Design of Tri-frequency Mode Transducer

78 Design of Tri-frequency Mode Transducer V. K. Singh, S. B. Chakrabarty Microwave Sensors Antenna Division, Antenna Systems Area, Space Applications Centre, Indian Space Research Organization, Ahmedabad-3815, India, emails: vksingh@sac.isro.gov.in, soumya@sac.isro.gov.in S. B. Sharma Antenna Systems Area, Space Applications Centre, Indian Space Research Organization, Ahmedabad- 3815,India, email: drsbs@sac.isro.gov.in Abstract This paper presents the design, modal analysis and measured results of a mode transducer operating at three frequency bands at 18.7, 23.8 and 36.5 GHz. In this device, cascaded circular waveguides excited by rectangular waveguides through co-axial line fed probes have been used. Mechanisms of cascading of waveguides have been investigated in order to achieve the intended performance at all the three frequency bands. The simulated results of the trifrequency mode transducer are compared with the measured results. Index Terms Circular waveguide, Mode transducer, Probe coupling. I. INTRODUCTION The multi-frequency mode transducers (MTs) and feeds covering more than two bands of frequencies are widely used in antennas for radiometers [1], [2], earth stations and satellite communication payloads. The literature in [3]-[6] elaborates on the design of single frequency mode transducer including the bandwidth enhancement techniques. There are very few literatures on the MTs operating at more than two frequency bands. A tri-frequency corrugated feed with groove discontinuity at input is presented in [7]. This paper [7] did not include the design details of the mode transducer. A work on four-frequency ortho-mode transducer is reported in [8] by the present authors. In this paper, although the approach of design is similar to [8], the frequency bands are taken different than the frequency bands of [8]. A different frequency at Ka-band frequency is taken in the present design to investigate the effects of the discontinuities of the lower frequency mode transducers. Since, the effect of discontinuities is envisaged to be more stringent at the highest frequency at Ka band (36.5 GHz in this design), it is worthwhile to investigate the modal behavior at the highest frequency in the presence of lower frequency ports and to arrive at an optimum design which yields optimum performance at all the frequency bands. The prime objective in the design of a multifrequency MT is to achieve frequency isolation, polarization isolation and purity of dominant mode at the output of MT at each frequency band as mentioned in [8] and [9]. Multi-frequency MTs can be realized by cascading circular waveguide sections and probe coupling mechanisms at individual frequency bands. As a result of cascading, discontinuities in the form of multiple probes or posts [9] and waveguide junctions, excite undesired higher-order modes and deteriorate the performance of the multi-frequency MT. These discontinuities need to be modeled and investigated in order to analyze

79 higher-order mode scattering properties, which is prerequisite to the design and realization of multifrequency MTs. In this paper, a mode transducer operating at three widely separated frequency bands is presented. Various configurations having different relative positions of power coupling ports are studied in terms of return loss, power distribution in various modes, isolation of orthogonal ports and isolation of polarization matched ports. A configuration of the MT yielding optimum result is fabricated and measured for its electrical performance. The design steps, configuration, simulation and measured results are presented. II. DESIGN The design complexity of mode transducers working at multiple frequency bands increases due to the presence of multiple discontinuities (symmetrical and asymmetrical) in the path of propagating signals at different frequency bands. A suitable method to design such transducers would be to cascade the mode transducers at individual frequency bands such that the cross-sectional dimensions at higher frequency bands are at cutoff for the lower frequency bands. But, the waveguide sections at lower frequency bands (supporting only the dominant mode) become over-sized at higher frequencies and support higher-order modes, which are excited because of structural discontinuities. The polarization matched power sensing mechanisms have to be used in the cascaded sections for multifrequency operation. In this case, the higher frequency dominant mode signal gets coupled to the lower frequency power sensing ports thereby increasing the insertion loss of the device. In such mode transducers probes, which sense power at lower frequency, band acts as a radial discontinuity and coupler for higher frequency signal. Since, the design goal is to ensure dominant mode purity at each frequency band, the higher-order modes have to be suppressed and all the frequency ports have to be decoupled. The dominant mode purity at each frequency band of MT will ensure the desired radiation patterns of a corrugated horn antenna if fed with such mode transducers. Hence, it is worthwhile to estimate the modal amplitude of different higher-order modes generated because of the discontinuities. Finite element method (FEM) based Ansoft s HFSS software has been used for modeling, analysis, design-optimization and estimation of amplitudes of different higher-order modes. In the MT design, the selected frequencies correspond to the frequencies of a radiometer. The present design is presented at 18.7 ±.2 GHz, 23.8 ±.3 GHz and 36.5 ±.5 GHz. The requirement is of single linear polarization at these three frequency bands. Since, the effect of discontinuities is envisaged to be more stringent at the highest frequency of 36.5 GHz, it is worthwhile to investigate the modal behavior at the highest frequency in the presence of lower frequency ports and to arrive at an optimum design which yields optimum performance at all the frequency bands. The present design of the tri-frequency common mode transducer is based on the coupling from primary cascaded circular waveguide sections to output rectangular waveguides WR-42 ( 1.66 mm

