Download CHAPTER 11 R-F COMPONENTS BY A. E. WHITFORD 11.1. The

CHAPTER
11.1.
The
R-f
11
R-F
COMPONENTS
BY
A.
Transmission
E.
WHITFORD
the
Problem.—In
block
diagram
of
a
basic radar system shown in Fig. 14, the parts shown as heavy double
lines transmit the radio-frequency
(r-f) energy from the magnetron to the
antenna,
and
receiving
book
carry
the faint
apparatus
this
function
considerable
around
echo
is located.
is
body
performed
of theory
this class of transmission
circuit
Although
elements;
space
duction
terminals
lines
The
function
good
where
k
between
conductors
is
here,l
A
uniform
behaves
as
introduced
-...
(1) l?llly
Micrwave
Circuits,
the
at
losses
1 Xfl-....)1",c
line
if
Wavegwid.
the
in
r,
of
were
input
the
introradar.
to
the
line
the
known
impedance,
input
as its
ZO, is
a
of the line, and
is almost
purely
inner
at
material
radius
resistive.
in
the
of
the
outer
its
characteristic
annular
space
and
conductor,
conductor.
any
point
infinitely
long;
in
there
disappears
any
is
into
no
the
discontinuous
impedance
reflection.
Power
termination
change
with
along
the
-.l
:_.Ill"1,,,~~""KSUlLILISSSlltX)~h~CL_.
L-- L__,..
.L:.
--:.-.. -;.
11..
--L.lJ~f...
b Uii,,
,UU,
,U
vol.
Vol.
of
the
inner
Transmission
Handbook,
applied
and insulators
of
is the
However,
--2.2
.-... l,,edLII,
—.-.
CALC,,Utl,
e,,
wavelengths.
sufficient
line, 'the current which flows
dielectrics
terminals
line.
is,
size of
losses,
the
terminated
line
A
grown up
in microwave
characteristic
constant
dielectric
radius
practices
property
The
low-loss
conductors,
outside
waveguides.
have
and to make under-
is suddenly
of the conductors
and
the
in this
from the far end arrive to confuse
the
line, neglecting
the
the
the
by
impedance. "
For a concentric
and
be many
treatment
transmission
before reflections
of the geometry
where
treated
essential new feature
of the line may
a limited
a voltage
is determined
"characteristic
(3)
circuits.
of a long and uniform
situation,
small
only
Waves.—When
in the initial interval,
TI is
coaxial
some of the reasons for current
Standing
for
by
will be given to show the general approach,
standable
the
the T-branch
of radar
is of the same order as the physical
the length
permits
into
the types
and a new set of techniques
of course, that the wavelength
the
signals
For
10; (4)
Principles
391
9;
(z)
of
Microwave
Microwave
~01.
hpk%rs,
Circuik?,
Vol.
8.
14;
RF
392
[SEC.11.1
COMPONENTS
line, such as might be introduced
by a change in dimension of one of the
conductors,
or any change in geometry introduced
by a sharp bend, or
by
a dent
or
an
obstacle
in
the
line,
will
produce
reflected energy travels back toward the source.
waves in the line.
These can be observed
a reflection.
The
This results in standing
by sliding a small probe along
a slot in the line after the manner shown for a concentric line in Fig. 11.1.
Only
a negligible
fraction
of
power
but,
is abstracted
fed
into
by
the
a suitable
the
probe,
indicator,
this M sufficient to regw,ter the voltw~o*~
age;ariati;nsalong~heline.
~~.
11"1.—slot~d VWfi~ ~ne "~
In
general there will be voltage maxima
for observing utandmgwaves (cutaway view).
probe
spaced
at
half-wavelength
intervals
with minima halfway bet ween them.
Only if the line is perfectly
matched will the voltage reading be constant
the probe moves along.
The ratio of the maximum
age is called the "voltage
criterion
of the
standing-wave
of how well a line is matched.
line,
circuit,
such as would
results
infinity.
in
zero
at
property
reflection
an open
half-wave
between
PSWR
SWR,
db
and Half-wave
Quarter-wave
from
volt-
and is the usual
at the end
circuit
intervals
or a short
and
a VSWR
of
may also be expressed as a power ratio (PSWR),
The relations
less transmission
long is terminated
(VSWR)
Complete
be expected
voltage
The mismatch
or in decibels.
