A single vertical element, a quarter-wavelength tall, fed at its base against a ground system. It is the workhorse omnidirectional antenna: it radiates equally in every compass direction, which suits repeaters, mobile and base stations, and low-angle HF work.
Half a dipole, plus a mirror
The ground supplies the missing half. Pattern shown over ideal (perfectly conducting) ground; real soil is lossier and lifts the peak a little.
A quarter-wave element only has half the length of a half-wave dipole. A conducting ground makes up the rest: it reflects the antenna's field as if a mirror-image element continued below the surface. Element plus image behave like a full half-wave dipole, with the current peaking at the base and zero at the tip.
Two things follow from that:
Only the upper half is real, so it needs a good ground for the missing half to exist. How good, and how to build it, is the subject of Radials and ground planes.
Because the image sits at the feed point, the antenna sees only half of a dipole's impedance: about 36 Ω rather than 73 Ω.
Length
Same band, three lengths drawn to scale. The 19-inch whip is one quarter of the wave.
The rule is the dipole rule halved:
Quarter-wave monopolelength (ft) = 234 ÷ f(MHz)
The 234 is slightly less than a true quarter of a free-space wavelength, because the element is electrically a little longer than it looks.
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
Band
Frequency
Element length
40 m
7.15 MHz
32.7 ft
20 m
14.2 MHz
16.5 ft
10 m
28.4 MHz
8.2 ft
6 m
50.2 MHz
4.7 ft
2 m
146 MHz
19.2 in
70 cm
446 MHz
6.3 in
Why choose one
Low-angle radiation. Energy leaves close to the horizon, which is good for long-distance HF contacts and for local VHF/UHF FM. A horizontal dipole has to be high, about half a wavelength or more, to send much energy at low angles on the low bands.
One support, small footprint. A single pole needs less room than a dipole on 40 m or 80 m.
Matches vertically polarized stations. FM repeaters, handhelds and mobile rigs use vertical polarization, so a vertical avoids cross-polarization loss; see Polarization.
The price is that a vertical receives more local man-made noise than a horizontal antenna, and on HF its performance depends heavily on the ground system. Short, loaded versions trade efficiency for size; see Loaded and short antennas.