Antenna Bandwidth Explained: SWR Curves, Q and Narrow Antennas

Every ham has stood at the radio and watched the SWR climb as the frequency moves a few tens of kilohertz from the center of the band. That is the visible face of antenna bandwidth: the range of frequencies over which an antenna works acceptably. Some antennas, such as a thick-wire dipole on 20 meters, cover the whole band without a tuner. Others, such as a short loaded vertical on 80 meters, are only usable over a narrow slice. Knowing why helps you to choose an antenna, to set it up correctly and to understand what your analyzer is telling you. This guide explains what bandwidth means for an amateur antenna, how it is measured, how it is linked to the quality factor Q, and what actually makes an antenna narrow or wide.

What Bandwidth Means for an Antenna

Bandwidth is not a single number. An antenna has several characteristics that change with frequency: its impedance, its gain, its radiation pattern and its polarization. The Wikipedia article on antennas lists impedance, gain, radiation pattern, bandwidth, resonant frequency and polarization among the main electrical characteristics, and it notes that most of them are the same whether the antenna transmits or receives. For most amateurs, the useful definition is the impedance bandwidth: the range of frequencies over which the standing wave ratio stays below a chosen limit, usually 2:1, or 1.5:1 if your transmitter is picky.

The other characteristics matter too. A Yagi can have a perfectly good SWR over a wide range while its front-to-back ratio collapses at the edges of the band, and a wire array can radiate in a different direction as the frequency changes. When we talk about the bandwidth of a beam, the pattern bandwidth is often narrower than the SWR bandwidth. This is why a beam that appears to work from one end of a band to the other may still be a poor performer at the edge.

Reading the SWR Curve

A sweep from an antenna analyzer or a NanoVNA draws the SWR as a curve against frequency. A resonant antenna shows a minimum near its resonance and rises on both sides. The bandwidth is the distance between the two points where the curve crosses your limit. A narrow, deep dip means a high-Q antenna; a broad, shallow curve means a low-Q antenna. To make a sweep, calibrate the instrument at the end of the cable that you will use, and sweep a little wider than the band that you care about. Our guide to the NanoVNA, calibration and sweeps shows how to do it correctly.

Reading the curve also means understanding what SWR does and does not say. A low SWR does not mean that the antenna radiates well, only that the load is matched to the feed line. Our article on what an SWR meter tells you and what it hides explains the difference. Still, SWR gives a direct measure of how much power is reflected, and the following figures show how little a moderate mismatch actually costs.

These figures are calculated from the reflection coefficient and apply to a lossless line. They show that an SWR of 2:1, which many transmitters tolerate, delivers nearly 90 percent of the power to the antenna, and that the loss is about half a decibel, much less than one S-unit. The practical limit is not the loss but the transmitter: many solid-state radios reduce their power when the SWR rises above 1.5:1 or 2:1, so the useful bandwidth is often defined by the radio and not by the antenna. A tuner can extend the usable range, but if the feed line has loss, a high SWR makes that loss worse.

The Link Between Q and Bandwidth

An antenna near resonance behaves like a resonant circuit, and the quality factor Q describes how sharp the resonance is. The Wikipedia article on the Q factor gives the relationship between Q and bandwidth: the two-sided bandwidth relative to a resonant frequency F0 is F0 divided by Q. Its example is an antenna tuned to a Q of 10 and a center frequency of 100 kHz, which would have a 3 dB bandwidth of 10 kHz. So a high Q means a narrow bandwidth, and a low Q means a wide one.

You can use that relation as a rough rule. An antenna with a Q of 10 on 3.6 MHz would have a bandwidth of about 360 kHz, which is wider than the whole 80 meter band in Europe, while an antenna with a Q of 50 would have only about 72 kHz. This is why a short, loaded antenna that has a Q of several tens needs retuning every time you move across the band, while a fat dipole covers it easily. The numbers are only approximate, since the SWR bandwidth depends on the limit that you choose, but the principle holds.

Why Short Antennas Are Narrow

The size of an antenna and its bandwidth are tied together. A half-wave dipole is large enough to radiate efficiently, and it stores relatively little energy in its near field. If you make the antenna much shorter than a resonant length, you must cancel the capacitive reactance of the short radiator with an inductor, and the radiation resistance falls at the same time. The Wikipedia article says that a loaded antenna has a higher Q factor and thus a reduced bandwidth, and that resistive losses in the loading coil, relative to the lower radiation resistance, reduce efficiency. The article also notes that bandwidth becomes a major constraint on antenna size at 1 MHz and lower.

That fits the experience of anyone who has tried a short loaded antenna on a low band. A mobile whip on 80 meters may have a useful bandwidth of only a few tens of kilohertz, and it requires a movable tap or a motor to follow the frequency. A capacity hat or a better coil reduces the problem a little, as explained in our guide to shortened antennas, loading coils and capacity hats. On the other hand, narrow bandwidth is not always bad: a high-Q antenna rejects out-of-band signals, and for a fixed-frequency link the narrow response is an advantage. The disadvantages are practical: any change in temperature, height or nearby objects shifts the resonance away from the frequency that you use.

What Makes an Antenna Wider

If you want more bandwidth from a wire antenna, you have several tools. They are listed here from the simplest to the most elaborate.

