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90 Handbook of Intercom Systems Engineering
Special Considerations
Wireless communications are here to stay. They have become an integral part of the total
professional communications package. There are, however, many factors associated with
wireless that need to be understood and addressed that do not come into play with
hardwired communications systems. In this section, we look at the special considerations
that must be considered when deciding whether or not to implement a wireless system.
The first area of study is the RF spectrum and how it can be used to implement a wireless
intercom system. Traditionally, wireless intercoms have been a function of broadcast
television productions, and as such have used, at least in part, a spectrum that falls under
FCC Code 47 CFR, Part 74 in addition to itinerant frequencies. The spectrum most
commonly used falls into two areas: VHF systems from approximately 154 MHz to 216
MHz, and UHF systems from 460 MHz to 608 MHz and 614 - 806 MHz. As mentioned in
an earlier section, large chunks of this spectrum have either been reallocated, or will soon
be reallocated. The FCC has found that auctioning spectrum is a good way for the
commission to move from an expense center to a profit center, and they are pursuing it
with a passion.
Wireless intercoms, like any other wireless system, require at least one transmitter to
function. Under FCC rules, all transmitters must be licensed prior to operation (there are
some very low power transmitters that can operate under Part 15 and do not need to be
licensed, but that doesn’t apply to any modern RF intercom systems). There are different
forms to obtain various types of licenses depending on what area of the spectrum your
system will operate in, who will be operating the system, and what the system will be used
for. The law is very clear in that no one is permitted to operate a transmitter typically used
for wireless intercom systems without first obtaining an FCC license.
Wireless equipment often operates in areas of the RF spectrum that are designated for TV
channels, but are unused in a given area. In all cases low power transmitters used by
wireless intercoms and wireless mics must operate on a secondary, non-interfering basis.
This means that wireless users must not cause harmful interference to television or other
receivers, and must accept all interference sources. In keeping with this, the FCC rules
state that VHF systems must not be operated within 50 miles of a television transmitter
occupying a similar spectrum. The rules further state that UHF systems must not be
operated within 75 miles of a television transmitter occupying a similar spectrum. See
Figure 6.4, for a depiction of what a television station’s assigned spectrum looks like.
Refer to Table 6.1 for the standard frequency allocations of television transmitters.
Figure 6.4
NTSC channel configuration.
TV Channel
Start Frequency
0 MHz
1.25 MHz
4.8295 MHz
5.75 MHz
6 MHz
123451
Guard Band
Chroma Sub
Audio Sub
TV Channel
End Freguency
Video Carrier
NTSC Channel Configuration
Slot Area