Development of a Smooth Taper Double-Ridge Waveguide Orthomode Transducer for a New 100 GHz Band Z-Machine Receiver for the NRO 45-m Radio Telescope

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1 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 125: , 2013 February The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. Development of a Smooth Taper Double-Ridge Waveguide Orthomode Transducer for a New 100 GHz Band Z-Machine Receiver for the NRO 45-m Radio Telescope SHIN ICHIRO ASAYAMA 1 NAOJ Chile Observatory, National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo , Japan; shinichiro.asayama@nao.ac.jp AND TAKU NAKAJIMA Solar-Terrestrial Environment Laboratory, Nagoya University Furo-cho, Chikusa-ku, Nagoya, Aichi , Japan Received 2012 November 26; accepted 2013 January 10; published 2013 January 28 ABSTRACT. A smooth taper double-ridge waveguide orthomode transducer (OMT) has been designed for the new 100 GHz band receiver system on the Nobeyama Radio Observatory (NRO) 45-m radio telescope. The OMT consists of a smooth taper double-ridge waveguide followed by a Bøifot type junction with a main arm and two side arms. The main arm output is a smoothed tapered transformer followed by an E-plane bend and an oval waveguide. This design facilitates the fabrication of the OMT using split blocks machined on a CNC (computer numerical control) machine. The OMT shows a return loss of better than 18 db, a polarization isolation of better than 28 db, and an insertion loss of less than 0.5 db across the GHz, and also demonstrated the excellent performance on the NRO 45-m radio telescope. 1. INTRODUCTION A low-noise wide bandwidth detector and a spectrometer are key instruments for millimeter wave radio telescopes to unravel the redshift distribution and physical nature of submillimeter galaxies (SMGs) (Hatsukade et al. 2011). With the purpose of searching highly redshifted spectral lines from galaxies of unknown redshift, a new receiver system (called Z-Machine) has been developed for the Nobeyama Radio Observatory (NRO) 45-m radio telescope in 100 GHz band (Nakajima et al. 2013). The atmospheric contribution to the system noise temperature at NRO is more than K around 100 GHz. Under this circumstance, the sensitivity cannot be improved by making lowernoise receivers since it is already limited by atmospheric noise. To improve the sensitivity, we need to build a dual-polarized receiver system. Whenpboth ffiffiffi polarizations are received simultaneously, there is a 2 improvement in the signal-to-noise ratio (S/N), or a reduction factor of 2 in observing time on a telescope (Chattopadhyay et al. 2000). Therefore, a wide-band dual-polarization operation is required for the NRO Z-Machine receiver system. One of the major components for a waveguidebased dual-polarization receiver is an orthomode transducer (OMT). An OMT is a passive microwave device which separates two orthogonal, linearly polarized signals within the same frequency band. It is worth mentioning that there has been 1 ALMA Senior RF Engineer, Joint ALMA Observatory (Chile). substantial progress in the field of applications for millimeter and submillimeter wavelength radio telescopes in the past year (Wollack & Grammer 2003; Narayanan & Erickson 2003; Moorey et al. 2006; Navarrini & Plambeck 2006; Pisano et al. 2007; Navarrini & Nesti 2009; Dunning et al. 2009; Navarrini et al. 2011). Double ridge Bøifot-type junction waveguide OMTs based on the design presented by Moorey et al. (2006) have been developed for the Atacama Large Millimeter/Submillimeter Array (ALMA) project (Wootten & Thompson 2009) in the National Astronomical Observatory of Japan. A stepped double-ridge waveguide transition has been adopted for the ALMA Band 4 ( GHz) and Band 8 ( GHz) (Asayama & Kamikura 2009; Kamikura et al. 2010) because it is particularly suitable for a split-block wave guide construction on a CNC milling machine using end-mills. To meet the ALMA Band 4 and 8 specifications of about 26% fractional bandwidth, we used 3-step Chebyshev transformers for our OMTs because dimensional tolerances are critical for ALMA mass production. Further, wide-band operations can easily be achieved with an increasing number of steps demonstrated by Dunning et al. (2009) and Moorey et al. (2006). Recently, quadruple-ridge waveguide OMTs with smooth taper transitions have been developed and demonstrated in low frequencies (L-Band: Coutts 2011a; X-Band: Coutts 2011b). Thanks to CNC Machining advances in recent years, smooth taper ridge transitions became possible for split-block construction in millimeter-wave range. 213

