undefined | unit 3 antenna fundamental and definitions

AWP

Unit 3

Antenna Fundamentals and Definition

Q1) Write a short note on antenna pattern?

The radiation pattern or antenna pattern is the graphical representation of the radiation properties of the antenna as a function of space.

That is, the antenna's pattern describes how the antenna radiates energy out into space (or how it receives energy).

It is important to state that an antenna radiates energy in all directions, at least to some extent, so the antenna pattern is actually three-dimensional. It is common, however, to describe this 3D pattern with two planar patterns, called the principal plane patterns.

These principal plane patterns can be obtained by making two slices through the 3D pattern through the maximum value of the pattern or by direct measurement.

It is these principal plane patterns that are commonly referred to as the antenna patterns.

Chart, radar chart

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(a):dipole azimuth radiation pattern (b):dipole elevation radiation pattern

Figure 1. Antenna patterns

In discussions of principal plane patterns or even antenna patterns, you will frequently encounter the terms azimuth plane pattern and elevation plane pattern. The term azimuth is commonly found in reference to "the horizontal" whereas the term elevation commonly refers to "the vertical".

The fig.(a) shows horizontal pattern and fig.(b) shows vertical pattern.

Q2) What is half power beam width?

Half power beam width

Definition

The 3 dB, or half power, beamwidth of the antenna is defined as the angular width of the radiation pattern, including beam peak maximum, between points 3 dB down from maximum beam level (beam peak).

Indication of HPBW

When a line is drawn between radiation pattern’s origin and the half power points on the major lobe, on both the sides, the angle between those two vectors is termed as HPBW, half power beam width. This can be well understood with the help of the following diagram.

Half Power Point

Figure 2 half-power points on the major lobe and HPBW.

Mathematical Expression

The mathematical expression for half power beam width is

Half power Beam Width = 70 λ /D

Where

λ is wavelength (λ = 0.3/frequency).

Dis Diameter.

Units

The unit of HPBW is radians or degrees.

Q3) Explain Radiation resistance?

It is that part of an antenna's feedpoint electrical resistance that is caused by the radiation of electromagnetic waves from the antenna.

The radiation resistance {\displaystyle R_{\text{R}}}RR can be defined as the value of resistance that would dissipate the same amount of power as radiated as radio waves by the antenna with the antenna input current passing through it.

It is equal to the total power {\displaystyle P_{\text{R}}}PR radiated as radio waves by the antenna divided by the square of the rms current {\displaystyle I_{\text{rms}}}Irms into the antenna terminals:

The radiation resistance is determined by the geometry of the antenna and the operating frequency.

Q4) Explain reflection co-efficient?

Definition:

In the context of antennas and feeders, the reflection coefficient is defined as the figure that quantifies how much of an electromagnetic wave is reflected by an impedance discontinuity in the transmission medium. The reflection coefficient is equal to the ratio of the amplitude of the reflected wave to the incident wave.

Calculating reflection coefficient

There are many ways in which the reflection coefficient can be calculated.

Using the basic definition of the reflection coefficient, it can be calculated from a knowledge of the incident and reflected voltages.

Diagram

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Figure 3. Reflection coefficient

r =

Where,

r= reflection co-efficient

Vref= = reflected voltage

Vfwd = forward voltage

It is also useful to be able to calculate the reflection coefficient in terms of the forward and reverse power levels. As the power is proportional to V2 [ watts = V2 / R], we can deduce that:

r =

Where:

Γ = reflection coefficient

Pref = reflected power

Pfwd = forward power

Q5) What is meant by polarization of an antenna?

When the direction is not stated, the polarization is taken to be the polarization in the direction of maximum gain.‖ In practice, polarization of the radiated energy varies with the direction from the center of the antenna, so that different parts of the pattern may have different polarizations

Polarization may be classified as linear, circular, or elliptical.

If the vector that describes the electric field at a point in space as a function of time is always directed along a line, the field is said to be linearly polarized.

In general, however, the electric field traces is an ellipse, and the field is said to be elliptically polarized.

Linear and circular polarizations are special cases of elliptical, and they can be obtained when the ellipse becomes a straight line or a circle, respectively.

The electric field is traced in a clockwise (CW) or counterclockwise (CCW) sense. Clockwise rotation of the electric-field vector is also designated as right-hand polarization and counterclockwise as left-hand polarization.

Linear Polarization For the wave to have linear polarization, the time-phase difference between the two components must be

Circular Polarization Circular polarization can be achieved only when the magnitudes of the two components are the same and the time-phase difference between them is odd multiples of π/2.

||=||

Elliptical Polarization Elliptical polarization can be attained only when the time-phase difference between the two components is odd multiples of π/2 and their magnitudes are not the same or when the timephase difference between the two components is not equal to multiples of π/2 (irrespective of their magnitudes). That is

||||

Q6) Explain the different types of patterns in antenna?

Isotropic Pattern - an antenna pattern defined by uniform radiation in all directions, produced by an isotropic radiator (point source, a non-physical antenna which is the only nondirectional antenna). Directional Pattern - a pattern characterized by more efficient radiation in one direction than another (all physically realizable antennas are directional antennas).

Omnidirectional Pattern - a pattern which is uniform in a given plane.

