```Advanced Radio
Part 6
Introduction
We know a simple aerial
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
the radio wave needs to be concentrated
into a single beam
"look" and "listen"
in one specific direction at a time.
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.
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
Whatever the different
shape, type or design,
there is some form of ‘reflector’ at the centre.
This is similar to the reflector
focusing the light
into a narrow beam.
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.
To obtain vertical information
the aerial is moved up and down through 90°.
accurate range and bearing information
can be measured.
Beam patterns for differing aerial types.
Single Wire
Aerial
Yagi Array
Aerial
Parabolic Dish
Aerial
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
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
CRT
10 12
The instant the pulse is transmitted
a spot travels across the CRT screen
moving at a constant speed.
This is known as
“Base Velocity".
CRT
If a target is detected
a "blip" appears.
Because the screen is calibrated in miles
we know the distance (range) to the target.
CRT
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
To get the bearing and height
we must make some alterations
to the CRT display screen,
Starting with the bearing –
CRT
To find the bearing
(direction)
of a target
we need to find its Azimuth
(bearing measured from North).
CRT
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
It is also possible
to display range and bearing
to aid in range finding.
N
W
E
S
CRT
The display starts
from the centre of the screen,
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
Ө
Height
N
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
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
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
Capacitor Resistance
Translator
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
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
It can only display ranges
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?