DIGITAL TELEVISION (DTV)
DTV requires a stronger signal than analog TV. Additionally, most DTV transmitters are in the Ultra High Frequency UHF band (channels 14 - 69). These signals are much higher in frequency and are Line-of-Sight -- do not pass through or around objects as signals in the Very High Frequency VHF band (channels 2 - 13). Also, over-the-air transmission losses are greater at UHF frequencies. Antennas that picked up analog TV signals will also pick up DTV signals, if the signal is strong enough.
DTV Channel vs Analog Channel
The old analog TV channel identification used the transmit frequency channel number, technically called the Radio Frequency RF channel, DTV does not. A station's DTV channel may or may not correspond to it's RF channel. Most analog TV stations changed their RF channel for DTV. Most stations that broadcasted in the VHF band moved to UHF. All analog stations were allowed to keep their old channel as their DTV or Virtual Channel identification.
Because multiple programs (stations) can be broadcast within one RF channel, DTV identifications are virtual channel with a dot or dash for sub-channel (e.g. 69.1 or 69-1). Also see St Louis DTV Stations.
IMPROVE SIGNAL RECEPTION
ANTENNA POINTING DIRECTION: UHF antennas are directional, and must be pointed in the direction of the transmitter tower for best reception. A UHF antenna beam is relatively wide, but less forgiving than VHF antennas.
ANTENNA ELEVATION: The antenna should be located as high as possible and have a clear line-of-sight (no hills, structures, trees, etc.) to the transmitting towers. The higher the receiving antenna is above the ground level the greater the signal density.
CABLES: Replace all twin-lead (300 ohm) cables with RG-6 coax (75 ohm) cables. The more shielding (up to quad shielding), the better the immunity from interference (power lines, transmitters, etc), and the less flexible the cable. Check all outside connectors for corrosion, replace if necessary. Cable runs should be as short as possible.
|Cable Length (feet)||10'||25'||50'||75'||100'||150'||200'||300'||400'|
|Loss in Range||-5%||-15%||-30%||-40%||-50%||-65%||-75%||-85%||-95%|
SIGNAL SPLITTERS: A 1 input, 2 outputs (2 port) signal splitter is equivalent to adding about 50 feet of cable.
BOOSTER AMPLIFIERS: Long cable runs and splitters may require a booster amplifier. A booster-amp will not improve a weak antenna signal, because it amplifies signal and noise, but it will allow for longer cable runs and more splitters.
PRE-AMPS: Pre-amps are low noise amplifiers (amplifies signal but not noise) that mount to an indoor or outdoor antenna to improve reception. Outdoor pre-amps have a separate power unit inside the home that supplies power to the pre-amp through the coax center conductor. Most have an adjustable gain, most plug into a standard power outlet (110 Vac), some are powered from a USB connection.
HIGH GAIN ANTENNA: High gain antennas have extended range but narrow beams. These antennas may require a rotor to move the beam (rotate antenna) between stations with a wide angle separation.
ADAPTERS: Older antennas and TV's use a 300 ohm twin-lead connection. In these cases a coax to twin lead adapter (75 to 300 ohm) is required to connect twin-lead to coax. The adaptors use an impedance matching transformer to keep signal loss minimal. An adapter works both ways, signals go from coax to twin-lead or twin-lead to coax.
ATTENUATORS: A signal that is too strong can overwhelm (saturate) a TV receiver, causing a loss of signal. Saturation occurs when a receiver is too close to the tower, or antenna gain is too high and cable run too short. An attenuator can reduce the signal to an acceptable level. Some attenuators have a fixed reduction, some can be adjusted.
Television antennas come in 3 basic frequency band configurations.
|VHF||2 - 13||54 - 216 MHz|
|VHF/UHF||2 - 69||54 - 216 MHz
470 - 806 MHz
|UHF||14 - 69||470 - 806 MHz|
|Most DTV transmitters are in the UHF band./tr>|
ANTENNA GAIN: Reception range doubles for a gain increase of +6 dB, and antenna size will quadruple. Antenna gain is expressed in dBi or dBD. Gain expressed in dB is usually really dBD, makes gain look 2 dB higher.
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DTV Antennas and Accessories
Outdoor Antennas -- Indoor Antennas -- Amplifiers -- Signal Splitters -- Coax -- Installation Hardware
RECEIVER SIGNAL LEVELS
Analog receivers require a minimum signal to noise ratio (S/N) of 10 - 15 dB, receive signal must be at least 10 - 15 dB above the noise floor. DTV receivers require a S/N of 25 - 30 dB. The noise floor varies with atmospheric conditions, time of day (higher during daylight), and increases in noisy electronic environments; close proximity to transmitters, high voltage lines, transformers, and motors.
dBm = decibels with respect to 1 milliwatt
dBmV = decibels with respect to 1 millivolt
mV = millivolts
RF CHANNEL FREQUENCIES
Each RF channel is 6 MHz wide.