Loop Antenna : Design, Working, Types, Radiation Pattern & Its Applications

An antenna is an aerial or metallic structure that converts electric current into EM (electromagnetic) waves. These are mainly used for transmitting & receiving non-ionizing electromagnetic fields, which comprise microwaves, radio waves, IR radiation & visible light. Antennas are available in different types, sizes, and shapes. Small antennas are used on the roof of your home for watching TV whereas big antennas are used for capturing signals from satellites. Similarly, a loop antenna in radio communication is a very popular transducer because of its unique characteristics as well as its versatile applications. By knowing this antenna’s basic concepts & also benefits, we can know their significance within modern communication systems. So, this article provides brief information on loop antenna, their types, working, and their applications.

What is a Loop Antenna?

A loop antenna definition is; a type of radio antenna that is formed with a coil of wire or loop by bending into a loop form. The loop of this antenna carries a radio frequency current. The loop of the antenna can vary in shape, size & orientation which influence the performance of the antenna. The loop antenna symbol is shown below.

The loop antenna efficiency is mainly determined through its dimensions & the operation frequency. Smaller types are sensitive, particularly to the electromagnetic field’s magnetic component, so these antennas are known as magnetic loop antennas. This antenna’s characteristic will make them less vulnerable to interference & electrical noise, so it is very beneficial for applications like amateur radio communications & shortwave radio reception.

Loop Antenna Design & Working

This antenna is designed with a coil that carries a radio frequency current. This loop may be in any form by bending the wire into different shapes, so loop antenna shapes are; rectangular, circular, square, hexagonal, or triangular based on the convenience of the designer. The current throughout this antenna will be within phase and the magnetic field is perpendicular to the complete loop that carries the current.

These antennas normally operate based on the electromagnetic induction principle which means whenever RF energy is present then it generates an alternating magnetic field, which in turn induces a current within the loop, thus enabling the transmission (or) reception of signals. The current within the loop will create an electric field that emits energy like an electromagnetic wave by allowing radio communication. The loop antenna frequency ranges from 300MHz – 3GHz. This antenna’s performance can be enhanced through numerous measures like improvements within the antenna’s shape & construction material of the loop.

Loop antenna characteristics are the same as monopoles & dipoles in that they are low-cost & very easy to construct. These antennas are available in different shapes although the fundamental characteristics of radiation pattern are mainly independent of the shape of a loop. Similar to monopoles & dipoles electrical length affects the antenna’s efficiency, the loop’s electrical size will determine the antenna’s efficiency.

Types of Loop Antenna

There are different types of loop antennae based on different factors like; the size of the loop, configuration & orientation which are discussed below.

Small Loop Antennas

Small or magnetic types

have a smaller circumference that is < 1/10th of the operating frequency’s wavelength. These types of antennas are very compact, so used in portable applications & installations with limited space. These antennas can give good performance despite their tiny size; gain, and radiation efficiency & particularly when tuned through a capacitor.

These are mainly used as receiving antennas, although sometimes these are used for transmission rarely despite their decreased efficiency and smaller circumference loops. The best example of a smaller loop antenna is a ferrite antenna or loop stick antenna which is used nearly in AM broadcast radios. The radiation pattern of these antennas is highest at directions in the loop plane, thus it is perpendicular to the maximum of large loops.

Large Loop Antennas

These antennas are also known as full-wave loops or self-resonant loop antennas where these antennas have >1/10th circumference of the operating frequency’s wavelength. These antennas are very efficient as compared to other types of antennae in terms of both transmission & reception. These antennas have a higher gain as compared to small loop antennas.

So these can be used in various applications like; radio direction finding, radio astronomy & long-distance communication. The radiation pattern of this antenna is a two-lobe radiation pattern at their opening, peaking in both directions and full-wave resonances are perpendicular to the loop’s plane.

Multi-turn Loop Antennas

These antennas have various turns of wire (or) other conducting material which increases the efficient region of the loop. These antennas have improved gain & higher radiation resistance as compared with single-turn-based loops. These antennas are utilized in different applications like; finding radio direction, low-frequency communication & sensing magnetic field.

