Document text
FeperaL Communications Commission
OMMUG Ye x
FCC 65-130
63396
ae Information Bulletin =- B
WASHINGTON 25, D. C. 20554 l February 19, 1965
POTENTIAL INTERFERENCE TO TELEVISION RECEPTION FROM THE
OPERATION OF FM BROADCAST STATIONS ON CERTAIN FREQUENCIES
FM Broadcast stations are assigned and operate on frequencies from
88.1 to 107.9 Mc/s. The second harmonic of all these frequencies (twice
the fundamental frequency) fallswithin the high VHF TV band, Channels 7
through 13. Thus, radiation of any FM signal on the second harmonic of the
assigned fundamental can be a source of interferencé to reception of TV -
Channels 7 through 13, since the FM harmonic signal acts like an undesired
signal on the same channel. In view of the large number of FM and TV stations
already in operation and the potential of many additional stations in the
next few years, the possibility of such a second harmonic relationship in the
ae community or in different communities may exist anywhere in the United
ates.
The severity and extent of this type of interference will depend on
the distance between the stations involved, the relative strengths of the
two signals, the exact relationship of the FM harmonic to the TV station's
picture and aural carriers, the orientation of the receiving antennas and
other factors. The potential interference is greatest where the FM signal
is strong in relation to the TV signal such as would occur where the
desired TV signal comes from a distant station whereas the FM station is
locally situated with respect to the TV viewers.
The potential interference to TV reception from FM stations is of
two types. One is that caused by the direct second harmonic radiation of
the FM signal falling within the TV receiver bandpass. It usually can be
corrected by the installation of suitable filters at the station. Our
rules require rather severe second harmonic suppression on the part of FM.
stations. This type of interference rarely occurs when the two stations
are in the same city or nearby cities since the ratio of the TV signal
strength to the FM signal strength is large enough to permit satisfactory
TV service. It is most likely to occur where the TV station is at a great
distance and the FM station is nearby.
The second type of potential interference is attributable to the
generation of the second harmonic signals within the individual TV re-
ceivers in some part of the booster or RF amplifier caused
by overloading of the TV receiver due to the very strong fundamental signal
Erom the local FM station. Here the interference occurs mostly near the
site of the FM station and is most severe when the TV signal comes froma
ee cee
distant station. This type of interference cannot be cured at the FM station
and should not be blamed on the FM station licensee. It is basically a TV
receiver design problem, since the receiver does nov have sufficient selec-
tivity to reject an FM signal which is far removed from the TV signal fre-
quency. The overloading type of interference may be eliminated or at least
alleviated by the insertion of filters or "wave-traps" designed to eliminate
the FM stations's fundamental carrier frequency. Sometimes re-orientation of the
TV - receiving antenna may be necessary. The problem becomes more diffi-
cult as the TV signal becomes weaker (the desired station is farther removed)
and the closer the TV viewer is to the local FM station. It is particularly
troublesome where the TV viewer is using a highly directive antenna, with or
without a booster, and the FM station is on a line between the viewer end the
distant TV station,
Since the first type of potential interference (direct second harmonic
radiation from the FM station) lends itself to correction at the station and
the second type (overloading) is a matter of receiver design which in most
cases can be remedied only at the TV receiver, these types of interference
have not been taken into account in the allocation of FM stations, either
under the previous rules or under the present Table of Assignments. Shift-
ing an FM assignment to a city at a greater distance from a TV station may
actually worsen the situation at a later date since the TV signal will be
weaker than before. There have been a number of instances where all cases
of interference of the types described could not be eliminated either by
further reduction cf the secord harmonic of the FM station or by the instal-
lation of traps at the TV receivers. In some of these cases changes were
made in the frequency assignment of the FM station if a simple solution agree-
able to all the parties concerned could be found. However, with the rapid
growth of FM and the increase in the number of FM stations there may not be
a possibility of such frequency shifts in all instances.
Occasionally TV viewers have jumped to the conclusion that the FM
station was at fault when interference occurred at the same time the FM
station began operation. In almost all such cases, the fault has not been
with the FM station but rather with the TV receivers, and the efforts of the
viewers to watch TV signals beyond their normal service range. It becomes
necessary therefore for all concerned with this problem, including the TV
receiver manufacturers, station licensees, and the viewing public to take
whatever measures are needed to insure that both of these important broad-
cast services can exist without adverse effect on each other. FM station
licensees should make sure that the harmonic radiation is as low as the
state of the art will permit. They should also carefully select their sites
as far removed removed from built-up residential areas as possible. TV receiver
manufacturers should consider either including FM traps in all their sets or at
least making such traps available to customers in areas of possible interference.
Finally, the viewing public should cooperate to the extent that they re-orient
their antennas or install the necessary wave~traps where it is recommended by the
FM station licensee or a competent TV service man.
=- FCC -
Adopted: February 17, 1965