2 214 ASAYAMA & NAKAJIMA FIG. 1. Wire frame model of the OMT internal waveguide structure. This article presents the design and performance of a Bøifottype waveguide OMT for the 100 GHz band, based on a smoothtaper double-ridge waveguide in place of a stepped double-ridge waveguide (Moorey et al. 2006; Asayama & Kamikura 2009; Kamikura et al. 2010). Our OMT covers the GHz band ( 40% fractional bandwidth) and has excellent performance. 2. DESIGN AND FABRICATION Figure 1 shows a wireframe model of the internal waveguide structure of the smooth taper double-ridge waveguide OMT. The 2 mm band OMT design (Asayama & Kamikura 2009) has been modified with a smoothed double-ridged waveguide junction. All step structures have also been replaced with smooth and continuous structures to demonstrate the feasibility of machining technique except for the 90 E-plane bend. The OMT consists of a square to smooth taper double-ridge waveguide transition followed by a Bøifot junction of two side arms with a central guide. A shape of cosine-square taper was applied for the transition between square and double-ridge waveguides. The vertical polarization (Pol 0) goes through a tapered smooth impedance transformer and an E-plane bend allows it to exit through the top of the OMT. The horizontal polarization (Pol 1) is split equally in the two side ports, and recombined by using a power combiner. This OMT was designed and optimized with a commercial 3D EM simulator (High-Frequency Structure Simulator [HFSS] software from Ansoft Corp). For the 100 GHz band design, a 2:54 2:54 mm square waveguide for the input and WR-10 (2:54 1:27 mm) rectangular waveguides for the outputs were used. Figure 2 shows closeup views with dimensions (in mm) of the Smoothed-ridge OMT Junction. The final simulation results for the reflected amplitude of the two independent fundamental modes TE10 (Pol 0) and TE01 (Pol 1) at the square waveguide input of Figure 1 are illustrated in Figure 3. Because the OMT has been designed for a 4 K cryogenically-cooled superconducting lower-noise receiver system (Nakajima et al. 2013), it will undergo a thermal contraction when cooled at 4 K. Therefore, this simulation took into account the thermal contraction of copper from 300 to 4 K (0.324% [Ekin 2006]). The reflection coefficient is below 18 db for both polarizations over the entire band of interest ( GHz). From the electromagnetic simulations, mechanical errors of 30 μm have little effect on the OMT performance. The OMT is accomplished by splitting the structure along the E-plane of the side-coupled rectangular waveguide. Figure 4 shows the photograph of the two unassembled blocks of the

3 SMOOTH TAPER DOUBLE-RIDGE WAVEGUIDE OMT 215 FIG. 2. Close up views with dimensions (in mm) of the smooth taper double-ridge OMT Junction. FIG. 3. Simulated reflected amplitude of the two independent fundamental modes at the square waveguide input of Fig. 1. FIG. 4. Photograph of the two unassembled blocks of the OMT split blocks showing the internal waveguide circuitry.

4 216 ASAYAMA & NAKAJIMA FIG. 5. Measured Pol 0 (left panel) and Pol 1 (right panel) input return losses of the prototype OMTs. OMT split blocks showing the internal waveguide circuitry. The split-blocks were machined from tellurium-copper and then the machined surfaces were gold-plated. The overall dimension of the OMT is mm. 3. EXPERIMENTAL RESULTS The OMTs were measured with an Agilent vector network analyzer (WR-10: GHz). The network analyzer was calibrated with TRL (Through-Reflect-Line) calibration standards. Square-to-rectangular waveguide transitions were used at the input port to be coupled to the network analyzer head. Two back-to-back circular-to-rectangular transitions were first measured to calibrate the insertion loss ( 0:15 db across the band) of the single transition. Figure 5 shows the input return losses of the three prototype OMTs for both polarizations measured at the input waveguide flange. The OMTs exhibited better than 18 db return loss in both polarizations across the GHz range. It was duly confirmed that the measurements closely follow the simulation results. The losses in the measurement setup have been removed in the insertion loss plot shown in Figure 6. The insertion losses are better than 0.3 db across the GHz range. The cross polarization couplings were greater than 28 db for both polarizations as shown in Figure 7. The left panel represents Pol 0 input polarization when the output is measured at the Pol 1 FIG. 6. Measured Pol 0 (left panel) and Pol 1 (right panel) transmissions of the prototype OMTs.