Horizontal Loop Antennas

The loop antenna which is oriented horizontally is known as a horizontal loop antenna. These types of antennas are also called skywave antennas which are used commonly for long-range communication because of their capability to capture signals that are reflected from the ionosphere.

Vertical Loop Antennas

The loop antenna that is oriented vertically is known as a vertical loop antenna. These antennas are very effective while capturing ground wave signals. These types of antennas are frequently used for direction finding & short-range communication.

Radiation Pattern

The radiation pattern of the loop antenna is shown below. This antenna’s radiation pattern is similar to a short horizontal dipole antenna. In the following figure, radiation patterns for various looping angles are very clearly shown. The tangent lines at zero degrees point out vertical polarization while the line at 90° points out horizontal polarization.

The polarization of the loop antenna is horizontally or vertically polarized based on the position of the feed. The horizontal polarization is specified at the middle of the horizontal side whereas the vertical polarization is specified at the middle of the vertical side based on the antenna’s shape. Generally, a small loop antenna is a linearly polarized one, so whenever this antenna is located on top of a moveable receiver where the o/p of the receiver is connected directly to a meter then it becomes a large direction finder.

Advantages and Disadvantages

The advantages of loop antennae include the following.

• Loop antenna’s size is compact and simple to design which allows for flexibility & portability in placement.
• These are low-cost & not simply de-tuned with hand movements.
• These are capable of operating near other kinds of antennas without any interference.
• These antennas are lightweight.
• It is appropriate for portable applications like direction finding, etc.
• These antennas are available in different varieties like; Adcock antenna, cloverleaf antenna, Alford loop, Bellini-Tosi antenna, and many more.
• These antennas have particularly tiny loops which provide numerous key benefits in radio communication as they exhibit low electric-field noise and make them outstanding to use in high interference locations.
• These antennas show directional radiation patterns and are very helpful in certain situations.
• A small loop of this antenna is normally used as a magnetic dipole.
• This antenna has directional properties.
• In this antenna, the induced EMF around the antenna’s loop must be equivalent to the variation in between the two vertical faces only.
• The radiation pattern of this antenna does not depend on the loop’s shape.
• The currents are at similar phases and magnitudes throughout the antenna’s loop.

The disadvantages of loop antenna include the following.

• The transmission efficiency of the antenna’s loop is extremely poor.
• This antenna is not suitable for high frequencies.
• The pattern’s two nulls of this antenna can result in 180 degrees of ambiguity.
• These antennas have poor gain, hard to tune & are extremely narrowband.
• They have reduced portability.
• Loop antennas as direction finders are not capable of distinguishing between a distant transmitter bearing & its mutual bearing
• The efficiency of antennas with small loops is poor, so these are used at lower frequencies as receiving antennae.
• Antennas with small loops have extremely low radiation resistance values, thus there is a power loss in the form of heat because of the high level of current flow.

Applications/Uses

The uses or applications of loop antennae include the following.

• These antennas are used in wireless communication systems like wireless sensor networks (WSNs), wireless power transfer (WPT) systems & RFID systems.
• These antennas are used in a broad spectrum of applications.
• These are used in broadcast receiver systems and for AM radio, they serve as in-built antennas.
• Large loop antennas are used in direction-finding equipment in Aircraft receivers and low-frequency RFID systems.
• These are used in HF, MF, & Short wave receivers.
• These antennas are used within UHF transmitters.
• These antennas are used to receive high-frequency waves within radio receivers.
• These are used for the broadcast signal reception within the longwave & mediumwave bands.
• These antennas are very popular within the amateur radio community for high-frequency band use.
• These antennas are used in short-range wireless devices & RFID systems with the increase of wireless communication & Internet of Things (IoT).

Thus, this is an overview of the loop antenna, working, types, advantages, disadvantages, and its applications. These antennas are flexible radio communication devices that are well known for their directional properties, versatility, resistance to electrical interference & compact size. These antennas work based on electromagnetic induction and are used in modern communication systems, radio communication, radio detection finding, short-wave radio reception, etc. Here is a question for you, what is a dipole antenna?