SWR and antenna matching: how to read the chart and what to do about a high SWR

SWR is the first number anyone who has ever built or hung an antenna looks at. A good 2-meter antenna can cost more than the radio itself, while a crooked homebrew job with a high SWR can quietly kill an expensive power amplifier in a few minutes of transmitting. At the same time, the abbreviation itself scares many people off with academic math full of standing waves and complex impedances. In practice it is much simpler: SWR is an indicator of how well the transmitter has "agreed" with the antenna, and the whole point of tuning is to drive that number into the green zone. In this article we will explain in plain terms what SWR is, how to read the chart, what causes high values, and what exactly to do about them by hand.

What SWR is in plain words

When the transmitter delivers power into the cable, that power reaches the antenna. An ideal antenna radiates all of it. A real one radiates part and reflects part back, because its feed-point impedance does not perfectly match the 50 Ω that the radio and cable expect. This reflected wave runs back down the cable, adds to the forward wave, and creates a "standing wave" in the line — hence the name: SWR — Standing Wave Ratio (VSWR, Voltage Standing Wave Ratio).

The number itself is a ratio. An SWR of 1.0 (written as 1:1) means there are no reflections at all, all the power went into the antenna — the ideal. The larger the number, the more energy came back. It is important to understand the physical meaning: reflected power does not disappear — it returns to the transmitter and heats the output transistor. Here are rough benchmarks for how much power is reflected back:

The key point is clear: going from 1.5 to 2.0 nearly triples the reflected power (from 4 to 11 %), and at an SWR of 3 a quarter of the power never reaches the antenna at all and bounces around the line. This barely affects reception — an antenna with an SWR of 3 receives almost as well as one with an SWR of 1.2. The problem is specifically on transmit: loss of radiated power and heating of the power amplifier.

How to read an SWR chart

A modern instrument (NanoVNA, antenna analyzer) plots SWR not as a single number but as a curve over frequency — that is the "SWR chart." It looks like a V-shaped dip, a "check mark":

• The bottom of the dip is the resonance. The frequency where the SWR is at its minimum and the antenna is matched best. The first thing to check is whether that bottom lands on your working frequency. If the resonance is at 138 MHz but you need to work on 145, the antenna is tuned to the wrong place, however low the SWR may be at the bottom of the dip.
• The width of the dip is the bandwidth. The wider the "check mark," the larger the portion of the band the antenna covers with an acceptable SWR. Bandwidth is usually measured at the level where the SWR rises to 1.5 or 2.0: you look at which lower and upper frequencies the curve crosses that threshold, and the difference between them is the working bandwidth.

An example of reading it: the marker at the bottom of the dip shows 145.300MHz  SWR 1.18, and the curve crosses the SWR 2.0 threshold at 143.6 and 147.1 MHz — meaning the antenna rings at 145.3 MHz and holds an SWR <2 across a bandwidth of about 3.5 MHz. For the amateur 2-meter band, 144–146 MHz, that is more than enough. A narrowband antenna (a short whip with a coil, balcony compromises) gives a narrow sharp dip — it has to be tuned more precisely, because the edges of the band easily slip past the threshold.

Causes of a high SWR

When the chart shows a high SWR, the cause is almost always in one of a few typical places. It is worth going down the list from top to bottom — from the most common to the rarest:

• Wrong radiator length. The most common cause with homebrew and new whip antennas (they are often supplied with extra length that you need to trim). The antenna resonates on the wrong frequency — the dip is shifted up or down from your working frequency.
• No ground / radials. A quarter-wave whip, GP, or mobile antenna without proper radials or with poor contact to ground (the body, the chassis) gives a high SWR even if the radiator itself is the perfect length. The antenna physically has "nothing to push against."
• Poor contact and an oxidized connector. A loose, under-tightened, or greened-over SMA/PL connector is a break in the match right inside the line. Oxidized contacts introduce a mismatch and can easily cause an SWR jump out of nowhere.
• Damaged cable. A coax that is crushed, broken near the connector, or has a flattened shield. Also in this category — a short between the center conductor and the shield (common with a homebrew-soldered connector).
• Moisture. Water that has gotten into the cable through a non-sealed outdoor connector "wicks" up along the shield and ruins the feed line's characteristics. Damp at the antenna feed point is a special scourge of outdoor installations.

What to do about a high SWR

The cure depends on which cause you found. In order:

1. Resonance on the wrong frequency → trim or lengthen the radiator. The basic rule for whips and wire antennas: resonance below the target → the antenna is too long → shorten it (shorter length = higher frequency); resonance above the target → too short → lengthen it or add a loading coil. Work in small steps and re-measure after each one: it is better to under-trim than to over-trim — adding metal back is harder.
2. High SWR with no clear resonance / on a whip → radials and ground. Add or lengthen the radials, check the antenna's contact to ground. On mobiles — a solid electrical contact of the base to the body; on a GP — proper radials. Sometimes it is enough to angle the radials downward to raise the feed-point impedance closer to 50 Ω.
3. Poor contact → check and re-solder the connectors. Re-solder or replace the suspect connector, clean off the oxide, tighten the PL/SMA to the proper torque (do not over-tighten an SMA — the torque is small). Replace a faulty adapter, do not just "trust" it.
4. Cable → replacement. A crushed, broken, or soaked coax cannot be "repaired" — it gets replaced. Do not skimp on the feed line: a cheap or old cable is itself a source of loss and an unstable SWR.
5. Moisture → drying and sealing. Replace a soaked cable, seal outdoor connectors (heat-shrink, self-amalgamating tape, mastic). The damp will come back if the joint is not protected from rain.

For typical builds — the quarter-wave GP, balcony compromises, collinear antennas — we have separate detailed write-ups with all the dimensions and the nuances of radials (links below).

What to measure SWR with

To see the whole chart rather than a single point, you need an instrument. There are two options:

• SWR meter. Simple and cheap. It is inserted in-line between the radio and the antenna and shows the SWR on the frequency you are currently transmitting on. The downside — it measures while transmitting: you are actually radiating into the air and heating the PA while tuning, and you have to build the chart by hand, retuning across frequencies. You may only transmit in a permitted band and with a callsign.
• NanoVNA / antenna analyzer. It measures passively, with its own micro-power, and immediately plots the entire SWR curve over frequency — you see both the resonance and the bandwidth in a single sweep. You do not need to connect and power up the radio for this (in fact you must not, see below). This is the most convenient and safest way to tune an antenna. How to use it, calibrate it, and read the dip is in our write-up on the NanoVNA.