User's Manual Part 2

March 29, 2006 Part ######A revision 00a-DRAFT 97
Chapter 5: Antenna Installation Procedures
Measure VSWR and Return Loss
Voltage standing wave ratio (VSWR) is a ratio of the maximum to minimum voltage as measured
along the length of a mismatched RF transmission line. VSWR indicates the level of impedance
matching between RF equipment (such as amplifiers, cabling, and antennas). When the impedances of
the RF equipment are mismatched, some of the RF energy is reflected back along the transmission
line. Reflected energy causes inefficiencies in the transmission power output.
A VSWR of 1:1, as measured from antenna cable to the antenna, indicates that 100% of the power
output is being radiated by the antenna. During a cable sweep, SOMA SoftAir RF equipment should
show a VSWR of 1.5:1 or less in the supported spectrum. A VSWR greater than 1.5:1 indicates
potential problems with the RF equipment.
A high VSWR may be caused by one or more of the following conditions:
 Moisture in the external cables or connectors
 Faulty equipment
 Poor connections between components
 Damaged cables or connectors
 An open- or short-circuit in RF equipment or cables
Return Loss
Return loss is closely related to VSWR. Return loss is a measure in decibels (dB) of the ratio of forward
to reflected power. For example, if a load has a return loss of 10 dB, then 1/10 of the forward power is
reflected. The higher the return loss, the less energy is being reflected.
Table 5.1 shows the correlation between VSWR, return loss, and the percentage of reflected power.
 To measure the VSWR of RF equipment
1 Power on and calibrate your cable sweep analyzer.
2 Power off the RFSS module connected to the RF equipment you want to test.
See page 108 for a list of circuit breakers.
VSWR Return Loss (dB) Power Being Reflected (%)
1:1 N/A (infinite value) 0
1.25:1 19.1 1.2
1.5:1 14 4.0
1.75:1 11.3 7.4
2:1 9.5 11.1
5:1 3.5 44.7
N/A (infinite value) 0 100.0
Table 5.1 VSWR, Return Loss, and Reflected Power Conversions