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Reissue of Broadcast Policy Statements 621
FCC 67-1012
PUBLIC NOTICE
FM INTERFERENCE TO TV RECEPTION
(Adopted: August 30, 1967; Released: September 1, 1967)
By THE COMMISSION: COMMISSIONER WADSWORTH ABSENT.
The Commission is encountering an increasing number of cases of
interference to television reception which develop when new FM
permittees begin operation or existing licensees increase their effective
radiated power by a substantial amount.
The following are four of the most common situations in which FM
interference to TV reception occurs:
(a)
(b)
(c)
(d)
VHF-TV stations with transmitters located 20 miles or
more from the city.
Another FM or a VHF-TV transmitter is located within two
miles of an FM transmitter and either/or both transmitters
are located in areas where substantial numbers of people
reside.
TV Channel #6 is viewed on a regular basis within the
service area of the FM station.
The: FM transmitter is located in a residential area and a
considerable number of TV viewers reside within one-half
mile of the FM transmitter.
The interference occurs for one of the following reasons: ;
a)
(2)
(3)
(4)
(5)
A TV receiver, in the presence of a strong FM signal, may
develop, within itself, a second harmonic of that signal. If
this receiver developed second harmonic falls within a TV
channel which is in use, the TV signal strength may be
insufficient to overcome the presence of the receiver
developed second harmonie signal.
Compliance with the Commission’s Rules on spurious emis-
sions, which include second harmonics, may be insufficient
to avoid interference to TV signals where the radiated
second harmonic of the FM station falls within a TV
channel that is being received in the area. In such cases the
radiated second harmonic of the FM station should be at
least 40 to 45 db below the level of the desired TV signal.
A TV receiver, intercepting strong signals from two FM
transmitters or an FM and a TV transmitter, may combine
the signals of the two transmitters to produce a third
frequency. This third frequency may fall within a TV
channel which is viewed in the area.
Some TV receivers may be incapable of rejecting a strong
FM signal while tuned to Channel #6 if the FM station
frequency is near the low end of the FM band.
A TV receiver within the “blanket” area of an FM station
622 Federal Communications Commission Reports
may be “overloaded” so that it is susceptible to various
forms of interference.
The Commission is advising all FM permittees and prospective
permittees that if an examination of the TV viewing habits within the
service area of the FM station and location of other FM and TV
transmitters within the vicinity reveals the possibility of interference
to TV reception, an FM permittee should proceed as follows:
(1) Conduct equipment tests when other stations which may be
involved are in operation, especially during daytime hours.
{2) Make special interference tests with two or more different
types of TV receivers (including color) with these receivers
located within the transmitter building and also at selected
locations throughout the city.
(3) If interference is indicated, determine the various types of
FM traps and filters which, when installed at the TV
receiver, will cure the problem.
(4) Communicate with as many TV retailers, wholesalers and
servicemen as possible and demonstrate to them the steps
necessary to alleviate the interference.
(5) When filing the license application and request for program
ı test authority, advise the Commission of the nature of
interference which may result when operation begins and
the steps which have been taken to anticipate complaints.