Bird Technologies 73-67-25 Series - Manual (Page 28)

TX RX Systems Inc. Manual 7-9120-5 07/26/10 Page 22
GENERAL DESCRIPTION
Isolators perform two important functions. Their pri-
mary function is to keep other RF frequencies out
of the transmitter so that intermodulation products
cannot be generated. Isolators have a substantial
amount of reverse isolation. They also insure that
the transmitter never sees any significant reflected
power so it will always operate with maximum sta-
bility at full-power output. Isolators prevent energy
from getting into the transmitters output stage by
dumping reflected RF energy entering the output of
the isolator into a dummy load.
The UHF (compact style) isolators available from
TXRX Systems are broad-band and do not require
tuning. The isolators are available as either single
section or dual section models. Dual section mod-
els consist of two single sections mounted in the
same case with a load permanently attached to the
load port of the first section. Table A1 lists the UHF
isolators available from TX RX Systems along with
their performance specifications.
INSTALLATION
The isolators can be mounted on most types of
surfaces but should not be physically located
where they will be exposed to moisture or very high
h u m i d i t y. T XR X S y s t e m s i s o l a to rs a r e we l l
shielded magnetically and may be mounted on
steel cabinets or panels.
The isolators can get quite hot
during operation. This can occur
when an antenna system compo-
nent fails causing high reflected
power which is then dissipated by
the isolator load. These loads can
get hot enough to burn skin so
use caution when servicing these
systems.
VERIFYING ISOLATOR FUNCTIONALITY
If you suspect there may be a problem with an iso-
lator you can verify the functionality of the device
by measuring its reverse isolation and insertion
loss. It is important to electrically remove the isola-
tor from the system before testing. This is easily
accomplished by disconnecting the input and out-
put cables.
WARNING: Do not make or break
cable connections to the isolator
while the circuit is under transmit
power. Shut down the transmitter
before servicing.
RECOMMENDED TEST EQUIPMENT
The following equipment or it’s equivalent is rec-
ommended when verifying isolator functionality.
1) Spectrum Analyzer. Bird Technologies Signal
Hawk.
2) A pair of double shielded coaxial cable test
leads (RG142 B/U or RG223/U).
3) 50 Ohm load with at least -35 dB return loss
(1.10 : 1) VSWR.
Measuring Reverse Isolation (S12)
The reverse isolation of your isolator can be veri-
fied by performing the following procedure in a
step-by-step fashion.
1) Make sure the transmitter associated with the
isolator is turned off.
2) Disconnect the input and output cable to the
isolator.
3) Connect a spectrum analyzer and tracking gen-
erator to the input and output ports of the isola-
tor respectively, as shown in Figure A1.
4) Make sure that a 50 Ohm load is connected to
the load port of the isolator. If you are testing
the isolator on the bench make sure you con-
nect a load. If you are testing the isolator while it
is still mounted on the system rack/cabinet
leave the existing load connected.
UHF Isolators (Compact Style)
Appendix A