Bird Technologies 21-87A-11-xx-T Series-Manual - Manual (Page 7)

TX RX Systems Inc. Manual 7-9100-5 10/17/11 Page 1
GENERAL DESCRIPTION
The 73-90-11-NN Series T-Pass Transmit Combin-
ers are designed to connect up to 21 transmitters
to a common antenna. They use three-port band-
pass filters (called T-Pass cavities) and ferrite iso-
lators to provide low channel insertion loss, high
isolation between transmitters, high antenna-to-
transmitter isolation, high intermodulation suppres-
sion, and excellent transmitter noise suppression.
T-Pass transmit combiners are broadband and
easily adaptable to the most difficult duplex system
design requirements.
An interconnect diagram of a typical transmit com-
biner is shown in Figure 1. The T-Pass filter
passes one narrow band of frequencies and atten-
uates all others with increasing attenuation above
and below the pass frequency. The T-Pass filter
has a “dual-port” output loop plate which allows the
filter to be easily connected to other T-Pass filters.
Connections between the filters are made with a
“thru-line” cable that behaves like a low loss 50
Ohm transmission line. The thru-line cables are
individually optimized to their own channel fre-
quency. No compromises are necessary to accom-
modate other channel frequencies. Each channel
can therefore be anywhere in a very broad fre-
quency range.
An isolator is added at the input to each T-pass
channel to increase channel isolation. The ferrite
isolators will isolate the transmitter from unwanted
signals that enter the system via the antenna. The
transmitter sees an excellent impedance match on
its output, because the isolator absorbs reflected
power that would otherwise enter the transmitters
output stage. This improves the stability, spectral
purity and long-term reliability of the transmitter.
The TX combiners can be expanded one channel
at a time with factory-tuned, easy-to-install expan-
sion channel assemblies. Expansion is usually
accomplished without modifications to the existing
system, and usually amounts to nothing more than
placing a new channel assembly, or several, on top
of the existing system. New channel frequencies
can be above, below, or between existing channel
frequencies.
The number of channels in the combiner is indi-
cated by the last two digits of the model number in
place of the NN designation. All of the information
for both installation and expansion is included in
this manual. The combiner is easy to install and
S
TX5
TX4
TX3
TX2
TX1
Transmitter Combiner (T-Pass)
Figure 1: Interconnect diagram of a typical Trans-
mit T-Pass Combiner. Typical five channel system
shown as an example.