In broadcasting, a transposer is a device in the service area of a transmitter which rebroadcasts electromagnetic signal to the receivers which can’t properly receive the signals of the transmitter because of a physical obstruction (like a hill). A high altitude transposer receives the signals of the transmitter and rebroadcasts the signals to the area of poor reception. That’s why sometimes the transposer is also called relay transmitter or rebroadcast transmitter.[1] Since transposers are used to broadcast a small shadow area their output powers are usually lower than that of transmitters.
Physical obstruction
Reception of RF 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 wavelenth is inversely proportional to frequency, it follows than that the higher frequency broadcasting is more sensitive to objects between the transmitter and receiver. Supposing the transmitter and the receiver are at the opposite sides of a hill, while MW radio signals can be received, UHF TV signals can’t be received at all. That’s why the transposers are mostly employed for VHF and UHF broadcasting. (ie, TV and FM radio)
Transposer circuitry
The transmitters have the following stages:
The transposers have the following stages.
- RF input stages (RF amplifiers with AGC and band-pass filter)
- Input mixer (RF → IF)
- IF stages
- Output mixer (IF → RF)
- RF output stages ( RF amplifiers and Band-pass filter).
It should be noted that, 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. But instead of a modulator an input mixer is used to convert RF signal to 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 diodes [2] 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 in transposers, the frequency of the output RF signal is always chosen to be different from the frequency of the input RF signal.[3] Furthermore, an input filter which is a part of input stages and an output filter which is a part of the output amplifier isolate the input from the output.
The future of the transposers
Transposers were very common before satellite broadcasting days. But beginning by the satellite broadcasting (TVRO and RRO), most operators tend to use low power transmitters instead of transposers because of higher boadcast quality of the transmitters. But transposers are less expensive than the (some power) transmitters and they can still be attractive to some operators.