1552 Chapter 42
- Carrier current system radiation limits are set differ-
ently. The radiation from the electrical system must
not exceed 15μV/m at a distance of 157,000/Fft,
where F is the frequency in kHz.
An examination of these radiation restrictions indi-
cates that the lowest operating frequency results in the
greatest coverage area. AM systems typically operate on
carrier frequencies below 700 kHz.
The sound quality and noise level of an AM broad-
cast signal are better than from a magnetic induction
loop but inferior to an FM system. In general, if a
pocket AM radio receives a local station well in the
space, the low-power broadcasting system will work as
well. Systems operating in the commercial broad-
casting band (540–1600 kHz) may be picked up on any
AM receiver. Systems operating below the standard
broadcast band are available with fixed-frequency,
nontunable receivers.
At first it may seem economically attractive to select
a system operating in the regular broadcast band
because the patrons could be expected to furnish their
own receivers. But on reflection, there is much to be
said in favor of the special nontunable receiver as the
patron is not required to find the signal among the many
public radio stations.
The coverage area from an AM system depends on
the type of antenna employed. The three types of
antenna are open wire, lossy coaxial cable, and carrier
current.
Open-wire antenna systems, with their restricted
power and antenna length, usually achieve coverage
ranging from 15–45 m (50–150 ft) depending on the
amount of steel in the building. Fixed-frequency,
below-the-broadcast-band systems usually employ an
open-wire antenna. These systems are used by many
churches.
Lossy coaxial cable systems employ a special type of
coaxial cable that has a very loose or open shield,
allowing a little radiation to occur all along the cable.
The usable reception range is 15–23 m (50–75 ft) from
the cable. The length of the cable may be quite long.
The lossy coaxial technique is found in drive-in
cinemas, stadiums, and arenas and can be used for
broadcasting flight arrival and departure information
along the highway approaches to airports.
The most common type of AM system is carrier
current. In this system the output of the transmitter is
capacitive coupled into the main power distribution
wiring of the building. The radio signal travels
throughout the building on the electrical wiring. This
system is widely used on college campuses for
limited-coverage, student-operated radio stations.
Carrier current is an inexpensive way to provide
program monitoring throughout a building.
The costs of the low-frequency, open-wire transmitter
and fixed-frequency receivers are about the same as FM
systems. The lossy coaxial and carrier-current systems
may cost more, depending on the power required and the
length of the coaxial cable. Any of these systems can be
installed easily by following the instructions from the
manufacturer. Both the lossy coaxial and carrier-current
systems should be planned through consultation with the
manufacturer of the transmitter.42.2.5 InfraredInfrared light can be used to broadcast a very
high-quality signal. Presently available systems can
broadcast up to twelve different programs on the same
emitter, making infrared very useful for large-scale
language translation systems. Infrared systems are also
used in museums and for lecturing and teaching on
auscultation, the listening of heart beats. Systems are
also produced for home listening, for both stereo and for
video (TV). It is the only system for the hearing
impaired that can transmit in stereo. Also unlike the
other systems, infrared broadcasts are completely
contained within the room because infrared behaves like
visible light; it cannot go through a wall; even a heavy
cloth is an opaque barrier. This control of the broadcast
range is a significant factor where confidentiality isFigure 42-11. Personal system including FM transmitter,
receiver, and battery charger. Courtesy Sennheiser
Electronic Corporation.