Why SWR Changes With Weather: Rain, Ice, Heat and Wet Coax
On a dry summer day the antenna shows an SWR of 1.3 on 20 metres. After a night of rain the same antenna reads 1.8, and by lunchtime, once the sun has dried everything, it is back to 1.3. Nothing was changed, and nothing is broken. Many radio amateurs meet this puzzle when they begin to watch an SWR meter regularly, and the first reaction is to suspect the coax. In truth, SWR changes with weather for several good physical reasons, most of them harmless. A few, however, are early signs of a fault that will get worse. This guide explains what rain, ice, wet ground, heat and wind do to an antenna and its feedline, gives rough figures for what is normal and shows how to tell the difference between detuning and damage.
What SWR Measures, in Brief
The standing wave ratio tells you how well the impedance of the antenna system matches the impedance of the feedline, usually 50 ohms. A perfect match gives 1:1. When the antenna impedance moves away from 50 ohms, part of the power is reflected, and the SWR rises. Anything that changes the electrical length of the antenna, its resistance or the losses around it can therefore change the reading. The article on what an SWR meter tells you and what it hides explains why the number on the dial is only a piece of the picture, and the table of return loss and mismatch loss shows how little power an SWR of 1.8 really wastes.
That second point matters for weather. A change from 1.3 to 1.8 means that the reflected power rises from under 2 percent to about 8 percent of the forward power. The mismatch loss is a fraction of a decibel, far below anything you can hear at the other end. The reading is real, but it is rarely a reason to panic.
Rain and the Wet Antenna
Water has a high relative permittivity, about 80 at radio frequencies compared with 1 for air, which is described in the article on relative permittivity. When rain forms a film on an insulated wire, on insulators, on the plastic of a trap or on the sheath of a ladder line, the electric field near the antenna passes through a layer with a higher permittivity. The effect is an increase of capacitance that lowers the resonant frequency slightly. The antenna becomes electrically longer. On a dipole made of bare wire the change is small, a shift of a few tens of kilohertz on 20 metres. On an insulated wire, whose coating already adds some capacitance, the shift is larger, and on a trap antenna or a loaded antenna with plastic housings and coils it can be larger still.
Water also adds loss. A damp film on an insulator or on the sheath absorbs some RF power, and in heavy rain the resistance at the feedpoint may rise. This is one reason why a wet antenna can show a slightly better SWR on one band and a worse one on another: the antenna is both detuned and more resistive, and the two effects add in different ways. The antennas that suffer most are those with a narrow bandwidth. A wideband dipole shifts a little and the SWR curve is flat, while a short loaded vertical or a mobile whip has a high Q and a narrow useful range, as the article on antenna bandwidth explains. A small shift of the resonance moves the operating point up the steep side of the curve.
Wet and Frozen Ground
For vertical antennas and for low horizontal wires, the ground is part of the antenna. The conductivity and permittivity of the soil depend on its moisture. Wet soil conducts better than dry soil, which changes the ground loss and the impedance at the feedpoint. After a long dry spell, the ground can have a high resistance, and a vertical with few radials shows a higher feedpoint resistance because of the ground losses. After rain, the resistance falls. The SWR may therefore move in either direction depending on where your antenna sits on its resonance curve. The article on vertical antenna radials gives the reason why a good radial system reduces this sensitivity: with many radials the ground matters less, and the antenna is more stable through the seasons.
Frozen ground is a special case. Ice is a poor conductor, so frozen soil behaves as dry ground at best, and the antenna may seem to change character in a hard winter. Snow cover adds another layer, and thick wet snow on wires acts like a coating with a high permittivity and a mechanical load. The lesson is that vertical and low-wire antennas are weather sensors, and their SWR in the winter can differ from that in July.
Ice, Snow and Mechanical Changes
Ice on a wire antenna has three effects. The ice has a permittivity of about 3, so the coated wire becomes electrically longer, with a lower resonant frequency. The weight pulls the wire down and the sag changes the height and the angle of an inverted V. And ice on insulators can create leakage paths. A glaze of a few millimetres on a 20 metre dipole can lower the resonant frequency by some tens of kilohertz, while a heavy accumulation may shift it more and also change the impedance by changing the height.
Temperature changes the length of the wire itself, but the effect is small. Copper expands by about 17 parts per million per degree Celsius, so a 40 degree swing changes the length of a 20 metre wire by about 14 millimetres, which is a fraction of a kilohertz on 20 metres. Far larger is the change in sag. A wire that tightens in the cold and relaxes in the heat changes the height of the centre and the shape of the antenna, and with it the feedpoint impedance a little. Wind adds a rhythmic movement, and in a gusty wind the SWR reading may wander slowly with the swing of the antenna.
Water in the Feedline
This is the weather effect that deserves attention. Water that gets into the coax or into a connector is a fault, not a normal variation. The path is usually a poorly sealed connector, a nick in the jacket or a cable end left open. In the cable, water that wets the braid and the dielectric increases the loss steeply and detunes the line. The signs are a loss that grows over weeks and an SWR that is high in the rain and does not recover when the weather clears.
