Orthomode transducer

An orthomode transducer (OMT), sometimes also called an orthomode coupler , is a passive component in microwave technology . It splits circularly polarized waves or joins orthogonally polarized waves together. The main areas of application are simple VSAT antenna systems. There it takes on the task of a diplexer or circulator if the received and transmitted signals are orthogonally polarized, and conducts both signals together via an antenna. The crosstalk attenuation between the received and transmitted signals is typically better than 40 dB.

Orthomode transducer as a waveguide section

Satellite radio antenna with a grooved horn radiator and an orthomode transducer

The term is made up of the following parts:

orthogonal : perpendicular ( in the sense of right-angled),
Mode : Form of the wave, see also modes in rectangular waveguides ,
Transducer: English for converter or emitter.

System description of the OMT

The Orthomode Transducer (OMT) for separating or merging orthogonally linearly polarized electromagnetic waves. The OMT forms the last stage in an antenna system before the actual antenna.

Four-door representation

From an electrical point of view, the OMT is a four-port . The physical structure, however, corresponds to that of a three-port, since the physically common port combines the two electrically independent polarization directions.

Gate1 is the "common gate". From an electrical point of view, it is composed of Tor1a for the vertically polarized wave ( -) and Tor1b for the horizontally polarized wave ( -). The gate into which the vertically polarized wave is coupled or decoupled is called Gate2 or “passage gate”. The gate into which the horizontally polarized wave is coupled or decoupled is called gate3 or "side gate". This naming anticipates the choice of the basic design for the OMT. ${\ displaystyle H_ {10}}$ ${\ displaystyle H_ {01}}$

Scatter matrix

The scatter matrix of an ideal OMT describes the desired system behavior.

{\ displaystyle S_ {OMT} = {\ begin {pmatrix} 0 & 0 & e ^ {j \ phi _ {1}} & 0 \\ 0 & 0 & 0 & e ^ {j \ phi _ {2}} \\ e ^ {j \ phi _ {1 }} & 0 & 0 & 0 \\ 0 & e ^ {j \ phi _ {2}} & 0 & 0 \ end {pmatrix}}}

The following properties of the system behavior result according to the scatter matrix.

The ideal OMT is a lossless system.
The OMT is a passive and thus reversible ( reciprocal ) system. This means that an OMT can be used both in the transmission branch of an antenna system and in the reception branch of an antenna system.
The ideal OMT is adapted to all gates . This property of the ideal OMT results in an important requirement for the design of a real OMT. In the event of a poor adjustment of the gates beyond the bandwidth considered, a large part of the energy is reflected into the coupling gate . This reflected energy is referred to as return loss and is given in dB as the ratio to the coupled energy.
Tor1a and Tor3 are decoupled as well as Tor1b and Tor2 and Tor2 and Tor3. This reflects the actual functionality of the OMT as a polarization switch. This property must also simulate the design of a real OMT as well as possible.
The OMT is a frequency-dependent system. Therefore all elements of the scattering matrix are functions of the frequency of the electromagnetic wave under consideration.
The system behavior of the OMT is frequency dependent. Therefore, the specification of the required frequency range is elementary for the description of the OMT.

Geometry of an OMT

General

The core of the simple general basic design is a common branching area in which the two directions of polarization are separated or brought together.

In addition, each gate has an adaptation network that fulfills two tasks. On the one hand, this ensures good broadband adaptation of the gate. On the other hand, the transition to the given geometrical interfaces (following waveguide cross section) is created. The figure shows the schematic structure of an OMT. The matching networks consist of waveguide transitions.

Simple basic geometries

As the name of the gates suggests, the original geometry of the OMT corresponds to a T-piece . These basic geometries are suitable for a relative bandwidth of 10% when adjusted to all gates of -25 dB. The relative bandwidth is the ratio of the absolute bandwidth to the center frequency .

In the simplest case, an OMT consists of a continuous waveguide section and a side gate placed on the side at a right angle . The branching area is constructed in such a way that the vertically polarized wave can pass it as unhindered as possible. The horizontally polarized wave, on the other hand, is reflected and decoupled into the side gate. A distinction is made here between the method by which the horizontally polarized wave is reflected.

Taper or branching

The first variant is the taper or branching OMT. Here the waveguide cross-section of the continuous waveguide section is reduced in height so that the horizontally polarized wave is no longer capable of propagation because it falls below the lower limit frequency. For the vertically polarized wave, the branching area basically only represents a transition from the cross section of the waveguide at port1 to that of the waveguide at port2. This transition can be stepped or continuous.

Septum or branching

In the second variant, the septum or branching OMT, the reflection of the horizontally polarized wave is achieved by a septum in the branching area. A septum is a thin metal plate, which in this case is placed horizontally in the middle of the common waveguide.

Ideally, the septum has no influence on the vertically polarized wave, the electric field components of which are perpendicular to the metal plate. The electric field components of the horizontally polarized wave are tangential to the plane of the septum. The septum thus represents a short circuit for the horizontally polarized wave. The wave is reflected and coupled out into the side gate.

Both variants have in common that the reflection of the horizontally polarized wave is frequency-dependent. In principle, the second variant is more suitable for broadband applications, since the septum, and thus the reflection point, act directly at the site of the side coupling.

Symmetrical geometries

In the branching area of an OMT, in which the two differently polarized waves are separated or brought together, many higher modes are excited. To reduce this, structures are being developed that are symmetrical for both polarization directions. However, these structures are very complex and therefore associated with a very large development effort and high production costs.

Although monolithic production is very expensive, it has the advantage that the fabricated structure does not have any seams or gaps. This is the only way to achieve optimal behavior with a view to the effects of passive intermodulation .

literature

N. Marcuvitz: Waveguide Handbook. Dover Publications, Inc., New York, 1965
J. Uhler, J. Bornemann, J., U. Rosenberg: Waveguids Components for Antenna Feed Systems. Theory and CAD. Artech House, Inc., Norwood, 1993, ISBN 0-89006-582-9
J.-K. Neske: Homogeneous Waveguides (continued). Lessons for distance learning at university. 1st edition, 2nd edition, Dresden, 1985
M. Kummer: Fundamentals of microwave technology . 1st edition, VEB Verlag Technik, Berlin, 1986. ISBN 3-341-00088-7
G. Lehner: Electromagnetic field theory for engineers and physicists. 3rd edition, Springer-Verlag, Berlin / Heidelberg, 1996. ISBN 3-540-60373-5
AJ Schwab: Conceptual world of field theory. 6th edition, Springer-Verlag, Berlin / Heidelberg, 2002. ISBN 3-540-42018-5
AM Bøifot, u. a .: Simple and broadband orthomode transducers. IEEE proceedings, Vol. 137, Pt. H, No. December 6, 1990
KJ Greene, GL James: An extended bandwidth feed sub-system for earth station applications. Chapter 2.1 Orthogonal Mode Transducers.