Transposer

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.^[1] Since transposers are used to cover a small shadowed 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 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).

Transposer circuitry

The transmitters have the following stages:

• Audio (AF) or video (VF) frequency buffer stages
• Modulator
• IF stages
• Mixer (IF → RF)
• RF output stages ( RF amplifiers and filters)

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 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 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 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, the transpower output frequency is always chosen to be different from the frequency of the input signal.^[3] Further, filters at the input and output isolate the input from the output.

The future of the transposers

Transposers were very common before satellite 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 transmitter
• Radial (radio)
• Earth bulge

References and notes

1. ^ R.Busi: High Altitude VHF and UHF broadcasting stations, European Broadcasting Union, Bruxxeles,Brussels, 1967, p.93
2. ^ The level of the incoming RF signal fluctiates greatly depending on weather.
3. ^ For example, in VHF TV a difference of at least two channels is required.