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If you are in doubt about the amount of gain required,
select an antenna that is slightly larger than you think
is actually needed. The performance of all
components deteriorates slightly during the years
they are exposed to rain, sun, wind, and corrosion.
Consequently, to ensure quality reception for a longer
period, choose an antenna with a little more gain than
is necessary. (Note: Excessive gain may cause
overloading. Therefore, choosing the antenna with
the highest gain might create more reception
problems than it cures.)
Sensitivity classifications are used to indicate the
type of reception area for which the antenna is
designed. Certain ranges of gain are best suited for
certain types of areas. The appropriate range of gain
for a specific area is what a manufacturer is indicating
when labeling an antenna. Outer Suburban or Near
Fringe Table 1-1 is a chart of typical sensitivity
classifications. It indicates in miles, how close an
antenna must be to the transmitter to produce the
best signal level. The classifications are somewhat
general because reception conditions at various sites
within a particular type of area may be different.
DIRECTIVITY
Direcitivity is the ability of an antenna to intercept
signals from only one direction and reject those from
other directions. Directivity indicates the antenna's
ability to intercept signals arriving at its front and
reject signals coming from the sides and rear.
Generally, the more highly directive an antenna, the
better it can reject signals from the sides and rear.
The front-to-back ratio of an antenna can be helpful
when attempting to determine its directivity.
Front-to-back ratio is expressed in decibels (dB) and
can be found in the literature accompanying a new
antenna. This ratio indicates an antenna's ability to
reject signals coming from the rear (rear rejection).
For example, an antenna with a front-to-back ratio of
25 dB will receive about 18 times more signal strength
from the front than from the back.
In most cases, an acceptably accurate estimate of an
antenna's directivity can be made by comparing its
specified front-to-back ratio with the relative sensitivity
classifications listed in Table 1-1.
The sensitivity classifications in Table 1-1 are intended
only as general guidelines for evaluating directivity.
Other factors, such as the antenna's beamwidth, can
affect its directivity. Beamwidth is related to an
antenna's overall gain and indicates how wide or
narrow the antenna's reception area is. For example,
if two antennas have the same front-to-back ratio, the
one with the highest overall gain will have the
narrowest beamwidth and consequently, will be the
most directive.
A
S
NTENNA
4
ELECTION
An antenna with a relatively narrow beamwidth
generally is best suited for areas where interference
from sides is a problem. An antenna with a broad
beamwidth is best suited for areas where a broad
beam is needed to capture the signals from widely
separated stations, and where interference is
minimal. Beamwidth information, if included by the
manufacturer, is usually displayed by use of polar
plots like the one in Figure 1-6.
Selecting the most suitable antenna becomes easy
with experience, but selecting quality materials
should be the first decision to make before starting
any installation. High-performance color reception
usually requires highly sensitive equipment. In the
long run, it pays to use the finest equipment
available, from the antenna down to the receiver end
of the transmission line. The slight additional cost
should be considered an investment in longer system
life and optimum performance.
Fig. 1-6. A polar plot like shown here reveals the directivity and
beamwidth of an antenna.
Area Designation
Deepest Fringe
Deep Fringe
Fringe
Near Fringe
Far Suburban
Suburban
Far Metropolitan
Metropolitan
Table 1-1. Typical Sensitivity Classifications
For VHF
For UHF
100 + miles
60 + miles
100 miles
60 miles
80 miles
45 miles
60 miles
35 miles
50 miles
35 miles
45 miles
30 miles
30 miles
25 miles
25 miles
15 miles
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