Bird Technologies 21-87A-11-xx-T Series-Manual - Manual (Page 18)

TX RX Systems Inc. Manual 7-9100-5 10/17/11 Page 12
Fine Cavity Tuning
Figure 7 shows a hookup suitable for fine tuning
any channel under power while installed in the mul-
ticoupler. The term fine tuning here refers to cavi-
ties that have already been tuned to frequency and
may only require adjustment of the fine tuning con-
trol (+/- 50 KHz). The transmitter is used as a sig-
nal source and the cavity is adjusted for minimum
reflected power.
Procedure
With the transmitter keyed, the cavity fine tuning
control is adjusted (pushed in or out) to obtain a
minimum meter reading. See Figure 8 for a detail
of the cavity tuning controls. If a minimum meter
reading is obtained with the fine tuning rod fully in
or completely out, do the following:
1) Set the fine tuning rod so that about 1/2 its
length is inserted into the cavity.
When tuning a cavity that has been in
service for some time it is not unusual
to find the main tuning rod hard to
move in or out. This occurs because
TX RX uses techniques borrowed
from microwave technology to provide large area
contact surfaces on our tuning plungers. These sil-
ver plated surfaces actually form a pressure weld
that maintains excellent conductivity. This pressure
weld develops over time and must be broken to
move the main tuning rod. This is easily accom-
plished by gently tapping the tuning rod with a plas-
tic screwdriver handle or small hammer so that it
moves into the cavity. The weld will be broken with
no damage to the cavity.
When adjusting the coarse tuning rod, it is easy to
put the cavity far off resonance and cause most of
the transmitter power to be reflected back into the
isolator output section load. This load should be
capable of dissipating this power or damage could
result. If in doubt about the loads capability, follow
the coarse tuning procedure outlined below. It is
based on the use of a spectrum analyzer and fre-
quency generator which avoids the need to con-
sider power levels.
2) Loosen the coarse tuning rod locking screw (5/
32"/4mm Allen/HexKey wrench required) and
move the rod in or out slightly to obtain mini-
mum meter reading. Small movements of the
coarse tuning rod are facilitated by tapping the
rod with the handle end of a screw driver while
gently pushing or pulling the main tuning rod.
Tighten the coarse tuning locking screw.
3) Adjust the fine tuning control for a minimum
meter reading.
4) Tighten the fine tuning locking mechanism.
NOTE
Cavity Resonator
Coarse Tuning Rod
Coarse Tuning Lock
10-32 Cap Screw
Calibration Index
Input/Output Port
Loop Plate Assembly
Loop Plate
Hold Down Screws
Loop Plate Assembly
Input/Output Port
Calibration Mark
Calibration Index
Fine Tuning Rod
Fine Tuning Lock
Knurled Thumb Nut
Figure 8: T-Pass cavity tuning controls.