The standing wave ratio (SWR) is an important topic to hams regardless if they are working HF, VHF, UHF or any other frequency range allocated to radio amateurs. Unfortunately it is technically involved and somewhat complex so is not intuitive or easy for non-technical folks to understand. We’ll give a simplified explanation of SWR here and give you a basic idea of its significance and how hams relate to it.
The simplest way to think of SWR is as a measure of impedance matching. Most commonly it is looking at the impedance differences between transceiver, transmission line (more often coaxial cable), and antenna.
Assuming that all modern radios and cable have 50Ω impedance, the real SWR of interest boils down to the match between the transmission line and antenna.
As we mentioned in a previous post, when impedance of a source equals load impedance the best possible signal coupling occurs. Conversely, when impedances are not the same, signals couple poorly. This is true of all electronics circuitry involving AC waveforms.
In amateur radio SWR focus is on transceiver to antenna coupling where we want to maximize RF power transfer in both transmit and receive modes. When impedances do not match, received signals will be weak or non-existent; when transmitting, power will not radiate well from the antenna. The ideal or “perfect” SWR for best possible transmit/receive coupling is 1:1, meaning a 50Ω radio/transmission line to a 50Ω antenna.
SWR is simply the ratio of two impedances being measured. It is commonly expressed in the X:X format and the larger value is always given first, regardless of which side is higher.
With a 50Ω radio and 50Ω coax cable, a 4:1 SWR could indicate either a 12.5Ω or a 200Ω load (antenna). Similar examples for 50Ω cable are contained in the General class exam pool:
A ham’s main concern with high SWR is significant power reflected back from the load, which stresses the transmitter power amplifier. While a 1:1 SWR is ideal, practically speaking, 1.5:1 or less is good. Many modern transceivers automatically reduce transmit power with a SWR greater than 2:1.
SWR can be measured Continue reading