TX RX Systems Inc. Manual 7-9487-3 05/05/11 Page 20
with high-quality 50-ohm coaxial cable such as
1/4-inch superflexible transmission line. Some
flexibility in the jumper cables will prevent strain
and possible damage to the connections. We
also recommend the use of quality BNC con-
nectors. Unused receiver outputs need not be
terminated. However an unused expansion port
should be terminated with 50 ohms until con-
nected to an expansion deck.
Interference and IM Considerations
Although TX RX Systems, Inc. TTA systems are
designed for maximum interference immunity,
there are many factors that can lead to harmful
interference when using a tower-mounted ampli-
fier. It is highly recommended that the receiving
and transmitting antennas be vertically separated
to maximize antenna isolation.
Although most 700/800 MHz transmitters are con-
nected to their antenna through a combiner, it is
quite likely that the combiner does not have
enough transmitter noise filtering to prevent desen-
sitization of the receivers unless there is significant
antenna space isolation. Large values of antenna
isolation are most easily realized when the anten-
nas are separated vertically. This antenna isolation
also helps reduce the possibility of intermodulation
interference in the receiving system.
One other important factor that can strongly con-
tribute to interference problems is excessive gain,
ahead of the receiver. Excessive gain can cause
overdrive to the station receivers when strong sig-
nals are present, making them more prone to inter-
modulation or carrier desensitization problems.
Receiver preamplifiers should not be used.
FEEDLINE DATA
As part of the installation process you will need to
determine the cable losses for your main and test
transmission lines. These loss values can be deter-
mined by sweeping the cables or they can be
looked up from the cable manufactures specifica-
tions. For your system these values will be fixed
once the cable type is chosen and cut to length.
Once you have determined the main and test line
cable loss for your system this information can be
recorded in system memory for future reference in
the Feedline Data menu selection. To save the
cable loss values in memory perform the following
steps.
1) From the default display press the DOWN
ARROW button on the front panel to scroll
through the menu choices until you reach the
FEEDLINE DATA menu
.
2) With the FEEDLINE DATA menu displayed
press the ENTER button to step down to the
MAIN LINE LOSS sub-menu.
3) Use the UP and DOWN ARROW buttons to set
the main line loss to the desired value. This
storage register works in a forward loop fash-
ion, starting at 0.0 and increasing to 9.9. A but-
ton press after 9.9 returns the setting back to 0.
4) After setting the main line loss value press the
ENTER button to return back to the FEEDLINE
DATA menu. This will save your setting choice.
5) With the FEEDLINE DATA menu displayed
press the ENTER button to step down to the
MAIN LINE LOSS sub-menu. Press the UP
ARROW button to move to the TEST LINE
LOSS sub-menu.
6) Use the UP and DOWN ARROW buttons to set
the test line loss to the desired value. This stor-
age register works in a forward loop fashion,
starting at 0.0 and increasing to 9.9. A button
press after 9.9 returns the setting back to 0.
7) After setting the test line loss value press the
ENTER button to return back to the FEEDLINE
DATA menu. This will save your setting choice.
Then pressing the CANCEL button while at the
FEEDLINE DATA menu will return you to the
default display.
OPTIMIZING THE SYSTEM
In the TTA system the first stage of amplification is
in the tower top box which is used to overcome the
main line loss, develop the noise figure, and the
reserve gain. The second amplifier, located on the
base control unit of MCU style systems is used to
overcome the losses associated with distribution.
The CMU style systems do not have base amplifi-
ers because signal distribution is done in the
GTR8000.
When the tower top amplifier system is installed
there are detailed adjustments and test procedures
which must be followed in order to insure optimum
performance of the system. The process includes:
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