Dipole Antenna | Electromagnetics - Electronics and Communication Engineering (ECE) PDF Download

Overview

The antenna is a communication, used to transmit & receive signals to represent some information. The first antenna was invented by Germans in the year 1888 and used for wireless communication purposes. These antennas can broadcast both microwave & radio signals. There are different types of antennas available that are classified based on their application like wire, log periodic, aperture, microchip, reflector, lens, array, and traveling wave. So knowing each antenna and its purpose is mandatory to use in suitable applications. For that, this article discusses an overview of one of the types of wire antenna namely dipole antenna, and its working with applications.

What is Dipole Antenna?

A dipole antenna is a type of RF antenna that includes two conductive elements like wires or rods where the metal wire length is half of the highest wavelength approximately in free space at the operation of frequency. At the center of the antenna, the conductive materials are separated through an insulator which is called an antenna section. The dipole antenna diagram is shown below.

The RF voltage source is given to the middle of the antenna then the voltage & current supplying throughout the two conductive elements generate an electromagnetic or radio signal and this signal is radiated outside of the antenna. At the center of this antenna, the voltage is minimum and current is maximum whereas the voltage is maximum & current is minimum at the dipole antenna two ends.

Dipole Antenna Design

A dipole antenna includes two conductive elements like wires and rods or wires where the feeder at the center & radiating sections of the antenna are on either side. The metal wires’ length is half of the highest wavelength that is λ/2 within free space at the frequency of operation. The basic dipole antenna diagram with the center feed point is shown below.

The conductive element in the antenna is split in the middle into two sections through an insulator which is called an antenna section. These sections are simply connected to a coaxial cable or feeder at the middle of the antenna. We know that wavelength is the distance among two consecutive highest or lowest points.

Here, the radiating element length can be determined by several properties of the antenna-like center operating frequency, feed impedance, etc. In this antenna, the length of the dipole is a significant parameter.
Any type of antenna will work for either transmitting or receiving. In various wireless applications, the antenna can be activated in between the transmitter & receiver.

Dipole Antenna Transmitter

• At the transmitter section, the dipole antenna generates the radio waves. A voltage at the preferred frequency is provided to the antenna. So the voltage across the elements of this antenna and the flow of current throughout them will create magnetic as well as electric waves.
• A transmitter in an antenna is the fundamental element that is created with a conductor. Here Conductor carries an electric current where the intensity of the current will fluctuate over time & change into RF radiation that transmits in space.

Dipole Antenna Receiver

• At the receiver section, the electromagnetic wave flowing over the dipole antenna will induce a little voltage. As a result, the antenna will become the source of signal for the input of the receiver.
• The receiver of an antenna performs the reverse operation of the transmitter. It receivers RF radiation & changes it into electric currents within an electric circuit which is connected to the antenna.

How does the Dipole Antenna Works?

• Once the RF voltage source is applied to the center of the two sections in the antenna then the flow of voltage & current throughout the two conductive elements can generate an electromagnetic or radio wave signal to be radiated outside of the antenna.
• At the center of this antenna, the voltage is minimum and the current is maximum. In opposition, the current is minimum & the voltage is maximum at the antenna’s ends. This is the current distribution of dipole antenna.
• The dipole antenna radiation pattern diagram is shown below which is vertical to the axis of the antenna. The radiation pattern is the graphical depiction of the antenna’s radiation properties. The antenna’s radiation pattern will describe how the antenna will emit energy into space.

So, this antenna converts the signals from electrical to RF electromagnetic & emits them at the transmitting end & changes RF electromagnetic signals into electrical at the receiving side.

Different Types of Dipole Antenna

Different types of dipole antennas available are used in different applications based on requirements like a half-wave, multiple half-waves, folded dipole, short dipole, and non-resonant.

1. Half-wave Dipole Antenna
A type of dipole antenna where the dipole length is half the wavelength at the operating frequency is called a half-wave dipole antenna. Sometimes, this antenna is also called the Hertz antenna. This antenna has a simple resonance structure as compared to other antennas, so used for transmission & reception purposes in different applications. The operating frequency of this antenna ranges from 3 kHz to 300 GHz.
The advantages of a half-wave dipole antenna are, they are not heavy, cost-efficient, its input impedance is similar to the transmission line’s input impedance, etc. The disadvantages of a half-wave antenna are is that it is an independent antenna, so it can be used as the fundamental element for other types of antennas that work at extremely high frequencies and the nature of the radiation pattern is omnidirectional. These antennas are mainly used in TV & radio receivers.

2. Folded Dipole Antenna
The collection of the two-dipole antenna which is simply connected separately to make a thin wire loop, then it is known as a folded type antenna. As the name suggests, the type of the dipole antenna is folded back and it includes two half-wave dipoles where one is continuous & the other is split at the middle. These are folded & connected in parallel together at the two ends.
The folded dipole antenna’s radiation pattern is similar to a normal dipole except the input impedance which is higher & the directivity is bi-directional. The main reason to use this antenna is the high value of feed impedance and wide bandwidth. So these antennas are used on their own, as a fundamental part of other antennas and also to provide high bandwidth. These antennas are available in two-wire and three-wire types.

