of a waveguide system should include a detailed
inspection of all support points to ensure that electro-
lytic corrosion is not taking place. Any waveguide
that is exposed to the weather should be painted and
all joints sealed.
Proper painting prevents natural
corrosion, and sealing the joints prevents moisture from
entering the waveguide.
Moisture can be one of the worst enemies of a
waveguide system.
As previously discussed, the
dielectric in waveguides is air, which is an excellent
dielectric as long as it is free of moisture. Wet air,
however, is a very poor dielectric and can cause serious
internal arcing in a waveguide system. For this reason,
care is taken to ensure that waveguide systems are
pressurized with air that is dry. Checking the pressure
and moisture content of the waveguide air may be one
of your daily system maintenance duties.
More detailed waveguide installation and mainte-
nance information can be found in the technical
manuals that apply to your particular system. Another
good source is the Electronics Installation and
Maintenance Handbooks (EIMB) published by Naval
Sea Systems Command. Installation Standards (EIMB)
Handbook, NAVSEA 0967-LP-000-0110, is the volume
that deals with waveguide installation and maintenance.
WAVEGUIDE DEVICES
The discussion of waveguides, up to this point,
has been concerned only with the transfer of energy
from one point to another. Many waveguide devices
have been developed, however, that modify the energy
in some fashion during the transmission. Some devices
do nothing more than change the direction of the
energy. Others have been designed to change the basic
characteristics or power level of the electromagnetic
energy.
This section will explain the basic operating
principles of some of the more common waveguide
devices, such as DIRECTIONAL COUPLERS,
CAVITY RESONATORS, and HYBRID JUNCTIONS.
Directional Couplers
The directional coupler is a device that provides
a method of sampling energy from within a waveguide
for measurement or use in another circuit. Most
couplers sample energy traveling in one direction only.
However, directional couplers can be constructed that
sample energy in both directions. These are called
BIDIRECTIONAL couplers and are widely used in
radar and communications systems.
Directional couplers may be constructed in many
ways.
The coupler illustrated in figure 3-55 is
constructed from an enclosed waveguide section of
the same dimensions as the waveguide in which the
energy is to be sampled. The “b” wall of this enclosed
section is mounted to the “b” wall of the waveguide
from which the sample will be taken. There are two
holes in the “b” wall between the sections of the
coupler. These two holes are
apart. The upper
section of the directional coupler has a wedge-of
energy-absorbing material at one end and a pickup
probe connected to an output jack at the other end.
The absorbent material absorbs the energy not directed
at the probe and a portion of the overall energy that
enters the section.
Figure 3-55.—Directional coupler.
Figure 3-56 illustrates two portions of the incident
wavefront in a waveguide. The waves travel down
the waveguide in the direction indicated and enter the
coupler section through both holes. Since both portions
of the wave travel the same distance, they are in phase
when they arrive at the pickup probe. Because the
waves are in phase, they add together and provide a
sample of the energy traveling down the waveguide.
The sample taken is only a small portion of the energy
that is traveling down the waveguide. The magnitude
of the sample, however, is proportional to the
magnitude of the energy in the waveguide. The
absorbent material is designed to ensure that the ratio
3-25
