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frequency range--380 Megahertz (MHz) to 399.9 MHz—as many unlicensed low-powered garage door openers, which have operated in this range for years. While DOD has been the authorized user of this spectrum range for several decades, their use of Land Mobile Radios between 380 MHz and 399.9 MHz is relatively new. With DOD's deployment of the new radios and increased use of the 380−399.9 MHz range of spectrum, some users of garage door openers have experienced varying levels of inoperability that has been attributed to interference caused by the new radios. Nevertheless, because garage door openers operate as unlicensed devices, they must accept any interference from authorized spectrum users. This requirement stems from Part 15 of the
Federal Communications Commission (FCC) regulations. Garage door openers and other unlicensed devices are often referred to as "Part 15 devices." Congress requested that GAO review the potential spectrum interference caused by DOD's recent deployment of land mobile radios. Specifically, Congress asked us to (1) determine the extent of the problem of spectrum interference associated with the recent testing and use of mobile radios at military facilities in the United States, (2) review the efforts made by DOD during the development of its land mobile radio system to identify and avoid spectrum interference, and (3) identify efforts to address the problem.
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220 MHz for LMR, but it sees little use. The 800 MHz band (851-866 MHz) is heavily used in most of the US. Frequencies are reserved for Public Safety and for
Industrial users. The 900 MHz band (935-940 MHz) is available solely for Industrial users. Finally, Public Safety entities are allotted an exclusive band of frequencies at 758-806 MHz. Low band has longer range capability, but requires mobile antennas as long as nine feet (2.7 m) tall. VHF bands works well in outdoor environments, over bodies of water, and many other applications. UHF bands typically perform better in urban environments and with penetrating obstacles such as buildings. Commercial and public safety users are required to obtain
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such as humidity and temperature can affect the performance of duplexers, so in most configurations a dryer is installed to keep humidity out of the duplexers and coaxial cable, along with heated buildings in which they are installed in. Excellent quality coaxial cable, connectors, and antennas must also be used, as a single-antenna is not as forgiving as a dual antenna system since any RF leakage or poor connection can greatly decay the reliability and performance of the repeater. In some applications, cables going from the repeater and duplexers must be tuned to mitigate these issues.
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build an array is available. The only issues with dual antenna systems is isolating the antennas so the receiver is not receiving what the transmitter is putting out. If this happens, it creates a loop, much like the feedback heard when a microphone is placed near a speaker. When this happens the repeater amplifies its own signal until it is either powered off or a TOT (time out timer) is expired.
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Repeater units utilize duplexers. These are notch filters usually in an array of six, eight, and sometimes four units. They separate the transmitter and receiver signals from each other so one antenna and coaxial line can be utilized. While this solution is very efficient and easy to install, factors
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tones, which allows the repeater to activate only if the station is sending a particular pre-programmed code, preventing unauthorized stations from using the repeater. Additionally, as repeaters are placed on high locations, it also prevents distant stations on the same frequency from interfering. A
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organizations such as police, fire, ambulance, and other governmental entities. They are allocated frequencies exclusively for their use. Commercial
Private Land Mobile systems are available for businesses in the Business, Industrial, and Land Transportation sectors. Most frequencies are shared with
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allowing them to transmit data as well as voice. Most (30-174 MHz) systems operate simplex, with multiple radios sharing a single radio channel. Only one radio can transmit at a time. The transceiver is otherwise normally in receiving mode so the user can hear other radios on the channel. To
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In dual antenna systems, there are two antennas and two lengths of coaxial cable running from the transmitter and receiver. Usually, triple shield coax and or low loss Heliax are used to keep the two systems isolated. Two antenna systems are usually used if tower space is not limited, or space to
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To address homeland defense needs and comply with government direction that agencies use the electromagnetic spectrum more efficiently, the
Department of Defense (DOD) is deploying new Land Mobile Radios to military installations across the country. The new Land Mobile Radios operate in the same
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polarization. Antennas that have a null spot directly above and below them are excellent choices since another antenna can be placed in the null zone and isn't affected as much. Antennas must also be polarized the same as the stations trying to access the repeater—usually vertical polarization.
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To solve this antennas must be placed several wavelengths from each other in opposite vertical planes. For example, the receiver antenna is vertically polarized, while the transmitter antenna is placed one wavelength (or more) below the receiver antenna, but rotated 180° as to maintain vertical
127:
frequency bands. 30−50 MHz (sometimes called "Low VHF Band" or "Low Band"), 150.8−174 MHz (sometimes called "High VHF Band" or "High Band"), 450−470 MHz "UHF". Many larger populated areas have additional UHF frequencies from 470 to 512 MHz. There is also a tiny segment at
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which is filtered to remove noise, and retransmits it on a second channel to avoid interference with the first signal. This is received by a second two way radio in the repeater's expanded listening area. When the second user replies on the second channel, representing the other half of the
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Land mobile radio systems are widely used by the military. Separate bands in the radio spectrum are reserved for their use. This includes portions of the 30-50 MHz band, and the entire 100-100, 100-100.8, and 540-2400 MHz bands, plus shared use of the 170- 170 MHz band.
