Neptune R450 MC, Installation And Maintenance Manual

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Appendix A
R450 Mini Collector Installation and Maintenance Guide 61
VSWR Calculation
Neptune recommends the following method of computing VSWR.
Taking the reading at the R450 MC end of the feed line without
compensating for cabling losses may give a false impression of the
antenna and feed line performance.
Using a Wattmeter and a Handheld 450 MHz Radio
The recommended procedure is to use a handheld commercial grade
transceiver that is tuned to the R450 MC's transmitter frequency and an
inline wattmeter, such as a Bird Model 43 Wattmeter with the
appropriate element (slug) for the frequency range and power output of
the transceiver.
The Bird Model 43 Wattmeter uses elements to set the frequency and
power ranges that the meter will read. The wattmeter element should be
a 400 - 1000 MHz model sized so that the forward power reading is
close to full scale. For a 5W output handheld, the Bird 5E element is
recommended. If the radio puts out more than 5W, a Bird 10E or higher
power element may be required.
Complete the following step to use a wattmeter and handheld to
calculate VSWR.
1 Connect the wattmeter and transceiver to the feed line in place of
the R450 MC.
2 Measure both the forward and reflected power.
3 Using the Power Measurement to Return Loss and VSWR Conver-
sion Table on page 59, find the return loss value.
4 Based on the coax type and the feed line length, find the loss
attributed to the coax using the Coax Cable Loss Table on page 60.
5 The adjusted return loss is calculated by the formula:
Return Loss (from Table) - (2 x Cable Loss) = Corrected Return
Loss
6 Using the Corrected Return Loss value, find the VSWR using the
Power Measurement to Return Loss and VSWR Conversion Table
on page 59.
7 Change the frequency on the handheld transceiver to the
R450 MC's receive frequency.
8 Measure the power and calculate VSWR using the procedure just
used for calculating transmitter VSWR.
9 Record both transmit and receive frequencies VSWR values. In
order for the antenna system to pass, both readings must be less
than 1.5:1.