Startup and Operation
20
Impedance trimming
Figure 12. Impedance-matching
transformer
The transformer has been factory-adjusted to 50 ohms at your frequency. You
will find a scribed line on each control rod shaft. It can be operated at that
setting, but it will give optimal performance on your tower if you readjust it
after installation.
Adjust the transformer as follows:
a. Loosen the clamps on the control rods enough to allow the rods to move.
b. Grasp either control rod and slide it in or out about 1/4 inch or 6 millimeters.
It will move stiffly because of O-ring friction.
c. Read the VSWR. If the reading went down, move the control rod again in the
same direction. If the VSWR went up, move the same rod in the opposite
direction.
d. Keep adjusting the same rod until no further improvement is seen. Adjust
the second rod in the same manner. If you get "lost," return both rods to the
factory setting (
Table 5
) and start over.
e. Return to the first rod, and so forth, until you have the lowest possible VSWR
or return power reading. This is the optimal transformer setting.
Table 5. Factory control rod settings
Nominal
Transformer Size
Factory Control Rod Setting
(
Figure 12
on page 20)
1-5/8”
3-3/4" ± 1/16" (95 ± 1.5 mm)
3-1/8"
2-3/4" ± 1/16" (69 ± 1.5 mm)
4-1/16"
3-1/2" ± 1/16" (89 ± 1.5 mm)
