INTRODUCTION TO RADAR ANTENNAS

ECE 514 – RADAR & SATELLITE ENGINEERING

Friday, 26 February 2021

1

HALF-WAVE DIPOLE

1. Half-wave dipole is the most common practical antenna.

2. It consists of a straight conducting rode of approximately a half-wave length long.

3. AC is usually fed to the aerial at the centre by a transmission line as shown.

2

SLOT ANTENNA

1. The slot antenna consists of a radiator formed by cutting a narrow slot in a large metal surface.

2. A slot antenna has exactly the same radiation pattern as a half-wavelength dipole.

3. Slot antenna are polarized at right angles to the slot while rod antennas are polarized parallel to the rods.

3

EXAMPLES OF SLOT AERIALS

1. Some types of aircraft suppressed aerials are based on the slot. In such aerials, the slot is cut from the aircraft skin.

2. Some designs use a slot cut from a wall of a waveguide.

4

QUARTER WAVE UNIPOLE

1. The physical length of a half-wave dipole can be very extensive at low frequencies (L =

𝑐

𝜆).

2. For example at 30MHz, 𝐿 =3 × 108

3 × 106= 100m

3. A quarter-wave unipole consists of a single conductor a quarter wave-length long mounted at right angle to a conducting surface.

4. The conducting surface acts as a reflector and creates an image of the unipole leading to performance like that of a half-wave dipole.

5. The radiation resistance of the unipole is about half that of a half-wave dipole.

5

USE OF PARASITIC ELEMENTS

1. Half-wave dipole aerial is omnidirectional and not directional.

2. To make it directional, parasitic elements are usually introduced to improve the directivity of the half-wave dipole.

3. Parasites are conducting rods placed near the energized (driven) dipole.

4. The signal transmitted from the dipole induces current in the parasites which in turn re-radiate the induced energy.

5. By adjusting the length of the parasites and their distances from the energized dipole, constructive interference can be induced in one direction and destruction in the opposite direction.

6

Directors provide constructive interference

Reflector provides destructive interference

Half-wave Dipole is energeized by RF Source

YAGI AERIAL

• Yagi aerial is formed from an energized dipole, a reflector and a number of directors.

• A folded dipole is often used as a driver in a Yagi aerial because it has a higher radiation resistance.

7

Folded Dipole

8

Energized Half-wave Dipole

Directors:Constructive interference

ReflectorDestructive interference

Cable connecting to Transmitter or Receiver

YAGI AERIAL

YAGI ANTENNA - DESIGN

9

1. Reflector: LR = 0.47 - 0.52λ2. Driven Element: L = 0.45-0.49λ3. Directors: LD = 0.4-0.45λ4. Separation between Directors: SD = 0.2-0.35λ5. Radii of directors: 0.15-0.25λ6. Separation between driven element and reflector: SR = 0.2-0.35λ

RADAR ANTENNAECE 514E – RADAR & SATELLITE ENGINEERING

Friday, 26 February 2021

10

ORIGIN OF THE WORD ANTENNA

1. During experiments with the electromagnetic waves Marconi used a wooden tent pole along which was carried a radiating wire.

2. This tent pole means in Italian language l'antenna.

3. The common use of this term in the description of the experiments of Marconi led to the popular name “antenna” for this component part of transmitter sites

11

FUNCTIONS OF A RADAR ANTENNA1. The antenna is one of the most critical parts of a radar system.

2. It performs the following functions:

a) To transfers the transmitter energy to signals in space at the required distribution and efficiency. This process is applied in an identical way on reception.

b) To ensure that the signal has the required pattern in space. Generally this has to be sufficiently narrow in azimuth to provide the required azimuth resolution.

c) To provide target position updates. In the case of a mechanically scanned antenna this equates to the angular speed.

• A angular speed can be a significant mechanical problem given that a radar antenna in certain frequency bands can have a reflector with immense dimensions and can weigh several tons.

d) To measure the pointing direction with a high degree of accuracy.

12

GENERAL CATEGORIES OF ANTENNAS

1. Isotropic Antenna: Energy radiate energy equally in all directions.

2. Anisotropic Antenna: a) Directional antennas where more energy is

propagated in certain directions than in others. b) The ratio between the amounts of energy

propagated in these directions compared to the energy that would be propagated if the antenna were not directional is known as its gain.

c) When a transmitting antenna with a certain gain is used as a receiving antenna, it will also have the same gain for receiving.

Isotropic

Omnidirectional

13

ANTENNA PATTERN

1. A radiation pattern is a plot of the radiated energy from an antenna.

2. This energy is measured at various angles at a constant distance from the antenna.

3. The shape of this pattern depends on the type of antenna used.

4. Patterns can be created:a) rectangular or

b) polar coordinates.

