Campbell Scientific LoggerNet Datalogger Support Software - Manual (Page 472)

Section 14. Troubleshooting Guide
If you are using a data radio that does not have a transmit button
built in, you can easily build a push to transmit button from the
documentation of the radio/RF modem interface connector. There
will be one pin that when pulled high or pulled low will initiate
radio communication. See the radio documentation to identify this
pin. Connect a momentary push-button to either raise or ground
that pin. Always make sure that the antenna is connected to
the radio before attempting to transmit. Serious damage to the
radio can occur if transmitting without an antenna.
Place the watt meter in series between the radio and antenna cable. Set the
watt meter to the 15-Watt range, or the next highest watt meter setting, and
point the directional arrow first toward the antenna cable to measure forward
power (Wf). Initiate radio communication, let the watt meter stabilize, and
record the watt meter reading. Reverse the directional arrow so it is pointing
back toward the radio, initiate radio communication, let the watt meter
stabilize, and record the watt meter reading. This second reading is the
reflected power (Wr). Take the square root of the reflected power divided by
the forward power to arrive at the square root ratio (R). Calculate the Voltage
Standing Wave Ratio (VSWR) with the following equation:
VSWR = [(1+R)/(1–R)]
Where, R = (Wr/Wf) ½
The impedance of the RF transmission cable (usually RG–8A/U) and antenna
combination should match the impedance (50 Ω) of the radio output circuit.
When the transmission cable or antenna does not match the impedance of the
output circuit of the radio, not all of the energy supplied to the cable will flow
into the antenna. Some of the energy supplied will be reflected back to the
radio, causing standing waves on the cable. The ratio of voltage across the line
at the high voltage points to that at the low voltage points is known as the
Voltage Standing Wave Ratio, or VSWR. The VSWR should be less than
1.5:1 for error-free radiotelemetry.
For example, if the forward power (Wf) is 5 Watts and the reflected power
(Wr) is 0.2 Watts, the VSWR is 1.5:1.
The VSWR will increase when:
• There is a problem with the connectors. Check for loose, corroded or
damaged connectors. (Connector problems are the most common source of
RF communications failures.) Pull gently on the cable to make sure the
connectors are still attached securely.
• The antenna is used in proximity of metal, which is reflecting the signal
back to the radio.
• Transmitting inside a building.
• The cable is worn, cut or damaged so that not all of the radio energy can
travel through to the antenna.
• The antenna design frequency does not match the radio frequency.
NOTE
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