Campbell Scientific RF300-Series DRL VHF/UHF Radio Transceivers - Manual (Page 52)

APPENDIX G. P50 RADIO
G-2
A problem has been found if the VSWR is greater
than 1.5:1. The VSWR will increase when:
•
The antenna is used in proximity of metal
•
Transmitting inside a building
•
The cable is bad
•
The antenna frequency does not match the
radio frequency
•
There is a bad connection
If the VSWR is below 1.5:1, then that
radio/cable/antenna link is good. However, be
sure the antenna is oriented properly.
While at the station also check the voltage on the
12 V port both with and without the transmit
button depressed. Regardless of the battery type,
the datalogger requires a minimum of 9.6 Volts.
G.3 TROUBLESHOOTING WITH
ATTENUATION PADS
If stations can be heard breaking squelch on
this base station radio, but communication
quality is poor or not being set up properly,
there many be a marginal or low signal power
inherent in the RF link. In this case, it is a good
idea to do a signal power check with attenuation
pads for each sublink in a complete RF link.
Every RF link has one or more sublinks. For
example, if there is one repeater in an RF link
then there is a sublink between the base station
and the repeater and a sublink between the
repeater and the field station. The sublinks
should be checked in both directions of
communication.
Before proceeding, it is a good idea to calculate
the theoretical signal power for each of the RF
links. Appendix C of the RF Telemetry manual
outlines the calculations.
Signal power must be greater than -95 dBm at
the standard 3.0K baud rate transmission rate,
or -80 dBm @ 2.4K baud. However, squelch
will break on the radios with a power greater
than -115 dBm. Therefore, there is a 20 dBm
range in which the radios are not working, but
may "sound" proper.
An attenuation pad inserted into the link
increases the power loss of the system. If a 20
dBm attenuation pad, or two 10 dBm pads in
series, is inserted into the link and subsequently
the radio will not break squelch, the signal
power is between -95 and -115 dBm which is
below the power limit for good data
transmission.
Similarly, if a 10 dBm attenuation bad is
inserted in the link and the radio subsequently
will not break squelch, the actual signal power is
between -105 and -115 dBm. In this case, the
signal power is far below the power limit. First,
test the sublink of the base station to the first
repeater or field station. Initially treat the base
station as the transmitting station and the first
field or repeater station as the receiving station.
Disconnect the radio's multicolored cable from
the RF modem. With somebody at each
station, depress the base station transceiver
button and listen at the receiving station to hear
if squelch is broken. If squelch is not broken,
then either the signal power is less than -115
dBm, or something is wrong with the power
supply, antenna orientation, or cable
connections. If squelch is broken on the
receiving radio, the site can be tested with the
attenuation pads to determine the approximate
signal power if it is between -115 and -95 dBm.
Insert the attenuation pad(s) (20dBm) between
the radio and antenna of the receiving station
ONLY (most attenuation pads have a limited
current capacity). Depress the base station
transceiver button. If squelch is broken at the
receiving station, this sublink is good in this
direction. If squelch is not broken, this sublink
has signal power between -95 and -115 dBm
which should be corrected. Corrections can
involve shortening distances, reorienting
antennas, providing a better power supply, or
shortening coaxial cable lengths.
If it did not break squelch with the 20 dBm
attenuation pad, it is possible to decrease the
attenuation to 10 dBm to determine if signal
power is between -95 and -105 dBm, or
between -105 and -115 dBm. This will identify if
the signal power is close or far away from -95
dBm.
If it did break squelch with the 20 dBm
attenuation pad, then that sublink is good in that
direction. The next sublink can now be tested.
Remember to place the attenuation pads at the
receiving station only! If all of the sublinks were
good, the same sublinks can be tested in the
opposite direction. If reversing directions in a
sublink gives bad results while the other
direction is good, be suspicious of the
transmitting radio in the bad direction and the
radio's power supply.