5 SMOOTH TAPER DOUBLE-RIDGE WAVEGUIDE OMT 217 FIG. 7. Measured Pol 0 (left panel) and Pol 1 (right panel) cross-polarization couplings of the prototype OMTs. output port, while the right plot represents Pol 1 input polarization when the output is measured at the Pol 0 output port. All OMTs show almost identical performance and the return loss measurements closely follow the simulation. It was not possible to measure the performances above 110 GHz due to the limitation of the test system. The performances above 110 GHz have been demonstrated with astronomical observations after installation on the NRO 45 m telescope (Nakajima et al. 2013; Iono et al. 2012). 4. CONCLUSION A variation of the double-ridge Bøifot-type waveguide OMT has been designed for the new 3 mm Band Z-Machine receiver for the NRO 45-m radio telescope. The new OMT design focuses on manufacturability of a wide-band smooth taper double-ridge waveguide for 3 mm Band. The new OMTs have been fabricated and tested. The OMT shows a return loss of better than 18 db, a polarization isolation of better than 28 db, and an insertion loss of less than 0.5 db across the GHz. These OMTs have been installed on the NRO 45-m telescope and have demonstrated excellent wideband performance. The wideband performance of the new OMT will allow us to determine redshifts of bright submillimeter galaxies without any prior redshift information. The authors would like to thank K. Kimura, T. Katase, J. Kizawa, M. Koyano from Osaka Prefecture University for testing OMTs, Advanced Technology Center (ATC), National Astronomical Observatory of Japan (NAOJ) for providing opportunities to use WR-10 vector network analyzer. This work was supported in part by a MEXT Grant-in-Aid for Specially Promoted Research (No ). REFERENCES Asayama, S., & Kamikura, K. 2009, JIMTW, 30, 573 Chattopadhyay, G., Miller, D., LeDuc, H. G., & Zmuidzinas, J. 2000, IEEE Trans. MTT, 48, 1680 Coutts, G. M. 2011a, IEEE Trans. Antennas Propagation, 59, b, IEEE Trans. Antennas Propagation, 59, 1902 Dunning, A., Srikanth, S., & Kerr, A. R. 2009, Proc. 20th ISSTT, 191 Ekin, J. W. 2006, Experimental Techniques for Low-Temperature Measurements. New York: Oxford University Press Hatsukade, B., Kohno, K., Aretxaga, I., Austermann, J. E., Ezawa, H., Hughes, D. H., Ikarashi, S., Iono, D., et al. 2011, MNRAS, 411, 102 Iono, D., Hatsukade, B., Kohno, K., Kawabe, R., Ikarashi, S., Ichikawa, K., Kodama, T., Motohara, K., et al. 2012, PASJ, 64, L2 Kamikura, K., Naruse, M., Asayama, S., Satou, N., Shan, W., & Sekimoto, Y. 2010, JIMTW, 31, 697 Moorey, G., Bolton, R., Dunning, A., Gough, R., Kanoniuk, H., & Reilly, L. 2006, Proc. Workshop on the Applications of Radio Science (WARS2006), Leura, NSW, Australia, Feb Nakajima, T., Kimura, K., Nishimura, A., Iwashita, H., Miyazawa, C., Sakai, T., Iono, D., Kohno, K., et al. 2013, PASP, in press Narayanan, G., & Erickson, N. 2003, Proc. 14th ISSTT, 508 Navarrini, A., Groppi, C., Lin, R., & Chattopadhyay, G. 2011, Proc. 22th ISSTT, 111 Navarrini, A., & Nesti, R. 2009, IEEE Trans. MTT, 57, 88 Navarrini, A., & Plambeck, R. L. 2006, IEEE Trans. MTT, 54, 272 Pisano, G., Pietranera, L., Isaak, K., Piccirillo, L., Johnson, B., Maffei, B., & Melhuish, S. 2007, IEEE MWCL, 17, 286 Wollack, E. J., & Grammer, W. 2003, Proc. 14th ISSTT, 169 Wootten, A., & Thompson, A. R. 2009, Proc. IEEE, 97, 1463