Monopole antenna
A vertical rod antenna using a ground plane, widely used for omnidirectional communication.
A monopole antenna is a type of radio antenna built from a single straight rod-shaped conductor. It is usually installed vertically above a conductive surface, known as a ground plane. The transmitter's current flows between the rod and the ground plane, or the received signal voltage is taken from between them. One side of the feedline connects to the bottom of the rod, while the other side connects to the ground plane, which can be the Earth itself. This setup differs from a dipole antenna, which uses two identical rod conductors with the transmitter's current applied between them. Mathematically, the monopole is related to the dipole. A vertical monopole is omnidirectional, has a low gain of 2 to 5 dBi, and sends most of its power out horizontally or at low angles. Common types include the whip, rubber ducky, umbrella, inverted-L, T-antenna, inverted-F, folded unipole, mast radiator, and ground plane antennas.
The monopole typically works as a resonant antenna, with the rod acting as an open resonator for radio waves, creating standing waves of voltage and current along its length. Because of this, the antenna's length depends on the wavelength of the radio waves it handles. The most common design is the quarter-wave monopole, where the rod is about one-quarter of the wavelength long. It is considered the most widely used antenna globally. Shorter versions, called electrically short monopoles, are also common because they are more compact. Five-eighths wavelength monopoles are popular too, as they radiate the most power horizontally at that length. A capacitively loaded or top-loaded monopole has horizontal conductors, like wires or screens insulated from the ground, attached to its top to boost radiated power. Large top-loaded monopoles, such as T, inverted-L, and umbrella antennas, serve as transmitting antennas for longer wavelengths in the LF and VLF bands. The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi, which is why it is also called the Marconi antenna, though Alexander Popov independently invented it around the same time.
Because of their omnidirectional radiation pattern, vertical monopole antennas are common in terrestrial radio communication systems where the direction to the transmitter or receiver is unknown or always changing.
- Invented
- 1895
- Inventors
- Guglielmo Marconi (also independently by Alexander Popov at about the same time)
- Typical gain
- 2 – 5 dBi
- Common resonant length
- quarter-wave (approximately one quarter of the wavelength)
- Other common length
- five-eighths wavelength (0.625 λ)
- Frequency bands used
- MF, LF, VLF, VHF, UHF, microwave
- Typical radiation pattern
- omnidirectional, radiating most power in horizontal directions or low elevation angles
Lore & Background
The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi, who found that connecting one side of his transmitter to a suspended wire and the other to a conductor buried in the Earth allowed him to transmit over longer distances than with dipole antennas. Alexander Popov independently invented it at about the same time for his lightning detection receiver. Marconi's early experiments led to transatlantic radio transmission in 1901 using a monopole transmitting antenna consisting of 50 vertical wires suspended in a fan shape from a support cable between 60-meter poles.
The monopole is usually used as a resonant antenna, with the rod functioning as an open resonator for radio waves. The most common form is the quarter-wave monopole, which is the smallest resonant antenna and an efficient radiator. Electrically short monopoles (shorter than one-quarter wavelength) are also widely used for compactness, while five-eighths wavelength monopoles radiate a maximum amount of power in horizontal directions. Large top-loaded monopoles such as the T, inverted-L, and umbrella antennas are used as transmitting antennas at longer wavelengths in the LF and VLF bands.
At higher frequencies in the VHF and UHF bands, artificial metal ground planes of screen or rods are used to allow mounting above ground. Common variants include the ground plane antenna, discone antenna, blade antenna, and the inverted-F antenna (IFA) used in wireless devices and cell phones. The quarter-wave whip and rubber ducky antennas used with handheld radios are also monopole antennas, though in portable devices without an effective ground plane they function more like asymmetrical dipoles.
Reader's Guide
The monopole antenna is significant as one of the most widely used antenna types globally, particularly in its quarter-wave form. Its omnidirectional radiation pattern makes it ideal for terrestrial radio communication systems where the direction to the transmitter or receiver is unknown or constantly changing, such as radio broadcasting, mobile two-way radios, beacons, cellphones, and Wi-Fi networks. The monopole radiates equal radio power in all horizontal directions while minimizing power radiated upward, making it efficient for ground-wave propagation.
Large monopoles serve as the main transmitting antennas for frequencies below 3 MHz (MF, LF, and VLF bands), where the Earth itself is used as the ground plane. The mast radiator, a vertical mast insulated from ground, is the most common design for AM radio broadcasting. At lower frequencies where practical masts are electrically short, capacitively top-loaded monopoles like the inverted-L, T, and umbrella antennas improve efficiency. At VHF and UHF frequencies, the monopole appears in forms such as the ground plane antenna, whip antennas on vehicles, and the inverted-F antenna in handheld wireless devices. The monopole's legacy includes Marconi's pioneering work that demonstrated radio as a practical communication method, and its continued ubiquity in modern wireless systems.
Did You Know?
- The monopole antenna was invented in 1895 by Guglielmo Marconi, and independently by Alexander Popov at about the same time.
- The quarter-wave monopole is said to be the most widely used antenna in the world.
- A five-eighths wavelength monopole radiates a maximum amount of its power in horizontal directions.
- Marconi achieved transatlantic radio transmission in 1901 using a monopole antenna made of 50 vertical wires suspended from 60-meter poles.
More in Antenna Types, Part 2 1-24
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