What is a Dipole Antenna?

A dipole antenna, also called a doublet or dipole aerial, consists of two conductive elements like rods or wires. This antenna type emits a radiation pattern similar to an elementary electric dipole, making it both simple and widely used.

The metal wires of a dipole antenna are each half the maximum wavelength (i.e.,) in free space at the operating frequency.

This wire or rod is split at the center, and the two sections are separated by an insulator, these sections are known as an antenna section.

These two sections of the antenna connect to a feeder, like a coaxial cable, at the center. The wavelength, defining the distance between consecutive peaks or troughs, influences where the feed point is placed.

The radio-frequency (RF) voltage source is applied to the center between the two sections of the dipole antenna. This voltage and a current flowing through the two conductive elements produce a radio signal or an electromagnetic wave to be radiated outwards from the antenna.

The current is maximum and voltage is minimum at the center of the dipole antenna. Conversely, the current is minimum and voltage is maximum at the ends of the dipole antenna.

The radiation pattern of the basic dipole antenna is shown in the figure below. It is perpendicular to the axis of the antenna.

Note that the radiation pattern is the graphical representation of the radiation properties of the antennas as a function of space i.e., the radiation pattern of the antenna describes how the antenna radiates energy out into space.

The dipole antenna is one type of transducer which converts electrical signals into RF electromagnetic waves and radiates them at the transmitting side and converts RF electromagnetic waves into electrical signals at the receiving side.

Dipole Antenna Design

Dipole antennas can be designed to operate across various bands—HF (high frequency), VHF (very high frequency), and UHF (ultra-high frequency)—of the radio frequency spectrum.

Let’s design a 1 MHZ dipole antenna.

Selection of Length of the Dipole Antenna

As we know that the wavelength of a radio wave or any other wave varies inversely proportional to the frequency. it is given by:

  

(1) 

Where, C = velocity of light =

f = frequency, in Hertz

= wavelength, in meter

Thus,

  

(2) 

Now, at a half-wavelength, the length of the antenna is given by,

  

(3) 

Thus, from equation (3) we can say that, if we used a 1 MHz radio transmitter, then the basic length of the antenna wire will be 150 meters or 492 feet or 5905 Inches.

This is correct if we neglect the “end effect”. This “end effect” is the dielectric effect of the air at the end of the antenna that increases the effective length of the antenna. Due to the end effect, an antenna wire acts as 5% longer than the actual length. This will produce interference between the exciting and oscillating currents and due to that oscillation amplitude may be weakened.

Hence, to counterbalance the “end effect” and to make the antenna works properly, it is necessary to cut the antenna wire about to 5% and makes its physical length approximately 95% of half of the wavelength.

Thus, to get the practical length of the antenna wire, the value multiplied by a factor K to the basic length of the antenna wire, i.e.,

  

(4) 

The value of K depends on the thickness of the conductor and the operating frequency. This value of K is accurate for antenna wire at a frequency of up to 30 MHz.

Selection of the Feed Impedance or Radiation Resistance

The feed impedance of a dipole is defined by the ratio of the voltage and the current at the feed point. It is typically fed at the voltage minimum and current maximum point.

To ensure the maximum transfer of energy from the feeder, or source/load, the feed impedance of the dipole antenna should be the same as that of the source or load. By matching the feed impedance to the source or load impedance, the antenna can operate to its maximum efficiency.

The radiation resistance or an input feed impedance of an ideal dipole antenna in free space can be approximately modelled by a 73 Ω impedance and under practical conditions it varies from 60 Ω to 70 Ω. The antenna impedance can be changed by varying the length or shape of the wires.

Many types of coaxial cable have a characteristics impedance of 75 Ω hence, the dipole antenna can be fed with coaxial cable of 75 Ω two wire which is a good match for a half-wave dipole antenna.

Also, the half-wave dipole can be fed with a transmission line of impedances 300 Ω, and 600 Ω open wire line with folded dipoles according to the power handling capabilities.

Use Balanced Feeder or Balun

The dipole antenna is the balanced antenna. Hence, it is necessary to use a balanced feeder. A balanced feeder consists of two parallel conductors. The currents flowing in both the conductors are equal in magnitude but opposite directions. Hence, the radiating field from them cancels out and no power dissipated. The spacing between the conductors has normally kept about 0.01 wavelengths. If a coaxial feeder needs to be used, then the balanced balun must be used.

Coaxial Cable

The most common feeder that is used to feed the antenna is coaxial cable or coax cable. It is often referred to as RF cable.

A coaxial cable carries current in both the conductors. These currents are equal in magnitude but opposite directions. Due to that, all the radiating fields are linked within the cable and hence they are canceling out each other.

Thus, there is no radiating field outside the cable hence it is not affected by nearby any objects. Therefore, it is best suitable as a feeder to the dipole antenna.

Strain Insulator

A strain insulator is an electrical insulator that is designed to withstand the pull of a suspended electrical cable or wire.

It is inserted between two lengths of the conducting wire, to isolate them electrically from each other. It is used in overhead electrical wire, to support radio antennas and overhead power lines.

The overall design of a 1 MHz dipole antenna is illustrated in the figure below.

Types of Dipole Antennas

The most common types of a dipole antenna are the half-wave dipole antenna. There are many types of dipole antennas can be designed. Let’s explain the major types of dipole antennas in detail.

Folded Dipole Antenna

A folded dipole antenna is an array of the two-dipole antenna. If two dipole antennas are connected in parallel to forms a thin wire loop, then it is called a folded dipole antenna.

As the name implies that the form of the dipole antenna is folded back on itself. In the folded dipole antenna, two half-wave dipoles – one continuous and the other split at the center are folded and joined together in parallel at the ends. The split dipole is fed at the center by a balanced transmission line. Hence, the two dipoles have the same voltages at their ends and two identical currents are generated.

The radiation pattern of a folded dipole is the same as to an ordinary dipole but the input impedance of the folded dipole is higher and the directivity of a folded dipole is bi-directional.

Two-Wire Folded Dipole Antenna

If two dipole antennas are connected in parallel to form a thin wire loop, then it is known as a two-wire folded dipole antenna.

If the radius of both the conductors is equal, then equal currents flow in both the conductors in the same direction i.e., currents are equal in magnitude and phase. If the total current fed at the terminal is ‘I’ then each dipole will have current ‘I/2’. Thus, with the same power applied, only half of the current flows in the first dipole, and hence the input impedance increases and it becomes four times. The folded dipole antenna is shown in the figure below.

The general formula for impedance calculation for a folded dipole is given by,

  

Where, n = no. of  antenna wire

Thus, for a two-wire folded dipole with equal radius, the input impedance or radiation resistance is given by

  

Therefore, the two-wire folded dipole can be fed with a conventional 300 Ω open-wire transmission line without any matching device.