Bandwidth and efficiency

What it is

Bandwidth is the range of frequencies over which an antenna does its job acceptably. The usual yardstick is SWR below a chosen limit, often 2:1. Efficiency is the fraction of the power delivered to the antenna that is actually radiated rather than turned into heat. They are separate properties, and small antennas tend to be poor at both.

Bandwidth

A resonant antenna has one frequency where its reactance is zero, and the SWR climbs on either side as the reactance grows (see Antenna impedance and radiation resistance). How wide the usable stretch is depends on Q. A low-Q antenna, such as one made of thick conductors or a The folded dipole, covers more spectrum than a high-Q one, such as a short loaded whip or a small Small magnetic loops.

Compare with what the band demands: a US 40 m band is about 4% wide, 80 m about 13%, 2 m under 3%. A simple dipole may cover 40 m or 2 m but only part of 80 m; at the band edge you retune, or add a tuner.

A narrow bandwidth is not automatically bad. It also filters out strong signals far from your frequency.

Efficiency

The antenna's feed-point resistance is the sum of two parts, the radiation resistance and the loss resistance. The same current flows through both, so the power splits in the same ratio as the resistances. With 12 Ω of radiation resistance and 8 Ω of loss, efficiency is 12 ÷ 20 = 60%: 40% of your power heats the antenna and ground, a loss of about 2.2 dB.

Typical places the losses hide:

• Ground and radials under a vertical: more radials, lower loss; see Radials and ground planes.
• Loading coils and traps, which add resistance in the current path.
• Thin or poor conductors, and corroded joints.
• Lossy surroundings such as nearby buildings and wet foliage.

The trade-off

Make an antenna much smaller than a wavelength and its radiation resistance falls while its loss resistance does not, so efficiency drops. At the same time its Q rises, so the bandwidth narrows. There is no free lunch: a small antenna can be efficient or broadband, but is rarely both. See Loaded and short antennas and Mobile whips and loading coils.

Related

• Antenna impedance and radiation resistanceWhat the feed line sees at the feed point.
• Gain and directivitydBi, dBd and where the energy goes.
• Resonance and QTuned circuits, bandwidth and selectivity.
• Loaded and short antennasTrading size against efficiency.
• The folded dipoleWider bandwidth and four times the impedance.
• SWR and reflectionsStanding waves, reflection coefficient and return loss.