Waveguide Junction

- details of RF waveguide junctions including E-Type, H-Type and Magic T waveguide junction types.

Waveguide junctions are used when power in a waveguide needs to be split or some extracted. There are a number of different types of waveguide junction that can be use, each type having different properties - the different types of waveguide junction affect the energy contained within the waveguide in different ways.

When selecting a waveguide junction balances between performance and cost need to be made and therefore an understanding of the different types of waveguide junction is usedful.


Waveguide junction types

There are a number of different types of waveguide junction. The major types are listed below:

  • H-type T Junction:  This type of waveguide junction gains its name because top of the "T" in the T junction is parallel to the plane of the magnetic field, H lines in the waveguide.
  • E-Type T Junction:   This form of waveguide junction gains its name as an E- type T junction because the tope of the "T" extends from the main waveguide in the same plane as the electric field in the waveguide.
  • Magic T waveguide junction:   The magic T waveguide junction is effectively a combination of the E-type and H-type waveguide junctions.
  • Hybrid Ring Waveguide Junction:   This form of waveguide junction is another form of waveguide junction that is more complicated than either the basic E-type or H-type waveguide junction.

E-type waveguide junction

It is called an E-type T junction because the junction arm, i.e. the top of the "T" extends from the main waveguide in the same direction as the E field. It is characterized by the fact that the outputs of this form of waveguide junction are 180° out of phase with each other.

Waveguide E-type junction
Waveguide E-type junction

The basic construction of the waveguide junction shows the three port waveguide device. Although it may be assumed that the input is the single port and the two outputs are those on the top section of the "T", actually any port can be used as the input, the other two being outputs.

To see how the waveguide junction operates, and how the 180° phase shift occurs, it is necessary to look at the electric field. The magnetic field is omitted from the diagram for simplicity.

Waveguide E-type junction E fields
Waveguide E-type junction E fields

It can be seen from the electric field that when it approaches the T junction itself, the electric field lines become distorted and bend. They split so that the "positive" end of the line remains with the top side of the right hand section in the diagram, but the "negative" end of the field lines remain with the top side of the left hand section. In this way the signals appearing at either section of the "T" are out of phase.

These phase relationships are preserved if signals enter from either of the other ports.


H-type waveguide junction

This type of waveguide junction is called an H-type T junction because the long axis of the main top of the "T" arm is parallel to the plane of the magnetic lines of force in the waveguide. It is characterized by the fact that the two outputs from the top of the "T" section in the waveguide are in phase with each other.

Waveguide H-type junction
Waveguide H-type junction

To see how the waveguide junction operates, the diagram below shows the electric field lines. Like the previous diagram, only the electric field lines are shown. The electric field lines are shown using the traditional notation - a cross indicates a line coming out of the screen, whereas a dot indicates an electric field line going into the screen.

Waveguide H-type junction fields
Waveguide H-type junction electric fields

It can be seen from the diagram that the signals at all ports are in phase. Although it is easiest to consider signals entering from the lower section of the "T", any port can actually be used - the phase relationships are preserved whatever entry port is ised.


Magic T hybrid waveguide junction

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.

Magic T waveguide junction
Magic T waveguide junction

The diagram above depicts a simplified version of the Magic T waveguide junction with its four ports.

To look at the operation of the Magic T waveguide junction, take the example of whan a signal is applied into the "E plane" arm. It will divide into two out of phase components as it passes into the leg consisting of the "a" and "b" arms. However no signal will enter the "E plane" arm as a result of the fact that a zero potential exists there - this occurs because of the conditions needed to create the signals in the "a" and "b" arms. In this way, when a signal is applied to the H plane arm, no signal appears at the "E plane" arm and the two signals appearing at the "a" and "b" arms are 180° out of phase with each other.

Magic T waveguide junction signal directions
Magic T waveguide junction signal directions

When a signal enters the "a" or "b" arm of the magic t waveguide junction, then a signal appears at the E and H plane ports but not at the other "b" or "a" arm as shown.

One of the disadvantages of the Magic-T waveguide junction are that reflections arise from the impedance mismatches that naturally occur within it. These reflections not only give rise to power loss, but at the voltage peak points they can give rise to arcing when sued with high power transmitters. The reflections can be reduced by using matching techniques. Normally posts or screws are used within the E-plane and H-plane ports. While these solutions improve the impedance matches and hence the reflections, they still reduce the power handling capacity.


Hybrid ring waveguide junction

This form of waveguide junction overcomes the power limitation of the magic-T waveguide junction.

A hybrid ring waveguide junction is a further development of the magic T. It is constructed from a circular ring of rectangular waveguide - a bit like an annulus. The ports are then joined to the annulus at the required points. Again, if signal enters one port, it does not appear at allt he others.

The hybrid ring is used primarily in high-power radar and communications systems where it acts as a duplexer - allowing the same antenna to be used for transmit and receive functions.

During the transmit period, the hybrid ring waveguide junction couples microwave energy from the transmitter to the antenna while blocking energy from the receiver input. Then as the receive cycle starts, the hybrid ring waveguide junction couples energy from the antenna to the receiver. During this period it prevents energy from reaching the transmitter.

Waveguide junctions are an essential element within waveguide technology. Enabling signals to be combined and split, they find applications in many areas as discussed in the text. The waveguide T junctions are the simplest, and possibly the most widely used, although the magic-T and hybrid ring versions of the waveguide junction are used in particular applications where their attributes are required.

By Ian Poole


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