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Wireless Data Acquisition and Control How Antennas Work Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 3 Element Yagi Yagi Horizontal Beam Pattern Omni-Directional
ANTENNA GAIN Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 2 ABOUT ANTENNAS The antennas in a wireless system are extremely important to the reliable performance of the system. The transmitter antenna radiates a magnetic field that must couple to the receiver antenna with sufficient strength to create a power level above the minimum threshold of the receiver. The gain of the antennas, the polarization of the elements, the height above ground or other objects, and the lack of obstructions between the two antennas play an important role in achieving satisfactory performance.
REFERENCE ANTENNAS GAIN RECIPROCITY Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 3 Antennas have a gain that is usually referenced to an isotropic source or a dipole*. The gain is the ratio of the power level out of the antenna of interest to the power level out an isotropic source or a dipole. Gain referenced to an isotropic source is expressed in dBi while gain referenced to a dipole is commonly expressed in dB or dBd. All antennas in this document have their gain expressed in dBi. Antennas have perfect reciprocity. The have the same characteristics whether used as a transmitting or receiving antenna. The gain is identical for both applications. *A dipole is 2 elements end to end with the RF signal fed to the ends of the elements at the center of the 2 elements.
4 Gain is expressed as the ratio of the antenna power out (transmitting) or in (receiving) to the reference antenna power out or in. Assume a reference antenna is used to receive a signal from a transmitter. Then the antenna of interest is used. The gain of the antenna of interest is: Gain = 10Log(P2/P1) dBi P2 = Power level of signal out of antenna of interest P1 = Power level of signal out of reference antenna Examples: A power ratio of 2 yields Gain = 10Log(2) Log 2 = 0.30 Gain = 3dBi A power ratio of 10 yields Gain = 10Log(10) Log 10 = 1 Gain = 10dBi GAIN Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved
HOW ANTENNAS GET THEIR GAIN (Reference Isotropic Source) An isotropic source antenna radiates a signal in a perfect sphere from a point source. At a given distance from the antenna, the signal level is the same at any point on the sphere. There is no isotropic antenna. It is a mathematical model only. Gain in an antenna is achieved by shaping the magnetic field to reduce the field in some directions by forcing an increase in other directions. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 5 Isotropic Source Perfect Solid Sphere Radiation Pattern
6 VERTICAL ANTENNAS This image is the vertical beam pattern of a 2.1dBi gain vertical antenna. The radiation angle has been changed by the design so it radiates more in a horizontal direction with a reduction of radiation vertically. It has a doughnut shaped pattern horizontally which gives it the name "omni-directional antenna". *Wavelength is the distance between 2 zero points on the RF signal (sine wave). Wavelength = Velocity of wave / Frequency ¼ wave at 900MHz = 3.1 inches All antenna design is frequency dependent. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved The simplest antenna is a vertical wire 1/4 wavelength* long with a metallic plane under the bottom. It has a gain of 2.1dBi.
7 VERTICAL ANTENNAS This image is the vertical beam pattern of a 6dBi gain vertical antenna. The radiation angle is much narrower than the 2.1dBi gain antenna. The narrow radiation angle puts more power out in a horizontal direction and less vertically. A longer mechanical length creates the greater gain by modifying the radiation pattern. It has a doughnut shaped pattern horizontally. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved
YAGI ANTENNAS Yagi antennas have a greater gain capability than a simple vertical antenna. This is possible due to the ability to reduce radiation in a vertical as well as horizontal direction and direct more energy in a single direction. The horizontal beam pattern shows the reduction in energy to the rear and sides of the antenna. The vertical beam pattern shows the reduction in energy to the top and bottom as well as the rear of the antenna. This 3 element yagi has a gain of 8dBi. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 8
YAGI ANTENNAS This 10 element yagi has a gain of 14dBi. Note the narrower beam width in the horizontal and vertical planes. The higher gain is achieved by narrowing the energy into this narrower beam in the horizontal and vertical planes. The insensitivity of the yagi to signals from the rear makes it a good choice for reducing interference from the back direction. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 9 The difference in gain for the 2 directions is labeled the front to back ratio. This antenna has a front to back ratio of 20dB to 30dB, depending on the polarization used.
