Movies for TV - Early Television Foundation

PRINCIPLES OF TV 43
good. Unfortunately, this is not always so, and it appears to boil
down to the fact that the higher the antenna, the better the signal,
even though the antenna power has to be reduced to keep within
the FCC limits. For instance, in the case of WNBT on channel
four in New York, the antenna is on top of the Empire
State Build-
ing and is about 1300 feet above the ground. The power required is
only about seven kilowatts to give the same coverage that 50 kilo-
watts would give at 500 feet !
Also, and a most important point, the higher the antenna, the
less the chances of surrounding buildings causing ghosts in the pic-
ture. Usually the ghosts are caused by nearby buildings which re-
flect the signal so that it arrives a few microseconds after the main
signal and shows as a pale image to the right of the main figures on
the screen. The distance between the antenna and the reflecting
building has a definite bearing in the spacing between the ghost
and the main image.
From the foregoing remarks, it might be inferred that television
waves act rather like light waves. This is quite true : in fact, the two
are very similar, and the higher the frequency, or the shorter the
wave length, the more like light waves they behave. Although figures
of 27 and 20 miles were mentioned above, it must be under-
stood that these are not the limits of propagation but the distance
to which the FCC says first-class service should be rendered and also
the area which was supposed to be kept free from interference from
other stations according to the old allocation plan for television
stations. One of the things which caused the current hearings before
the FCC was the fact that interference was being experienced
where it was not expected to be. Most stations provide usable serv-
ice to 50 miles or even more, but above that distance reception is
usually due to abnormal conditions. It is not power which generally
limits the range of television signals but the height of the antenna,
for since the waves travel in straight lines they cannot very easily
bend around the curve of the earth. Consequently, places only
a few miles behind the horizon seen from the transmitting antenna
are often out of range of the station.
The television band is divided into two parts, the upper and