VSWR Calculation
Neptune recommends the following method of computing VSWR.
Taking the reading at the R450 DC 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 450MHz Radio
The recommended procedure is to use a handheld commercial grade
transceiver that is tuned to the R450 DC'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 steps to use a Wattmeter and handheld to
calculate VSWR.
1. Connect the wattmeter and transceiver to the feed line in place of
the R450 DC.
2. Measure both the forward and reflected power.
3. Find the return loss value using Table 15 on page 90 and Table 16
4. Find the loss attributed to the coax using Table 17 on the previous
page based on the coax type and feed line length.
The adjusted return loss is calculated by the following formula.
Return Loss (from Table) - (2 x Cable Loss) = Corrected Return
Loss
5. Find the VSWR using Table 15 on page 90 and Table 16 on
page 91 using the Corrected Return Loss value.
6. Change the frequency on the handheld transceiver to the
R450 DC's receive frequency.
7. Measure the power and calculate VSWR using the procedure just
used for calculating transmitter VSWR.
8. 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.
R450 Data Collector Installation and Maintenance Guide
95
Appendix A: RF Antenna Installation
Summary of Contents for R450
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