Output power of an analog TV transmitter
Encyclopedia
Output power of a TV transmitter is the electric power
applied to antenna
system. There are two definitions: nominal (or peak) and thermal.
For a system where the voltage and the current are in phase, the output power can be given as
In this equation
is the resistance and 
is the output voltage
where
is the rms value
of the output voltage.
To measure the nominal output power, measuring devices with time constant
s much greater than the line time are used. So the measuring equipments measure only the highest level (sync pulse) of a line waveform which is 100 %.
(VSB modulation or C3F). The modulation polarity is negative . That means that the higher the level of the video signal the lower the power of the RF signal. The lowest possible modulating signal during the synchrone interval yields 100 % of the carrier. (The nominal power of the transmitter.) The blanking level (300 mV) yields 73 % (in an ideally linear transmitter). The highest modulating signal at white (1000 mV) yields only 10 % of the carrier. ( so called residual carrier
). So the output power applied to the antenna system is considerably lower than the nominal power. The thermal power which can be measured by a microwave power meter
depends on the program content as well as the residual carrier and sync depts.
For system B
, the duration of the black level 300 mV (together with the front and back porches), is 59.3 μs and it corresponds to 73 % of maximum voltage level.The duration of the sync pulse is 4.7 μs. The total duration of the line is 64 μs.
So the maximum thermal power applied to the antenna system is 57 % of the nominal power, even in the black scene. In normal program content this ratio may be around 25 % or less.
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
applied to antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
system. There are two definitions: nominal (or peak) and thermal.
Power defined in terms of voltage
The average power for a sinusoidal drive isFor a system where the voltage and the current are in phase, the output power can be given as
In this equation
is the resistance and is the output voltageNominal power of a TV transmitter
Nominal power of a TV transmitter is given as the power during the sync interval. (For the sake of simplicity aural power is omitted) Since, the voltage during the sync interval is a fixed value,where
is the rms valueRoot mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
of the output voltage.
To measure the nominal output power, measuring devices with time constant
Time constant
In physics and engineering, the time constant, usually denoted by the Greek letter \tau , is the risetime characterizing the response to a time-varying input of a first-order, linear time-invariant system.Concretely, a first-order LTI system is a system that can be modeled by a single first order...
s much greater than the line time are used. So the measuring equipments measure only the highest level (sync pulse) of a line waveform which is 100 %.
The thermal power
In analogue TV broadcasting, the video signal modulates a carrier by a kind of amplitude modulationAmplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
(VSB modulation or C3F). The modulation polarity is negative . That means that the higher the level of the video signal the lower the power of the RF signal. The lowest possible modulating signal during the synchrone interval yields 100 % of the carrier. (The nominal power of the transmitter.) The blanking level (300 mV) yields 73 % (in an ideally linear transmitter). The highest modulating signal at white (1000 mV) yields only 10 % of the carrier. ( so called residual carrier
Residual carrier
In analogue TV technology, residual carrier is the ratio of carrier level which is modulated by the maximum video signal to the unmodulated carrier level.- Video signal :...
). So the output power applied to the antenna system is considerably lower than the nominal power. The thermal power which can be measured by a microwave power meter
Microwave power meter
A microwave power meter is an instrument which measures the electrical power at microwave frequencies.Usually a microwave power meter will consist of a measuring head which contains the actual power sensing element, connected via a cable to the meter proper, which displays the power reading...
depends on the program content as well as the residual carrier and sync depts.
Ratio of thermal power to nominal power
Since the program content is variable, the thermal power varies during the transmission. However, for testing purposes a standard line waveform can be applied to the transmitter. Usually line waveforms corresponding to 350 mV or 300 mV black image (and without field sync) are applied to the input of the transmitter.For system B
System B
System B is the analog broadcast television system used in the most countries. There are several systems in use and letter B is assigned for the European system which is also used in the majority of Asian and African countries. - Specifications :Some of the important specs are listed below.Frame...
, the duration of the black level 300 mV (together with the front and back porches), is 59.3 μs and it corresponds to 73 % of maximum voltage level.The duration of the sync pulse is 4.7 μs. The total duration of the line is 64 μs.
So the maximum thermal power applied to the antenna system is 57 % of the nominal power, even in the black scene. In normal program content this ratio may be around 25 % or less.