8 X 4.32 mm ) for 18.7 and 23.8 GHz and WR-28 ( 7.2 mm X 3.6 mm ) for 36.5 GHz. The diameters of straight circular waveguide sections are chosen as 5.4, 8.1, 1.1 mm for the propagation of dominant TE 11 mode at 36.5, 23.8 and 18.7 GHz, respectively. The lengths of the straight circular waveguide sections have been selected as 33.5, 45 and 27 mm at 36.5, 23.8 and 18.7 GHz, respectively. The higher-order propagating modes supported at the outermost waveguide section are TM 1, TE 21, TE 1, TM 11 at 36.5 GHz and TM 1 at 23.8 GHz, respectively. There are no higher order modes supported at 18.7 GHz. The taper angle and length of the tapered sections between straight waveguide sections(main arm) are optimized in order to minimize the power in the higher-order modes and maximize the power in the desired dominant TE 11 mode. The optimum flare angles for the tapered sections of the geometry have been kept within 5 degree. It is found that in the absence of side coupled rectangular waveguide arms the power coupling in higher-order modes is negligible and 99 % (.44 db) power is confined in the desired TE 11 mode in the outermost circular waveguide section at all the three frequency bands. The radiation patterns have been computed at 36.5 GHz with pure TE 11 mode at the aperture of the mode transducer. The co-polar radiation patterns have 5 degree amplitude taper of -13.8 db and -8.5 db in the E- and H-planes respectively. The cross-polar radiation level in the diagonal palne(45-degree plane) with respect to the co-polar peak is of the order of -2 db. These co- and cross-polar radiation patterns are shown in Figure 1.. Also, the reflected power in the axial direction of cascaded circular waveguides is -23 db at 18.7 GHz, -27 db at 23.8 GHz and -29 db at 36.5 GHz. The power coupling and return loss deteriorates if probe coupled, side rectangular waveguide arms are introduced. -5 H-plane Co-pol. E-plane Co-pol. Diagonal Plane Cross pol. -15 Power (db) -2-25 -3-35 -4-45 -5-18 -15-12 -9-6 -3 3 6 9 12 15 18 Theta (Degree) Figure 1. Radiation pattern at 36.5 GHz with maximum power in the TE 11 mode, i.e., without higher-order modes. In the present design of tri-frequency mode transducer, the main arm circular waveguide is coupled to rectangular waveguide through coaxial probes. The impedance seen by these probes in circular waveguide is matched to rectangular waveguides using multistepped transitions in the rectangular waveguide as shown in Figure 2.. This transition consists of two parts- a rectangular-to-ridged