POLARIZATION A vertical omni-directional antenna has vertical polarization. Any antenna sending or receiving signals to/from it must have the same polarization. For the magnetic field around an antenna to be received by another antenna, the magnetic field must be at a right angle to the receiving antenna. Since the magnetic field is at a right angle to the transmitting antenna, the 2 antennas must be parallel to each other. A yagi must have its elements parallel to another yagi or an omni-directional antenna. When antenna elements are cross polarized, the signal at the receiving antenna can approach zero. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 10
POLARIZATION Yagi antennas must be aligned pointing at each other and with the same polarization. Yagi antennas must be aligned pointing at omni-directional antennas and must have the same polarization. Omni-directional antennas at different heights must be tilted to make the elements parallel and the elements should be in the same plane. Treat them as a single element yagi and point them at each other. Antennas can provide a wide choice of gain to insure reliable operation. The cables that connect the antennas to the receiver and transmitter have a loss in RF energy. The resultant system gain is the antenna gain minus the coaxial cable loss. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserve d 11
SHORT RANGE ANTENNAS Small vertical antennas are used for short range communications 7” vertical dipole antenna for indoor mounting. Mounts by screwing on RPSMA bulkhead connector. Articulated 90 degree joint allows horizontal or vertical connector to be used. Line of sight range about 1500 to 2000 feet. 2.1dBi gain 2 3/4” vertical antenna for outdoor mounting. Mounts on a special bulkhead connector. A bracket is available for mounting on a flat vertical surface or a mast. This type antenna must mount on a “ground plane” such as a metal enclosure or a metal disk. Line of sight range about 1500 to 2000 feet. 2.1 dBi gain Vertical omni-directional antenna for outdoor mounting. Type N female connector to accept coaxial cable. Has bracket for mounting on 1 ½” mast. Line of sight range several miles. 6dBi and 8.5dBi gain models. 27” and 60” lengths. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 12
915MHz Band Antennas LONG RANGE ANTENNAS Yagi antennas with high gain are used for long range communications. 8 dBi gain 18” length 11 dBi gain 21” length 12 dBi gain 26” length 14 dBi gain 48” length 2 Stacked Yagi 17 dBi gain 48” length Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 13 MAST
RECEIVER SENSITIVITY Receiver sensitivity is usually expressed as the threshhold power level at which the receiver detects a useable signal level. The level is expressed in dBm (decibels referenced to 1 milliwatt across 50 ohms). Antennas and receiver input impedance is standardized at 50 ohms. An outstanding receiver may have a sensitivity of -110dBm, while many will have a sensitivity of -90dBm. The 20 dB difference is a factor of 100 difference in power sensitivity. A wireless system should be designed for a 20dBm signal safety margin. 1 milliwatt across 50 ohms = 0.223V. -90dBm = 22.3µV. A 20 dBm Safety margin = -70dBm = 223µV -110dBm = 2.23µV. A 20 dBm Safety margin = -90dBm = 22.3µV Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 14
TRANSMITTER POWER The FCC regulations for the 900MHz ISM band limits radiated power to 4 watts. If a 100mW transmitter is used, the antenna gain minus cable losses cannot exceed 16dB. In a one way point to point system, receiver antenna gain can be anything possible. If a transmitter is involved, the 16dB rule applies. Transmitters radiate a sine wave AC signal. The sine wave is not perfect, therefore harmonics of the fundamental frequency are radiated. The FCC requires the antenna to appear to be, within limits, a 50 ohm resistor to the transmitter to keep down excessive radiation of harmonics. To prevent improper antennas from being easily connected to ISM transmitters, they are fitted with reverse polarity coax connectors. The reverse polarity connectors are not easily found. A common connector is the SMA series. On a transmitter, one will find an RPSMA connector. Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 15
MORE INFORMATION Wilkerson Instrument Company is creating a group of Power Point presentations for wireless applications, and other instrumentation problem solving applications. We are also creating application notes in a pdf format for the same subjects. The slide presentations and application notes can be viewed on our web site www.wici.com.www.wici.com These aids can be downloaded. The slide presentations and application notes will also be on our CD Wireless Data Acquisition Products and Instrumentation Products catalogs. These catalog CD’s can be downloaded or they can be requested from email@example.com. Tel.800 234 firstname.lastname@example.org The application notes will be added to our email broadcast list. If you would like to be added to our list, please send a note to email@example.com@wici.com Richard Huffman President Wilkerson Instrument Company, Inc firstname.lastname@example.org Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 16
Need More Information! Wilkerson Instrument Co., Inc. 2915 Parkway Street Lakeland, FL 33811 Toll Free: 800-234-1343 Phone: 863-647-2000 Fax: 863-644-5318 Email: email@example.com@wici.com Complete documentation is available for all Wilkerson Instrument Co. products at www.wici.com.www.wici.com Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 17
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