Transposer
Encyclopedia
In broadcasting
, a transposer is a device in the service area of a transmitter
which rebroadcasts signals to the receivers which can’t properly receive the signals of the transmitter because of a physical obstruction (like a hill). A transposer receives the signals of the transmitter and rebroadcasts the signals to the area of poor reception. Sometimes the transposer is also called a relay transmitter or rebroadcast transmitter. Since transposers are used to cover a small shadowed area, their output powers are usually lower than that of transmitters.
signals is sensitive to the size of obstruction in the path between the transmitter and the receiver. Generally speaking, if the size exceeds the wavelength
the reception is interrupted. Since the wavelength is inversely proportional to frequency
, it follows than that the higher frequency broadcast is more sensitive to objects between the transmitter and receiver. If the transmitter and the receiver were at the opposite sides of a hill, MW
radio signals may be received, but UHF TV signals won’t be received at all. That’s why the transposers are mostly employed for VHF and UHF broadcasting (television
and FM radio
).
The transposers have the following stages.
The output stages of both devices are similar, but the input stages are quite different. There is no AF or VF input to the transposer. The transposer receives input RF signals by means of an antenna
just like a home receiver. Since received signal is already modulated there is no modulator. Instead an input mixer converts the RF signal to an IF signal. A second mixer (known as output mixer) converts the IF signal to output RF signal.
s If the frequency of the output signal is set equal to the frequency of the input RF signal, the output RF which feedbacks (thru output and input antennas) the input stage overloads the input stage and completely blocks out the transposer. So, the transpower output frequency is always chosen to be different from the frequency of the input signal. Further, filters at the input and output isolate the input from the output.
broadcasting. With availability of satellite broadcasting (TVRO and RRO), most operators tend to use low power transmitters instead of transposers because of higher broadcast quality of the transmitters. Transposers are less expensive than the (same power) transmitters and they can still be attractive to some operators.
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
, a transposer is a device in the service area of a transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
which rebroadcasts signals to the receivers which can’t properly receive the signals of the transmitter because of a physical obstruction (like a hill). A transposer receives the signals of the transmitter and rebroadcasts the signals to the area of poor reception. Sometimes the transposer is also called a relay transmitter or rebroadcast transmitter. Since transposers are used to cover a small shadowed area, their output powers are usually lower than that of transmitters.
Physical obstruction
Reception of RFRadio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
signals is sensitive to the size of obstruction in the path between the transmitter and the receiver. Generally speaking, if the size exceeds the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
the reception is interrupted. Since the wavelength is inversely proportional to frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, it follows than that the higher frequency broadcast is more sensitive to objects between the transmitter and receiver. If the transmitter and the receiver were at the opposite sides of a hill, MW
Mediumwave
Medium wave is the part of the medium frequency radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz...
radio signals may be received, but UHF TV signals won’t be received at all. That’s why the transposers are mostly employed for VHF and UHF broadcasting (television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
and FM radio
FM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
).
Transposer circuitry
The transmitters have the following stages:- Audio (AFAudio frequencyAn audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...
) or video (VFComposite videoComposite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
) frequency bufferBuffer amplifierA buffer amplifier is one that provides electrical impedance transformation from one circuit to another...
stages - Modulator
- IFIntermediate frequencyIn communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
stages - MixerElectronic mixerAn electronic mixer is a device that combines two or more electrical or electronic signals into one or two composite output signals. There are two basic circuits that both use the term mixer, but they are very different types of circuits: additive mixers and multiplying mixers...
(IF → RF) - RF output stages ( RFRadio frequencyRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
amplifiers and filters)
The transposers have the following stages.
- RF input stages (RF amplifiers with AGCAutomatic gain controlAutomatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
and band-pass filterBand-pass filterA band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
) - Input mixer (RF → IF)
- IF stages
- Output mixer (IF → RF)
- RF output stages ( RF amplifiers and filters).
The output stages of both devices are similar, but the input stages are quite different. There is no AF or VF input to the transposer. The transposer receives input RF signals by means of an 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...
just like a home receiver. Since received signal is already modulated there is no modulator. Instead an input mixer converts the RF signal to an IF signal. A second mixer (known as output mixer) converts the IF signal to output RF signal.
Relation between input and output RF signals.
In order to stabilize the output power, the amplification of the input RF signal is automatically controlled by PIN diodePIN diode
A PIN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts....
s If the frequency of the output signal is set equal to the frequency of the input RF signal, the output RF which feedbacks (thru output and input antennas) the input stage overloads the input stage and completely blocks out the transposer. So, the transpower output frequency is always chosen to be different from the frequency of the input signal. Further, filters at the input and output isolate the input from the output.
The future of the transposers
Transposers were very common before satelliteSatellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
broadcasting. With availability of satellite broadcasting (TVRO and RRO), most operators tend to use low power transmitters instead of transposers because of higher broadcast quality of the transmitters. Transposers are less expensive than the (same power) transmitters and they can still be attractive to some operators.
See also
- TV transmitters
- Transmitters
- Output power of an analog TV transmitterOutput power of an analog TV transmitterOutput power of a TV transmitter is the electric power applied to antenna system. There are two definitions: nominal and thermal.- Power defined in terms of voltage :The average power for a sinusoidal drive is...
- Radial (radio)Radial (radio)In RF engineering, radial has two different meanings, both referring to lines which radiate from a radio antenna.-Planning:...
- Earth bulgeEarth bulgeEarth bulge is a term used in telecommunications. It refers to the circular segment of earth profile which blocks off long distance communications.- Distance to horizon :...