VSWR in RF Design

In any RF application if you have a mismatch in impedance between the transmitter and the antenna then not all of the power output from the transmitter will be radiated by the antenna, some will be reflected by the point of mismatch. In fact, the antenna is a effectively a matching device that couples the transmission line impedance to the characteristic impedance of free space (which I believe is about 377 ohms). For efficient power transfer in any system the load must be impedance matched to the source.

Hi venki hindu,

Voltage Standing Wave Ratio (VSWR) is simply the ratio to measure the efficiency of a transmission line under test. Under the maximum power transfer, meaning all the incident power is dissipated by load or antenna and nothing is reflected back, VSWR value is 1. So the higher the value of your VSWR, the lower the efficiency of your transmission line and the more faults can be found on this transmission line. Infinite VSWR means all incident power are reflected back means an open transmission line or no load or antenna termination.

this clears my doubt in VSWR

Yes it's true that the power will be reflected from the mismatch point. But that alone isn't a real problem if the power levels are low because the reflected power will reflect again from the transmitter and some of it will again be transferred to the antenna (using the antenna example).

However, that having be said, it's generally not desirable to have a large VSWR because:

- the incident power flow and reflected power flow constructively add up on the transmission line, and the resulting voltage may exceed the transmission line's breakdown voltage (i.e. arcing between the traces in your microwave amp).

- the VSWR causes your calculation of peak voltage to be

Epeak = sqrt (power x Zo x VSWR) x sqrt(2) which indicates that the voltage on the transmission line increases with the sqrt of the VSWR

For example: 100 watts into a 50 ohm matched line with swr=1

RMS voltage = sqrt (100 x 50 x 1), E peak = RMS x sqrt (2) = 100 volts

100 watts into 50 ohm matched line with swr=10

E peak = sqrt(100x50x10)xsqrt(2) = 316 volts