Standing wave ratio measures how well an impedance matches a transmission line. It does not measure radiation, efficiency, range or antenna quality. A resistor of the right value produces a flawless reading and radiates nothing, and a badly deteriorated feedline can improve the reading while making the station worse.
What the number is derived from
When a transmitter drives a line into a load that does not match the line’s characteristic impedance, part of the wave reflects at the load and travels back toward the source. Forward and reflected waves superimpose along the line, producing a standing pattern of voltage maxima and minima. The ratio of the maximum to the minimum is the standing wave ratio. A meter in the line samples forward and reflected power and computes it. All of that concerns the interface between line and load. Whether the load converts its absorbed power into radiation or into heat is outside the measurement entirely, because a resistive loss and a radiation resistance look identical to a meter at the feedpoint. That single fact is the source of most of the confusion around this reading.
Feedline loss flatters the reading
The reflected wave has to travel back along the feedline to reach the meter, so it passes through the cable twice. Every decibel of cable loss attenuates it, and the meter therefore sees less reflected power than actually exists at the antenna. On a short run of good cable this is a minor correction. On a long run of aging coax at high frequency, the reflected signal can be attenuated so heavily that a genuinely mismatched antenna reads close to perfect. A reading that improves over the years without anyone touching the antenna is a strong hint that the cable is deteriorating. Measuring at the antenna feedpoint, or with an analyzer that accounts for line loss, separates the two effects and turns a comforting number into a useful one.
Why moderate mismatch costs very little
The additional loss caused by a mismatch is often overstated. On low-loss cable at modest frequencies, a two-to-one ratio wastes only a small percentage of the transmitted power, which is far below what anyone can notice at the receiving end. The real reason to care is the transmitter. Solid-state output stages are designed for a specific load impedance and typically include protection that reduces power as the reflected level rises, so the visible symptom of a mismatch is a power fold-back rather than lost radiation. On lossy cable the calculus changes, because the mismatch causes the wave to traverse a lossy path repeatedly. Judge tolerable mismatch by the cable loss and the transmitter’s tolerance, not by a superstition about a perfect reading.
Things that produce a good reading for bad reasons
Several failure modes look excellent on a meter. Water in coax raises loss substantially and flattens the reading. A corroded connector adds series resistance that absorbs power and behaves like a partial load. A shorted or open element in a multi-element array can leave the driven element still roughly matched while the pattern collapses. Some wideband antennas incorporate resistive loading by design, giving a broad flat match and modest efficiency. Common-mode current on the outside of a coax shield can also change the reading depending on where the meter sits, which is why moving the meter along the line and seeing the number change is a diagnostic for a missing choke rather than an instrument fault.
Measure the things the meter cannot see
Because match and performance are separate, a useful station check looks at more than one indicator. Received signal strength on a known distant beacon or repeater, compared over time, tracks real performance. An antenna analyzer sweep shows the shape of the resonance, and a shape that becomes unusually broad and shallow suggests growing loss. Time-domain reflectometry, available on many modern analyzers, locates the distance to a fault in the line. A simple continuity and insulation check at the connector finds water ingress. Physical inspection of terminations, sealing tape and strain relief catches most problems before they become electrical. The meter in the shack remains valuable as a trend indicator, provided you never mistake it for a report card on the antenna.
What to check before you act
- Measure at the antenna feedpoint when the result matters.
- Treat a steadily improving reading as a possible cable failure.
- Inspect connectors and sealing for water ingress and corrosion.
- Track received signal levels as an independent performance check.
Common questions
Is a reading above two to one harmful?
Not inherently. Most modern transmitters simply reduce output as reflection rises. The loss penalty depends mainly on how lossy the feedline already is.
Why does the reading change when I touch the coax?
That indicates common-mode current flowing on the outside of the shield. A choke at the feedpoint usually stabilizes it.
The practical takeaway
Use the meter to watch for change rather than to grade an antenna. If the number looks suspiciously good, suspect the feedline, measure at the antenna, and confirm performance with an independent signal check before you congratulate yourself.
References and further reading
- FCC: transmitter operation and radio frequency energy fundamentals
- FCC: radio frequency safety and technical frequently asked questions
- OSHA: electrical safety requirements relevant to antenna and mast work
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
