Below are factors often overlooked that may explain why a signal is stronger or weaker than expected.
Terrain masking is always a concern, but is especially problematic for UHF channels. Terrain, and relatively close large structures, can completely block (mask) a signal. Broadcast tower Antenna MSL (height above Mean Sea Level) is used to trace signal path for terrain interference. In the below illustration the broadcast antenna is listed as 1200 meters MSL. Elevated terrain (a mountain) blocks a direct path to the receive antenna. There is some signal reduction and fringing past the mountain, but not enough fringing for UHF channels. A VHF channel may be receivable.
The tower antenna height Above Ground Level (AGL) is antenna MSL (1200 meters) minus ground MSL (900 meters), or 300 meters (984 feet) AGL.
The area between the broadcast and receive antenna's should be clear of obstructions for best reception, the free space region. The region is shaped like an ellipsoid, or a cartoon cigar shape. Near either antenna the region's radius is a couple of wavelengths, or about 4 to 30 feet radius (UHF to VHF). The area near and all around either antenna should be clear.
At the ellipsoid mid point the radius (r) is largest, and depends on frequency (f) and distance (d) between antennas. The lower the frequency and the greater the distance, the larger the radius. Hills, mountains, radio horizon, antenna close to ground, or distances over about 20 miles will introduce a terrain loss of 4 - 12 dB or more.
The broadcast tower ground elevation and tower height (above ground level) determine the radio horizon, locations outside the horizon don't get a signal. The radio horizon is greater than the visual (optical) horizon. In the atmosphere radio waves bend slightly upward increasing the range, light waves do not bend (very much). In a free space vacuum radio and light waves propagate in a straight line.
Using a Smooth Earth Model (over water at 0 meters MSL) the radio horizon varies from about 10 miles for a 15 meters (50 feet) tower to over 60 miles for a 600 meters (about 2000 feet) tower. Broadcast towers are usually located on the highest ground possible, increasing horizon range. One tower in Denver is 6000 feet, over a mile, above the mile high city. Elevating the receive antenna can increase reception range, if the signal is strong enough.
|Calculate Radio Horizon|
Antenna Height (ht or hr) |
|Calculate Antenna Height|
Radio Horizon |
Radio horizon calculations are based on line-of-sight equations using a larger earth radius (the 4/3's earth radius model).
Large relatively close structures could cause significant signal loss, structures in the distance will cause less loss. Trees will reduce a signal, the longer the distance the signal travels through a tree or multiple trees, the greater the loss. Trees without foliage (in winter) may have slightly less loss (about 1 dB) at UHF frequencies.
Wire or metal mesh (wired glass window, chicken wire, chain link fence, etc.) will completely block a signal if the largest open space in the mesh is less than or equal to a quarter wavelength. The mesh acts exactly like solid metal for signals that are a quarter wavelength or longer (lower frequency). Openings greater than a quarter wavelength will reduce signal strength, but not block the signal completely.
Beam loss occurs when the receive antenna is not directly aligned to the broadcast signal. Signals aligned directly (0°) to the antenna get the maximum gain (no beam loss). Gain decreases slightly with angle up to the beam edge. At the beam edge gain is down by -3 dB. Past the beam edge gain drops dramatically. The beam edge for an outside antenna with moderate gain is about ±45° off center (a 90° beam). A lower gain antenna could have a beam as wide as 120°. A high gain antenna could have a beam as narrow as 30°. Also see TV Antenna Types.
Antenna Gain Varies
Antenna gain is not constant and varies, up or down, with frequency (RF channel). Advertised gains are usually the average gain over the frequency band, some are the maximum gain. The gain maximum to minimum difference (spread) can be as low as 2 dB (gain ±1 dB), or as high as 6 dB (gain ±3 dB) or more.
INDOOR ANTENNA FACTORS
Attic mount loss is at least -3 dB for a 3/4 inch plywood roof covered with roofing paper and one layer of 3 tab asphalt shingles. Metal backed insulation in the attic or walls and metal exhaust vents and air ducts all block signals.
Walls, floors, ceilings, roofs, doors, windows, appliances, furniture, and partitions will reduce a signal. Wall insulation without a metal backing has a minor loss, a fraction of a dB. Metal backed insulation, metal siding, awnings, doors. screens, air ducts, and water pipes will block / reflect a signal. An outside wall with vinyl siding, or an inside wall, will have a loss of about 6 dB or more, A brick wall loss is about 8 dB or more.
Loss estimates are for UHF frequencies. VHF is less lossy, by 1 or 2 dB or more.
An antenna mounted 30 feet above the ground in a flat open field with a clear line-of-sight and directly aligned to the broadcast tower could receive a signal near or greater than expected. In practice a 3 - 6 dB additional loss is not uncommon. Signal path clutter and indoor antenna's will have greater loss. Terrain could introduce more loss.
Beam loss, terrain masking, and radio horizon loss can be estimated. Attic mount and indoor antenna loss are more difficult to predict. Ground clutter, terrain loss, and antenna gain variation are often difficult or impossible to estimate.Top
Over-the-Air Digital TV (OTA DTv)
Television Signal Strength Factors
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