SWR
Related wiki pages Antennas, Gain, Propagation, Bands
Standing Wave Ratio - (SWR)
What is SWR?
When there is a mismatch between the output impedance of a transmitter and input impedance of an antenna, some of the signal - in the form of a wave - is reflected back down the transmission line to the transmitter. Standing waved of electrical energy will "appear" in the transmission line. SWR is the ratio:
Amplitude (height) of the standing wave : Amplitude of an adjacent node (central point on the wave form)
The most commonly used SWR measurement is Voltage SWR or VSWR. Current SWR or IVSWR is rarely used but provides the same information. The diagram above can be thought of as representing either voltage or current.
If the feedline has no loss, and has the same impedance as the transmitter output and the antenna input impedance, then maximum power will be delivered to the antenna. In this case the VSWR will be 1:1 and the voltage and current will be constant over the whole length of the feedline.
The proportion of power to the antenna that is reflected back down the feedline is known as reflected power. It is determined by the reflection coefficient <math> \rho </math> at the antenna, given by:
<math> \rho = \frac{Z_1 - Z_0} {Z_1 +Z_0} = \frac {E_r}{E_f} </math>
where:
<math> Z_1 = </math> antenna impedance
<math> Z_0 = </math> feedline impedance
<math> E_f = </math> forward voltage - towards the antenna
<math> E_r = </math> reflected voltage
How is SWR calculated?
Online SWR calculator from VE3KL