Figure 3-66.—Magic-T with input to arm d.
When an input signal is fed into the a arm as
shown in figure 3-67, a portion of the energy is
coupled into the b arm as it would be in an E-type
T junction. An equal portion of the signal is
coupled through the d arm because of the action of
the H-type junction. The c arm has two fields
across it that are out of phase with each other.
Therefore, the fields cancel, resulting in no output
at the c arm. The reverse of this action takes place
if a signal is fed into the c arm, resulting in
outputs at the b and d arms and no output at the a
arm.
Figure 3-67.—Magic-T with input to arm a.
Unfortunately, when a signal is applied to any
arm of a magic-T, the flow of energy in the output
arms is affected by reflections. Reflections are
caused by impedance mismatching at the
junctions. These reflections are the cause of the
two major disadvantages of the magic-T. First, the
reflections represent a power loss since all the
energy fed into the junction does not reach the
load that the arms feed. Second, the reflections
produce standing waves that can result in internal
arcing. Thus, the maximum power a magic-T can
handle is greatly reduced.
Reflections can be reduced by using some
means of impedance matching that does not
destroy the shape of the junctions. One method is
shown in figure 3-68. A post is used to match the
H plane, and an iris is used to match the E plane.
Even though this method reduces reflections, it
lowers the power-handling capability even further.
Figure 3-68.—Magic-T impedance matching.
HYBRID RING.— A type of hybrid junction
that overcomes the power limitation of the magic-
T is the hybrid ring, also called a RAT RACE. The
hybrid ring, illustrated in figure 3-69, view A, is
actually a modification of the magic-T. It is
constructed of rectangular waveguides molded
into a circular pattern. The arms are joined to the
circular waveguide to form E-type T junctions.
View B shows, in wavelengths, the dimensions
required for a hybrid ring to operate properly.
The hybrid ring is used primarily in high-
powered radar and communications systems to
perform two functions. During the transmit
period, the hybrid ring couples microwave energy
from the transmitter to the antenna and allows no
energy to reach the receiver. During the receive
cycle, the hybrid ring couples energy from the
antenna to the receiver and allows no energy to
reach the transmitter. Any device that performs
both of these functions is called a DUPLEXER. A
duplexer permits a system to use the same
antenna for both transmitting and receiving.
SUMMARY
This concludes our discussion on transmission
lines and waveguides. In this volume you have
been given a basic introduction on wave
propagation from the time it leaves the
transmitter to the point of reception. In volume 8
you will be introduced to a variety of electronic
support systems.
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