Re-emitters in iso-frequency mode
When a re-emitter operates in iso-frequency mode, the DTT signals are broadcasted on the same frequencies that are received. In this type of configuration is essential to avoid the coupling between the transmitting and the receiving antenna. The lack of isolation between antennas causes that the receiving antenna receives the broadcasted signal.
If when using a re-emitter, which is working in iso-frequency mode, occurs a coupling between antennas, then the receiving antenna receives two overlapped signals: One coming from the transmitter away from the centre and the signal itself transmitted by the re-emitter.
When the re-emitter is regenerative, due to the processes of demodulation, regeneration of the stream and modulation, the coupled signal has a delay higher than the guard time.
In these cases, the DVB-T demodulator is unable to recover the transport stream. In order to prevent this situation it is necessary to minimize coupling between antennas.
The isolation between antennas basically depends on the type of antennas used and the location thereof. On the one hand antennas of high directivity must be used and on the other hand antennas must be separated the most. The effect of separating the antennas is more important in the first 3 or 4 meters. Generally, the isolation increases from 15 to 20 dB when the distance between antennas increases from 1 to 10 meters. The use of different polarisations between transmission and reception also increases the decoupling between antennas.
To measure the isolation of the radio-link station in Prades, it is added a frequency pilot tone to a DT-722 amplifier module. That tone has a frequency similar to the re-emitted channels. Then the tone is amplified and radiated from the re-emitter. Due to the lack of isolation, the receiving antenna captures a fraction of the broadcasted signal.
Knowing the radiated and the received power the isolation can be determined:
Ixy = Pradiated – Preceived
The installation at the left gives an isolation of 103 dB and the one at the right gives an isolation of 65 dB.
Determination of Maximum Emitted Power
Knowing the power received from the remote transmitter (Rxy) and the isolation (Ixy) it is obtained the value of the maximum power that can be emitted from the re-emitter.
As the DTT demodulator requires a MER of between 27 and 30 dB to operate properly, then it can be assumed that the coupled signal should be at least 30 dB below the signal received from the remote transmitter (Rxy). That is:
Coupled Signal: Pxy – Ixy
Margin: 30 dB
Pxy – Ixy = Rxy – 30 dB
Therefore, maximum power:
Pxy = Rxy + Ixy – 30 dB
So, for an isolation (Ixy) of 103 dB and a signal received (Rxy) of 48 dB, the re-emitter may broadcast 21 dBuV (14 dBm) at the most. If the decoupling is 65 dB, then the maximum power will be 83 dBuV (-24 dBm).
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