UNIT –I
RADIATION FROM WIRE ANTENNAS
PART – A
1.What is a Short Dipole?
A short dipole is one in which the field is oscillating because of the oscillating voltage and current. It is called so, because the length of the dipole is short and the current is almost constant throughout the entire length of the dipole. It is also called as Hertzian Dipole, which is a hypothetical antenna and is defined as a short isolated conductor carrying uniform alternating current.
2.How radiations are created from a short Dipole?
The dipole has two equal charges of opposite sign oscillating up and down in a harmonic motion. The charges will move towards each other and electric filed lines were created. When the charges meet at the midpoint, the field lines cut each other and new field are created. This process is spontaneous and so more fields are created around the antenna. This is how radiations are obtained from a short dipole.(See Figure from John. D .Kraus Book)
3.Why a short dipole is also called an elemental dipole?
A short dipole that does have a uniform current will be known as the elemental dipole. Such a dipole will generally be considerably shorter than the tenth wavelength maximum specified for a short dipole. Elemental dipole is also called as elementary dipole, elementary doublet and hertzian dipole.
4.What is a Infinitesimal Dipole?
When the length of the short dipole is vanishing small, then such a dipole is called a infinitesimal dipole. If dl be the infinitesimally small length and I be the current, then Idl is called as the current element.
5.Why a short dipole is called a oscillating dipole?
A short dipole is initially in neutral condition and the moment a current starts to flow in one direction, one half of the dipole require an excess of charge and the other a deficit because a current is a flow of electrical charge. Then ,there will be a voltage between the two halves of the dipole. When the current changes its direction this charge unbalance will cause oscillations. Hence an oscillating current will result in an oscillating voltage. Since, in such dipole, electric charge oscillates ,it may be called as Oscillating electric dipole.
6.What do you understand by retarded current?
Since, the short electric dipole is so short, the current which is flowing through the dipole is assumed to be constant throughout its length. The effect of this current is not felt instantaneous at a distance point only after an interval equal to the time required for the wave to propagate over the distance r is called the retardation time.
The retarded current [I]=Io exp(j w(t-r/c)) Where wr/c is the phase retardation.
7.Define induction field
The induction field will predominate at points close to the current element ,where the distance from the center of the dipole to the particular point is less. This field is more effective in the vicinity of the current element only. It represents the energy stored in the magnetic field surrounding the current element or conductor. This field is also known as near field.
8.Define Radiation field
The radiation field will be produced at a larger distance from the current element, where the distance from the center of the dipole to the particular point is very large. It is also called as distant field or far field.
9.At what distance from the dipole is the induction field equal to the radiation field?
As the distance from the current element or the short dipole increases, both induction and radiation fields emerge and start decreasing. However, a distance reaches from the conductor at which both the induction and radiation field becomes equal and the particular distance depends upon the wavelength. The two fields will thus have equal amplitude at that particular distance. This distance is given by r = 0.159l
10.Define Radiation Resistance
It is defined as the fictitious resistance which when inserted in series with the antenna will consume the same amount of power as it is actually radiated. The antenna appears to the transmission line as a resistive component and this is known as the radiation resistance.
11.Give the expression for the effective aperture of a short dipole
The effective aperture of a short dipole is given by Ae = 0.119l2
12.What is a dipole antenna?
A dipole antenna may be defined as a symmetrical antenna in which the two ends are at equal potential relative to the midpoint.
13.What is a half wave dipole?
A half wave antenna is the fundamental radio antenna of metal rod or tubing or thin wire which has a physical length of half wavelength in free space at the frequency of operation
14.Give the expression for the effective aperture of a Half wave Dipole
The effective aperture of a half wave dipole is given by Ae = 0.13l2
15.What is the radiation resistance of a half wave dipole
The radiation resistance of a half wave dipole is given by Rr=73 ohm
16.What is a loop antenna?
A loop antenna is a radiating coil of any convenient cross-section of one or more turns carrying radio frequency current. It may assume any shape (e.g. rectangular, square, triangular and hexagonal)
17.Give an expression of radiation resistance of a small loop
Radiation resistance of a small loop is given by Rr=31,200 (A/l2) 2
18.How to increase the radiation resistance of a loop antenna
The radiation resistance of a loop antenna can be increased by:
1. increasing the number of turns
2. inserting a ferrite core of very high permeability with loop antenna’ s circumference which will rise the magnetic field intensity called ferrite loop.
19.What are the types of loop antennas?
Loop antennas are classified into:
A.Electrically small (circumference <l/10)
B. Electrically large (dimension comparable to l)
20.What are Electrically Small loop antennas?
Electrically Small loop antennas is one in which the overall length of the loop is less than one-tenth of the wavelength. Electrically Small loop antennas have small radiation resistances that are usually smaller than their loop resistances. They are very poor radiators and seldom employed for transmission in radio communication.
21.What are Electrically large loop antennas?
Electrically Large loop antennas is one in which the overall length of the loop approaches the wavelength.
22.List out the uses of loop antenna
Various uses of loop antenna are:
1) It is used as receiving antenna in portable radio and pagers
2)It is used as probes for field measurements and as directional antennas for radio wave navigation
3)It is used to estimate the direction of radio wave propagation
23. What is capacitance hat?
The capacitance hat is circular in shape with mast at the center of the circle. There are number of horizontal conducting wires with their ends joined together by means of a ring. The capacitance hat is used to increase the electrical length of low frequency antennas.
24. Define top loading
Top loading is a method to increase the effective capacitance at the top of the antenna. This is accomplished by mounting one or more horizontal conductors at the top of the antenna.
25. Define retardation time
It is the time required for the wave to propagate over the distance r. It is given by r/c where c is 3*108m/s
PART – B
1. Derive the expression for the radiated field from a short dipole? (16)
2. Starting from first principles obtain the expression for the power radiated
by a half wave dipole? (16)
3. Derive the expression for power radiated and find the radiation resistance
of a half wave dipole? (16)
4. Derive the radian resistance, Directivity and effective aperture of a half
wave dipole? (10)
5. Derive the fields radiated from a quarter wave monopole antenna? (8)
6. Find the radiation resistance of elementary dipole with linear current
distribution? (8)
7. Derive the radian resistance, Directivity and effective aperture of a
Hertzian dipole? (10)
8. Derive the power radiated and radiation resistance of current element. (10)
9. Explain in detail assumed current distribution for wire antennas (8)
10. Write in brief about the use of capacitance hat and loading coil for
short antennas. (10)
