What is SWR?

Standing Wave Ratio (SWR) is (in basic terms) a way to measure the impedance matching of loads on a transmission line (coaxial cable). SWR essentially indicates the ratio of forward RF power to reflected RF power in the antenna system. An SWR measurement shows how much of your transmitter’s power is potentially getting to the antenna to be radiated and how much is being reflected back toward the transmitter.  Excessive reflected power is wasted in heat instead of being radiated fand too much refkected power can cause damage to your transmitter..  

SWR is sometimes called VSWR (Voltage Standing Wave Ratio).  Lower reflected power is better.  Therefore an SWR of 1 to 1 (expressed as 1:1) is ideal and unlikely to happen in the real world unless you are using a test bench setup with a dummy load instead of a live antenna.

For typical VHF and UHF operation it is optimal to have an SWR of 1.5:1 or less, although a higher SWR of up to 2:1 can be acceptable.  Operating a radio when the SWR is too high can damage the final RF output stage of the radio (“The Finals”) and cause you to have to send your radio to the shop for a potentially expensive repair.  Generally anything 3:1 or over is considered too high and you should not use your transmitter under those circumstances.

An SWR of 1.1:1 indicates that you are losing .2% or your power.  For example, your radio puts out 50 watts at the antenna port. If your feedline and antenna have an SWR or 1.1:1 you theoretically have about 49.9 watts making it to the antenna (not considering cable and connector loss).

An SWR of 1.5:1 indicates 96% efficiency, so a 50 watt transmitter with an antenna system SWR of 1.5:1 will yield 48 watts (again, not taking into account other losses).

SWR is not the only factor in measuring antenna performance.  Other factors such as resonant frequency, antenna gain and many other things must be considered. For VHF and UHF operation antenna height is important since those frequencies generally require "line-of-sight" between transmitter and receiver (VHF is a little more forgiving).

For more information about SWR and antennas check these web sites as a start, then use Google and check YouTube.

http://www.hamuniverse.com/swr.html

http://www.hamuniverse.com/testingswr.html

http://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf

By the way… one doesn’t measure “SWRs”.  SWRs is the equivalent of saying “standing wave ratios”.  You are concerned with a ratio, not multiple ratios.