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Though the signal can
usually be "heard" more than 10 miles away over open
country, the strongest signal area is normally 3-5 miles in radius,
regardless of the frequency being used. Most operators position
their "tune-to" signs within that distance. Signal
distances can be less in areas with very hilly or mountainous
terrain or very poor soil. Distances can also appear to be less in
areas where interference is present from other radio stations on or
near the operating frequency or where high voltage overhead power
wires or AM-broadcast towers are present. Another factor that
affects the listeners' ability to hear the signal at greater
distances is the car radio s/he is using. Sensitivity of
automobile radios vary, and different cars produce different amounts
of their own engine interference, which can affect radio reception
in areas where signals are not strong.
Regarding hearing the signals in homes: Most communities do not advertise this specifically, because home reception varies widely with many factors, which include: Distance from the home to the AM transmitting antenna (AM signal level); quality of the AM receiver in the home; presence of an AM receiver that will work on loss of power (battery operated); structure of the building and interference sources that it may contain. Solutions: Some communities point residents to Radio Shack for their $29 battery/AC table radio, which works rather well. Other cities make the receiver available to anyone who asks, as a community service. Finally, for reception in critical buildings (schools, hospitals & care facilities) which have notoriously poor AM penetration, ISS now offers and external AM antenna which can bring the AM signal indoors at high quality, and can feed up to four receivers via coax links.
See also "Travelers
Information Station/ALERT AM Signal Penetration in Homes."
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The antenna-height
restriction is 15 meters (49.2 feet) from tip to ground. So don't
select structures too tall. To function well, however, the antenna
must be mounted at the top of the structure you do choose.
Also, due to
propagation characteristics and antenna interactions in low-power AM
frequencies, we do not recommend side-mounting the antenna. The
antenna's coil and tip must be above other objects nearby to prevent
extremely poor results.
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Generally speaking, this class of service is voice only, noncommercial in nature. You are not allowed to transmit music, mention commercial names (except in a noncommercial way, e.g., air carrier names can be used to provide directions at airport facilities) or provide any kind of commercial announcement. Although these radio stations operate in the AM broadcast band, they are licensed under public safety radio regulations; and decision about content must be made with that in mind. The stations are not in any way meant to compete with full-power broadcast services.
Regarding events and announcements: provided such broadcasts meet the regulatory limitations on broadcast content, they can be aired. One question to ask in determining this is whether an announcer will be able to maintain a constant flow of information, so there is little or no "dead air," critical to insuring that would-be listeners hear something within a few seconds; or, chances are, they will tune elsewhere.
Regarding emergency information to people in their homes: while theoretically it is possible, the 10-watt power limitation of these systems means that the signal strength developed is much lower throughout a given area than that of a local commercial broadcaster. The noise sources in and near the home are myriad, including dimmer switches, electric fences, computers and other common devices. Considering the combined effect of these factors, we do not recommend it. it is good to keep mind that the nature of low-power AM service is to transmit directions and other information to motorists. To learn about ISS systems designed for emergency management, see
ALERT AM Emergency Advisory Radio.
Regarding sponsorships: They are not permitted. Sponsorships and advertising are banned from this class of service, explicitly, based on noncommercial service, and implicitly, due to the license class' public safety regulation.
For more on this
topic, please see our page: What May Be Broadcast on
AM Advisory Radio.
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There is no single right answer
for everyone, partly because, in different areas of the country, all AM
frequencies are not available due to FCC separation regulations. Generally,
however, the conventional wisdom is as follows:
The favorite is 1610. Here's
why:
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It is known by listeners as
a common information radio frequency.
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Its short antenna and
smaller groundplane are somewhat more convenient to locate and install.
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There is only one commercial
radio station in the US on that frequency, which means that interference,
especially at night, is usually less.
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The short wavelength is
interfered with less by power lines.
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Almost every car radio
receives the frequency.
The second favorite is 530.
Here's why:
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It is known by listeners as
a common information radio frequency.
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Almost every car radio
receives the frequency.
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There are no commercial
radio stations in the US on 530 to cause interference, the only source of
night interference being Caribbean and Canadian stations in the eastern US
after dark.
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530's long wavelength signal
fades slowly and can produce a large fringe coverage area.
The third favorite is
1620-1700. Here's why plus one negative:
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Very few commercial radio
stations are permitted on these frequencies, which lessens interference
and produces good signal range for information radio stations using them.
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Short antenna and smaller
groundplane are somewhat more convenient to locate and install.
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The short wavelength is
interfered with less by power lines.
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Negative: At
this time, not all car radios receive these frequencies, especially 1630
and above. This situation, however, improves every year and someday will
not be an issue. Of this group, 1620 is preferred, because it appeared on
many car radios before the other frequencies.
The fourth favorite is
540-1600. Here's why plus one negative:
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Almost every car radio
receives the frequencies, being in the original AM band.
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Negative:
Nighttime coverage is usually less.
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5. May I use
low-power FM instead?
At present, the FCC is not
taking applications for a low power FM service (LPFM), though it did as
recently as 2001. At that time, the Commission granted a handful of FM low
power stations, each with about 100 watts power (about 5 miles of range). The
FCC has not indicated that it will again offer the service for another round
of applications. Of special note: first-round grantees discovered that LPFM
stations are not ideal as pure information sources because of FCC-imposed
operating parameters, which, for example, mandated that LPFM station operation
be more like commercial FM stations in terms of hours of operation, content
restrictions and equipment requirements.
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6. How
many messages may play at once on RealTIME Transmitter Synchronization stations?
Only one recording is advisable for
RealTIME Transmitter Synchronization stations, because they are on the same frequency and would interfere with each other. However, a community may have more than one station, as long as the stations are on different frequencies.
An instance of 2 different stations with different messages is, drivers who cross the Macinack Bridge (in Michigan) hear one message on k530 and a different message when returning, transmitted on k1610. The stations are not
synched.
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7.
Can any field tests be done to see how well a system would
perform; if so, how far will it cover?
Yes, but these tests are commonly done only where the exact coverage pattern or range needs to be determined in advance of purchase. They are not often required, because the transmitter power can be set anywhere in the 0-10 watt range to produce a standardized signal level at the FCC-mandated 1.5 km distance from the antenna. This means that unless there is severe terrain, very poor soil or an interference potential, almost all ALERT AM stations will produce the same coverage pattern. As a result,
pre-testing isn't often required.
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8. Can this system be used for the
AMBER Alert Program?
AMBER Alerts were not anticipated by the FCC when they instituted this service in the
1970s. It is generally thought, however, that AMBER Alerts are a perfect addition to the content that's allowed, perhaps under the heading "travel
advisories." Of course, if the AMBER Alert is broadcast across the NOAA/EAS radio in the area, it may automatically be broadcast on
the ISS ALERT AM Emergency Advisory Radio System.
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10. How can ALERT AM tie in with our outdoor warning
sirens?
There are 3 contacts in the ALERT AM system that you can use to make a specific pattern of prerecorded messages trigger in conjunction with a specific siren activation. Some communities simply assign a siren controller to the ALERT AM system, and the radio system gets the same cue that the sirens get when they are activated. Other communities put pushbuttons in the EOC which the dispatcher can push when certain situations occur, to immediately broadcast messages prerecorded for those situations.
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This page was last updated:
May 06, 2008. |
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