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Mobile and portable stations have a fairly limited range, usually three to twenty miles (~5 to 32 km) depending on terrain. Repeaters can be used to increase the range of these stations. They are usually placed upon hills and buildings to increase range.
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Many businesses and industries throughout the world use these radios as their primary means of communication, especially from a fixed location to mobile users (i.e. from a base site to a fleet of mobiles). Commercial radios are typically available in the
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Repeaters have one or more receivers and a transmitter, with a controller. The controller activates the repeater when it detects a carrier on one of its incoming channels, representing a user talking. The repeater receives the radio signal,
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are small enough to mount on vehicles or handheld transceivers. Transmitter power is usually limited to a few watts, to provide a reliable working range on the order of 3 to 20 miles (4.8 to 32 km) depending on terrain.
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Morse code or a synthesized voice module may produce station ID to comply with station identification regulations. Large users assigned use of their own frequencies may be exempted from this requirement.
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systems are available for use by commercial businesses offering communications service to the public for a fee. This would include mobile telephone and paging service, as examples.
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conversation, his signal is received by the repeater and similarly translated and retransmitted on the first channel back to the first user. Most controllers also decode
187:, Canada, had suddenly stopped working, due to a powerful radio signal that appeared to be interfering with the remote controls that open them.
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installed on tall buildings, hills or mountain peaks can be used to increase the coverage area. Older systems use
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band overpowered the garage door openers. One technician likened it to a whisper competing with a yell.
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311:"Telecommunications: Potential Spectrum Interference Associated with Military Land Mobile Radios"
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units), mobile (installed in vehicles), or portable (handheld transceivers e.g. "
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In the summer of 2004, garage door operators noticed similar phenomena around
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23:(LMRS) is a person-to-person voice communication system consisting of
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other users. Land mobile radio systems use channels in the
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293:"New military radio system battles garage-door openers"
270:"Garage doors work after mystery signal vanishes"
183:In November 2005, many automatic garage doors in
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248:"Mysterious signals jamming garage door openers"
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71:bands, since the antennas used at these short
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194:bases. The strong radio signals on the 390
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88:modulation, while some recent systems use
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130:U.S. Federal Communications Commission
38:in one unit) which can be stationary (
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160:Continuous Tone-Coded Squelch System
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291:Hoffman, Lisa (December 8, 2005).
276:. November 7, 2005. Archived from
254:. November 4, 2005. Archived from
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742:
458:Common traffic advisory frequency
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58:systems are available for use by
209:Government Accountability Office
132:licenses in the United States.
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1:
567:Maritime mobile amateur radio
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710:Voting (diversity combining)
448:Aircraft emergency frequency
395:General Mobile Radio Service
179:Interference in the spectrum
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7:
463:Mandatory frequency airport
297:SCRIPPS HOWARD NEWS SERVICE
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10:
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655:Automatic vehicle location
680:Dynamic range compression
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597:Dual-tone multi-frequency
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511:Professional mobile radio
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473:Single Frequency Approach
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97:button that turns on the
93:talk, the user presses a
56:Private land mobile radio
705:Radiotelephony procedure
521:Specialized Mobile Radio
50:Public land mobile radio
21:land mobile radio system
415:Multi-Use Radio Service
731:Military radio systems
487:Land-based commercial
375:Amateur radio repeater
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489:and government mobile
436:(aeronautical mobile)
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526:Trunked radio system
390:Public Radio Service
385:Family Radio Service
363:Amateur and hobbyist
101:of the transceiver.
16:Communication system
557:Coast radio station
443:Air traffic control
380:Citizens band radio
223:Land mobile service
540:Marine (shipboard)
425:UHF CB (Australia)
211:report GAO-06-172R
90:digital modulation
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579:Selective calling
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636:System elements
562:Marine VHF radio
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228:Astro (Motorola)
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317:. Dec 1, 2005.
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44:walkie-talkies
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531:Walkie-talkie
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25:two-way radio
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617:Quik-Call II
607:Push-to-talk
577:Signaling /
506:Mobile radio
496:Base station
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278:the original
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256:the original
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151:audio signal
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105:Military use
95:push to talk
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40:base station
28:transceivers
20:
18:
700:Tone remote
690:Link budget
685:Fade margin
612:Quik-Call I
315:Gao-06-172R
156:half-duplex
147:demodulates
99:transmitter
73:wavelengths
32:transmitter
410:Mobile rig
234:References
30:(an audio
670:DC remote
660:Call sign
434:Aviation
149:it to an
136:Repeaters
86:frequency
82:amplitude
78:Repeaters
725:Category
675:Dispatch
602:MDC-1200
547:2182 kHz
468:MULTICOM
274:CBC News
252:CBC News
217:See also
206:—
36:receiver
645:Antenna
622:Selcall
552:500 kHz
453:Airband
400:KDR 444
627:SELCAL
592:D-STAR
478:UNICOM
420:PMR446
405:LPD433
587:CTCSS
46:").
650:APRS
123:and
34:and
665:CAD
196:MHz
125:UHF
121:VHF
69:UHF
67:or
65:VHF
727::
313:.
295:.
272:.
250:.
19:A
348:e
341:t
334:v
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