This
ratio"
as
to the minimum
these three measures are
=
(VSWR)Z
=
10 log,,
(PSWR).
Lines.—It
can be shown that when a loss-
line of characteristic
impedance
in an impedance
2,, the input
is widely
wavelength
(1)
Zi =
Z~/Zl.
For
used.
Z, a quarter
impedance is
example,
two
transmission
lines
of
differing impedance can be matched to each other by joining them through
a quarter-wave
line whose characteristic
impedance
is the geometric
mean
Or,
of that
if
of the two
a quarter-wave
impedance
is infinite,
an open-circuited
a short circuit.
lines.
line
i.e.,
This
equivalent
quarter-wave
to
of the
has many
(Sec. 118).
uses
by
an
a matching
a short
open
circuit.
line appears
at the input
a wavelength
characteristic
also,
particularly
=
impedance
in
transformer.
circuit,
For a lossless line half
2,
irrespective
is called
is terminated
the
input
Conversely,
terminal
to be
long
2,,
(2)
of the line.
duplexers
(Sec.
This
11.5)
and
principle
mixers
SEC.11.2]
COAXIAL
LINES
Why
a Matched Line?—The
fraction
to the source from a section of transmission
Power
reflection
coefficient
of the incident
systems
or unpressurized
is a possibility,
[(VSWR)
[(VSWR)
=
airborne
+
reflected
is
1]'
1]2
(3)
1.5, it is seen that the power
surely not serious.
In high-
systems,
where line breakdown
the strain is of course higher for a high VSWR.
of 1.5 means that for a given breakdown
gradient,
can be delivered
be delivered
This
power
line of given VSWR
For the usual upper design limit of VSWR
=
reflection loss is only 4 per cent, or 0.3 db,
power
393
to the load than
could
A ratio
33 per cent less power
in a matched
load.
can be a limitation.
The
strongest
properties
netron
load
requirement
for
of the magnetron.
exhibits
into
lower than
mum.
an output
which
it
a well-matched
Like
frequency
works.
20 at voltage
mismatched
minimum,
or negative.
component
in Chap.
from
the
the mag-
dependent
upon
represents
a resistance
line
and higher than
As explained
arises
oscillators,
and a stability
A
At other phases it has a reactive
positive
line
all self-excited
the
ZO at voltage
maxi-
which maybe
either
10, magnetrons
are in gen-
eral designed to be stable against a VS WR of 1.5 in any phase.
the origin of the commonly
specified upper limit for mismatch.
There
is a further
long line-for
as the frequency
and
limitation
example
so also
changes the number
does the phase
seen at the magnetron
magnetron
of the
of wavelengths
of the standing
the phase happens
line with
frequency
itself, and of opposite
netron
has no stable
netron
is
This
wave.
The
function
known
Then
line impedance
of frequency.
to be such that the variation
is more
In another,
is
at the end of a
in the line changes,
sign, a condition
frequency.
stabilized.
occurs
from the magnetron.
is therefore a rapidly varying
If at a certain frequency
of reactance
if the mismatch
50 to 100 wavelengths
This
as
rapid
than
that
of the
results where the magfavorable,
the
phase
"long
line
the mageffect.'
The
result is that for long lines either (1) VSWR'S lower than 1.5 are necessary
to
guarantee
stable
the effective
be
included
stretcher"
in the
not unlike
or a dielectric
outer
usually
,I
Micrmuaw
antenna
line.
of
or
a trombone,
The
are
These
cables
not
with
Ma#ndrorz.s,
aMicrowave Transrntision Circuits, Vol.
(2)
a method
latter
may
be
new.
consisting
At
a solid
Vol.
6,
9, Radiation
lower
dielectric
wave
done
must
by
a squeeze
add an undesirable
lines
of changing
of the standing
or in wave guide by
Lines.-Coaxial
conductors
consisted
operation,
and hence the phase
phase shifter.z
1102. Coaxisl
and
magnetron
line length
a
"line
section
adjustment.
of
concentric
frequencies
and
inner
they
have
a braided
outer
Radiation
Laboratory
Laboratory
Series.
Series.
is