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Fig. L Channel Master's Antenna Laboratory. Left is v.h.f. range showing broadband corner reflector transmitting antenna with receiving
tower (large circle) in background. The u.h.f. horn transmitting antenna is on roof wi'h receiving mast (small circle) at rear of building.
HE performance characteristics of
any TV receiving antenna, irre-
T spective of type or frequency range,
depend upon three properties: gain,
directivity, and impedance.
This month's cover shows a scene
in a modern TV antenna test laboratory
where these three characteristics
are measured. At the engineer's left
is the vb.. transmitter panel containing
a v.h.f. transmitter, the associated
modulator, and a selsyn system
which controls the rotation of the
antenna under test. The engineer is
adjusting the tuned output meter
Fig. 2. Setup where both Channels 2 and
13 are transmitting from the same point
with the same effective radiated power.
62
TRANSMITTERS FOR CHANNELS 2 A 13
which gives direct gain readings in db.
To his right is a polinear recorder
which plots polar diagrams as the test
tower is rotated.
The purpose of this article is to explain
these three antenna properties
(gain, directivity, and impedance) and
to describe how they are measured at
Channel Master.
The first of the antenna properties
is gain. The gain of a television receiving
antenna is a measure of its
ability to provide a signal. There are
many ways to measure the gain of a
television receiving antenna and,
therefore, many opportunities for
misconception or actual misrepresentation.
The commonly accepted way,
and the method used by most reliable
firms, is to compare the gain of an
antenna with a tuned dipole at
each particular frequency under consideration.
In other words, if we
speak of the gain of a broadband
antenna, its performance on each
channel must be compared to a tuned
dipole for that specific channel. If we
are speaking.of the gain of a yagi, it
must be compared to a tuned dipole
on the channel for which the yagi is
cut.
The Decibel
The unit of measurement used to
make this comparison between an antenna
and a reference tuned dipole is
the "decibel" (db). The decibel is not
a fixed quantity like a watt, or a volt,
or an ohm. Rather, it is a ratio between
two quantities. For example,
assume that our reference dipole at a
given channel will provide a signal of
100 microvolts. This, then, becomes
the 0 decibel level. Table 1 is a chart
showing the total signal in microvolts
resulting from various db gains.
There is an interesting and important
point to bear in mind when comparing
the gain of an antenna with
a tuned dipole for its specific channel.
The field strength of a television
transmitting station is generally
measured in microvolts - per - meter.
This means that a length of wire one
meter long will intercept a signal of
100 microvolts on the 100 microvolt
field strength contour. Fig. 2 shows
the transmitting setup where both
Channel 2 and Channel 13 are transmitting
from the same point with
the same effective radiated power.
Their field strength contours coincide.
Assume that on the 100 microvolt
contour (where a wire one meter long
will intercept 100 microvolts) we have
a Channel 2 dipole and a Channel 13
dipole. The Channel 2 dipole is approximately
two meters long and will,
therefore, intercept approximately 200
microvolts. The Channel 13 dipole is
only about one -half meter long and it
will intercept only 50 microvolts. In
other words, a Channel 2 dipole will
pick up approximately four times as
much signal. These factors should be
borne in mind when comparing high -
band and low -band performance in
antennas. In choosing a broadband
antenna, the high -band gain (in decibels)
should be much higher than the
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