Antenna Tutorial: Functions, Types, Principles, and Applications

Introduction

Antennas play a pivotal role in modern communication systems by converting electrical signals into electromagnetic waves and vice versa. From broadcasting and satellite communication to radar and wireless networking, antennas are integral to transmitting and receiving data over various distances. This tutorial provides a comprehensive overview of antenna basics, covering essential topics such as functions, types, working principles, terminologies, near field vs far field, radiation pattern, applications, advantages and disadvantages.

Antenna Functions

As an antenna resides between a cable/waveguide and the medium of air, the main function of the antenna is to match the impedance of the medium with the cable/waveguide impedance. Hence, the antenna is an impedance transforming device. The second and most important function of an antenna is to radiate the energy in the desired direction and suppress it in unwanted directions. An antenna serves two primary functions: transmission and reception.

• Transmission: When used as a transmitter, an antenna converts electrical signals from a transmitter into electromagnetic waves, which then propagate through the air.
• Reception: As a receiver, an antenna captures electromagnetic waves and converts them back into electrical signals for the receiver to process.
• These functions make antennas crucial in enabling wireless communication by allowing signals to travel through free space without the need for physical connections like cables.

Types of Antennas

There are two main types: non-directional antenna and directional antenna.

1. Non-directional antenna: It radiates energy in all directions and is also called an isotropic antenna. In fact, there is nothing like an ideal isotropic antenna; each antenna will have some directivity. A simple half-wave dipole falls into this category. It is also called a Hertz antenna. This antenna has an omni-directional radiation pattern in one direction. It has a figure-eight shape radiation pattern in the other two dimensions.

2. Directional antenna: It transmits/receives energy to/from a particular direction. Parabolic antennas and horn antennas fall into this category. This parabolic reflective antenna is widely used in microwave and satellite applications. They focus energy in a particular direction.

   

Antennas come in various shapes and sizes, each suited for specific applications and frequency ranges. Here are some common antenna types based on their shapes and sizes:

• Dipole Antenna: The most basic form of antenna, often used in TV and radio applications.
• Monopole Antenna: Similar to a dipole but with a single pole; often used in ground-based communication.
• Loop Antenna: Used for low-frequency applications like AM broadcasting.
• Horn Antenna: Widely used in microwave frequencies and in radar applications due to its high directivity. A horn antenna is shown in figure-1 (above).
• Parabolic Dish Antenna: Known for high gain, it’s commonly used in satellite communication.
• Patch Antenna: Compact and flat, ideal for applications like mobile devices and GPS receivers.
• Yagi-Uda Antenna: A directional antenna used in television reception and point-to-point communication.

Each antenna type has unique characteristics, making it suitable for particular use cases based on frequency, directionality, and power handling capacity.

How Antennas Work

Antennas operate based on the principle of electromagnetic wave propagation. When an alternating current (AC) flows through the antenna’s conductive elements, it generates oscillating electric and magnetic fields. These fields propagate outward as electromagnetic waves. The wavelength and frequency of these waves depend on the dimensions of the antenna and the frequency of the applied AC signal. Key points which illustrate the working of antennas are as follows:

• Resonance: An antenna must be resonant at the frequency of the signal to efficiently transmit or receive energy.
• Polarization: Antennas emit electromagnetic waves with specific polarizations (horizontal, vertical, or circular), which must match the receiving antenna’s polarization for optimal signal transfer.

Antenna Terminologies

Understanding common antenna terminologies is vital for interpreting antenna specifications.

