WA VEGUIDE
113)
SEC.
types
of metal
hose or conduit
403
are produced.
A molded rubber sheath
the adjacent turns
small compared
with
pressurizes and protects the piece, as well as holding
in tight contact.
For short lengths, convolutions
the wave-length
can be formed
to a continuous
metal tube.
Transitions
form
between
of a quarter-wave
the waveguide
FIG.
to
Fig.
11 12a.
The
endplate
back
in phase
with
that
the
an open circuit
single unit
transitions.
probe
reflects
going
lowers
the
short-circuited
the
of Fig.
Then
breakdown
against
each
of the waveguide.
much as possible.
Rotary
potential
joint
guide
at
1~-in.
OD
between
of the type
each
line
end.
and
pieces
described
Thk
the
is
direction
line
in terms
of
guide
in series.
guide.
The
of
reduce
It can be
it presents
no loading
the electrical
gradient
may
consist
11.2 with transitions
in
the
10-cm
just
a
Such a
"doorknob"
a special form of stub support
and inductive effect have been
transitions
to
presents
is only
breakdown.
of waveguide
common
doorknob
in that
seen at the probe
as a support
in Sec.
coaxial
Expressed
two units
of the
contours
the
into
transition,
section
to minimize
so that
Rounded
joints
rotary
other
going
guide.
th~ load
rather than
take
as shown in
Probe
quarter-wave
11. 12b is designed
usually
line projecting
transition.
energy
the
thought
of as a quarter-wave
probe with
for the tip of the probe in which capacitive
balanced
lines
(a)
Doorknob
down
at the probe.
give flexibility
from an endplatej
(b)
of guide impedance,
transition
cooxial
on the coaxiai
waveguide
waveguide.
impedances,
and
antenna
guide-wavelength
line
and thereby
11.1 la! shows an example.
waveguide
stub
a quarter
11.12.—Coaxial
hydraulically,
Figure
of a coaxial
to wave-
region.
mentioned
as
The
are used
large
404
R-F COMPONENTS
[SEC.113
where powers of the order of a megawatt
are handled.
In the 3-cm
region transition is made to round guide, where the next-lowest
mode has
axial symmetry.
Currents flow across the junction between the rotating
and nonrotating
parts of the round tube by means of the same folded
choke arrangement
used for the outer conductor
of the coaxial
rotary
joint described in Sec. 11.2.
Figure 11.13 shows a typical
joint.
The
lowest mode in round guide has diametral
rather than axial symmetry,
FIG. 11.13.—Waveguide rotary joint usingroundwaveguideand axially symmetrical
mode.
and if present to an appreciable
extent will cause serious variation
in
voltage
standing-wave
ratio as a function
of angle of rotation.
The
transitions from rectangular to round guide are designed to avoid exciting
the undesired
mode as far as possible.
The guide wavelengths
of the
desired and undesired modes are sufficiently
different so that a suitable
choice of length of round guide will minimize coupling
of the undesired
mode from one rectangular guide to the other.
Various absorbers for the
undesired mode are sometimes used.
The power-handing
ability
of a waveguide, calculated from the electric
fields involved, is approximately
twice that of the largest coaxial line that
SEC. 11.4]
RESONANT
CAVITIES
405
Since, howcould be used for the wavelength
carried by the waveguide.
ever, the distortion of the normal field by the stub supports of the coaxial
line makes it impossible
to realize the calculated
limit, the factor of two
does not represent the full superiority of waveguide.
In either waveguide
or coaxial line, small nicks, burrs, or solder fillets can easily cause breakdown at a fifth of the calculated
maximum
power.
Table
11.1 summarizes the properties of the waveguides widely used in microwave radar,
and for comparison
those
of some standard
coaxial
lines and cable.
Attenuation
in waveguide is seen to be about
half that in the largest
coaxial line suitable for a given wavelength.
For smaller coaxial lines
of
a given
impedance
the
attenuation
is inversely
proportional
t6
diameter.
The conclusion
to be drawn is that waveguides
are superior
electrically
to coaxial line in nearly every respect.
They are easier to
fabricate
because
the inner conductor
and its precisely
machined
stub
supports
are simply
omitted.
For these reasons, waveguide
is almost
universally used for wavelengths below 8~cm.
In the lo-cm region where
the size and weight of the Ii- by 3-in. waveguide
are awkward and the
power-carrying
capacity
is not needed, coaxial
line is frequently
used.
For '(long wave"
radars (wavelengths
of 50 cm and greater) wave guide
is never used because of its relatively enormous size.
11.4. Resonant
Cavities.-If
both
ends of a waveguide
are
closed is introduced
by a short-circuiting
plate, and energy
by a probe so small
that it does not appreciably
change the properties
of the enclosure, the
amplitude
of the standing-wave
pattern
in the waveguide
will show a
sharp maximum
when the frequency
is such that
the length
of the
enclosure is an integral number of half guide-wavelengths.
The reflections will then be in the proper phase to reinforce each other and cause a
resonant
buildup.
(This
is the property
used in wavemeters.)
For
standard rectangular
dimension
is taken
guide, reference to Eq. (4) shows that if the broad
to be 0.707~ then half a guide-wavelength
is also
0.707A.
The shortest resonant
piece of such a waveguide
is therefore
The height does not affect the resonant wavelength,
though if
square.
it is greater than A/2, modes polarized at right angles to the desired mode
become
possible.
Rounding
off the corners of the square box shortens
the resonant
wavelength
slightly;
exact
calculation
shows that
for a
cylindical box the resonance occurs when X = 1.30 times the diameter, as
opposed to 1.41 times the side of the square.
These round and square boxes are examples of resonunt
cam"ties, which
play the same role in microwave
transmission
circuits as do resonant circuits involving
lumped
inductance
and capacity
in traditional
circuit
theory.
Any hollow metal enclosure is capable of supporting oscillations
in a large number of modes.
In practice the geometry is usually chosen
so only a single mode,
often the lowest,
is excited.
For simple
geometn-