles, mast mount, pillar, and patch antennas. • The standard 2.14 dBi "Rubber Duck" is the most commonly used omni-directional antenna. Rick Graziani graziani@cabrillo.edu

30. Dipole Antenna Radiation Pattern Side View (E) • The radiation patterns will be shown as a horizontal, looking down (H-plane) radiation pattern, an Elevation, looking across (E-plane), or Vertical radiation pattern, or both. Top View (H) Side View (E) Side View (E) Rick Graziani graziani@cabrillo.edu

31. 2.2 dBi Dipole “rubber duck” antenna(s)(AIR-ANT4941) • Indoor diversity dipole antennas with a base are designed to extend the range of Aironet LMC client adapters and has two MMCX (2) connectors instead of the RP-TNC connector. Rick Graziani graziani@cabrillo.edu

32. Cisco 2.2 dBi ceiling mount diversity patch antenna Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

33. Cisco 5.2 dBi ceiling mount omni-directional antenna Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

34. 5.2 dBi Mast Mount Vertical Omnidirectional indoor/outdoor antenna Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

35. 12 dBi Omnidirectional antenna (outdoor only) Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

36. 5 GHz outdoor wireless bridge 9-dBi omnidirectional antenna Used with 1400 Bridge Side View (E Plane) Vertical Radiation Top View (H) Rick Graziani graziani@cabrillo.edu

37. 5.14 dBi Pillar Mount Diversity Omni Designed to be mounted to the side of a pillar Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

38. Integrated antennas - 1100 Rick Graziani graziani@cabrillo.edu

39. Integrated Antennas - 1200 Rick Graziani graziani@cabrillo.edu

40. Directional Antennas

41. Directional Antennas • Directional antennasdo not offer any added power to the signal, and instead simply redirects the energy it received from the transmitter. • By redirecting this energy, it has the effect of providing more energy in one direction, and less energy in all other directions. • As the gain of a directional antenna increases, the angle of radiation usually decreases, providing a greater coverage distance, but with a reduced coverage angle. • Directional antennas include Yagis, patch antennas, and parabolic dishes. • Parabolic dishes have a very narrow RF energy path and the installer must be accurate in aiming these at each other. Rick Graziani graziani@cabrillo.edu

42. Cisco 6 dBi patch antenna Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

43. 6 dBi diversity patch antenna Indoor/outdoor antenna with two RP-TNC connectors. It is similar to the above patch, but providing diversity antennas in the same package for areas where multipath problems exist Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

44. Cisco 8.5 dBi antenna Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

45. 13.4 dBi Yagi (outdoor/indoor) Linear array of parallel dipoles • The Yagi is constructed of at least three elements, which are metal bars that supplement the wave energy transmitted. • In a Yagi antenna, there is at least one driven element, one reflector element, and usually one or more director elements. • The Yagi antenna is also known as a linear end-fire antenna or a Yagi-Uda array, has a linear array of parallel dipoles. • Yagi antennas are directional and designed for long distance communication. Rick Graziani graziani@cabrillo.edu

46. 13.4 dBi Yagi (outdoor/indoor) • The Cisco Yagi provides 13.5 dBi of gain and features a range of up to 10 km (6.5 miles) at 2 Mbps, and 3.2 km (2 miles) at 11 Mbps. • Most Yagi antennas are mounted with U-bolts, to a sturdy mast. Top View (H Plane) Horizontal Radiation Rick Graziani graziani@cabrillo.edu

47. 21 dBi Parabolic Dish • Distances of up to 40 km (25 miles) may be possible. • It is important to evaluate how well the dish will withstand icy conditions and high winds. • Equally important is the sturdiness of the mast and tower the antenna will be mounted on. • The Cisco high gain parabolic dish is designed to be used as a bridge antenna between two networks or for point-to-point communications Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

48. 5 GHz 28-dBi dish antenna antenna • Operates in the UNII-3 band (5725 to 5825 MHz) • Can be extended up to 12.9 miles (20.7 kilometers) at 54 Mbps. Top View (H Plane) Horizontal Radiation Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

49. 9.5-dBi sector antenna • Used with the Cisco Aironet 1400 Series Outdoor Wireless Bridge • The antenna is not compatible with other Cisco Aironet radio products operating in the 5-GHz frequency band. Top View (H Plane) Horizontal Radiation Side View (E Plane) Vertical Radiation Rick Graziani graziani@cabrillo.edu

50. Cable and Accessories