A deceptive effect arises here. Loss in the feedline attenuates the reflected wave on its way back to the transmitter, so a lossy line makes the SWR at the radio look better than it is at the antenna. If your SWR gets lower in the rain, you may be losing power in the coax and not enjoying a better match. The correct way to check is to measure at the antenna end, or to compare with a known good state, using a tool such as a NanoVNA. The guide to coax fault finding shows how to locate water ingress and other faults with a sweep. The advice of the article on weatherproof connectors is simple: tape, mastic and a drip loop keep water out of the connector at the lowest cost.
Ladder Line, Twin-Lead and Baluns
Balanced feedlines react to weather in their own way. Open-wire line, with two bare conductors held apart by spacers, is almost immune to rain, since there is mostly air between the wires and the loss stays very low. Window line, with a plastic web, is a little more sensitive, because water collects in the windows and ice bridges the gaps. Solid 300 ohm twin-lead, with a continuous plastic dielectric, is the most affected: a wet film can double its loss on the higher bands, and the impedance seen at the tuner moves. If you use any of these with an antenna tuner, expect to touch the tuner after a downpour, and keep the line away from metal and from the ground. A balun or unun at the feedpoint should be in a sealed case, because moisture in the winding or on the core makes both the loss and the SWR drift.
Normal Shifts and Faults Compared
| Observation | Probable cause | What to do |
|---|---|---|
| SWR rises a little in rain and recovers when dry | Water film and wet ground, normal detuning | Accept it, or widen the bandwidth if it matters |
| SWR higher in winter on a vertical | Frozen or dry ground, changed ground loss | Add radials, retune in season |
| SWR rises on ice and snow, recovers in a few days | Ice coating and sag, lower resonance | Wait or retune; check supports |
| SWR grows over weeks and stays high after the rain | Water in coax or connector | Inspect, reseal or replace connector and cable |
| SWR jumps erratically in wind | A loose connection or a failing balun | Check solder joints, connectors and wire ends |
| SWR improves while signal reports fall | Loss in the feedline masks the mismatch | Measure at the antenna end with an analyzer |
How Much Variation Is Acceptable?
A change of 0.2 to 0.5 in SWR between a dry day and a wet day on a resonant wire antenna is ordinary. The transmitter, with its protection circuit that starts to reduce power at about 2:1 or 3:1, has no problem with it. For a mobile or portable antenna, a larger variation is normal because of the high Q. It makes sense to look for a cause only when the change is bigger than about 1.0 on a wide-band antenna, when it does not recover, when it is accompanied by a drop in signal reports or when it is a new behaviour in an antenna that was stable for a year.
Ways to Reduce the Effect
Several design choices make an antenna less sensitive to weather. A wider-bandwidth antenna, such as a fan dipole or a thick-wire dipole, has a flatter SWR curve, so a small shift in resonance changes the reading less. Bare wire is less affected by rain than insulated wire, because the water film adds less capacitance than a coating plus water. Antennas with many radials or an elevated radial system are less dependent on the soil. Traps, coils and baluns in sealed housings with drain holes resist moisture better than open ones. And a feedline that is well sealed at both ends, with the connectors protected and the cable arranged to drip away from them, removes the one weather problem that is really harmful.
A practical way to set up an antenna is to tune it for the weather that you meet most often, rather than for the driest day of the year. If the antenna is used all year, set the resonance at a time when the ground is moderately damp. Keep a log with the date, the weather and the SWR at the main operating frequencies, and a measurement from an antenna analyzer, such as the one described in the NanoVNA guide, once a season. After a year of notes, you will know what a normal rainy-day reading is for your own station and will spot an abnormal one at once.
The Bottom Line
SWR changes with weather because water, ice and temperature change the capacitance around the antenna, the ground losses and the sag of the wire. Shifts of a few tenths are normal, recover when the weather clears and cost almost no signal. A reading that grows week after week, stays high after the rain or is accompanied by weaker signals points to water in the connector or the coax and needs inspection. Measure at the antenna end when in doubt, seal every outdoor connection, favour wider-bandwidth antennas and keep a log of your own readings so that you can tell a seasonal shift from a fault.
Frequently Asked Questions
Why does my SWR go up when it rains?
A water film raises the capacitance around the wire and insulators, lowers the resonant frequency and adds some loss. Wet ground also changes the feedpoint impedance of verticals and low wires.
Is a higher SWR in the rain dangerous for my radio?
A change from 1.3 to 1.8 is not dangerous. Modern transceivers start folding back the power at about 2:1 to 3:1, and a small rise wastes only a few percent of the power.
Why is my SWR lower when it is wet?
Possibly the antenna moved to a better match, but it can also be that water in the coax adds loss and hides the reflected power. Measure at the antenna to find out.
How can I tell if water is in my coax?
Look for an SWR or loss that grows over time, damp or green braid at the connectors, and use a sweep with an antenna analyzer to see a changed response or a fault location.
Does ice change the SWR of a dipole?
Yes, a coating of ice lowers the resonant frequency and its weight changes the sag, so the SWR shifts until the ice melts.
Should I retune the antenna every season?
Only if the shift matters to you. A wide-bandwidth antenna tuned for average conditions seldom needs it, while a narrow loaded or mobile antenna may be worth adjusting.