Folded Dipole Antenna

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

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) If I1=I2

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(3Z11)
Z=V/I1=3x3Z11=32 Z11=9x73=657 Ω
 V/n= I1(nZ11)
Z=V/I1=nxnZ11=n2 Z11=n2 x73 Ω

Where r2=r1=radii of elements

2 2
 r   2r 
Z  Z11 1  2   731  1  =73x9=657 Ω
 r1   r1 

2
 log a / r1 
Z  Z11 1    Z11 * Z ratio
 log a / r 2 
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
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.
They are −
If the specifications given above are followed, one can design an 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
 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 I2 I1
I2 I1 I3
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
 Voltage and Current Relations in Parasitic Antenna

V1=I1Z11+I2Z12;
V2=I1Z21+I2Z22; [ As Z12= Z21, Z13= Z31, and V2=0 being parasitic ]
As V1=I1Z11+I2Z12;
So 0=I1Z12+I2Z22
Z 
So I1Z12 = - I2Z22 I 2   I 1  12 
 Z 22 

Z   Z12 
2
V1=I1Z11  I 1  12  Z12 V1  I 1  Z11  
 Z 22   Z 22 

   
    V1  
2
  
Z12

I2   
V1 V1 Z1   Z11 
I1      I 1  Z 22 
 Z 11  Z 12
2
  Z 12 
Z 11Z 22


   Z 12 
 Z 22  V2  Z Z 
Z2    Z12  11 22 

I2  Z12 
Advantages, Disadvantages and Applications of Yagi-Uda 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.