(a) Antenna Pattern in Polar Coordinates

(b) Antenna Pattern in Rectangular Coordinates14

ANTENNA BEAM WIDTH

1. Beam width is the angular range of the antenna pattern in which at least half of the maximum power is still emitted.

2. It is described by Bordering points of this major lobe at which the field strength has fallen to around 3dB from the maximum.

3. Beam width can be defined in both the horizontal and vertical planes

15

Beam width and Side Lobes

ANTENNA APERTURE1. The effective aperture of an antenna Ae is the

surface presented to the radiated or received signal.

2. The antenna gain is related to the effective area by the following relationship:

𝐺 =4𝜋𝐴𝑒

𝜆where 𝐴𝑒 = 𝐾𝑎𝐴

and

𝐴𝑒 = effective antenna aperture

λ = Wavelength

A = Physical area of antenna

𝐾𝑎 = Antenna Aperture efficiency

16

ANTENNA MAIN AND SIDE LOBES

1. The radiation intensity in one lobe is considerably stronger than in the other.

2. The strongest lobe is called major lobe;

3. The others are side lobes.

Beam Width3dB

17

ANTENNA FRONT-TO-BACK RATIO

1. Front-to-back ratio is the ratio of power gain between the front and rear of a directional antenna.

2. A high front-to-back ratio is desirable in radar engineering because this means that maximum energy is directed at the target.

18

ANTENNA POLARIZATION1. The radiation field of an antenna is composed

of electric and magnetic lines of force.

2. These lines of force are always at right angles to each other.

3. The electric field determines the direction of polarization of the wave.

4. When a single-wire antenna is used to extract energy from a passing radio wave, maximum pickup will result when the antenna is oriented in the same direction as the electric field.a) Linear polarization: The oscillations of the electric

field is oriented in a single direction

b) Circular or elliptical polarization: the oscillation direction of the electric field rotate as the wave travels.

19

LINEAR HORIZONTAL AND VERTICAL POLARIZATION

1. An antenna is said to be vertically polarised (linear) when its electric field is perpendicular to the Earth's surface. • An example of a vertical antenna is a

broadcast tower for AM radio or the "whip" antenna on an auto-mobile.

2. Horizontally polarised (linear) antennas have their electric field parallel to the Earth's surface. • Television transmissions use horizontal

polarisation.

20

PARABOLIC ANTENNA

1. The parabolic dish antenna is the form most frequently used in the radar engineering.

2. A dish antenna consists of one circular parabolic reflector and a point source situated in the focal point of this reflector.

3. This point source is called primary feed or simply feed.

21

FRONT-VS OFFSET-FED DISH ANTENNAS (1)

1. One problem associated with feedhornsis the shadow introduced by the feedhorn.

2. The shadow is a dead spot directly in front of the feedhorn.

3. Normally the feed horn constitutes an obstruction for the rays coming from the reflector at a parabolic antenna.

22

1. To solve this problem the feedhorn can be offset from center.

2. In an offset feed, the feed is outside the path of the wave so there is no pattern deterioration due to aperture blocking.

23

FRONT-VS OFFSET-FED DISH ANTENNAS (2)

OFFSET DISH RADAR ANTENNA

24

PHASED ARRAY ANTENNA

1. A phased array antenna is composed of lots of radiating elements each with a phase shifter.

2. Beams are formed by shifting the phase of the signal emitted from each radiating element, to provide constructive/destructive interference so as to steer the beams in the desired direction.

25

1. Two radiating elements are fed with the same phase.

2. The signal is amplified by constructive interference in the main direction.

3. The beam sharpness is improved by the destructive interference.

PRINCIPLE OF PHASED ARRAY RADARS

26

1. Two radiating elements are fed with the same phase.

2. The signal is amplified by constructive interference in the main direction.

3. The beam sharpness is improved by the destructive interference.

1. The signal is emitted by the lower radiating element with a phase shift of 10 degrees earlier than of the upper radiating element.

2. As a result, the main direction of the emitted sum-signal is moved upwards.

NO PHASE SHIFT 𝟏𝟎𝒐 PHASE SHIFT

Main Lobe

Main Lobe

Back Lobe

Back Lobe

ELECTRONIC CONTROL OF BEAM DIRECTION

1. The main beam always points in the direction of the increasing phase shift.

2. Therefore, if the signal to be radiated is delivered through an electronic phase shifter giving a continuous phase shift, the beam direction will be electronically adjustable.

27

FREQUENCY SCANNING ANTENNA

1. A frequency scanning Antenna is a special type of the phased array antenna where the main beam steering occurs by the frequency scanning of the exciter.

2. As a result, beam steering is a function of the transmitted frequency.

3. The normal arrangement is to feed the different radiating elements from one folded waveguide.

4. Height information is generated using the following philosophy:

1. If the transmitted frequency rises then the beam travels up the face of the antenna;

2. If the transmitted frequency falls then the beam travels down the face of the antenna.

5. The radar set is designed so that it keeps track of the frequencies as they are transmitted and then detects and converts the returned frequencies for 3D display data.

28