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Radio-frequency transducers
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The term radio-frequency (RF) transducer is a fancy name for an antenna. Antennas are so common that you probably don t think about them very often. Your car radio has one. Your portable headphone radio, which you might use while jogging on a track (but never in traffic), employs one. Cellular and cordless telephones, portable television receivers, and handheld radio transceivers use antennas. Hundreds of books have been written on the subject. There are two basic types of RF transducer: the receiving antenna and the transmitting antenna. A receiving antenna converts electromagnetic (EM) fields, in the RF
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614 Wireless and personal communications systems range from about 9 kHz to several hundred gigahertz, into ac signals that are amplified by the receiving apparatus. A transmitting antenna converts powerful alternating currents into EM fields, which propagate through space. There are a few significant differences between receiving antennas and transmitting antennas designed for a specific radio frequency. The efficiency of an antenna is important in transmitting applications, but not so important in reception. Efficiency is the percentage of the power going into a transducer that is converted into the desired form. If the input power to a transducer is Pin watts and the output power is Pout watts, the efficiency in percent, Eff%, can be found using the following equation: Eff % 100 Pout /Pin
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In a transmitting antenna, 75 W of RF power are delivered to the transducer, and 62 W are radiated as an EM field. What is the efficiency of the transducer To solve this problem, plug the numbers into the formula. In this particular case, Pin 75 and Pout 62. Therefore, Eff% 100 62/75 100 0.83 83 percent
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Another difference between transmitting and receiving antennas is the fact that, for any given frequency, transmitting antennas are often larger than receiving antennas. Transmitting antennas are also more critical as to their location. Whereas a small loop or whip antenna might work well indoors in a portable radio receiver for the frequencymodulation (FM) broadcast band, the same antenna would not function well at the broadcasting station for use with the transmitter. Still another difference between transmitting and receiving antennas involves power-handling capability. Obviously, very little power strikes the antenna in a wireless receiver; it can be measured in fractions of a microwatt. However, a transmitter might produce kilowatts or even megawatts of output power. A small loop antenna, for example, would get hot if it were supplied with 1 kW of RF power; if it were forced to deal with 100 kW, it would probably melt.
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Infrared transducers
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Many wireless devices transmit and receive energy at infrared (IR) wavelengths, rather than at radio wavelengths. Infrared energy has a frequency higher than that of radio waves, but lower than that of visible light. Infrared is sometimes called heat radiation, but this is a misnomer. Some wireless devices transmit and receive their signals in the visible-light range, although these are encountered much less often than IR devices. The most common IR transmitting transducer is the infrared-emitting diode (IRED). You learned about this type of diode in chapter 20. A fluctuating direct current is applied to the IRED. The current causes the device to emit IR rays; the fluctuations in the current constitute the modulation, and produce rapid variations in the intensity
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Wireless local area networks 615 of the rays emitted by the semiconductor junction. The modulation contains information, such as which channel your television set should seek, or whether the volume is to be raised or lowered. Infrared energy is not visible, but at some wavelengths it can be focused by ordinary optical lenses and reflected by ordinary optical mirrors. This makes it possible to collimate IR rays (make them essentially parallel) so they can be transmitted for distances up to several hundred feet. Infrared receiving transducers resemble photodiodes or photovoltaic cells, which were also discussed in chapter 20. The only real difference is that the diodes are maximally sensitive in the IR, rather than in the visible, part of the electromagnetic spectrum. The fluctuating IR energy from the transmitter strikes the P/N junction of the receiving diode. If the receiving device is a photodiode, a current is applied to it, and this current varies rapidly in accordance with the signal waveform on the IR beam from the transmitter. If the receiving device is a photovoltaic cell, it produces the fluctuating current all by itself, without the need for an external power supply. In either case, the current fluctuations are weak, and must be amplified before they are delivered to whatever equipment (television set, garage door, oven, security system, etc.) is controlled by the wireless system. Infrared wireless devices work best on a line of sight, that is, when the transmitting and receiving transducers are located so the rays can travel without encountering any obstructions. You have probably noticed this when using television remote control boxes, most of which work at IR wavelengths. Sometimes enough energy will bounce off the walls or ceiling of a room to let you change the channel when the remote box is not on a direct line of sight with the television set. But the best range is obtained by making sure you and the television set can see each other. You cannot put an IR control box in your pants pocket and expect it to work. Radio and IR control boxes are often mistaken for one another because they look alike to the casual observer.
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