Principal Plane Patterns - the E-plane and H-plane patterns of a linearly polarized antenna.

E-plane - the plane containing the electric field vector and the direction of maximum radiation.

H-plane - the plane containing the magnetic field vector and the direction of maximum radiation.

Q7) What is beam angle?

Beam Area (or beam solid angle)

In polar two-dimensional coordinates an incremental area dA on the surface of a sphere is the product of the length r dθ in the θ direction (latitude) and r sin θ dφ in the φ direction (longitude), as shown in Fig. Thus,

dA = (r dθ)(r sinθ dφ) = r 2 dΩ (1)

Where dΩ = solid angle expressed in steradians (sr) or square degrees ( ) dΩ = solid angle subtended by the area dA

The beam area or beam solid angle or ΩA of an antenna is given by the integral of the normalized power pattern over a sphere (4π sr)

Q8) Explain beam effeciency?

Beam Efficiency

The (total) beam area ΩA (or beam solid angle) consists of the main beam area (or solid angle) ΩM plus the minor-lobe area (or solid angle) Ωm. Thus,

ΩA = ΩM + Ωm

The ratio of the main beam area to the (total) beam area is called the (main) beam efficiency εM. Thus,

Beam Efficiency = (Dimensionless)

The ratio of the minor-lobe area (Ωm) to the (total) beam area is called the stray factor. Thus

= stray factor

It follows that

+ = 1

Q9) Explain directivity and gain?

Directivity (D)

The directivity D and the gain G are probably the most important parameters of an antenna. The directivity of an antenna is equal to the ratio of the maximum power density P(θ, φ)max (watts/m2 ) to its average value over a sphere as observed in the far field of an antenna. Thus,

D =

The directivity is a dimensionless ratio ≥1. The average power density over a sphere is given by

P =

= dΩ (W )

Therefore, the directivity

D = =

And

D =

This is the diversity from beam area, where Pn(θ, φ) dΩ = P(θ, φ)/P(θ, φ)max = normalized power pattern.

Gain

The gain G of an antenna is an actual or realized quantity which is less than the directivity D due to ohmic losses in the antenna or its radome (if it is enclosed). In transmitting, these losses involve power fed to the antenna which is not radiated but heats the antenna structure. A mismatch in feeding the antenna can also reduce the gain. The ratio of the gain to the directivity is the antenna efficiency factor. Thus, G = kD

Where k= Efficiency factor (0< k< 1), dimensionless

Q10) Explain antenna aperture?

The concept of aperture is most simply introduced by considering a receiving antenna. Suppose that the receiving antenna is a rectangular electromagnetic horn immersed in the field of a uniform plane wave as suggested in Fig.

Let the Poynting vector, or power density, of the plane wave be S watts per square meter and the area, or physical aperture of the horn, be Ap square meters. If the horn extracts all the power from the wave over its entire physical aperture, then the total power P absorbed from the wave is

Diagram

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Figure 5. Plane wave incident on electromagnetic horn of physical aperture AP

P =

Figure 6. Radiation over beam area

Thus, the electromagnetic horn may be regarded as having an aperture, the total power it extracts from a passing wave being proportional to the aperture or area of its mouth. But the field response of the horn is NOT uniform across the aperture A because E at the sidewalls must equal zero.

Thus, the effective aperture Ae of the horn is less than the physical aperture Ap as given by

(Dimensionless) Aperture efficiency

where εap =aperture efficiency.

Q11) Write a short note on radio communication link?

The Friis transmission formula is used in telecommunications engineering, equating the power at the terminals of a receive antenna as the product of power density of the incident wave and the effective aperture of the receiving antenna under idealized conditions given another antenna some distance away transmitting a known amount of power

Pr = Received power, W

Pt = Received power, W

Aet = Effective aperture of transmitting antenna, m2

Aer = Effective aperture of receiving antenna, m2

r = Distance between antennas, m

= Wavelength, m

Friss transmission is given by

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Q12) Write a note on fields on oscillating dipole?

It is a normal dipole antenna, where the frequency of its operation is half of its wavelength. Hence, it is called as half-wave dipole antenna.

The edge of the dipole has maximum voltage. This voltage is alternating (AC) in nature. At the positive peak of the voltage, the electrons tend to move in one direction and at the negative peak, the electrons move in the other direction. This can be explained by the figures given below.

Working Half-Wave Dipole

Figure . Haly wave dipole.

The figures given above show the working of a half-wave dipole.

Fig 1 shows the dipole when the charges induced are in positive half cycle. Now the electrons tend to move towards the charge.

Fig 2 shows the dipole with negative charges induced. The electrons here tend to move away from the dipole.

Fig 3 shows the dipole with next positive half cycle. Hence, the electrons again move towards the charge.

The cumulative effect of this produces a varying field effect which gets radiated in the same pattern produced on it. Hence, the output would be an effective radiation following the cycles of the output voltage pattern. Thus, a half-wave dipole radiates effectively.

Radiates Effectively

Figure . Current distribution

The above figure shows the current distribution in half wave dipole. The directivity of half wave dipole is 2.15dBi, which is reasonably good. Where, ‘i’ represents the isotropic radiation.

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