81 waveguide transition and a ridged waveguide-to-coaxial transformer. For making compact design the stepped ridge transformer has been designed to follow a cosine profile for impedance. Cosine profile offers impedance matching with a very compact design of the transition. Three cases with different orientations of the rectangular waveguide ports have been studied for the design of the tri-frequency mode transducer, which are discussed in order in thefollowing section. Case 1: The configuration for case-1 is shown in Figure 2.(a). The performance is simulated at 36.5 GHz in the presence of mode transducers at 23.8 GHz and 18.7 GHz. Here, all ports at 18.7, 23.7 and 36.5 GHz are aligned, i.e., polarization matched. In this case, the return loss at 36.5 GHz, is 15.14 db. The signal at 36.5 GHz is isolated from the 23.8 GHz port by 17.61 db and from 18.7 GHz port by 21.46 db. From the data presented in Table 1., it is clear that more power couples in the higherorder modes at the aperture at 36.5 GHz. Due to polarization matching of coaxial probes and closeness of frequencies, the signal at 23.8 GHz is poorly isolated from 18.7 GHz. This isolation is of the order of -11 db only. In the presence of coaxial probe discontinuities and hence higher-order modes, the computed cross-polar radiation from the outermost aperture of cascaded circular waveguides is of the order of -13.3 db, as compared to -2 db for pure TE 11 mode(without higherorder modes) at 36.5 GHz. The co-polar radiation patterns have 5 degree amplitude taper of -11. db and -7.9 db in the E- and H-planes respectively at 36.5 GHz. Case 2: The configuration for case-2 is shown in Figure 2.(b). The performance is simulated at 36.5 GHz in the presence of mode transducers at 23.8 GHz and 18.7 GHz. Here, ports at 18.7, 23.7 are aligned and orthogonal to 36.5 GHz. In this case return loss at 36.5 GHz is 11.4 db. The signal at 36.5 GHz is isolated from the 23.8 GHz port by 7.98 db and from 18.7 GHz port by 58.85 db. The Table 1. shows that more power couples in the higher-order modes at the aperture at 36.5 GHz. In this case also, the signal at 23.8 GHz is poorly isolated (-11 db) from 18.7 GHz. In this configuration, the computed cross-polar radiation from the outermost aperture of cascaded circular waveguides is of the order of -11.9 db at 36.5 GHz in the presence of coaxial probe discontinuities. The co-polar radiation patterns have 5 degree amplitude taper of -14.1 db and.1 db in the E- and H-planes respectively at 36.5 GHz. Case 3: The configuration is shown in Figure 2.(c). The performance is simulated at 36.5 GHz in the presence of mode transducer at 23.8 GHz and 18.7 GHz. Here, ports at 36.5, 23.7 are aligned and orthogonal to 18.7 GHz as shown in Figure 2. (c). In this case return loss at 36.5 GHz is 14.29 db. The signal at 36.5 GHz is isolated from the 23.8 GHz port by 18.45 db and from 18.7 GHz port by 38.65 db. The Table 1., shows that maximum power (-1.15 db) couples in the dominant TE 11 mode and minimum power couples in the higher order modes at the aperture at 36.5 GHz in the presence of

