Two equal lengths of wire, a quarter-wavelength each, fed at the centre. Together they span half a wavelength, hence the name. It is the simplest efficient antenna, and the yardstick other antennas are measured against.
How it works
Current and voltage on the wire swap places in time. Averaged over a cycle: current is strong in the middle, voltage is strong at the ends.
A wave travels to the end of the wire and reflects, setting up a standing wave. At the ends the current has nowhere to go, so it is zero and the voltage is highest. In the middle it is the other way round: high current, low voltage.
That is why the centre is the natural place to feed it: it presents a low impedance, close enough to 50 Ω coax. Feeding at an end instead means high voltage and low current, a very high impedance; see End-fed half-wave antennas.
Length
Bars are drawn to the same scale. Half the wavelength, half the wire: the monopole is exactly half a dipole.
Dipole 468 ÷ f32.8 ft
Monopole 234 ÷ f16.4 ft
Radio waves travel slightly slower along a wire than through free space, and the wire's ends add a little capacitance. Both effects make the antenna electrically longer than it looks, so the wire is cut about 5% short of a true half wavelength. That is the 468 in the formula, rather than 492.
Band
Frequency
Total length
40 m
7.15 MHz
65.5 ft (about 20 m)
20 m
14.2 MHz
33.0 ft (about 10 m)
2 m
146 MHz
38.5 in
Pattern
The wire is the hole of the donut. Signal is strongest straight out from its side, weakest off its ends.
The antenna radiates best at right angles to the wire and not at all along it, like a donut with the wire through the hole. Seen from above, that is a figure-8.
Height above ground matters as much as the antenna itself:
Left: pattern looking along the wire. Right: pattern from above at the chosen elevation. Low antenna, high angle, nearly round.
Max minus min (azimuth)1.8 dB
Low (well under a quarter wavelength) it sends most energy upwards, good for contacts within a few hundred miles. Raised toward a half wavelength or more, more energy goes out at low angles, better for distant stations.
In practice
Feed with coax through a 1:1 balun or common-mode choke, so RF stays off the outside of the cable.
It works as a flat line, or drooped into an The inverted V if you have one tall support.
It is naturally narrow-band: a few percent of the frequency. Multiband versions use fan or trap designs.
Nearby trees, metal and buildings pull the resonant frequency down, so a real antenna is often a bit shorter than the formula says.