A folded dipole is an antenna, with two conductors connected on both sides,

and folded to form a cylindrical closed shape, to which feed is given at the

center. The length of the dipole is half of the wavelength. Hence, it is called

as half wave folded dipole antenna. The directivity of Folded dipole

Antenna is bi-directional. The input impedance is higher.

Folded Dipole Antenna

Frequency range

The range of frequency in which half wave folded dipole operates is around

3KHz to 300GHz. This is mostly used in television receivers.

If the Radii of the 2 conductors are equal, then equal currents in both the

conductors, in the same direction, i.e currents are equal in magnitude and

phase in the 2 dipoles.

The total power developed in folded dipole is equal to that developed in the

conventional dipoles, therefore the input or terminal impedance of folded

dipole is greater than that of the conventional dipole. It can be proved that

the input impedance at the terminals of a folded dipole antenna is equal to

the square of number of conductors comprising the antenna times the

impedance at the terminals of a conventional dipole.

V/2= I1Z11+I2Z12

V/2= I1(Z11+Z12)

Z11=Z12

V/2= I1(2Z11)

Z=V/I1=2x2Z11=22Z11=4x73=292 Ω [Z11= 73 Ω for dipole antenna]

V/3.= I1Z11+I2Z12+ I2Z13

V/3= I1(Z11+Z12+ Z13)

V/3= I1(3Z11)

Z=V/I1=3x3Z11=32 Z11=9x73=657 Ω

If I1=I2

ratioZZra

raZZ *

2/log

1/log1 11

2

11

V/n= I1(nZ11)

Z=V/I1=nxnZ11=n2 Z11=n

2 x73 Ω

Where r2=r1=radii of elements

If r2=2r1 then

2

1

1

2

1

211

21731

r

r

r

rZZ =73x9=657 Ω

Advantages The following are the advantages of half-wave folded dipole antenna −

Reception of balanced signals.

Receives a particular signal from a band of frequencies without losing

the quality.

A folded dipole maximizes the signal strength.

High input impedance

Wide band in frequency

Acts as built in reactance components network

Disadvantages The following are the disadvantages of half-wave folded dipole antenna −

Displacement and adjustment of antenna is a hassle.

Outdoor management can be difficult when antenna size increases.

Applications The following are the applications of half-wave folded dipole antenna −

Mainly used as a feeder element in Yagi antenna, Parabolic antenna,

turnstile antenna, log periodic antenna, phased and reflector arrays, etc.

Generally used in radio receivers.

Most commonly used in TV receiver antennas.

Yagi-Uda antenna is the most commonly used type of antenna for TV reception over the last

few decades. It is the most popular and easy-to-use type of antenna with better performance,

which is famous for its high gain and directivity

Frequency range

The frequency range in which the Yagi-Uda antennas operate is around 30 MHz to 3GHz

which belong to the VHF and UHF bands.

Yagi-Uda Antenna

ELEMENT SPECIFICATION

Length of the Driven Element 0.458λ to 0.5λ

Length of the Reflector 0.55λ to 0.58λ

Length of the Director 1 0.45λ

Length of the Director 2 0.40λ

Length of the Director 3 0.35λ

Spacing between Directors 0.2λ

Reflector to dipole spacing 0.35λ

Dipole to Director spacing 0.125λ

They are − If the specifications given above are followed, one can design an Yagi-Uda antenna.

General Characteristics of Yagi-Uda Antenna

If 3 elements array (i.e. one reflector, one director and Driven element) is used, then

such type of antenna is generally referred to as beam antenna.

It has unidirectional beam of moderate directivity with light weight, low cost and

simplicity in feed system design.

With spacing of 0.1 λ to 0.15 λ a frequency band width of the order of 2% is obtained

It provides gain of the order of 8 db or front to back ratio of about 20 db

It is also known as super directive or super gain antenna due to its high gain and beam

width per unit area of the array. An antenna or array which provides directive gain,

appreciable greater than that obtainable from uniform distribution is known super

directive or super gain antenna.

If greater directivity is desired, further elements may be used. For example, five or six

elements are used with case and arrays up to 40 can be constructed.

It is essentially a fixed frequency device i.e frequency sensitive and a band width of

about 3% is obtainable. This much band width is sufficient for television reception.

Voltage and Current Relations in Parasitic Antenna

V1=I1Z11+I2Z12+I3Z13+-----------------+InZ1n

V2=I1Z21+I2Z22+I3Z23+-----------------+InZ2n

V3=I1Z31+I2Z32+I3Z33+-----------------+InZ3n

--------------------------------------------------

Vn=I1Zn1+I2Zn2+I3Zn3+-----------------+InZnn

V1, V2, V3------- Vn = Voltage applied to antenna no. 1,2,3 ----- n

I1, I2, I3----------- In = Current flowing in antenna no. 1,2,3 ----- n

Z11, Z22, Z33----- Znn= Self impedance of antenna no. 1,2,3, ----n

Z12, Z21, Z13----- Z31= Mutual impedance of antenna no. 1,2,3, ----n

I1 I2 I1 I2 I3

V1=I1Z11+I2Z12;

V2=I1Z21+I2Z22; [ As Z12= Z21, Z13= Z31, and V2=0 being parasitic ]

As V1=I1Z11+I2Z12;

So 0=I1Z12+I2Z22

So I1Z12 = - I2Z22

22

1212

Z

ZII

22

121

Z

ZIV1=I1Z11 Z12

22

2

121111

Z

ZZIV

22

2

1211

11

Z

ZZ

VI

12

221112

12

Z

ZZZ

VI

22

2

1211

1

11

Z

ZZ

I

VZ

12

221112

2

22

Z

ZZZ

I

VZ

Voltage and Current Relations in Parasitic Antenna

Advantages The following are the advantages of Yagi-Uda antennas

High gain is achieved.

High directivity is achieved.

Ease of handling and maintenance.

Less amount of power is wasted.

Broader coverage of frequencies.

Disadvantages The following are the disadvantages of Yagi-Uda antennas

Prone to noise.

Prone to atmospheric effects.

Applications The following are the applications of Yagi-Uda antennas

Mostly used for TV reception.

Used where a single-frequency application is needed.

Advantages, Disadvantages and Applications of Yagi-Uda Antenna