Orthomode Transducer at 43GHz

Transcription

1 Electromagnetic Design Orthomode Transducer at 43GHz Author: Renzo Nesti The electromagnetic (EM) design of the Orthomode transducer is based on a symmetric structure to prevent higher order mode excitation. The polarization splitting junction has no septum or pins which are difficult to be fabricated at these frequencies. The vertical (V) polarization comes out basically undisturbed from the ThroughPort (TP), which is on axis with the common port (CP), while the horizontal (H) polarization is equally (-3dB) backward coupled into two side waveguides by means of 2x4 thick windows and recombined in the output SidePort (SP).The EM model is given in Fig. 1. Fig. 1 : EM model of the 43GHz OMT. Common port (1=H, 2=V) and transduced port (SP e TP) Requirements are as follows: o Bandwidth: 36-50GHz; o Return loss (V, H): >20dB; o Isolation:>35dB; Another model has been studied in order to optimize mechanical criticalities. In particular: o the corners in the coupling junction have been smoothed. o The groove in the recombination junction for the H polarization has been made smaller. These modifications led to the model in Fig. 2, which is the final one.

2 Fig. 2 : Final 43GHz OMT model. The expected input matching performances are given for the two models in Fig. 3. It has to be noticed that the behavior of the two models is basically the same, although the curves in the plots are slightly different. Return loss better than 20dB is expected in the whole 36-50GHz band. Fig. 3 : Input matching comparison between the two 43GHz OMT models. TP (left); SP (right). Fabrication In the following the fabrication process of the 43GHz OMT model n. 2 is briefly described. The electroforming technique is adopted. An aluminum mandrel has been machined, electroformed and subsequently dissolved so that the resulting product can be re-machined to allow the soldering of wellaligned flanges.

3 The drawing of the basic mandrel is given in Fig. 4. This drawing has been optimized in order to better obtain several parts, shown in Fig. 5, to be assembled. Some parts have been fabricated in standard machining, the others by electroerosion. Fig. 4 : 3D view of the OMT mandrel to be electroformed. Fig. 5 : Mandrel parts to be assembled and electroformed. The mandrel parts have been assembled and electroformed. After this phase we have both the mandrel preserved and the electroformed material grown on it (Fig. 6). The mandrel has been then chemically removed and the port regions were machined to mount the flanges (Fig. 7).

4 Fig. 6 : OMT after the electroforming process with the mandrel still inside. Fig. 7 : OMT after dissolving the mandrel and further machining.

5 The flanges have been finally soldered to have the end product in Fig. 8. EM Test Fig. 8 : Final OMT. EM test have been done by using a Vector Network Analyzer with a suitable calibration kit (Fig. 9). Fig. 9 : OMT and test setup parts. Results are given in the following plots, showing very good results and agreement with expected performances.

6 Fig. 10 : Input matching: TP (left) and SP (right). Coupling measurements between TP and SP (Fig. 11) shows an isolation better than 40dB. Fig. 11 : Coupling between TP and SP port. Furthermore the OMT shows 0.15dB and 0.25dB loss in the path, respectively, TP<->CP and SP<->CP.