3. Short Dipole Antenna

• A dipole antenna that has a small length as compared to half a wavelength is known as a short dipole antenna. So this is a simple wire antenna where one end is open-circuited and the other is fed through an AC source. The frequency range of this antenna ranges from 3 KHz – 30MHz. so it is applicable in low frequency-based receivers.
• The length of this antenna is smaller than the wavelength. A voltage source is simply connected at one end while making a dipole shape that is the lines are ended at the remaining end.This antenna diagram is shown above including length ‘L’. The original antenna size does not matter but the wire that leads to the antenna should be low than 1/10 of the wavelength.
  So L< λ/10, Where the wire length of the short dipole is ‘L’ and the wavelength is ‘λ’.

4. FM Dipole Antenna
The FM dipole antenna is very simple to make in different ways with less cost. So these antennas are the ideal solution in the roof space or attic in an internal FM antenna. They may be used once a temporary antenna is required. This is a half-wave half dipole vertically polarized antenna which is most frequently used to provide better reception of VHF FM broadcast. Generally, these antennas are used mainly for FM broadcasting where its frequency ranges from 88 MHz – to 108 MHZ.

5. Fan Dipole Antenna
This type of antenna is also called fanned or parallel dipole antenna. It is a multi-band wire antenna, so that is very simple to design. Generally, this antenna includes several dipole antennas which share a similar coaxial feed line. Every dipole must be slash for the approximate band centers where you want to resonate.

Once the signal is transmitted, then only the resonating element for that band is observed through your radio, because the remaining dipoles present at higher impedance. This antenna’s mechanical design is not critical because it can be arranged as an inverted V shape even in a horizontal way.
Some arrangements take benefit of the adjacent spread wires in different directions. Generally, some nearby elements can interfere with each other particularly are tightly united. Generally, this antenna tuning needs cautious trimming of elements to attain resonance on different bands.

Dipole Antenna Gain

• Antenna Gain is one type of parameter used to measure the amount of directivity of the radiation pattern of the antenna. An antenna with high gain will especially radiate within a specific direction. The antenna gain is a passive occurrence where power is not included through the antenna, however, simply reallocated to give additional radiated power in a specific direction. The gain of the antenna can be measured in dBi & dBd.
  Isotropic Antenna gain is measured in dBi
  Dipole Antenna gain is measured in dBd
• There is the main relation between both the dBd & dBi which is given below.
  dBi = dBd + 2.15
• While determining the antenna gain, the designer must take the application of the antenna into consideration.
• Antennas with high gain include many benefits like the quality of the signal is better, longer range, etc.
• Antennas with less gain have a shorter range; however, the antenna’s orientation is comparatively insignificant.
• Dipole antenna gain can be calculated in dBd with respect to a reference dipole antenna. A reference dipole antenna gain is 2.15 dBi. So changing between dBi & dBd is very simple by including or deducting 2.15 based on this formula: dBi = dBd + 2.15.

Wavelength of Dipole Antenna
The wavelength of an antenna can be calculated from the frequency formula.
Frequency (f) = C/λ
From the above equation, the wavelength can be derived
λ = C/f
Where:
‘C’ is the light speed.
‘f’ is the frequency.

Difference between Dipole Antenna and Monopole Antenna

The difference between dipole and monopole antenna include the following.

Advantages

The advantages of a dipole antenna include the following.

• The simplicity is the main advantage of this antenna.
• These are extremely efficient once used at the resonant frequency.
• The gap sizes of dipole don’t impact the antenna’s performance.
• These are Omnidirectional while transmitting & receiving signals.
• As compared to telescoping monopole antenna, telescoping dipoles allow you to get a wider frequency range.
• The radiation pattern of this antenna is donut-shaped by the antenna sitting within the center of the hole.
• These antennas are simple to design and install in different configurations like sloping, drooping, inverted V, etc
• Any signal can be received almost without worrying about its direction.
• These antennas can be reduced & resonated with a loading coil. When this coil is arranged in the center then we can get good results.

Disadvantages

The disadvantages of dipole antennae include the following.

• The wavelength is long for very less frequencies to make an antenna
• Folded type antennas are somewhat complicated as compared to monopole antennas, although they are not complicated like the quad patch or yagi UDA antennas.
• For below 27 MHz frequencies, the antenna’s efficiency will start reducing when the length of the antenna decreases.
• As compared to a typical antenna, the efficiency of Inverted-V type antennas has reduced.
• The size of indoor antennas is very small as compared to outdoor.
• Outdoor antennas are large, so installing and transporting are very difficult to handle.

Applications

The applications of dipole antennae include the following.

• These antennas are used as a basic part in complicated antennas and are also used in different areas where they can form the radiating element.
• This antenna is applicable in radio & telecommunications.
• The same antenna is used in two-way communication, for both transmission & reception.
• A half-wave type antenna can be used in TV & radio receivers.
• Generally, the antenna is used as either a transmitting or receiving antenna because the transmitting antenna changes the signals from electrical to electromagnetic & emits them whereas a receiving antenna can be used for changing the signals from electromagnetic to electrical.
• A parabolic reflector type antenna is used in radio astronomy, satellite communications & for several types of radio communication links.
• VHF & UHF type antennas are mainly used in land mobile communication within public safety, coastal areas, industrial & public communication applications.
• A Folded type antenna can be used within in Yagi-Uda antennas for global TV reception through the balanced line like Z0 = 300 Ω because the input impedance of this antenna is high, so it is matching simply through the transmission line impedance.
• FM type antenna is mainly used as FM broadcasting-receiving antenna, especially for the FM broadcast band which ranges from 88 MHz – to 108 MHz.