Viavi CAA06M, User Manual

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25
Reflection Test
Introduction
This chapter provides instructions on how to perform Reflection measurements in VSWR or Return Loss.
Topics discussed in this chapter are as follows:
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Introducing reflection measurements
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Preparing for a measurement
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Making a measurement
Introducing reflection measurement
Reflection measurements in VSWR or Return Loss are used to characterize cable and antenna system in
order to ensure a cell site's impedance matching and signal reflection characteristics.
A proper RF emission in cell sites is achieved with a maximum power transfer from the transmitter to the
antenna, where all the transmission media should have an impedance match. A mismatch at the antenna
system produces a reflective 'traveling wave’ that goes in the opposite direction from the incident wave.
As the two traveling waves cross each other in opposite direction, it is produce an interference pattern
called a "standing wave". VSWR is the ratio between the power that is sent forward to the cable and/or
antenna and the amount of the power that is reflected back to the transmitter.
Some of the consequences of having a high VSWR condition in cellular services are dropped calls, poor
reception, and an overall unacceptable performance in the cell (or section of cell) coverage covered by
the base station antenna. Therefore, the VSWR of the antenna system including the feed line is one of
the most critical factors in the service and maintenance of the RF transmitter systems.
VSWR
In telecommunications, standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave
at its maximum amplitude and at its minimum, in an electrical transmission line. The SWR is usually
defined as a voltage ratio called the VSWR, for voltage standing wave ratio. For example, the VSWR
value 1.2:1 denotes the maximum standing wave amplitude that is 1.2 times greater than the minimum
standing wave value.
Return loss