TV Signal Factors

• Radio Horizon
• Antenna Ground Loss
• Terrain
• Ground Clutter
• Indoor Loss
• Summary

There are multiple factors that influence and can reduce signal strength. The lower to the ground your antenna is located, the shorter the radio horizon range. Also, antennas less than 30 feet above the ground have a ground reflection loss, the higher the antenna the lower the loss. Terrain and ground clutter can reduce signal strength or block signals. Indoor antennas have additional losses.

Antenna height determines the radio horizon range, the higher the antenna the greater the range. Broadcast signals go past the Radio Horizon, but not over it. Receive antennas cannot get signals that are not in the receive antenna radio horizon.

The radio horizon is greater than the visual (optical) horizon. In the atmosphere radio waves bend slightly with the earth curvature increasing the range about 13%, 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. The model is based on the average earth radius ((average sea level) as ground elevation. Actual ground elevation is a minor factor and has virtually no effect on calculation accuracy.

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.

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 above any clutter or terrain at the tower's radio horizon.

A few times a year atmospheric conditions cause a ducting effect extending radio horizon greatly. The duct reception range can be as great as 1000 miles, but a couple hundred miles is more common. The condition typically last minutes to hours, and usually occurs on cool dry mornings around a season change.

The higher the antenna is above ground level (AGL), the greater the signal density and the lower the ground loss. Antennas 30 feet or higher capture the full signal. Below 30 feet ground reflections cause multi path interference. The lower the antenna the greater the loss. Antennas in a city have more loss than in a residential area, which has more loss than a rural area. Except for rural areas UHF signals have greater loss than VHF signals.

Average Loss

Calculate Antenna Height Loss
Antenna Height feet meters Area City Residential Rural ANTENNA Height Area LOSS UHF dB LOSS VHF dB LOSS Calculate or Re-calculate

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

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 Loss
The free space region between the broadcast and receive antennas should be clear of obstructions. The region is shaped like an ellipsoid (a cartoon cigar shape) and depends on range and frequency. The lower the frequency and the greater the distance, the larger the radius and volume. 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
Mid Point Radius (r)
r = 273.85 ( d_km / f_MHz )^0.5

r dkm fMHz

= Mid Point Radius in meters = Distance (Range) in kilometers = Frequency in Megahertz

Calculate Mid Point Free Space Radius

Band or RF Ch: VHF-Lo VHF-Hi UHF RF 2 RF 3 RF 4 RF 5 RF 6 RF 7 RF 8 RF 9 RF 10 RF 11 RF 12 RF 13 RF 14 RF 15 RF 16 RF 17 RF 18 RF 19 RF 20 RF 21 RF 22 RF 23 RF 24 RF 25 RF 26 RF 27 RF 28 RF 29 RF 30 RF 31 RF 32 RF 33 RF 34 RF 35 RF 36 RF 38 RF 39 RF 40 RF 41 RF 42 RF 43 RF 44 RF 45 RF 46 RF 47 RF 48 RF 49 RF 50 RF 51 RF 52 RF 53 RF 54 RF 55 RF 56 RF 57 RF 58 RF 59 RF 60 RF 61 RF 62 RF 63 RF 64 RF 65 RF 66 RF 67 RF 68 RF 69 Range: Miles Kilometers

Range Frequency and Band. ------------------------------------ Mid-Range / Free Space Radius English Metric Calculate or Re-calculate

Hills and mountains, or distances over about 20 miles, have some of much of their free space region intersecting the ground. This will introduce a terrain loss of 4 - 12 dB or more.

Any object near or above your antenna elevation can cause signal reduction. Structures and trees can measurability reduce or block signals. 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.

Attic Antenna
Attic antennas where the roof is 3/4 inch plywood covered with roofing paper and one layer of 3 tab asphalt shingles has a -3 dB loss or more. 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 1 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 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.

OTA DTv TV Signal Factors