Monopole Antenna : Design, Working, Types & Its Applications

A radio pioneer named “Guglielmo Marconi” invented a monopole antenna in 1895 and patented the year 1896 during his first historical experiments in radio communication. So this antenna is also known as the Marconi antenna. This is half of a dipole antenna arranged above a conducting ground plane. So the quarter-wave monopole antenna is the most common type where this antenna is around 1/4 of radio waves wavelength. These antennas are used in internet networks & mobile communications. So this article discusses an overview of a monopole antenna – working with applications.

Monopole Antenna Definition

A type of radio antenna that includes a straight rod shape conductor that is perpendicularly mounted above a ground plane is known as a monopole antenna. This antenna is a simple and single-wire antenna, mainly used for both transmitting & receiving signals, so broadly used in wireless communication systems.

In a monopole antenna, the conductor rod works like an open resonator mainly for radio waves & oscillates by standing voltage & current waves through its length. The antenna’s length is simply determined depending on the desired radio wave wavelength. The monopole antenna frequency range is from 1.7- 2 GHz, with a 3.7 dBi average gain.

Monopole Antenna Design

A monopole antenna is ½ (one-half) of a dipole antenna which is nearly mounted on top of some type of ground plane. So, this antenna is mounted on an infinite ground plane with an ‘L’ length as shown below. By image theory, the fields on the ground plane can be found through the antenna within free space which is shown in the second diagram. This is just a dipole antenna with twice a monopole antenna’s L (length).

In the first figure, the fields on the ground plane are equal to the fields. The fields of the monopole antenna under the ground plane within this figure are zero. The monopole antenna’s directivity is related to a dipole antenna directly. If a dipole antenna’s directivity with length 2L is D1, then the monopole antenna’s directivity with length ‘L’ will have D1+3 which means, the monopole antenna’s directivity is double the dipole antenna’s directivity. The main reason for this is only because no radiation takes place under the ground plane; thus, the antenna is efficiently doubled as “directive”.

Monopole Antenna Working Principle

The working principle of a monopole antenna is; when the power is fed to a monopole then it is radiated similarly in all directions vertical to the antenna’s length above the ground plane on which it is mounted. The radiation pattern of this antenna is omnidirectional, so it radiates with equivalent power within all directions at right angles to the antenna. The radiated power from the antenna changes with elevation angle through the radiation dropping off to zero at the peak on the axis of the antenna.

Monopole Antenna Radiation Pattern

The radiation pattern is the representation of the antenna’s wavefront emission or reception to specify its strength so that one can easily recognize the antenna’s function & directivity.

When the power is emitted from the antenna then it has an effect on far & near field regions. The radiation pattern can be plotted graphically as an angular position & radial distance function from any antenna. The radiation pattern of the monopole antenna is shown below.

The radiation pattern of the monopole antenna is omnidirectional, so it emits equal power in all directions which are vertical to the monopole antenna. The radiated power of the antenna changes with the elevation angle by the dropping-off radiation to zero at the peak on the axis of the antenna. It vertically radiates polarized radio waves.

In the above diagram, the vertical radiation patterns of three monopole antenna graphs with different lengths are mounted above a completely conducting ground.

The line’s radial distance from the source at any elevation angle is simply proportional to the radiated power density at that elevation. The radial axis is adjusted within power density that is relative to a quarter wave monopole & decibels-isotropic. So, the 0.25λ blue color quarter-wave monopole wave has a model equal to the higher half of a dipole antenna pattern. The 0.5λ green color half-wave monopole radiates more power within horizontal directions. The monopole antenna’s highest horizontal radiation can be simply achieved at a length of the red color wave with 0.625λ.

The radiation pattern at above 0.5λ half-wavelength lengths separates into two lobes through a second conical lobe that is directed into the sky. The highest at 0.625λ occurs because, at high angles, the reverse phase radiation of the two lobes cancels compressing more power into the vertical lobe.

Monopole Antenna Types

There are different types of monopole antennas like a whip, helical, rubber ducky random wire, umbrella, mast radiator, inverted-L, T-antenna, ground plane, folded unipole & inverted-F which are discussed below.

Whip Antenna

A whip antenna is a type of monopole antenna and it is very flexible so that it does not crack simply. The name of this antenna was derived from the whip-like motion that exhibits once disturbed. This antenna simply includes a straight flexible rod or wire and the bottom of this antenna is simply connected to the radio transmitter or receiver.

For portable radios, these antennas are frequently designed with a set of interlocking telescoping metal tubes, thus they can be pulled back once not in use. Longer whips are mainly designed to mount on vehicles as well as structures which are designed with a flexible fiberglass rod approximately a wire core & can be up to 11 m long. The perfect length of this antenna can be determined through the radio waves’ wavelength.

These are the most frequently used monopole antennas in the HF, UHF & VHF radio bands. These are extensively used for FM radios, cordless phones, hand-held radios, Wi-Fi-enabled devices, walkie-talkies, and boom boxes. These are connected to vehicles for car radios as well as 2-way radios for vehicles & aircraft.

Helical Antenna

A helical antenna includes a minimum of one or above conducting wires which are wounded in a helix form. When a helical antenna is designed with one helical wire then this antenna is known as monofilar whereas antennas designed with a minimum of 2 or 4 wires within a helix are then called quadrifilar/bifilar. Please refer to this link to know more about – Helical Antenna.

Random Wire Antenna

A random wire antenna includes a long wire that is suspended over the ground where the wire is straight or may be strung back & forth in between walls or trees just to get the sufficient wire into the air. Because of the huge variability in the antenna structure, efficiency can change from one fix to another.

