Structure of DTV Networks and Transmitting Stations - Digital Video and Audio Broadcasting Technology - page 530

Thus, 5 UHF channels are coupled together and conducted up to the
transmitting antenna via a coax cable. VHF channel 10 only needs a mask
filter and no combiner and is connected to the VHF transmitting antenna
via its own cable. The length of the line to the antenna is about 280m. De-
pending on frequency, the cable attenuation can be assumed to be about
0.5 dB per 100 m length of transmitting cable.

31.3.1.1 Transmitter Technology Used

All of the 6 DVB-T transmitters on the Mt. Wendelstein are liquid-cooled
high-power transistor transmitters of the NV/NM7000 series by
Rohde&Schwarz. Such a transmitter can be imagined simply as a high-
power amplifier driven by an exciter and produced by connecting together
a large number of power amplifier modules. Each transistor in these single
power amplifier stages generates a average power of about 25 W. The out-
put signals of the amplifiers are added together via 3 dB couplers. Using
3 dB couplers, more and more amplifiers are coupled together one by one
so that a total power of about 450 W per amplifier module is obtained in
the case of the NV/NM7000 series by Rohde&Schwarz. The output pow-
ers of the various amplifier modules are then combined again via couplers
to give the total output power of the transmitter of about 5 kW (mean
power) in the case of the Mt. Wendelstein transmitter. These couplers can
be implemented as Wilkinson coupler or again as 3dB coupler. A Wilkin-
son coupler is a 0 degree coupler, whereas the 3dB coupler is a 90 degree
coupler.

R Source

50 Ohm 50 Ohm

-3dB 0 o

R Load

/4

R Term

50 Ohm

50 Ohm

-3dB -90 o

R Load

Fig. 31.4. 3 dB splitter