82 all the ports. In this configuration (see Figure 2.(c)), the computed cross-polar radiation from the outermost aperture of cascaded circular waveguide is of the order of -12.4 db. Poor cross-polar radiation is the result of the higher order modes generation due to the coaxial probe discontinuities at 36.5 GHz. The co-polar radiation patterns of the mode transducer have 5 degree amplitude taper of - 13.77 db and -7.7 db in the E- and H-planes respectively at 36.5 GHz, as shown in Figure 3.. The configuration (see Figure 2.(c)) gives the optimum performance at all the frequencies and it has been considered as final design configuration. The modal analysis (see Table-1) results for this configuration show that the return loss at 23.8 GHz is -17.49 and the isolation of 23.8 GHz signal with 18.7 GHz is -58.61 db. At 23.8 GHz, maximum power couples in the TE 11 mode, which is of the order of.95 db. At 18.7 GHz also, the maximum power is in the desired TE 11 mode, since 18.7 GHz circular waveguide section does not support higher-order modes at 18.7 GHz. Simulated and measured performance of the cascaded mode transducer (see Figure 2.(c)) operating at all the three frequency bands is given in the next section. III SIMULATED AND MEASURED RESULTS The measured and simulated performance of the combined three-frequency mode transducer (see Figure 2. (c)), which gave optimum performance in terms of return loss, isolation and higher-order mode coupling have been found out. The results are compared with the performance at individual mode transducers. Figures 4., 5. and 6., show the return loss performance at 18.7, 23.8 and 36.5 GHz. The performance at 23.8 GHz frequency band is slightly affected due to the presence of 18.7 GHz port in front of 23.8 GHz port as compared to the performance of individual mode transducer at 23.8 GHz. Figure 2. Different configuration to combine waveguide sections at 3 frequencies.

83 The performance at 36.5 GHz is more affected by the presence of two lower frequency ports at 18.7 GHz and 23.8 GHz in front of the 36.5 GHz port. Measured isolation of 36.5 GHz signal with 23.8 GHz port is of the order of 2 db over 8 percent of the desired frequency band and the isolation with 18.7 GHz port is of the order of 25 db as shown in Figure 7.. The measured isolation of 23.8 GHz signal with 18.7 GHz port is of the order of 5 db as shown in Figure 8.. TABLE I. MODES AT THE APERTURE OF THE CASCADED MODE TRANSDUCER AT 36.5 GHZ. Mode names TE 11 TM 1 TE 21 TE 1 TM 11 Modal Amplitude -1.37-8.82-11.4-48.21-55.46 (db) for Fig.2. a Modal Amplitude -1.36-5.41-7.33-38.1-52.99 (db) for Fig.2. b Modal Amplitude (db) for Fig.2. c -1.15-23.33-7.74-37.6-48.11-5 H-plane Co-pol. E-plane Co-pol. Diagonal Plane Cross pol. -15 Power (db) -2-25 -3-35 -4-45 -5-18 -15-12 -9-6 -3 3 6 9 12 15 18 Theta (Degree) Figure 3. Radiation pattern at 36.5 GHz with power in dominant TE 11 mode and the higher-order modes. Figure 4. Simulated and measured return loss at 18.7 GHz.

84 Return Loss(dB) -5-15 -2-25 -3-35 -4-45 Simulated ( with 18.7 and 36.5 GHz ports) Simulated (without 18.7 and 36.5 GHz ports) Measured (without 18.7 and 36.5 GHz ports) Measured (with 18.7 and 36.5 GHz ports ) -5 23.5 23.6 23.7 23.8 23.9 24. 24.1 Frequency(GHz) Figure 5. Simulated and measured return loss at 23.8 GHz. Return Loss(dB) -5-15 -2-25 -3-35 -4-45 Simulated (without 18.7 and 23.8 GHz ports) Measured (without 18.7 and 23.8 GHz ports) Measured (with 18.7 and 23.8 GHz ports ) Simulated (with 18.7 and 23.8 GHz ports) -5 36. 36.1 36.2 36.3 36.4 36.5 36.6 36.7 36.8 36.9 37. Frequency(GHz) Figure 6. Simulated and measured return loss at 36.5 GHz. -2-3 Isolation(dB) -4-5 -6-7 -8-9 Simulated (isolation with 18.7 GHz port) Measured (isolation with 18.7 GHz port) Measured (isolation with 23.8 GHz port) Simulated (isolation with 23.8 GHz port) 36. 36.1 36.2 36.3 36.4 36.5 36.6 36.7 36.8 36.9 37. Frequency(dB) Figure 7. Measured and simulated isolation performance of 36.5 GHz signal with 18.7 and 23.8 GHz ports. -2-3 Isolation(dB) -4-5 -6-7 -8-9 Simulated Measured 23.5 23.6 23.7 23.8 23.9 24. 24.1 Frequency(GHz) Figure 8. Measured and simulated isolation performance of 23.8 GHz signal with 18.7 GHz port.

