Waveguide Junctions

You may have assumed that when energy traveling down a transmission line and reaches waveguide junctions, it simply divides and follows the whatever route happens to be available. This is not strictly true. Different types of waveguide junctions affect the energy in different ways. Since they are used extensively in most systems, you need to understand the basic operating principles of those most commonly used.

The T JUNCTION is the most simple of the commonly used type of waveguide-junction. T junctions are divided into two basic types, the E-TYPE and the H-TYPE. HYBRID JUNCTIONS are more complicated developments of the basic T waveguide junctions. The MAGIC-T and the HYBRID RING are the two most commonly used hybrid junctions.

E-TYPE T JUNCTION

An E-type T waveguide junctions are illustrated in the figure below, view (A). They are called E-type T waveguide junctions because the junction arm extends from the main waveguide in the same direction as the E field in the waveguide.

View (B), illustrates cross-sectional views of the E-type T waveguide junctions with inputs fed into the various arms. For simplicity, the magnetic lines that are always present with an electric field have been omitted. In view (K), the input is fed into arm b and the outputs are taken from the a and c arms. When the E field arrives between points 1 and 2, point 1 becomes positive and point 2 becomes negative. The positive charge at point 1 then induces a negative charge on the wall at point 3.

The negative charge at point 2 induces a positive charge at point 4. These charges cause the fields to form 180 degrees out of phase in the main waveguide; therefore, the outputs will be 180 degrees out of phase with each other. In view (L), two in-phase inputs of equal amplitude are fed into the a and c arms. The signals at points 1 and 2 have the same phase and amplitude. No difference of potential exists across the entrance to the b arm, and no energy will be coupled out.

However, when the two signals fed into the a and c arms are 180 degrees out of phase, as shown in view (M), points 1 and 2 have a difference of potential. This difference of potential induces an E field from point 1 to point 2 in the b arm, and energy is coupled out of this arm. Views (N) and (P) illustrate two methods of obtaining two outputs with only one input.

E fields in E type T waveguide junctions.

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H-TYPE T WAVEGUIDE JUNCTIONS

H-type T waveguide junctions are illustrated in view (A) of the figure below. It is called an H-type T junction because the long axis of the "b" arm is parallel to the plane of the magnetic lines of force in the waveguide. Again, for simplicity, only the E lines are shown in this figure. Each X indicates an E line moving away from the observer. Each dot indicates an E line is moving toward the observer.

In view (1) of the figure below (B), the signal is fed into arm b and in-phase outputs are obtained from the a and c arms. In view (2), in-phase signals are fed into arms a and c and the output signal is obtained from the b arm because the fields add at the junction and induce E lines into the b arm. If 180-degree-out-of-phase signals are fed into arms a and c, as shown in view (3), no output is obtained from the b arm because the opposing fields cancel at the junction. If a signal is fed into the a arm, as shown in view (4), outputs will be obtained from the b and c arms. The reverse is also true. If a signal is fed into the c arm, outputs will be obtained from the a and b arms.

E fields in an H-type junction. H-TYPE T WAVEGUIDE JUNCTIONS.

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E fields in an H-type junction. FIELDS FOR VARIOUS INPUTS.

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MAGIC-T HYBRID WAVEGUIDE JUNCTIONS

A simplified version of the magic-T hybrid junction is shown in the figure below. The magic-T is a combination of the H-type and E-type T junctions. The most common application of this type of junction is as the mixer section for microwave radar receivers. Its operation as a mixer will be discussed in later NEETS modules. At present, only the fields within the magic-T junction will be discussed.

Magic T hybrid WAVEGUIDE JUNCTIONS.

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If a signal is fed into the b arm of the magic- T, it will divide into two out-of-phase components. As shown in the figure below view A, these two components will move into the a and c arms. The signal entering the b arm will not enter the d arm because of the zero potential existing at the entrance of the d arm. The potential must be zero at this point to satisfy the boundary conditions of the b arm.

This absence of potential is illustrated in views B and C where the magnitude of the E field in the b arm is indicated by the length of the arrows. Since the E lines are at maximum in the center of the b arm and minimum at the edge where the d arm entrance is located, no potential difference exists across the mouth of the d arm.

Magic T with input to arm B.

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Magic T with input to arm B.

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Magic T with input to arm B.

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In summary, when an input is applied to arm b of the magic-T hybrid junction, the output signals from arms a and c are 180 degrees out of phase with each other, and no output occurs at the d arm.

The action that occurs when a signal is fed into the d arm of the magic-T is illustrated in the figure below. As with the H-type T junction, the signal entering the d arm divides and moves down the a and c arms as outputs which are in phase with each other and with the input. The shape of the E fields in motion is shown by the numbered curved slices.

As the E field moves down the d arm, points 2 and 3 are at an equal potential. The energy divides equally into arms a and c, and the E fields in both arms become identical in shape. Since the potentials on both sides of the b arm are equal, no potential difference exists at the entrance to the b arm, resulting in no output.

Magic T with input to arm D.

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When an input signal is fed into the a arm as shown in the next figure below, 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.

Magic T with input to arm A.

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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 which the arms feed. Second, the reflections produce standing waves that can result in internal arching. 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 the next figure below. 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.

Magic T impedance matching.

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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 the figure below 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

Hybrid ring with wavelength measurements.

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Hybrid ring with wavelength measurements.

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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. Since the only common application of the hybrid ring is as a duplexer, the details of hybrid ring operation will be explained in later NEETS modules concerning duplexers.

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