Microwave TEE Junction

Microwave Waveguide Tee junction is used for the purpose of connecting a branch or a section of a waveguide in a series or parallel to the main waveguide. The intersection of waveguides in the shape of the English capital letter “T” is called a T junction. E-plane Tee and H-plane Tee are examples of three-port waveguide  T junctions. Normal reciprocal three-port junctions have one drawback, that is, lack of isolation between the output ports. This results in the dependence of the power consumed at one port on the termination at the other output port. This lack of isolation between the output ports limits the usefulness of the three-port junctions, particularly in power monitoring and divider applications.

E-plane Tee

The E-plane Tee is a voltage or series junction. A sidearm is attached to a waveguide by cutting a rectangular slot along the broader dimension of the waveguide as shown in the figure below.

If the E-plane junction is completely symmetrical and if waves enter through the sidearm. The waves that leave the main arms are equal in magnitude and opposite in phase. Since the electric field lines change their direction when they come out of ports 1 and 2, it is called an E-plane Tee. Any signal that is to be split or any two signals which are to be combined will be fed to the E arm.

As power divider

• If the amplitude of the input wave at port 3 is A, the amplitudes of the waves at ports 1 and 2 are the same and equal to A/\sqrt{2}. They are out of phase when its collinear arms lengths are the same.
• When the power incident at port 3 is P. The powers that appear at ports 1 and 2 are P/2 each.
• i.e.   P₃ = P,    P₂ = P/2,   P₁ = P/2,    P3= P₁+ P₂ = 2P₁ =2 P₂
• The amount of power coming out of ports 1 and 2 in decibels is

=10\log_{10}\frac{P_{1}}{P_{3}}=10\log_{10}\frac{P_{1}}{2P_{1}}10\log_{10}\frac{P_{1}}{2P_{2}}10\log_{10}\frac{1}{2}=-3dB

Why it is called a 3dB splitter?

• As power combinerWhen equal input signals are given at both the collinear ports, the output signal appears at the side arm port whose power is the sum of the powers of the input signals provided the collinear arm lengths are same and the sources are out of phase.
• The output power is zero. When the sources are equal, in the phase and collinear arms lengths are same.

H-plane Tee

H-plane Tee is a current, shunt, or parallel junction. Since the axis of the sidearm is parallel to the plane of the H field of the main waveguide, it is called an H-plane Tee. A rectangular slot is cut along the narrow dimension of a long waveguide, and a sidearm is attached as shown in the figure below.

If the H-plane junction is completely symmetrical and waves enter through the sidearm, the waves that leave the main arms are equal in magnitude and phase.

As power divider

• They are inphase when its collinear arm lengths are same. If the amplitude of the input wave at port 3 is A, the amplitudes of the waves at ports 1and 2are same and equal to A/\sqrt{2}.
• It is called a 3db splitter, because when the power incident at port 3 is P, the powers that appear at ports 1 and 2 are P/2 each.

As power combiner

• When equal input signals are given at both the collinear ports, the output signal appears at the side arm whose power is the sum of the powers of the input signals provided the collinear arm lengths are same and the sources are in phase.
• The output power is zero. When the sources are equal, in the out of phase and collinear arms the lengths are same.

Applications:

Rectangular waveguide Tees are used

• As tuners by placing a short circuit in the symmetrical arm
• As power dividers and adders
• In the duplexer assemblies of radar installations

Magic Tee

The combination of an E-plane Tee and an H-plane Tee is called as Magic Tee. A Magic Tee can be formed by attaching arms to the slots made in the broad and narrow walls of a waveguide. It is also called as a hybrid tee in which the power distributes equally between the output ports. The outputs may have 0° or 180° phase difference. Magic Tee is a 3db hybrid coupler which is also called an anti-symmetric coupler. If one of the coplanar arms is terminated, then the power delivered to another coplanar arm is independent of the terminated port. The hybrid (Magic) Tee Junction is shown in the figure below.

Characteristics of Magic Tee

• Let us consider two waves whose magnitude and phase are equal. If we feed these waves to ports 1 and 2, the outputs at port 3 and port 4 will be zero and additive respectively.
• The power distributes equally at ports 1 and 2 if a wave is incident at port 4 and no power will appear at port 3.
• The power at ports 1 and 2 appears with an equal magnitude and the opposite phase if a wave is incident at port 3 and no power will appear at port 4.
• If a wave is incident on any one of the coplanar arms i.e., port 1 or port 2, then no power will appear at other coplanar arm i.e., port 2 or port 1. This is due to the occurring of phase delay and phase advance in E arm and H arm respectively.

• In Magic Tee, the imaginary plane bisects arms 3 and 4 symmetrically. There will not be any reflections in the junction if ports 1 and 2 are terminated with matched loads.
• Since all the ports are the collinear arm ports in Magic Tee, they are perfectly matched to the junction. The E and H arm ports are decoupled individually.
• The signal distributes equally between the E and H arms, if signal is incident on collinear arm and output signal is given  as P_{out}=P_{in}/2 and A_{out}=A_{in}/\sqrt{2}.
• A signal into the H arm splits equally between the collinear arms. The outputs being in phase, equidistant from the junction.
• A signal into the E arm splits equally between the collinear arms. The outputs being out of phase, equidistant from the junction.
• For signals into both collinear arms.

1. The signal output from the E arm is equal to 1/\sqrt{2}  times the phasor difference of the input signals. (Difference arm)

2. The signal output from the H arm is equal to 1/\sqrt{2}  times the phasor sum of the input signals. (Sum arm)

Advantages of magic tee

1. Due to the decoupling property of output ports, the power delivered to one of the output ports becomes independent of the termination at the other output port.
2. In the E- or H-plane Tee, the power division between ports depends on terminations existing at the respective output ports; but in Magic Tee (in which all the ports are perfectly matched), power division between the ports is independent of terminations.

Disadvantages of magic tee

There is an impedance mismatch at the junctions, when a signal is applied to any arm of the Magic Tee. Because of this impedance mismatch the flow of energy in the output arms is affected by reflections. These reflections cause the following two disadvantages of Magic Tee:

1. When all the energy that is fed into the junction does not reach the load due the reflections, it results in power loss.
2. The standing waves that are produced due to reflections can result in internal arcing. Thus, it results in reduction of the maximum power that a Magic Tee can handle.

Applications of magic tee junction

Depending on the above-explained properties, a magic tee has many applications. It can be used as an isolator, as a matching device, as a phase shifter, as a duplexer, as a mixer.

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