• Thicker conductors. A fat radiator has a lower Q than a thin one at the same length. For HF wire, using several wires in parallel, or a cage, raises the diameter without much weight. Tubing is used on VHF for the same reason.
• Folded or fan elements. A fan dipole with several wires of different lengths at one feed point gives low SWR on several bands, and the spread between wires broadens the range on each band.
• Different shapes. The discone, the log-periodic and the terminated folded dipole are built for very wide ranges. Our articles on the T2FD antenna and the discone antenna cover two of them. The price is usually some loss or lower gain.
• Matching networks and traps. A matching network can flatten the SWR, though it does not make the radiator itself broader. Traps give multiband operation at the cost of narrower bandwidth on each band.

A key point is that gain and bandwidth often trade against each other. The Wikipedia article on antennas notes that a Yagi-Uda antenna is highly directional but has a rather limited bandwidth, and that a log-periodic dipole array can be wideband but is less directional. The same trade-off appears in your own station: a monoband beam for one band is more efficient than a trap beam, and a high-gain narrow antenna is no good if you cannot tune it to the frequency you want.

Percent, Not Kilohertz

A helpful habit is to think of bandwidth as a percentage of the center frequency. A dipole with a 2:1 SWR bandwidth of 4 percent will cover 144 kHz on 3.6 MHz, 560 kHz on 14 MHz and more than 1 MHz on 28 MHz, but the width of the amateur bands shrinks in percentage as the frequency rises. The 80 meter band, 3.5 to 3.8 MHz in much of Europe, spans roughly 8 percent of its center frequency, the 40 meter band about 3 percent, the 20 meter band about 2.5 percent and the 10 meter band about 6 percent. That is why a single dipole covers 20 meters easily, but may need a tuner or an adjustment to cover the whole of 80 meters and 10 meters. The calculation takes seconds, and it tells you before you build whether the design has any chance of covering the range that you want.

It also explains why advice about antennas differs from band to band. On 160 meters, even a reasonably thick dipole covers only part of the band, and operators often accept a tuner or choose a segment where they operate most. On the upper bands, bandwidth is rarely the problem and attention shifts to the pattern, the height and the noise. When you compare two antennas of different bands, remember to compare the percentage, not the number of kilohertz.

Pattern Bandwidth and Beams

A beam has two bandwidths that do not match. The first is the SWR bandwidth, which shows how well the feed point is matched across the band. The second is the pattern bandwidth, the range over which gain and front-to-back ratio stay close to their best values. For a Yagi the pattern bandwidth is usually narrower, because the parasitic elements are tuned for a particular frequency. A beam designed for the CW segment of 20 meters may have a good SWR on SSB but a weaker rear rejection, which you will notice as more interference from behind. If you are choosing a beam for a contest, check both curves in the manual or the model, and consider optimizing for the part of the band that you will actually use.

Practical Steps for a Dipole

For a simple wire dipole, the process is the same each time. Cut the wire about 5 percent longer than the formula result, hang it at the height that you will use, and sweep it. The resonance will be lower than you planned, and you can trim a little at each end until the minimum SWR is near the frequency that you most use. Note that the resonance changes with height, with the ground and with the wire insulation, which is why the velocity factor and end effects matter; our article on velocity factor and wire length explains the details.

Decide where in the band you want the minimum. If you work mostly CW at the bottom of the band, set it there; if you work mostly SSB near the top, set it higher. If the SWR is still too high at the far ends of the band, try a thicker wire, a fan or a different feed. Many hams on 80 meters use two dipoles at slightly different lengths, or a dipole with a tuner, to cover the whole band. On higher bands, such as 20 meters and above, the whole band is narrow in percentage terms, and a single dipole covers it with no problem. The percentage is the useful measure: the 80 meter band is about 8 percent wide in Europe, while 20 meters is only 2.5 percent.

The Bottom Line

Antenna bandwidth is the range of frequencies over which an antenna, in practice its SWR, stays acceptable. It is tied to the quality factor Q: bandwidth is about the center frequency divided by Q, so a high-Q antenna is narrow and a low-Q antenna is wide. Short and loaded antennas have a high Q and a narrow range, while thick, folded or special-shape antennas are wider. A moderate SWR costs only a fraction of a decibel, so the limit is usually set by the transmitter and by what you want to do. Sweep your antenna, understand the curve, and choose your tools to put the resonance where you operate.

Frequently Asked Questions

What is antenna bandwidth?

The range of frequencies over which an antenna meets a chosen requirement. For amateurs it is usually the range where the SWR is below 2:1 or 1.5:1.

How is Q related to bandwidth?

The two-sided bandwidth is approximately the center frequency divided by Q. A higher Q gives a narrower bandwidth.

Why is my short antenna so narrow?

Shortening an antenna and loading it with a coil raises its Q and reduces its radiation resistance, which narrows the bandwidth and adds loss.

Does a wider antenna have more gain?

No. Gain and bandwidth often trade against each other. Directional antennas such as Yagis are narrower than simple dipoles.

How much power is lost at 2:1 SWR?

About 11 percent of the power is reflected, a mismatch loss of roughly 0.5 dB on a lossless line. Feed line loss makes the real figure higher.

How can I widen a dipole?

Use a thicker conductor, such as a cage or several wires, use a fan arrangement, or choose a wideband design such as a T2FD.