Detailed coverage planning identifies the gaps in the DTT networks. Such areas with poor signal are covered with low power transmitters TERX or repeaters ARX and HALO. Mainly are used in complex outdoor environments or complex terrains such as mountains, underground locations, highways, villages, railways, etc. Nowadays omnidirectional antennas are used (i.e. OMNI-1 or OMNI-2), mounted on the top the towers.

Two network types are used: MFN, there is no technical limitation in radiated power; SFN, more common, but to prevent oscillation, the gain of the fill-in transmitting equipment must be lower than the measured feedback. Measured isolation values range from 60 to 110dB. A safety margin of 10dB is applied to the isolation value in order to allow time variations.

Modern fill-in gap-fillers equipment allows amplification up to 25dB above the isolation value, which can be improved by larger separation between receiving and transmitting antenna, increased receiving and transmitting antenna directivity or if orthogonal polarization between input and output signal is used.

In the case ERP is not sufficient to cover the area either MFN network should be applied (if frequency is available), or the fill-in should be fed by microwave link, satellite or cable. In this case, the fill-in transmitter TERX should be equipped with GPS/GLONASS receiver.

A sufficiently strong input signal at level above -55dBm is commonly needed to ensure a good quality output signal. Line of sight between the main transmitting antenna and the receiving antenna of the fill-in station is likely to be required.

For SFN case it has to be considered sufficient isolation between receiving and transmitting antenna, moreover the output power depends on the isolation value. For that reason measurements and possibly adjustments are needed at each site and at each frequency before the fill-in station becomes operational.