Basic Television Reception
Concepts
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Local Area Television Broadcasts |
Local area television stations
broadcast their signal “over the
air” to conventional TV
antennas. Major networks (ABC,
CBS, NBC, FOX, PBS) and local
programming (sports, weather,
news and local interest
programs) are broadcast to
transmitters, which then send
signal to surrounding areas.
Picture quality depends on the
television set, type of antenna
and distance from station
transmitter.
Costs include antenna purchase
and any accessories needed —
preamplifiers, distribution
amplifiers, or splitters for
more than one television set —
plus a professional installation
fee if necessary.
A conventional outdoor TV
antenna is the most economical
choice for local area
broadcasting and local network
programming. There is a onetime
investment for the antenna,
accessories and installation (if
necessary). You pay no monthly
subscription cost. |
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Antenna Components |
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The main components of an outdoor
antenna consist of: Elements, Boom
and Phasing Lines. |
There are three types of
elements:
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Director elements: At
the front of the antenna;
smaller elements which
initially pick up the
signal.
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Driven element: The
element where the coax is
connected; “Drives” the
signal down the coax to the
TV.
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Reflector element:
Longest element; always
located at the back of the
antenna; reflects unwanted
signals away and reflects
desired signal from the
front of the antenna back to
the driven elements.
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The Boom is the center
section the elements are
fastened to. The Phasing
Lines may be small aluminum
wires or rugged aluminum braces
— whatever form they are, these
phasing lines pass the signal
from the driven elements to the
coax downlead. Sometimes the
boom will act as phasing lines,
and is called a “hot boom.” |
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Antenna Downleads (connection to
television) |
Coax cable is used for downlead
on outdoor antennas. It can be
run through pipes, stapled to
walls, taped to the mast —
making it easy to install. Coax
is 95% shielded, excellent noise
rejection, durability and is not
affected by weather. RG-6 is the
preferred coax because it has
less loss than RG-59 coax.
Television broadcasting stations
operate on three bands and
frequencies — VHF (very high
frequency) low band, channels
2-6 plus FM; VHF high band,
channels 7-13; UHF (ultrahigh
frequency), channels 14-69.
The
FM band is located between
channels 6 and 7 on VHF, and is
closest to channel 6. A strong
FM station might cause
interference on channel 6
because of its close proximity. |
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Signal Strength |
The broadcast signal strength at
your location depends on four
variables:
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- Distance from
transmitter. The farther
away you are, the weaker the
signal, resulting in reduced
picture and sound quality.
UHF signals are harder to
receive than VHF.
With TV signals, there are
two carriers in transmission
— video and audio. The video
(picture) will dissipate
faster than audio, so
sometimes you receive sound
but no picture.
- The terrain between
your location and the
transmitter. Unlike AM
signals, which follow the
curvature of the earth, TV
and FM signals travel in a
tangent to the earth. Other
obstructions (high
buildings, hills, etc.) can
also interfere with
broadcast signals.
Interference caused by
buildings, water towers etc.
often causes “ghosting”
problems (ghosting is
multiple images on your
screen) — the signal from
the transmitter is reflected
by obstacles; the signal
reflected arrives at your TV
set a split second later
than the main signal from
the transmitter and causes
multiple imaging. A
directional antenna with
good side and rear rejection
can eliminate or greatly
reduce ghosting. Some
ghosting cannot be solved
without moving to another
location.
- Type and size of
antenna. The size of
your antenna is determined
by the distance from the
transmitter — antenna size,
including length and number
of elements, increases as
distance between location
and transmitter increases.
In extreme fringe areas,
stacking antennas (using
multiple antennas on the
same mast) is suggested.
- Yagi: Single
channel antenna; usually
with high gain.
- Broadband:
Picks up all available
channels — VHF broadband
(2-13), VHF/ UHF
broadband (2-69), UHF
broadband (14-69)
- Area Special:
One or more antennas
mounted together to
receive channels in a
specified area. For
distributors only.
- Amount of signal loss
in your system. In every
system, there is some amount
of loss from antenna to TV
set or FM receiver.
- Cable Loss:
There is some loss for
every foot of cable
used, depending on the
size of the cable
(Bigger cable = smaller
loss); the length
(longer length = bigger
loss); signal frequency
(more loss on channel 69
than channel 2).
- Splitter Loss:
Occurs every time the
signal coming from the
antenna is split. To
operate two TV sets, the
two-way splitter used
reduces signal 30%; four
TV sets, the four-way
splitter reduces the
signal 60%.
- Feed-thru Loss
and Isolation Loss:
Usually occur in large
MATV systems (motels,
apartment buildings). In
98% of home systems,
these losses will not
occur.
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