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, 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.
Older TV antennas incorporated a VHF antenna with a UHF antenna, and combined the signals from both antennas into a single cable. Today, most TV antennas are UHF only, and usually smaller.
DTV Channel vs Analog Channel
The old analog TV stations identified themselves by Radio Frequency RF channel, DTV does not. A station's DTV channel may or may not correspond to it's RF channel. Stations that transmitted analog changed their RF channels for DTV, but were allowed to grandfathered in the old channel designation as it's DTV or Virtual Channel.
Because multiple programs (stations) can be broadcast within one RF channel, DTV designations are virtual channel with a dot or dash for sub-channel (69.1 or 69-1). Also see St Louis DTV Stations.
IMPROVE SIGNAL RECEPTION
ANTENNA POINTING DIRECTION: Most DTV signals are in the UHF band and UHF antennas are directional. A UHF antenna must be pointed in the direction of the transmitter tower for best reception. A UHF receive antenna beam is relatively wide, but less forgiving than VHF antennas.
ANTENNA ELEVATION: The antenna should be located as high as possible and clear of obstructions (hills, structures, trees, etc.). The higher the receiving antenna is above the ground level the greater the signal density. In some scenarios range could more than double or triple.
CABLES: All twin-lead (300 ohm) cables should be replaced with RG-6 coax (75 ohm) cables. Double or triple shielded RG-6 should be used in noisy electronic environments; transmitters, high power lines, transformers or motors. Cable runs should be as short as possible.
|RG-6 Cable Loss for UHF Band|
|Cable Length (feet)||10'||25'||50'||75'||100'||200'||300'||400'|
|Loss in Range||-7%||-16%||-29%||-40%||-50%||-75%||-87%||-94%|
Signal splitters (1 input, 2 outputs) are equivalent to adding about 55 feet of cable. Long cable runs and multiple splitters may require a booster amplifier. A booster-amp will not improve a weak antenna signal, it amplifies signal and noise, but it will allow for longer cable runs and more splitters.
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 when running coax. The adaptors use an impedance matching transformer to keep signal loss minimal.
PRE-AMP: Pre-amps are low noise amplifiers (amplifies signal but not noise) that mount to the antenna to improve reception. A separate control box inside the home supplies power to the pre-amp through the coax cable center conductor. The control box has a manual gain control and plugs into any power outlet (110 Vac). The pre-amp requires a straight cable run (no splitters) to the control box.
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.
Best performance is achieved when the home antenna has a clear line-of-sight to the transmitting antenna. The higher the home antenna, the stronger the signal density.
RADIO FREQUENCY (RF) BAND:
Television antennas come in 3 basic configurations or frequency band sets; VHF, VHF/UHF, or UHF. Very few DTV transmitters are in the VHF band.
|VHF||2 - 13||54 - 216 MHz|
|VHF/UHF||2 - 69||54 - 216 MHz
470 - 806 MHz
|UHF||14 - 69||470 - 806 MHz|
Reception range doubles for a gain increase of +6 dB, and antenna size will quadruple.
- Antenna gain is usually expressed in dBi -- decibel (dB) difference compared to a lossless isotropic radiator.
- Manufacturers measure gain in dBD -- dB difference compared to a standard dipole antenna with a gain of +2.15 dBi.
- Antenna gain expressed in dB is usually really dBD (makes gain look higher).
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 or motors, and atmospheric conditions.
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.