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CHAPTER THREE: M
I
deally, an antenna should be mounted at the point
where the signals are the strongest, though often
this is impractical or impossible. Also, the antenna
should be mounted where it can be easily serviced
should repairs or adjustments be required in the
future. So carefully "aiming" (orienting) the antenna
and adjusting its height often can overcome the
problems created by installing it in a slightly weaker
signal area.
In an area where there is a relatively short,
unobstructed signal path between the installation
and the transmitting towers, you can mount the
antenna just about anywhere and receive sufficient
signal. As you move deeper into the fringe areas
however, there may be significant differences in
signal strength at various points on the roof. This is
where careful antenna selection, precise orienting,
and accurate adjustment of the height become
essential. The careful installer should be able to
locate a spot where there is both a usable signal and
sufficiently easy access to the antenna.
Large trees can present reception problems and nearby
high-voltage power lines can cause interference that
cannot be completely eliminated. Consequently, the
further the antenna is from large trees and high-voltage
lines, the better the reception will be.
WALKING THE ROOF FOR OPTIMUM
SIGNAL STRENGTH
"Walking the roof" is usually the best way to find the
spot on the roof where the strongest signals are
present. First, attach a small TV antenna to short
mast (4-6 feet). Attach one end of a short transmis-
sion line to the antenna terminals. Attach the other
end of the transmission line to the input jack of a
field-strength meter. Then walk to the most suitable
mounting areas. Point the front of the test antenna in
the general direction of the transmitter tower(s).
Slowly, rotate the antenna while carefully watching
the field-strength meter. (Most field strength meters
are equipped with shoulder straps that free both of
your hands for manipulating the antenna.)
USING A FIELD-STRENGTH METER
A small, battery-powered portable TV is helpful for
determining general reception quality. However, there
is no better way to determine actual signal strength
than to use a field-strength meter (Figure 3-1). It is one
of the best investments you can make. The meter will
save you so much time and trouble that it will quickly
pay for itself.
M
OUNTING FOR
6
OUNTING
O
S
PTIMUM
IGNAL
O
PTIMUM
FOR
Fig. 3-1. A field-strength meter.
Field-strength meters come with detailed operating
instructions. Below are general characteristics and
operating procedures that apply to most field-
strength meters.
A field strength meter has an input impedance of 75
ohms. Most television antennas have an output
impedance of 300 ohms. For accurate measurements
these two impedances must be "matched." Attach
a balun (matching transformer) to the antenna
terminals and run coaxial cable from the balun to the
meter input jack. (TV coaxial cable has an impedance
of 75 ohms.)
Most field-strength meters will measure the strength
of both VHF and UHF signals and will separately
measure each channel. Most field strength meters will
also separately measure the picture and sound signals
of each channel.
The ideal is to get the strongest signal possible. When
this is not possible and you need to sacrifice a little
strength, connect the test antenna output to a battery
operated, portable TV set and check the picture and
sound to ensure that the signal levels are adequate
for acceptable reception. The TV will also help you
better explain reception possibilities and problems to
your customers.
A field-strength meter indicates signal strength
in microvolts (uV) and decibels relative to one milli-
volt (dBmV). One microvolt is one-millionth of a volt,
or 0.000001 volt. The microvolt reading is probably
the easiest to use. As a general guideline, a signal
level of 50 uV at the antenna output terminals is the
weakest usable signal; however, it will require strong
preamplification to provide acceptable TV reception.
S
TRENGTH
S
S
IGNAL
TRENGTH
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