Installation guide

ANTENNA SELECTION
2
CHAPTER ONE: ANTENNA SELECTION
B
asically, a receiving antenna is a device for
intercepting the electromagnetic waves or
signals, sent from a transmitter. Some antennas
are simple vertical poles; others are small wire loops
attached to the back of a TV set.
In this manual, we will discuss the outdoor TV antenna
design with which most of us are familiar: a central
horizontal boom with small elements attached at
right angles.
The main receiving element of an antenna is called
the dipole (Fig. 1-1). All of the other antenna parts are
designed primarily to help the dipole do its job. The
dipole consists of two half-elements to which the
transmission line is attached. It is the element around
which the other antenna parts are designed and
positioned.
BASIC TV ANTENNA TYPES
TV antennas can be grouped in four major functional
categories: VHF/FM, UHF, UHF/VHF/FM, and FM only
combinations.
VHF and FM Antennas
Most VHF (Very High Frequency) antennas are
engineered to receive TV channels 2 through 13. They
also will receive the FM radio band, which is located
between TV channels 6 and 7. FM only antennas are
available also (Figure 1-6). Channels 2 through 6 are
known as the low band. Channels 7 through 13 are
referred to as the high band. Some VHF antennas are
designed to receive only one band, either the low or
high band. Antennas designed to receive both the low
band and the high band are called VHF/FM broad-
band antennas (Figure 1-2).
TV channel 1 “disappeared” in the early days of TV
because of a change in frequency assignments by the
Federal Communications Commission (FCC). A
reshuffling of FM, Amateur, and other bands removed
TV from the frequencies previously reserved for TV
channel 1. Because the other 12 TV channels were
already numbered 2 through 13, the channel 1
designation simply was dropped.
UHF Antennas
UHF (Ultra High Frequency) antennas are designed to
receive TV channels 14 through 69, the UHF TV band.
The UHF TV band originally extended from channel 14
to channel 83. However, the FCC now has reassigned
channels 70 through 83 (also known as the translator
band) to mobile communications use. Although there
still are many antennas capable of receiving all 82
channels, the translator band (former TV channels
70-83) is useless to the TV viewer. UHF TV antennas
come in a wider variety of shapes and sizes than
VHF/FM antennas. The wider variety of UHF designs
(Figure 1-3) is possible because they don’t require the
long elements that VHF/FM antennas do.
Combination UHF/VHF/FM Antennas
Combination UHF/VHF/FM TV antennas are designed
to receive both the UHF and the VHF/FM bands.
Although the use of separate antennas for each band
is ideal for peak reception, high-performance
UHF/VHF/FM combination models are readily
available and are becoming increasingly more
popular. Examples of combination UHF/VHF/FM
antennas are shown in figures 1-4 and 1-5.
HOW TO SELECT THE RIGHT ANTENNA
There are many bands and hundreds of TV antenna
models available. However, choosing the right one
is relatively easy if you are aware of a few basic
reception and TV antenna characteristics.
First, “good reception,” or production of a snow-free
color TV picture, requires a signal level of about
1000 microvolts (µV) = 1 millivolt (mV). To deliver
this signal level to the receiver, the antenna requires
a certain antenna gain. The amount of gain required is
dependent on the distance between the station’s
transmitting antenna and the receiving antenna. The
required type of antenna therefore depends on the
channels to be received and the distance and direc-
tion of the customer’s home from the transmitting
antennas. These facts are readily available for any
area, but be sure your information is accurate and
complete. Call a local TV station if you have ques-
tions. Most TV stations are willing to help TV antenna
installers because they also benefit from the
improved reception to the station’s viewers.
Be sure to discuss with your customers the number
and types of channels that are receivable. This may
sound basic, but your customers may not understand
the difference between VHF/FM and UHF. They also
may not be aware that with the right equipment they
may be able to receive out-of-town channels, some of
which may carry sports programs that are locally
blacked out. The most important points to remember
when selecting an antenna are its gain, sensitivity
classification, directivity, and front-to-back ratio.
Gain
The gain of an antenna indicates the relative strength
of signal it can deliver to a receiver. Most manufactur-
ers list the gain of their antennas in decibels (dB). The
higher the antenna gain, the stronger the signal at the
antenna output terminals. In most cases, the larger the
antenna, the higher the gain.