Bird Technologies 4391A - Manual (Page 22)

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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 signal coming from the element, whereas the Model 4391A
uses peak and negative peak detector circuits to measure peak and minimum
square root of power and combines them using the equation:
With this technique, operation of CW mode is predictable regardless of enve-
lope shape (see Figure 4).
Reading Reflected CW Power
Operation of the reflected CW power mode is identical to that for forward CW
power described above with two exceptions: the readings are taken from the
element in the socket marked “REFLECTED” and the range of the element is
assumed to be 1/10 the range indicated by the range switches.
Figure 7 Reflected Power (CW)
Reading SWR
Two elements with a 10 to 1 power range ratio are required for this mode. Press
the SWR key momentarily. If average forward power is between 10% and 120% of
the scale and the average reflected power is less than 120% of the reflected ele-
ment range, SWR will be displayed. If any of the above conditions are not met, an
error message will be displayed. Two arrows pointing to the right — or “greater-
than” symbols — indicate over-range, while two left-pointing arrows — or “less-
than” symbols — indicate under-range or too little power. Refer to Table 2.
Figure 8 Standing Wave Ratio (SWR)
CW POWER
Peak Power Minimum Power +
2
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