Factors often overlooked that may explain why a TV signal is weaker than expected.
Antenna height determines the radio horizon. Broadcast signals go past the horizon, but not over it. 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 both propagate in a straight line.
The 4/3's Earth Radius model is used to estimate atmospheric radio wave bending. The Smooth Earth model is used to factor earth curvature. The Smooth Earth model uses flat terrain and average earth radius for ground elevation. On flat terrain actual ground elevation is a minor factor and has virtually no effect on calculation accuracy. However, broadcast towers are often located on the highest ground in the area, increasing horizon range. In this case the antenna height used for calculations should be the antenna height above average ground level.
The radio horizon range (R) in miles is approximately the square root of twice the antenna height (h) in feet above ground level (AGL). Radio horizon varies from about 4 miles for an 8 foot high antenna to 60 miles for an 1800 foot high antenna.
R = range to radio horizon in miles. |
h = antenna height AGL in feet.
|Calculate Radio Horizon|
Antenna Height (h) |
|Calculate Antenna Height|
Radio Horizon |
Locations outside a broadcast horizon can usually get a signal if the receive antenna is above ground clutter and high enough. The receive antenna needs to be high enough for its' horizon to extend up to and past the broadcast horizon point.
Note the earth surface range is slightly longer the the line-of-sight range. The difference is so small it can be ignored for practical purposes.
TV signals require a clear line-of-sight between broadcast and receive antennas. Large obstructions and terrain features like hills and valleys can completely block a signal.
The free space region between the broadcast and receive antenna's should be clear of obstructions for best transmission. The region is shaped like an ellipsoid (a cartoon cigar shape) and depends on range and frequency. The lower the frequency (the lower the RF channel) and the greater the distance, the larger the radius. Near an antenna the free space region's radius is a couple of wavelengths, or about 4 to 30 feet (UHF to VHF). The free space region is largest at the midpoint.
= Mid Point Radius in meters
= Distance (Range) in kilometers
= Frequency in Megahertz
Hills, mountains, antenna close to ground, or distances over about 20 miles have their free space region intersecting the ground. This will introduce a terrain loss of 4 - 12 dB or more.
Any object at or above your antenna elevation can cause signal reduction. Structures and trees can measurability reduce or block signals. Ground clutter signal loss is extremely difficult to predict.
Signal loss due to trees can be roughly estimated from empirical data. Trees without foliage (in winter) may have slightly less loss (about 1 dB) at UHF frequencies.
|Loss Due to Trees|
Wire or metal mesh (wired glass window, chicken wire, chain link fence, etc.) will completely block a signal if openings are too small. Openings smaller than a quarter wavelength will completely block a signal as if it were solid metal. Also see Quarter Wave Calculator.
|UHF||< 4 - 6 inches|
|VHF||< 1 - 4 feet|
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 and metal vents and air ducts block signals.
Metal backed wall insulation will block a signal. Wall insulation without a metal backing has a minor loss, less than a dB. Air ducts and metal pipes in the wall will reduce and can block signals. Metal outside fixtures such as siding, awnings, and doors will reduce and can block signals. Even inside walls, floors, ceilings, doors, appliances, furniture, and partitions will cause some signal loss.
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 expected. In practice a 3 - 6 dB or more additional loss is not uncommon.
Beam loss, terrain masking, attic mount, and radio horizon loss can be estimated. Room antenna loss is more difficult to predict. Ground clutter and terrain loss are often difficult or impossible to estimate.Top
Over-the-Air Digital TV (OTA DTv)