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Navigation:: Home >> Antennas and propagation >> this page

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

This waveguide tutorial is split:

[1] Waveguide basics
[2] Waveguide theory
[3] Waveguide impedance
[4] Flexible waveguide
[5] Waveguide couplers and transitions
[6] Waveguide dimensions and sizes
[7] Waveguide flanges
[8] Waveguide junctions
[9] Waveguide directional coupler
[10] Waveguide bends
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

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
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

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 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

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

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.

Summary
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.
Further pages from this tutorial
Page [ 1 ] >> [ 2 ] >> [ 3 ] >> [ 4 ] >> [ 5 ] >> [ 6 ] >> [ 7 ] >> [ 8 ] >> [ 9 ] >> [ 10 ] >>

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