Structure of DTV Networks and Transmitting Stations - Digital Video and Audio Broadcasting Technology - page 534

large because of their power rating and weigh about 100 kg. The mask fil-
ters are simply used for lowering or suppressing adjacent channel emis-
sions. The mask filters are tuned to the respective channel and have an at-
tenuation of about 0.3 to 0.6 dB in the passband which is also easily
noticed as heat.

31.3.1.3 Combiner

The combiner has the task of combining the various TV channels to form
one signal which is then supplied to the respective TV transmitting antenna
via one cable in the VHF band and one cable in the UHF band. The trans-
mitters themselves must be well decoupled from one another which is pre-
cisely what the filters in the combiner are doing. The combiner has an ap-
proximate passband attenuation of 0.3 dB.

0º,
3dB

0º, 0dB

50

90º, 0dB

90º,
3dB

50

Fig. 31.9. Series connection of two 3 dB couplers

f 2

Narrowband input

f 1

f 1

Broadband input

f 1 +f 2

50 Ohm

Output

f 1

Fig. 31.10. Principle of an antenna combiner

Each channel separating filter of the combiner consists of two channel
filters which are tuned to the respective TV channel to be supplied. This is
preceded and followed by a 3 dB coupler. To understand the operation,
two series-connected 3 dB couplers can first be considered. If a signal is
fed into the first coupler, it divides into 2 signals of 0 degrees and 90 de-