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Intermediate and Radiofrequency Sources and Exposures in Everyday Environments
Published in Ben Greenebaum, Frank Barnes, Bioengineering and Biophysical Aspects of Electromagnetic Fields, 2018
Transmitters are typically mobile or portable communication devices, either handheld or attached to vehicles. They are frequently used by police, fire, and other emergency services, but also by maintenance staff, security agencies, and other industrial and commercial activities. Portable systems include walkie talkies, cordless telephones, cellular phones, and marine and airplane communication systems. Transmitters commonly attached to vehicles include citizen band (CB) radio and other types of two-way radios. Analogical cordless telephones work with frequencies around 50 MHz, while cellular/mobile phones and modern DECT phones work in the range between 380 up to 3,500 MHz, depending on the technology used. Exposure levels depend on the power of the device and its frequency. Electric field strengths between 20 and 700 V/m have been measured near transmitters attached to vehicles working at 800 MHz. Hand-held transmitters or transceivers’ emissions occur near the head of the users, so recommended exposure limits can sometimes be exceeded (Hitchcock and Patterson, 1995; Lambdin and EPA, 1979; Ruggera, 1979). Table 2.2 summarizes the characteristics of the antennas and transmitters further described in this section.
Widget Deconstruction #1: Smartphone
Published in John D. Cressler, Silicon Earth, 2017
Radios transmit and receive your voice via a modulated (altered) “carrier” frequency. The voice information is “encoded” into the high-frequency EM signal traveling at the speed of light (186,000 miles/s) by radio 1 and sent, and is then “decoded” by radio 2 on arrival so you can hear it. This voice modulation might be accomplished by changing the frequency of the EM carrier wave (i.e., frequency modulation—FM) or by changing the magnitude (amplitude) of the EM carrier wave (i.e., the older, and often static-filled, amplitude modulation—AM). In the case of smartphones, a more complicated carrier modulation scheme is required. The “range” of the radio, how far it can communicate effectively, depends on the signal output power of the transmitter and sensitivity of the receiver. The range might be 1 mile for a walkie-talkie using a 0.25 W transmitter or 5 miles for a 10 W CB radio.
Everyman’s radio-telephone: how CB radio users mobilized West Germany’s communication culture
Published in History and Technology, 2019
Citizen Band (CB) radio relies on a simple two-way radio technology with rather inferior performance features. It lacks any operator or intermediary infrastructure, and any given channel can be listened to by anyone ‘online’ on his or her own station. As a short-range mobile communication medium, CB radio can only be used on a regional scale. The analog radio signals cover a radius of up to 15 or 20 kilometres, although in cities or when operated with low-power equipment this can shrink to a single kilometre. At the same time, these technical characteristics make CB radio an autonomous medium, open to anyone who owns a station. And indeed, users in the heyday of the CB era ultimately appropriated the medium in multiple ways, both on-the-go and domestically, precisely due to CB radio’s low-tech and low-cost characteristics.