Abstract
We report on advanced techniques for the accurate characterization of millimeter-wave Orthomode Transducers (OMTs) enabling the derivation of the fundamental parameters of such devices, i.e., the insertion loss, the return loss, the cross-polarization, and the isolation. These techniques include standard frequency-domain and time-domain VNA (Vector Network Analyzer) measurement methods, which can be applied to remove the effects of the waveguide transitions necessary to access the OMT ports and excite the desired modes. After reviewing the definition of the OMT parameters, we discuss the test equipment, the VNA calibration procedures as well as the VNA time-domain time-gating method for application in OMT characterization. We present simplified equations that relate the calibrated VNA measured quantities with the OMT S-parameters, illustrate various characterization methods, and examine various OMT experimental test setups. The advantages and disadvantages of each of the OMT characterization procedures are presented and compared among them. We provide a list of waveguide components required in the OMT test setups (adapters, loads, quarter-wave and longer waveguide sections, feed-horn, etc.), discuss the error terms introduced by such components and examine their impact on the measured values. Furthermore, we identify strategies to mitigate or remove the effects of the measured errors, to derive the desired OMT parameters. Different OMT configurations, with a distinct orientation of the waveguide input and outputs, are discussed. Although the presented techniques refer to the characterization of a specific configuration of a W-band OMT, the described methods can be applied to other OMT configurations and frequency ranges (from microwave to THz frequencies), therefore having a general validity.
Highlights
An Orthomode Transducer (OMT) is a passive device operating in a dual mode [1]: in receiver mode, it separates two orthogonal linearly polarized signals at its common input port to two independent physical ports at its output; in transmitter mode, it combines two independent signals at its two input ports into two separate polarizations at the common output port
We describe most of the OMT characterization technspiqeuctersaTlthhraaentgsceca.hnebme uasteicd diniapgraractmiceo, afnadnfOocMusTonasapapfliocuatrio-enlseicntrthiceaml-iplloimrtetdere-vwiacveeis show 1
We note that all Sij elements of the OMT S-matrix are used in the definitions in Equations (3)–(12), except for elements S12 and S21, which represent the coupling between the VP signal at Port 1 and the HP signal at Port 2, i.e., the cross-reflection at the input port
Summary
An Orthomode Transducer (OMT) is a passive device operating in a dual mode [1]: in receiver mode, it separates two orthogonal linearly polarized signals at its common input port to two independent physical ports at its output; in transmitter mode, it combines two independent signals at its two input ports into two separate polarizations at the common output port. We note that all Sij elements of the OMT S-matrix are used in the definitions in Equations (3)–(12), except for elements S12 and S21, which represent the coupling between the VP signal at Port 1 and the HP signal at Port 2, i.e., the cross-reflection at the input port (or common port isolation). The amplitude of these matrix elements should be zero in an ideal OMT. The electromagnetic simulators can be set to model the finite mechanical misalignment and tolerance accuracy of real OMTs, in which case they correctly predict their finite cross-polarization and isolation levels
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