85 36.5 GHz mode transducer 23.8 GHz mode transducer 18.7 GHz mode transducer Figure 9. Tri-frequency mode transducer at 18.7, 23.8 and 36.5 GHz. The photograph of the tri-frequency mode transducer is shown in Figure 9.. IV CONCLUSION A novel design of three-frequency band mode transducer has been presented to develop a mode transducer at 18.7, 23.8 and 36.5 GHz. In the multi-frequency environment, the methods of cascading the mode transducers at different frequency bands to yield minimum power in the higher-order modes were investigated. Modal analysis was performed to estimate the effects of taper and asymmetrical probe discontinuities in the main waveguide particularly at the higher frequency band in the presence of lower frequency mode transducers. An optimum configuration of multi-frequency mode transducer yielding desired the isolation of orthogonal ports, isolation of higher frequencies with lower frequency ports of same polarization and maximum power in the dominant TE 11 mode has been obtained. The simulation and measured results were presented for the optimum design of mode transducer. The slight deviation of the measured data from simulated data may be attributed to fabrication tolerances and minor assembly and alignment errors. The modal analysis based design approach presented in this paper may be applied to the design of multi-frequency mode transducers at other frequency bands. ACKNOWLEDGMENT The authors thank Dr. R. R. Navalgund, Director SAC, Ahmedabad for encouragement and support. The authors thank the Engineers of Microwave Sensors Antenna Division, Antenna Systems Area, SAC, Ahmedabad for extending the help and the necessary support. REFERENCES [ 1 ] E. G. Njoku, J. M. Stacey, and F. T. Barath, The Sea-Sat scanning multi-channel microwave radiometer (SMMR): Instrument description and performance, IEEE Journal of Oceanic Engineering, vol. 5, pp. 1-115, 198.

86 [ 2 ] Shashi Bhushan Sharma, Antenna system for the multi-frequency scanning microwave radiometer: MSMR, IEEE Antennas Propagat. Magazine, vol. 42, no. 3, pp. 21-29, June 2. [ 3 ] G. L. Ragan, Microwave Transmission Circuit. MIT Radiation Laboratory Series, New York: Dover, 1965, New York: McGraw-Hill, 1948, sections 6.17, 6.18, pp. 11, 364-369. [ 4 ] A. F. Harvey, Microwave Engineering. New York: Academic, 1963, sections 3.6, pp. 11, 157, 126-129, 119-12, 89, 919-92. [ 5 ] R. D. Tompkins, A broad-band dual-mode circular waveguide transducer, IRE Trans. Trans. Microwave Theory Tech., pp. 181-183, July 1956. [ 6 ] A. M. Boifot, Classification of ortho-mode transducers, European Trans. Telecommunications and Related Technologies, vol. 2, no.5, pp. 53-51, 1991. [ 7 ] S.B. Sharma and V. K. Singh, Multifrequency corrugated feed with groove discontinuity at input, Electron. Lett., vol. 37, no. 18, pp. 1121-1122, Aug. 21. [ 8 ] S. B. Sharma, V. K. Singh, and S B Chakrabarty, Multi-frequency waveguide orthomode transducer, IEEE Trans. Microwave Theory Tech., vol. MTT-53, no. 8, pp. 264-269, Aug. 25. [ 9 ] S. B. Sharma, Vijay Kumar Singh, Ranajit Dey, and S.B. Chakrabarty, Analysis of post discontinuity in an oversized circular waveguide, IEEE Trans. Microwave Theory Tech., vol. 57, no. 8, pp. 1989-1995, Aug. 29.