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Basic Television Reception Concepts

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.
 
Antenna Components
The main components of an outdoor antenna consist of: Elements, Boom and Phasing Lines.
There are three types of elements:
  1. Director elements: At the front of the antenna; smaller elements which initially pick up the signal.
  2. Driven element: The element where the coax is connected; “Drives” the signal down the coax to the TV.
  3. 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.
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.”
 
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.

 
Signal Strength
The broadcast signal strength at your location depends on four variables:
 
  1. 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.

  2. 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.

  3. 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.

  4. 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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