About SWR

By Danny Horvat, E73M / N4EXA — antenna design engineer and founder of MyAntennas.com.

SWR is the most-measured and most-misunderstood number in amateur radio. Hams chase 1.0:1 as if it were a signal report, panic over 1.8:1, and trust a flat curve that’s actually hiding a problem. After decades of designing antennas and answering thousands of SWR questions from customers, here is what the number really tells you, what it doesn’t, and when it genuinely calls for action.

What SWR Is

Standing Wave Ratio describes how well your antenna system’s impedance matches your transmission line — for us, 50 ohms. When the match is perfect, all the power your coax delivers is accepted by the antenna. When it isn’t, some power is reflected back down the line, and the interaction of forward and reflected waves forms a standing wave along the coax. The ratio of its maximum to minimum voltage is the SWR. A perfect match reads 1.0:1; the worse the mismatch, the higher the number.

What the Numbers Actually Mean

Here is the part that should lower everyone’s blood pressure. The reflected power at common SWR values:

1.5:1 reflects about 4% of your power. 2:1 reflects about 11%. 3:1 reflects about 25%.

Now translate that into what actually matters on the air: signal strength. Even the 11% at 2:1 is a loss of about half a decibel — one-tenth of one S-unit. No station on Earth can hear the difference between your signal at 1.2:1 and at 2:1. Anything under 2:1 is excellent, full stop. The obsessive pursuit of 1.0:1 across an entire band is chasing a number on a meter, not a better signal.

What SWR Does NOT Tell You

This is the most important section on this page. SWR measures match, not performance. A 50-ohm dummy load shows a perfect 1.0:1 on every frequency — and radiates nothing. Any antenna can be made to show beautiful SWR if enough of your power is converted to heat somewhere along the way.

This is exactly the trick behind many cheap end-fed antennas and broadband transformers on the market: a lossy transformer produces a deceptively flat, low SWR curve, because the reflected power gets absorbed in the ferrite instead of returning to your meter. The curve looks wonderful; your signal is weaker. It’s why we publish measured insertion loss for every transformer we make, not SWR plots alone — the loss figure is the honest number, and ours run 0.1–0.5 dB where similar-looking products lose 0.7–2 dB. When you compare antennas, ask for the insertion loss data. If a maker only shows you SWR, ask yourself why.

The Truth About Antenna Tuners

Your tuner changes the SWR your radio sees — it does not change the SWR at the antenna. The mismatch between the coax and the antenna is still there; the reflected power still flows on your feedline and still passes through the matching transformer. The tuner’s real job is to let your transmitter deliver full power into an imperfect load without folding back. That’s genuinely useful. But it is matching, not fixing.

Why this matters in practice: run a kilowatt into 3:1 SWR through a tuner, and roughly a quarter of that power is being bounced around the system — much of it heating your transformer’s ferrite cores. This is the mechanism behind a symptom we hear regularly: “my SWR slowly creeps up during long transmissions.” That is not a defect — it is a core warming up and telling you to fix the match or reduce power. The safe rules for our antennas: at SWR under 2:1, run full rated power. Between 2:1 and 3:1, reduce to 250–500W on high-power models. Above 3:1, stay at 200W or less. At 100W, any SWR your tuner can match is fine. And never combine high power, a tuner, and SWR above 2:1.

Why Your SWR Curve Differs From Our Published Charts

Every published curve on our product pages was measured with the antenna installed in the clear. Your backyard is not “in the clear,” and that’s not a criticism — it’s physics. Every antenna has a reactive near field extending roughly 0.159 wavelengths in all directions, and any conductive object inside that zone — buildings, metal roofs, towers, vehicles, other antennas, house wiring — couples to the antenna and shifts its feed point impedance, exactly like parasitic elements on a Yagi.

On 80m that zone is about 41 feet; on 40m, 22 feet; on 20m, 11 feet; on 10m, 5 feet. This is why a metal shed 20 feet from your wire raises SWR on 80m and 40m while leaving the higher bands untouched: the shed sits inside the near field on the low bands and outside it on the high bands. Your antenna isn’t defective. It’s measuring your yard. Height, wire bends, and ground quality shift the curve too — which is why every antenna we ship includes extra wire at the end insulator for fine-tuning on site (cut, never fold — see the trimming procedure in All About EFHW Antennas).

Troubleshooting a Sudden SWR Change

The antenna worked perfectly for months, and now the SWR is wrong. In our support experience across tens of thousands of installations, here is the actual order of likelihood:

1. A failed lightning arrestor. The gas discharge tube inside degrades or fails after surge events, often invisibly, and it is the number-one cause of sudden post-storm SWR changes. Bypass the arrestor and re-measure before touching anything else.

2. A degraded cable or connector. Water intrusion at a connector, a corroded PL-259, a damaged jumper. Substitute a known-good cable and compare.

3. Wire stretch. Months of tree movement gradually stretch the antenna wire, moving resonance down in frequency. If your SWR minimum has slowly drifted lower over a season, the wire has lengthened — trim it back using the procedure in the EFHW guide.

4. Something changed in the near field. New gutters, a parked RV, a neighbor’s addition, wet foliage in summer versus bare branches in winter. Seasonal SWR shifts on the low bands are normal.

Notice what’s last on the list: the transformer. In tens of thousands of units shipped since 2015, we have never seen one fail in normal use — the ferrite cores would need to reach 300°C, and the enclosure would melt first. (And a multimeter showing a “short” across the transformer is normal — that’s how every RF transformer of this type reads at DC.) When something breaks, it’s almost always the accessories around the antenna, in the order above.

Frequently Asked Questions

What is an acceptable SWR?

Anything under 2:1 is excellent — the difference from a perfect 1.0:1 is about half a decibel, inaudible at the other end. Under 3:1 is usable at reduced power. The number to watch isn’t “how close to 1.0” but “has it changed from what it was.”

Does low SWR mean my antenna is working well?

No — it means the impedance match is good, nothing more. A dummy load has perfect SWR. A lossy antenna or transformer can show a beautifully flat curve while wasting your power as heat. Insertion loss, not SWR, is the honest measure of a matching transformer.

Will an antenna tuner fix my high SWR?

It fixes what your radio sees, letting the transmitter run at full output. The mismatch at the antenna — and the reflected power heating your transformer — remains. Fine at 100W; at high power, fix the antenna instead of masking it.

Why does my SWR rise during long transmissions?

The transformer core is heating from reflected power — a sign you’re running too much power into too high an SWR. Improve the match or reduce power; the reading will return to normal as the core cools.

Why is my SWR different from the chart on the product page?

Published curves are measured in the clear. Objects within the antenna’s reactive near field — about 41 ft on 80m, shrinking to 5 ft on 10m — shift the feed point impedance in your particular installation. Fine-tune on site with the extra wire provided.

My SWR suddenly changed. Is the antenna broken?

Almost certainly not. Check in this order: lightning arrestor (bypass it), cables and connectors (substitute known-good), wire stretch (has the minimum drifted down?), and changes near the antenna. Transformer failure in normal use is essentially unheard of.

Learn more: the ARRL’s “Understanding SWR by Example” (PDF) is an excellent primer, and Dave Casler KE0OG’s video presentation below covers the fundamentals visually.