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Television Signal Strength Factors


Contents -- Antenna Factors | Terrain | Indoor Antenna's | Radio Horizon | Summary

Below are factors often overlooked that may explain why a signal is stronger or weaker than expected.

Beam Loss

ANTENNA FACTORS

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 (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.

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TERRAIN FACTORS

Terrain Loss
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 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. Sometimes it is unavoidable to have the free space region entirely clear of hills, mountains, radio horizon, or antenna close to ground. The loss can vary from 4 - 12 dB or more.

Free Space Ellipsoid

Free Space Ellipsoid
rmeters = 273.85 (dkm / fMHz)0.5

Calculate Ellipsoid Mid Point Radius
RF Band or Channel:
Range:

Ellipsoid Mid Point Radius in feet and meters.

Terrain Masking
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.

Terrain Masking
Terrain Masking

The tower antenna height Above Ground Level (AGL) is antenna MSL (1200 meters) minus ground MSL (900 meters), or 300 meters (984 feet) AGL.

Ground Clutter
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.

ground clutter
FOLIAGE LOSS
Distance
feet
VHF
dB
UHF
dB
20' 3 4
40' 4 6
60' 5 8
80' 6 9
100' 7 11
120' 8 12
140' 8 13
160' 9 14
180' 10 15
200' 10 16

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 ( 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).

metal mesh
Calculate Wavelength & Quarter Wave
RF Channel:

RF Ch
Frequency, Band
Wavelength (English and Metric Units)
Quarter Wave (English and Metric Units)



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Attic Antenna

INDOOR ANTENNA FACTORS

Attic Antenna
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.

Room Antenna
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.

Signal Loss
Attic
Asphalt Shingle Roof 3 dB
Glass
0.25 in. thick 1 dB
0.5 in. thick 2 dB
Glass Block 6 dB
Wood
Plywood 2 dB
Wood Door 3 dB
Walls
Plasterboard 2 dB
Drywall 3 dB
Marble 4 dB
Brick
3.5 in. thick 3 dB
7 in. thick 5 dB
10.5 in. thick 6 dB
7.5 in. Brick /Concrete 13 dB
Cinder Block
8 in. wide 11 dB
16 in. wide 15 dB
24 in. wide 25 dB
Concrete
4 in. thick 11 dB
8 in. thick 21 dB
12 in. thick 32 dB

Loss estimates are for UHF frequencies. VHF is less lossy, by 1 or 2 dB or more.

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RADIO HORIZON
Radio Horizon Equation 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.


Radio Horizon Calculators
Using a Smooth Earth Model
Calculate Radio Horizon
Antenna Height (ht or hr)

Horizon
Optical
Radio

Calculate Antenna Height
Radio Horizon

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).


SUMMARY
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.

Source and Approximate Loss
SOURCE LOSS (dB)
Antenna Beam Loss:
Main Beam (0° ± 45°) 0 - 3
Side Lobe ±(45° to 90°) 10 - 20
Back Lobe ±(90° to 180°) 30
Antenna Gain Variation: ± 2
Polarization Loss: 0 - 1
Attic Mount: 3
Indoor Antenna: 1 - 11
Ground Clutter: 3 - 15
Terrain Loss: 4 - 12
Terrain Masking: No Signal
Outside Radio Horizon: No Signal

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.

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Television Signal Strength Factors
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