Bird Technologies 28-69-04A - Manual (Page 7)

the interconnect cables whenever an inspection is
performed on other station equipment.
Because duplexers are passive devices, field re-
pairs are rarely required. Field repair of duplexers
is limited to the replacement or repair of damaged
cables. Cavity damage, when it occurs, is usually
due to catastrophic failure from lightning or power
far in excess of the duplexers rating. If cavity prob-
lems are suspected, the unit should be returned to
the factory for repair. Due to the critical alignment
of parts inside of the cavity resonators, field repair
is not recommended.
TUNING
Vari-Notch duplexers are originally pre-tuned at
the factory to the customers specification. To re-
tune the duplexer, each resonant cavity must be
separated from the group and adjusted individu-
ally. Then the individual cavities are re-connected
and each channel is fine tuned to peak it's overall
response. When reconnecting the assembly, it is
mandatory that each filter and cable be replaced in
it's original position.
There are two adjustable parameters found in a
Vari-Notch filter; the
pass frequency and the rejec-
tion notch . Adjustment of the coarse and fine tun-
ing rods will allow the filters passband to be
centered at the desired frequency. The rejection
notch frequency is adjusted by turning the variable
capacitor located on the loop plate assembly.
The insertion loss of a cavity is determined by the
position of the loop plate and is not field adjust-
able. The loop plate on a 6.625" cavity should
never be loosened or moved from it's factory
preset position.
It is also important to note that the insertion loss
specification in table 1 for each of the different
models, is the total insertion loss for each channel
of that model. For instance, the specification for
model 28-13-01F is 1.5 dB, this means both the
high and low frequency channels will each have a
TX RX Systems Inc. Manual 7-9177-1 09/19/97 Page 3
Coarse Tuning Rod
Fine Tuning Rod
Coarse Tuning Lock
10-32 Cap Screw
Variable Capacitor
Equipment
Port
Equipment
Port
Antenna
Port
Fine Tuning Lock
Knurled Thumb Nut
Loop Plate
Assembly
Loop Plate
Hold Down Screws
Access Barrel
Cavity
Loop Plate
Hole Cover
Mounting
Bar
Calibration
Index
Resonant
Interconnect
Cable
Interconnect
C abl e
Antenna
Cable
Antenna
Cable
Figure 2: Top view of a typical four-cavity Vari-Notch Duplexer (6.625" diameter cavities).