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Category : Antennas
VSWR (Voltage Standing Wave Ratio)
2011-08-02 16:33:14 - by : admin

Theory

VSWR is a ratio between the amplitude of standing waves the maximum (| V | max) voltage and minimum (| V | min) voltage
In the transmission line there are two components of the voltage waveform, ie voltage is sent (V0 +) and the reflected voltage (V0-). Comparison between the reflected voltage delivered by the voltage reflection coefficient is referred to as (), namely:

                  reflection coeficient formula        

ZL is the load impedance  and Z0 is a lossless impedance.
Voltage reflection coefficient (r) has a complex value, which represents
magnitude and phase of reflection. For some simple cases,
when the imaginary part of impedance is zero, then:
a) r = -1: maximum negative reflection, when the channel connected short
b) r = 0: no reflection, when the channel in a state of perfectly matched.
c) r = +1: positive reflection maximum, when the channel in an open circuit.

Formula to find the value of VSWR is

                  vswr formula

The best conditions are when the VSWR value is 1 (S = 1) which means no
reflection when the channel in a state of perfect matching. However, this condition on
practice it is difficult to obtain. Therefore, the value of the allowable standard VSWR
for fabricating the antenna is VSWR ≤ 2.

VSWR not only has function of voltage but also influenced by the working frequency. Here's an example of the VSWR measurement which is a function of VSWR vs frequency


vswr - frequency spectrum
 
From the figure above, each point frequency scale will cause the value of VSWR that is different, so that by determining what percentage of expected radiated power will we be able to determine antenna conditions or how big antenna bandwidth.

Each VSWR value correlate with power transmitted and also return loss. Here below is a table comparison between the values of VSWR, Return Loss and Transmitted Power
 vswr-returnloss-power
 
In the picture above, how to read as follows, as an example to the value of VSWR = 1.5 then the Return Loss = 14 dB and the radiated power is 96% x the input power.

The things that affect the value of VSWR is:
1. Impedances difference transmission line with load/antenna
2. Transmission line discontinued connection, which are caused by bad contact connectors, feeder too much bending or damage to the feeder itself.


Application

VSWR is a very influential parameters on the RF system. On the practice field, all the RF systems using a standard VSWR measurements to obtain the maximum RF emission results (maximum transmit power) such as:
- Aerial Communications HF / VHF / UHF
- Cellular Communication
- Wifi Communication
- Wimax Communication
- Etc

For cellular systems, in one sector antenna which has three essential antenna components : TX Antenna, RX Antenna and RX Diversity Antenna. And if the number of TRX in one sector more than 2 then it can be described in the diagram as follows:
             trx antenna connection

So if there are significant differences in the value of the VSWR between antenna 1 and antenna 2 will produce a poor performance in the sector because of two antennas in the same sector emits a different transmit power. Because of differences in the transmit power is what causes call drop during handover between time slots or between the TRX.
It is therefore advisable to within one sector, then the VSWR value should be a nice addition to also consider the difference in the value of VSWR that would affect the transmit power on each antenna. Transmit power differences in the same sector will produce 2 layer in the cell, and this should be avoid to give better performance in the cell.

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