Bird Technologies 4391A - Manual (Page 25)

13
Measuring Power in dBm
Operation of the forward and reflected dBm modes is identical to the forward and
reflected CW power modes, except that the resulting reading is converted to dB
above 1 milliwatt before it is displayed. It should be noted that in doing this con-
version, the range set on the slide switches is assumed to be watts rather than
kilowatts or milliwatts. If it is not, 30.0 must be added to all dBm readings when
the range is in kilowatts, or subtracted from all readings when it is in milliwatts. An
error message is displayed if CW power is more than 24 dB below, or peak power
is more than 6 dB above the nominal element range. Refer to Table 4.
Figure 11 CW Power in dBm
Table 4 Watts/dBm Equivalents
Measuring Return Loss
The measurement of return loss is the same as that of SWR except that the
result is displayed in dB. In other words a reading of 21.65 indicates that
reflected power is 21.6 dB down from forward power.
Figure 12 Return Loss, Insertion Loss, or Attenuation
Power dBm Power dBm
1 milliwatt 0 4 watts 36
10 milliwatts 10 10 watts 40
100 milliwatts 20 20 watts 43
1 watt 30 40 watts 46
2 watts 33 100 watts 50