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7-2.2 VSWR: - Voltage standing wave ratio
Standing wave :- If the impedance of the load (Antenna) and source ( Generator ) is not
equal, that is not matching then part of the energy is reflected back towards the source.,
some power is absorbed and rest is reflected .this interference form one set of waves V &
I traveling towards the load and reflected set travels back to generator. These two sets of
traveling wave moving in opposite direction 180 out of phase. This reflected wave which
varies in Voltage/ Current combines with transmitted wave and summation of both
produces a wave called standing wave. It forms a set of interference in the form of
standing wave. See Fig. 7-8
Fig. 7-8
SWR: - The ratio of maximum voltage to minimum voltage along with a transmission
line is called SWR. When impedance of source (Transmitter), media (Waveguide), and
load (Antenna) is matching perfectly then SWR = 1.
VSWR: - Due to the irregularities in waveguide, voltage tends to add to get maximum
voltage and due to phase difference voltage tends to minimum also.
Cause of worst VSWR: - Inside damage of waveguide, bends, inside water, loose
connections, inside holes etc.
If not within limit then required receive level is not possible at distant end , degradation
of services, poor coverage in case of GSM.
Return Loss: - It is a ratio of amplitude of reflected wave to the amplitude of incident
wave. This loss shows reduction in amplitude of reflected energy compared to
transmitted energy. Is is expressed in dB. It is a measure of irregularities in W/G. Return
loss should be always more to suppress echo distortion, inter modulation noise and to
maintain group delay and response within limit.
Vr
Return Loss = - 20 Log ρ (Reflection coefficient) = -----------------
Vi
Vr: - Reflected voltage wave and Vi: - Incident voltage wave.
How to measure Return loss:-
- Make set up of System Analyzer Transmitter, Receiver, Termination Bridge as
per diagram no. 7-9, 7-10, 7-11.
- adjust sweep oscillator for desired band of frequency, confirmed by frequency
counter.
- Calibrate level with open/ short and load connectors.
- remove Connectors and connect W/G to the test port.
- Read the trace and return loss over the required frequency band.
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