SEC.1.3]
COMPONENTS
firing of the modulator.
the magnetron,
This
which is the type of transmitting
used in modern radar.
which may typically
For the brief
1 ~sec, the
be
7
OF A
L9YSTEM
RADAR
sends a high-power,
high-voltage
tube almost
pulse to
universally
duration
of the modulator
pulse,
magnetron
oscillates
at the radio
frequency for which it is designed, usually
per second.
The r-f pulse thus produced
some thousands of megacycles
travels down the r-f transmis-
sion line shown
switches
by
designated
of a very
special
double
lines in Fig.
1.4, and passes through
as TR
and ATR.
These
sort.
The
gas discharge
the two
are gas-discharge
is started
by
devices
the high-power
Rotatingantenna
—1
4!
hr!!lI
High.
voltaga
pulses
1==1
1
Scanner base +
)
J
tRf
echo
pulses
/'JMixer
Echo
l:-fecey
I-f echo pulses
Video
amplifier
t
I
l-f
amplifier
Echopulses
o
Angledata from scanner
Indicator
zwming
pUIS.Sindicating
imtant
of transmission (i.e. Zerorange)
FIG.1.4.—Blockdiagramof
r-f pulse
pulse;
from
during
the transmitter
the transmitter,
this
time
the
and
(for
r-f line to the antenna,
receiver
anti-TR)
when fired, simply
mitter
these
switch,
to
pass
connects
shown
it
switches
with
are
the rnher to the antenna,
tron to prevent
After
travels
through
gas-discharge
for the duration
transmit-receive)
and disconnects
below
the TR
permits
in
an
switch.
loss.
unfired
and the ATR
the
through
these
r-f
to
line
two
the
Between
state,
where
ATR
(for
the transpulses,
the
disconnects
switches,
antenna,
connects
The
the r-f pulse from
negligible
of that
switch
the mixer and the
loss of any part of the feeble received
passing
down
simpleradar.
maintained
TR
rest of the radar
a
TR
when
switch
the magne-
signal.
the
it
transmitter
is
radiated.
pulse
The
J'
8
INTRODUCTION
[SEC.1.4
antenna is designed in such a way that the beam shape it produces is
suitable for the requirements the radar set must meet.
It is mounted on
a scanner which is arranged to sweep the beam through
space in the
manner desired; simple azimuth rotation is indicated in Fig. 1"4.
After the transmission
of the pulse, the discharges
in the TR
and
ATRstitches
cease andthesystem
isready
to receive echoes.
Echoes
are picked up by the antenna and sent down the r-f line to the mixer.
The mixer is a nonlinear device which, in addition to receiving the signals
from the antenna, is supplied c-w power from a local oscillator operating
at a frequency
only a few tens of megacycles
per second away from the
magnetron frequency.
The difference frequency that results from mixing
these two signals contains
the same intelligence
as did the original r-f
echoes, but it is at a sufficiently low frequency
(typically,
30 Me/see)
to
be amplilied by more or less conventional
techniques in the intermediatefrequency
amplifier
shown.
Output
signals from the i-f amplifier
are
demodulated
by a detector, and the resulting unipolar signals are further
amplified
by a video-frequency
amplifier
similar to those familiar
in
television technique.
The output signals of the video amplifier are passed to the indicator,
which displays them, let us say for definiteness,
in plan-position
form.
In order to do this, it must receive a timing pulse from the modulator,
to
indicate the instant at which each of the uniform range sweeps out from
the center of the PPI tube should begin.
It must also receive from the
scanner information
on the direction
in which the antenna is pointing,
in order that the range sweep be executed in the proper direction from the
center of the tube.
Connections
for accomplishing
this are indicated
in
the Fig. 1.4.
In Chaps. 9 to 14, inclusive,
the detailed design of each of the components shown in Fig. 1.4 is treated.
In addition, consideration
is given
to the problem of supplying primary power in a form suitable for use with
a radar set; this is especially difficult and important in the case of airborne
radar.
1.4. The Performance
of Radar.-In
discussing the performance
of
radar, one usually refers to its range pwforrnunce-that
is, the maximum
distance at which some target of interest will return a sufficiently strong
signal to be detected.
The factors that determine range performance are
numerous and they interact in a rather complicated
way.
Chapter 2 is
devoted
to a discussion of them, and Chap. 3 deals with the important
matter of the properties of radar targets.
The usual inverse-square law which governs the intensity of radiation
from a point source acts to determine the range dependence of the fraction
of the total transmitted energy that falls on a target.
So far as the echo
is concerned, the target can also be thought of as a point source of radia-
SEC.1.4]
THE
PERFORMANCE
9
OF RADAR
tion, so that the inverse-square
law must be applied again to determine
the range dependence of the amount of echo energy reaching the receiver.
In consequence,
the echo
inverse fourth
power
factors
constant.
being
To
energy
received
of the range from
be detectable,
us call the minimum
a signal
must
detectable
of a radar set on a target
according
to the expression
have
signal
of
is a constant
a given
and Pt
a target
varies
with
minimum
Then
the maximum
range
will
be determined
by
type
=
the
other
a certain
tlti~.
S.n
where K
from
the radar set to the target,
power;
let
Smi.,
g,
is the power
in the transmitted
pulse,
which the received signal power will clearly be proportional.
to
Rearranging,
?'4
R-=
Equation
(2) displays
~
() m.
the difficulty
of increasing
of a radar set by raising its pulse power.
required to double the range.
10,CQO
the range performance
increase in power
A=locm,
A
80
/
:
E
g60
~
100
5
~
/
<10
cm
40
1
g
:
10
/
/
'
1940
20
1944
1941
1942
1943
0
<=3.2
/
/
0
is
100
s
j
(2)
A 16-fold
10?m
3.2
cm
~ 1000 -—+s
.s
5
s
.
'
1940 1941
1945
cm
I
A=l.25cm
1942 1943 1944 1945
Date
Date
FIG.1.5.—Historicaldevelopmentof
microwavemagnetrons.
However
remarkable
formidable
facts
this
of the wartime
requirement
years
appears,
one
most
of development
practicable
pulse powers in the microwave
frequency
range
Me/see
and above) have increased by a factor of hundreds
short time.
This
rapid improvement
Chap.
10.
stupendous
advance
of the multicavity
Figure 1.5 shows the history
respect to pulse power and efficiency,
wave bands
exploited
drawn,
only
and
resulted
magnetron,
during the war.
their
general
trend
from
the
of radar is that
(about
1000
in a relatively
the invention
which
of magnetron
of
and
is described
development,
at the three most important
in
with
micro-
The curves are rather arbitrarily
is significant.
Not
every
upward