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TV Signal Factors

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Factors often overlooked that may explain why a TV signal is weaker than expected.

RADIO HORIZON
Radio Horizon

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 earth radius is increased by a third (4/3) to account for beam bending and the earth curvature. The model assumes flat terrain and ground elevation of the average earth radius (average sea level). 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 = ( 2 h )0.5

R = range to radio horizon in miles.
h = antenna height AGL in feet.

Calculate Radio Horizon
Antenna Height (h)

Horizon
Optical
Radio

Calculate Antenna Height
Radio Horizon

Antenna Height
feet
meters

Radio Horizon Equation

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.

ANTENNA HEIGHT LOSS

The higher the antenna is above ground level (AGL), the greater the signal density. Antenna's 30 feet or higher capture the full signal. Below 30 feet ground reflections cause multi path interference, reducing the signal strength. The lower the antenna the greater the loss. Antenna's in a city have more loss than in a residential area, which has more loss than a rural area. Except for rural areas, UHF (RF 14-69) signals have more loss than VHF (RF 2-13) signals.

UHF Loss VHF Loss

Calculate Antenna Height Loss
Antenna Height


Area

ANTENNA
Height
Area


LOSS
UHF dB LOSS
VHF dB LOSS

The L-R model (Longley-Rice propagation algorithm) is used to estimate antenna height ground loss.

Loss (dB)
(A/6) x 20 Log10(h/30)
= 3.33 A Log10(h/30)
h = Antenna Height in feet.
1.5 ≤ h ≤ 40 ft
A = Area Factor
Area Factor (A)
Area VHF UHF
Rural
Residential
City / Town
4
5
6
4
6
8

TERRAIN FACTORS

Terrain Masking
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.

Terrain Masking
Terrain Masking

Terrain Loss
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.

Free Space Ellipsoid
r = 273.85 (dkm / fMHz)0.5
r
dkm
fMH
= Mid Point Radius in meters
= Distance (Range) in kilometers
= Frequency in Megahertz

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

Frequency and Band.

Range / Radius
English / Metric

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.

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GROUND CLUTTER

Clutter 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.

ground clutter
Loss Due to Trees
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

metal mesh 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.

RF
Ch
Quarter
Wave
Band
51 4.2 in UHF
14 6,2 in
13 1.2 ft VHF
2 4.3 ft
Blocks Signals
Band Openings
UHF < 6.2 inches
VHF < 4.3 feet


Calculate Wavelength & Quarter Wave
RF Channel:

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


in - inches
ft - feet
m - meters


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INDOOR LOSS
Attic Antenna

Attic Antenna
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.

Room Antenna
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.

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
Insullation < 1 dB
Plasterboard 2 dB
Drywall 3 dB
Marble 4 dB
Vinyl Siding 2 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.

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 expected. In practice a 3 - 6 dB or more additional loss is not uncommon.

Source and Approximate Loss
SOURCE LOSS (dB)
ANTENNA Main Beam: 0 - 3
Side Lobe:
Back Lobe:
10-20
30
Gain Variation: 0 - 6
Polarization Loss: 0 - 3
Home Antenna Height Loss: 0 - 20
Broadcast Pattern Loss: 0 - 10+

INDOOR Attic Antenna: 3+
Room Antenna: 1 - 11+

TERRAIN Ground Clutter: 0 - 15+
Terrain Loss: 0 - 12
Masking: No Signal
Over Radio Horizon: No Signal

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.

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TV Signal Factors