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11
Chapter 4 Operating Instructions
Operating
Modes
The Model 4391A has nine modes of operation which are
selected by pressing the mode keys momentarily. In
addition, each mode has three output options selected by
pressing the modifier keys. Detailed descriptions of the
modes and output options follow.
Reading
Forward CW
Power
For this measurement only a forward element is needed.
Install the meter and element according to the preceding
paragraphs and move the power switch to LINE or BAT
depending on the power source desired. When powered up,
the Model 4391A always goes into the forward CW power
mode. If the unit is already operating, the forward CW power
mode is selected by pressing the FWD CW key momentarily.
If the applied power exceeds 120 percent of the range, two
right-facing arrow heads (i.e., “greater-than” symbols) will
be displayed. The operation of this error message does not
depend on the correct setting of the range switches by the
operator, nor will the meter or its elements be damaged if
the switches are incorrectly set.
The Model 4391A arrives at values of CW power by a method
quite different from analog meters such as the Model 43, also
manufactured by Bird Electronic. While the two instruments
will agree when the measured wave is of constant amplitude,
AM or SSB waves will result in different indications (in the
CW mode). This is because the analog instrument uses the
inertia of the microammeter to “time-average” the varying
CAUTION
Never apply RF power to the Model 4391A Wattmeter
unless both line section sockets are filled with either an
element or a dust plug. If an element is used it is
advisable to place the element with the arrow at a 90°
angle to the coaxial line.
CAUTION
Always disconnect ac power before opening the Model
4391A enclosure. Removal of at least one battery cell is
also recommended when servicing the instrument.
Figure 6
Forward
Power (CW)