Introducing Reflection Measurements; Vswr; Return Loss - JDS Uniphase CellAdvisor JD720C Series User Manual

Chapter 5 Reflection Testing

Introducing reflection measurements

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

In telecommunications, return loss is a measure of power reflected from imperfections in an
electrical or optical communication link. The ratio (PR / PT), represents the wave power
reflected from the imperfection (PR) to that of the incident, or transmitted, wave, (PT). For
maximum transmitted power, the return loss should be as small as possible, meaning the
ratio PR / PT should be as small as possible.
Return loss is usually expressed in dB and the return loss value describes the reduction in
the amplitude of the reflected energy, as compared to the forward energy. It will always be a
loss and therefore a negative dB. However, you can write -3 dB simply as 3 dB of loss,
dropping the negative sign and adding loss. For example, if a device has 15 dB of return loss,
the reflected energy from that device (PR), is always 15 dB lower than the transmitted energy
(PT). When expressed in dB, larger (in magnitude) negative numbers represent larger return
losses and thus smaller reflected power (PR).
In electrical systems, return losses often occur at junctions between transmission lines and
terminating impedances. It is a measure of the dissimilarity between impedances in metallic
transmission lines and loads. For devices that are not perfect transmission lines or purely
resistive loads, the return loss value varies with the frequency of the transmitted signal.
CellAdvisor JD720C Series User's Manual 43