Bird Technologies 3129 Display - Manual (Page 69)

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Appendix 3 Reference Information
Determining VSWR Alarm Trip Point
Even though the VSWR trip point can be set to 2.5, the instrument cannot
always measure VSWR above 1.9. This is due to the large amount of reflected
power caused at higher VSWRs. The maximum VSWR that can be physically
measured is determined by the ratio of the forward and reflected couplers at full
scale. This ratio is 10:1. The maximum measurable VSWR occurs when the
reflected channel is near its limit. The following table shows the maximum mea-
surable VSWR as a function of percentage of full scale forward input power. Use
this table as a guide to determine the VSWR trip point. For example, using a
BPM-E 1 for the VHF low band, the full scale power measurement is 2 kW. If you
are measuring a transmitter that delivers 1.7 kW (1.7 Kw is 85% of the 2 kW full
scale), the maximum measurable VSWR is 2.24. If the transmitter power is
reduced to 1 kW (50 % of the 2 kW full scale), then the maximum measurable
VSWR is 3.00.
Percentage Full
Scale
Max VSWR
50% 3.00
55% 2.82
60% 2.68
65% 2.56
70% 2.46
75% 2.38
80% 2.31
85% 2.24
90% 2.19
95% 2.14
100% 2.09