• Gain: Antenna gain is the ratio of power radiated in the direction of interest with respect to the power radiated in the same direction by an isotropic antenna. In other terms, gain is the product of antenna efficiency and antenna directivity.
• Bandwidth: The range of frequencies over which an antenna can operate effectively.
• VSWR (Voltage Standing Wave Ratio): Indicates how well the antenna is matched to the transmission line or receiver.
• Impedance: The resistance to the flow of electrical current, typically 50 or 75 ohms for most antennas.
• Squint: The main beam of the radiation pattern is shifted from its normal position; that shift is called squint (degree of shift).
• Bore sight: The direction of maximum gain.
• Polarization: It is defined in terms of the direction in space of the electric field vector.
  • Elliptical Polarization: The polarization of an electric field which changes in both direction and amplitude synchronously with the radio frequency.
  • Circular Polarization: A special case of elliptical polarization in which the amplitude is constant and the direction rotates synchronously with the radio frequency.
  • Linear Polarization: The polarization of an electric field whose amplitude changes at the radio frequency rate and whose direction remains fixed.
• Beam width: The angle between half power points on the major lobe corresponding to maximum gain, usually in the principal plane.
• G/T: Ratio of antenna gain to receiving system noise temperature. It is called figure of merit.
• Cross polarization: The component of the electric field normal to the desired polarization.
• Sensitivity: The minimum level of the signal the antenna can receive is called its sensitivity.
• Efficiency: Ratio of total power radiated by antenna to the input power of antenna.
• Directivity: It is the ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when receiving.

Near field Vs Far field

The behavior of an antenna’s electromagnetic waves can be classified into two regions, viz. near and far field.

• Near field (Reactive Region): The distance up to $2D^2/λ$ is called the near-field region. Here, D is the antenna diameter and λ is the wavelength of free space. This region is further divided into two parts. The region up to $3λ$ is called the reactive or evanescent region, where the wave is of non-propagating type. The region above $3λ$ is called the radiating region or Fresnel region.
• Far field (Radiative Region): The distance beyond the limit of the near-field region is called the far-field region. This region is also called the Fraunhofer region. In these regions, power radiated from the antenna decays inverse to the square of the distance.

Understanding the near and far field regions is essential for antenna design and placement, especially in high-frequency applications.

Antenna radiation pattern

An antenna’s radiation pattern describes how it distributes energy in space. It is typically represented as a 2D or 3D plot, showing the strength of radiated or received signals at various angles. The figure depicts one such radiation pattern of an antenna. There are two main categories as follows:

1. Omnidirectional Radiation Pattern: Radiates energy uniformly in all horizontal directions, typical of dipole and monopole antennas.

   

2. Directional Radiation Pattern: Focuses energy in a particular direction, providing high gain in that direction. Examples include Yagi-Uda, horn, and parabolic antennas.

Radiation patterns help visualize an antenna’s performance and are crucial for determining its effectiveness in specific applications.

Applications of Antennas

Antennas have numerous applications across various industries:

• Broadcasting: TV and radio stations use antennas to transmit signals over wide areas.
• Telecommunication: Cellular networks rely on antennas for wireless communication between towers and mobile devices.
• Satellite Communication: Parabolic dishes enable high-gain transmission and reception for satellite links.
• Radar Systems: Horn and parabolic antennas are used in radar to detect and track objects.
• IoT Devices: Compact antennas like patch and monopole are used in IoT devices for data transmission.

Antennas are also used in applications like Wi-Fi, GPS, RFID, and more, depending on their specific characteristics.

Advantages of Antenna

Following are common benefits of antennas of all types.

• Enable wireless communication over long distances.
• Versatile with different designs to suit specific frequency ranges and applications.
• Capable of handling high power signals in some configurations (e.g., parabolic dish).
• Can be designed for directional or omnidirectional coverage as needed.

Disadvantages of Antenna

Following are generic limitations of antennas.

• Size and cost can vary significantly depending on application and frequency range.
• Susceptible to interference and environmental factors, which can degrade performance.
• Matching and tuning are required for optimal performance, especially in resonant antennas.

Conclusion

Antennas are a fundamental component in the field of wireless communication. For beginners, it’s essential to grasp the basic idea that antennas serve as a bridge between electrical circuits and electromagnetic waves, enabling the transmission and reception of signals over the air. They come in various shapes and sizes, each designed to operate at specific frequencies and cover desired areas.