Introduction
We know a simple aerial will radiate electromagnetic waves equally in all directions; it can also receive signals from all directions.
But this is of limited use when trying to determine the direction of a particular reflection.

Introduction
Instead of radiating, the radio wave needs to be concentrated into a single beam so that the radar can be made to
"look" and "listen" in one specific direction at a time.

Radar Aerials
In order to produce a beam of radiation we need to radiate from a shaped area, not a single wire.
To overcome this problem reflectors are used to modify the aerial’s pattern and reflect the waves in one particular direction.

Radar Aerials
In order to produce a beam of radiation we need to radiate from a shaped area, not a single wire.
But there are a number of different shapes, types and designs that we can use to produce this single beam of radiation.

Radar Aerials
Whatever the different shape, type or design, there is some form of ‘reflector’ at the centre.
This is similar to the reflector in a torch or headlight focusing the light into a narrow beam.

Radar Aerials
To detect bearings the aerial is rotated, sweeping a narrow beam in a complete circle.
(called Scanning ).
All reflections can be plotted around a circle – with the aerial at the centre.

Radar Aerials
To obtain vertical information the aerial is moved up and down through 90 °.
From the reflections received, accurate range and bearing information can be measured.

Radar Aerials
Beam patterns for differing aerial types.
Single Wire
Aerial
Yagi Array
Aerial
Parabolic Dish
Aerial
RADIATION PATTERNS EMITTED

Radar Display
Obtaining a target is only part of the detecting process.
The operator needs to "see" the target in a visual form.
For this we use a Cathode Ray Tube (CRT) which works on a similar principle to a television screen.
CRT

Radar Display
As the time interval between the radio pulses is short the actual CRT screen can be calibrated in miles to match the range of the pulse.
12 10 10 12
CRT

Radar Display
The instant the pulse is transmitted a spot travels across the CRT screen moving at a constant speed.
This is known as
CRT

Radar Display
If a target is detected a "blip" appears.
Because the screen is calibrated in miles we know the distance (range) to the target.
CRT

Radar Display
This radar, known as ‘ Type A ’, is good for determining the range of an object, but it cannot determine the object’s bearing, or its height.
CRT

Radar Display
To get the bearing and height we must make some alterations to the CRT display screen,
Starting with the bearing –
CRT

Radar Display
To find the bearing
(direction) of a target we need to find its Azimuth
(bearing measured from North).
CRT

Radar Display
By using a
Plan Position Indicator (PPI) that rotates a beam through 360 ° the azimuth
(bearing) of any target can be calculated.
N
W
E
S
CRT

Radar Display
It is also possible to display range and bearing by adding Range Rings to aid in range finding.
N
W
S
CRT
E

Radar Display
The display starts from the centre of the screen, and produces a radial trace moving in time with the rotation of the aerial, and ‘blipping’ the target contact.
N
W
E
S
CRT

Other Ranges
Having determined both range and bearing; by using the Slant Range
(distance from the radar to the target).
it is possible to determine object height
Height
Ө
N
Radar Angle
W E
Height = Slant Range x sin
Ө
S
CRT

Other Ranges
Having determined both range and bearing; by using the Slant Range
(distance from the radar to the target). the target’s ground range can also be calculated
Height
Ө Radar Angle
Ground Range = Slant Range x cos
Ө

Other Ranges
Using the Slant Range
Height = Slant Range x sin Ө and
Ground Range = Slant Range x cos Ө
Height
Ө Radar Angle

Other Ranges
To pinpoint a target by both height and bearing requires more than one aerial.
However, there is now a radar system that combines both facilities into one aerial, known as the 3-D.
It works by electronically selecting the various aerial arrays and passing the information to the Plan Position Indicator ( PPI) display.

Check of Understanding
What do the initials CRT stand for?
Cathode Ray Tube
Cathode Radiation Tube
Capacitor Resistance
Translator
Cathode Radio Tube

Check of Understanding
What is the purpose of the reflector and directors of an aerial?
To match the aerial to the transmission line
To modify the aerial’s radiation pattern
To alter the aerial’s electrical length
To allow the aerial to work at many frequencies

Check of Understanding
What does PPI stand for?
Plan Potential Indicator
Partial Plan Indicator
Plan Position Indicator
Partial Position Indicator

Check of Understanding
Which of these statements applies to a PPI radar display?
It uses a height finding radar
It has a circular trace using radial deflection
It can only display ranges
It has a radial trace rotating in time with the radar

Check of Understanding
What can be added to a PPI to aid in its operation?
Height Indicators
Sector Ranges
Range Rings
Bearing Rings

Check of Understanding
To change the range display on a radar screen . . .
A new set of markers is required
The transmitter power is changed
The timebase sweep velocity is changed
The display intensity is changed

Check of Understanding
What is a slant triangle composed of?
Slant range, target and ground range
Angle of elevation, height and target velocity
Slant range, height and ground range
Angle of elevation, height and ground range

Check of Understanding
The ground range of a target can be calculated by using . . .
Slant range x sin Ө
Slant range x tan Ө
Slant range x cos Ө
Slant range x height

Check of Understanding
A few more questions.
1.
What is meant by the term ‘scanning’?
2.
What is meant by the term ‘base velocity’?
3.
What is meant by the term ‘azimuth’?
4.
True or False ? ‘Type A’, radar cannot determine the object’s bearing, only its height.
5.
What do the initials PPI stand for?
6.
In the slant range calculations, what does the symbol ‘ Ө ’ signify?