Random wire antennas are extensively used as receiving antennas on the short wave, medium wave & long wave bands, and these antennas are used as transmitting antennas on these bands mainly for emergency or temporary transmitting stations, small outdoor & in places where more permanent antennas cannot be mounted.

Rubber Ducky Antenna

The rubber ducky antenna is a short monopole antenna that works fairly as a base-loaded whip antenna. This antenna includes a narrow helix-shaped springy wire closed within a plastic or rubber jacket to guard the antenna. These antennas are mainly used within handy handheld radio equipment at UHF & VHF frequencies.

These antennas are used in various portable radio devices like walkie-talkies, scanners portable transceivers and where security and robustness take priority above electromagnetic performance. This antenna is very flexible and appropriate for handheld operation, particularly when worn on the belt as compared to former rigid telescoping type antennas.

Mast Radiator

A mast radiator is a type of monopole antenna. This is a radiating tower or radio mast which is where the metal structure is energized & works as an antenna. This is normally used for transmitting antennas that function at low frequencies within the MF & LF bands, particularly used for AM radio broadcasting stations. This antenna’s base is normally mounted on a nonconductive support to protect it from the ground.

Umbrella Antenna

An umbrella antenna is a wire monopole antenna that can be used as transmitting antennas under 1 MHz in the LF, MF & mainly the VLF bands at sufficiently low frequencies that it is infeasible or impractical to make a complete size quarter-wave monopole antenna. The external end of every radial wire is sloping down from the pinnacle of the antenna and is connected to a supporting rope through an insulator attached to the earth. The radial wires will make this antenna the giant umbrella with a wire frame.

T-antenna

A T-antenna is a monopole radio antenna which is also known as a flat-top or T-aerial antenna. This antenna includes one or above horizontal wires which are balanced between two radio masts otherwise buildings & protected at the ends from them. A horizontal wire is simply connected to the middle of the horizontal wires, hangs down very close to the ground, and is connected to the transmitter (or) receiver. T-antennas are normally used in the MF, LF, VLF & shortwave bands. These antennas are broadly used as transmitting antennas mainly for amateur radio stations, medium wave & long wave AM broadcasting stations. For shortwave listening, these antennas are used as receiving antennas.

Folded Unipole Antenna

This is one kind of monopole mast radiator antenna mainly used as a transmitting antenna within the medium wave band for AM radio broadcasting stations. This antenna mainly includes a mast or vertical metal rod connected to its grounding system that includes buried wires. The mast is simply enclosed by vertical wires which are electrically connected at the pinnacle of the mast. These wires are simply connected by a metal ring close to the base of the mast & the feeding power of the feed line from the transmitter is attached between the ring & the ground. This antenna is the best choice whenever an AM radio station shares a tower with other antennas like FM broadcasting antennas.

Inverted-F Antenna

An inverted-F antenna is used mostly at microwave & UHF frequencies within wireless communication. It includes a monopole antenna that runs parallel to a ground plane & grounded at one end. This antenna is fed from a middle point from the distance of the grounded end. This antenna is more compact, allowing it to emit power efficiently, etc. These antennas are extensively used in wireless compact hand-held devices. These are using at military test ranges within telemetry applications.

Monopole Antenna Vs Dipole Antenna

The difference between monopole antenna and dipole antenna are discussed below.

Advantages and Disadvantages

The monopole antenna advantages include the following.

• Monopole antennas are very simple to build & also install
• These are rugged & not expensive to make.
• This antenna has a fairly high reactive impedance above most of its frequency range.
• When this antenna gets longer & the ground losses are decreased, then the antenna’s efficiency gets better.
• A vertical-type monopole antenna is used for any frequency shorter than 2/3rd of the wavelength.
• These are simple omnidirectional antennas that use less space as compared to an array of wheel antennas mounted on top of each other.
• These antennas handle communications within any path except vertically on top of the antenna.
• Printed monopole antennas are extensively used because of many benefits like low cost, low profile, simple fabrication, less weight & combination with other active devices.

The monopole antenna disadvantages include the following.

• It has poor radiation within all directions because radiating evenly in all directions.
• These are expensive.
• Signal reflections can be caused by metal objects & the ground, so you may get both vertically & horizontally polarized signals.
• The design & size constraints required for a ground plane in this antenna are frequently restrictive.
• This antenna’s radiation pattern mainly depends on the direction of the ground plane.

Applications

The uses/applications of monopole antennae include the following.

• Monopole antennas are used in various fields like space science, radar technology, biomedical, research, etc.
• The monopole is frequently used as a resonant antenna where the rod of this antenna works as an open resonator, particularly for radio waves & fluctuates through standing waves of current & voltage through its length. So, the antenna lengths are simply determined by the preferred radio waves wavelength.
• This type of antenna is extensively used within wireless communication systems.
• A monopole antenna with a low-profile pentagonal model is designed & used for wearable applications.
• This antenna is extensively used in automobiles, portable AM or FM radios, etc
• A very efficient, low-cost & low profile based monopole antenna is used in dual-band, multi-band, and UWB (ultra-wideband) applications.

Thus, this is an overview of a monopole antenna – working, types with applications. The monopole antenna characteristics mainly depend on the ground plane & radiator geometry. These antennas are extensively used in different fields like space science, radar technology, wireless communication systems & biomedical research because of many benefits like less cost, low profile, lightweight, easy fabrication & combination through other active devices. Here is a question for you, what is a dipole antenna?