Below list loss factors that may explain why a signal is weaker than expected.
ANTENNA ANGLE - BEAM LOSS
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. 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 next section Antenna / Gain and Beam.
ANTENNA FREQUENCY VARIATION
Antenna gain is not constant and varies, up or down, with frequency. Advertised gains are usually the average gain over the frequency band, some are the maximum gain. The gain spread can be as low as 2 dB, or as high as 6 dB or more, 4 dB (±2 dB) is typical for a moderate gain antenna.
ANTENNA POLARIZATION LOSS
Most television stations transmit a signal with horizontal polarization. Some stations transmit right hand circular elliptical (main axis in horizontal plane) for better reception in hilly and cluttered areas. Most receive antenna's are designed for horizontal polarization. When the receive and transmit antenna's polarizations are not the same, there is small polarization loss, about -1 dB.
The area between the broadcast and receive antenna's should be clear of all obstructions for best reception, the free space region. The region is shaped like an ellipsoid, or a 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 mid point the ellipsoid 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. Many times ground terrain, (hills, mountains, near radio horizon, antenna close to the ground) will get into the free space region, causing a signal reduction (terrain loss). The loss can vary from 4 - 12 dB or more.
Free Space Ellipsoid
rmeters = 273.85 (dkm / fMHz)0.5
Calculate Ellipsoid Mid Point Radius
RF Band or Channel:
Ellipsoid Mid Point Radius in feet and meters.
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.
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 at UHF frequencies (about 1 dB less loss).
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. Elevating the receive antenna can increase reception range, if the signal is strong enough.
Radio Horizon Calculators Using a Smooth Earth Model
Calculate Radio Horizon
Antenna Height (ht or hr)
Horizon Optical Radio
Calculate Antenna Height
Antenna Height feet meters
Radio horizon calculations are based on line-of-sight equations using a larger earth radius (the 4/3's earth radius model).
Attic mount loss is at least -3 dB for a 3/4 inch plywood roof with roofing paper and a single layer of 3 tab asphalt shingles. Metal backed insulation on the attic roof or walls and metal exhaust vents / pipes block signals.
Walls, floors, ceilings, roofs, and indoor objects (appliances, furniture, partitions, posts, metal racks) will reduce a signal. Metal objects (awnings, siding, etc.) will block signals. Also, the closer the antenna is to ground level, the lower the signal density.
Clear Glass Window
2 - 4 dB
Clear Glass Block Wall
Asphalt Shingle Roof
Metal Door / Inside Wall
Metal Door in Brick Wall
Concrete / Brick
Cinder Block Wall
2 - 5 dB
5 - 10 dB
Loss estimates are for UHF frequencies. VHF is less lossy, by 1 or 2 dB or more.
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 equal to or less than a quarter wavelength (Opening ≤ Quarter Wave). The mesh acts exactly like solid metal for signals that are a quarter wavelength or longer (lower frequency). Television broadcast quarter wavelengths' vary from 3.7 inches for RF channel 69 to 4.3 feet for RF channel 2 (93 mm to 1.3 meters).
Calculate Wavelength & Quarter Wave RF Channel:
RF Ch, Frequency, Band Wavelength (English and Metric Units) Quarter Wave (English and Metric Units)
Source and Approximate Loss
Antenna Beam Loss:
Main Beam (0° to ± 45°)
Side Lobe ±(45° to 90°)
Back Lobe ±(90° to 180°)
Antenna Gain Variation:
Outside Radio Horizon:
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.