Orthomode transducer
An orthomode transducer (OMT) is a waveguide component that is commonly referred to as a polarisation duplexer.
Orthomode transducers serve either to combine or to separate two orthogonally polarized microwave signal paths.
[2] For VSAT modems the transmission and reception paths are at 90° to each other, or in other words, the signals are orthogonally polarized with respect to each other.
Hence this device serves in an essential role as the junction element of the outdoor unit (ODU) of a VSAT modem.
The BUC is also connected to the feed horn through a wave guide port of the OMT junction device.
Orthomode transducers are used in dual-polarized VSATs, in sparsely populated areas, radar antennas, radiometers, and communications links.
When the transmitted and received radio signal to and from the antenna have two different polarizations (horizontal and vertical), they are said to be orthogonal.
The transmission signal of the up-link, of relatively high power (1, 2, or 5 watts for common VSAT equipment) originating from BUC and the very low power received signal power (in the order of μV) coming from the antenna to the LNB receiver unit, in this case are at an angle of 90° relative to each other, are both coupled together at the feed-horn focal-point of the parabolic antenna.
In the VSAT Ku band of operation case, a typical OMT provides a -40 dB isolation between each of the connected radio ports to the feed horn that faces the parabolic dish reflector (-40 dB means that only 0.01% of the transmitter's output power is cross-fed into the receiver's wave guide port).
The 40 dB isolation provides essential protection to the very sensitive receiver amplifier against burn out from the relatively high-power signal of the transmitter unit.
Further isolation may be obtained by means of selective radio frequency filtering to achieve an isolation of -100 dB (-100 dB means that only a 10−10 fraction of the transmitter's output power is cross-fed into the wave guide port of the receiver).
The following images show a Portenseigne & Hirschmann Ku band configuration, that highlights the horizontal the vertical, and circular polarized wave-guide ports that join to the feed-horn, the LNB or BUC elements of an outdoor unit.
An ortho-mode transducer is also a component commonly found on high capacity terrestrial microwave radio links.
, which is symmetrical for a reciprocal OMT (i.e. not including circulators, isolators or active components), thus leaving 10 independent terms for a general lossy device:
Characterization of a manufactured OMT (considered the device under test, DUT) is usually a delicate matter for both mechanical and theoretical reasons.
The ideal OMT splits the two polarizations at the dual-polarized port into two standard single-polarized ports and such arrangement allows the direct measurement of all the scattering parameters of the DUT (either by using a 4-port vector network analyzer (VNA) or a 2-port one with 2 single-polarized loads used in several combinations).
Such ideal setup is only prone to mechanical uncertainties related to the physical placement and alignment of the dual-polarized ports.
If, by proper phase recombination, the two contributions tend to cancel each other, the actual measured XPD can increase to infinity (possible only if
Depending on the expected XPD of the DUT, mechanical countermeasures should be introduced to guarantee that the artificial measurement uncertainty can be neglected.
If a dual-polarization matched load is available in place of the ideal OMT, this allows 2×2 measurements from the single-polarization ports, yielding only 2 of the reflection terms (
Other measurements aimed at gaining estimations of the other scattering parameters of the DUT involve the dual-polarized port and require additional components, such as dual-polarized to single-polarized transitions or tapers, which are often not matched on at least one of the two polarizations: this creates undesired reflections which propagate through the OMT and combine at the VNA ports thus preventing direct measurements.
Due to the increasing demand for high-capacity data links, the exploitation of dual-polarization has fostered research in design and characterization of OMTs to overcome the practical difficulties.
The literature concerning OMT modelling and practical characterization consists of works both by academic organizations such as the National Research Council (Italy),[3] Marche Polytechnic University and European Space Agency[4] and likewise by industrial teams such as CommScope[5] and Siae Microelettronica[6] with immediate impact on products for modern dual-polarized telecommunication systems, for instance in terrestrial microwave backhauling.
