High-Performance Full-band TM01 Mode Converter A.Mediavilla1, K. Cepero1 Abstract This paper describes a novel and compact stepped TM01 mode converter that exhibits extremely good performances over a 42% bandwidth with more than 98% mode conversion efficiency. This mode converter transforms energy from the rectangular TE10 port towards the first high order mode TM01 in a circular waveguide. This feature is required in certain applications such as motional joints that provide continuous rotation in both directions without deteriorating the electrical performance. A transition from rectangular to circular waveguide can be easily added to the design to complete a TM01 to TE11 mode converter. This feature is often used in High-power microwave (HPM) sources that generate a TM01 circular waveguide mode. In these cases the TE11 mode is needed since it has a converged radiation pattern able to drive conventional antennas.
1 INTRODUCTION Various high power microwave (HPM), generators such as the oscillators of virtual cathode (vircators), the relativists backward wave oscillators from reflected waves (BWOs) and the magnetically insulating transmission-line oscillators (MILOs), generate azumitally symmetric output modes with, including the TM01 circular waveguide mode and even the coaxial line transverse electromagnetic mode TEM. If these modes are directly radiated to the space from the waveguides output, a donutshaped radiation pattern will be produced, with a clear absence of signal in the axis (boresight) owing to the fact that the transverse electric field is null in the axis, fig.1a. Nevertheless, in multitude of applications the TE11 circular waveguide mode is used since it has a well defined polarization together with a convergent radiation pattern similar to “Gaussian shape” of a hybrid HE11 mode corresponding to a corrugated circular waveguide. In this sense, and in the necessary case, we can always transform, of high efficiency, the TE11 mode towards the HE11 mode in a relatively short physical convert length. A number of waveguide mode converters for generating TM01 mode in circular waveguide have been reported in [1] - [5]. In [6], a mode launcher exciting TE11 circular waveguide mode from the TEM coaxial mode has been presented. However, most of them are not fully satisfactory due to their performance limitation in term of frequency bandwidth and in term of conversion efficiency. 1
A. Tribak12
In the high power microwave equipment these TM01TE11 mode converters are often used based on the concept of dual bend [7] - [9]. Nevertheless, the output and the input are not aligned in the same axis which affects the compactness and robustness of the system and may impede many important applications. In this work a very wide band mode converter that has no bended waveguide is presented. It formed by a linear structure that transforms the TM01 circular waveguide mode to the dominant TE11 mode that provides high conversion efficiency, and especially with a very high power handling capacity. Fig. 1b shows the structure of the full band TM01 mode converter. This structure employs an intermediate mode TEM in the transition between TM01 and TE10. This coaxial line provides natural isolation while acting as a mode filter between the device ports. Assuming that the TM01 mode enters the structure from the left part of fig. 1b, the energy is transmitted into a short multi-stepped coaxially-shaped circular waveguide transformer that couples a stepped coaxial-to-rectangular magnetic waveguide launcher having three septum sections. The use of this new architecture assures high performance with good experimental results. The return loss was better than 35dB and the insertion loss was better than 0.05dB over a 42% bandwidth (10-15.3 GHz), thus drastically improving previous reported results. The original contribution in this design is the specific incorporation of two intermediate modes in order to isolate without ambiguity two TE11 circular modes in the ports of interest and to eliminate effectively the possible feedback between them across the higher order modes propagation. During the design of the mode converter we have to take into account several aspects and criteria [4]: bandwidth, good match in the access ports, input TM01 mode purity, Output TE11mode purity, Coalignment input-output in the same axis and other electrical and mechanical aspects that are highly desirable and must be considered during the design and manufacturing processes.
Department of Communication Engineering (DICOM) University of Cantabria Santander – SPAIN, e-mail:
[email protected],
[email protected], tel.: +34 942 200889, fax: +34 942 201488. 2 Institut National des Postes et de Télécommunication Rabat- Maroc, e-mail:
[email protected].: +34 942 200889, fax: +34 942 201488.
(a)
energy is transmitted firstly in a section of coaxial line formed by three transitions – fig. 2. This skill allows excellent characteristics of TM01-TEM conversion in a frequency bandwidth more 42%. Subsequently, a transition between the coaxial line and rectangular waveguide is designed using a septum of at least three sections, which allows a high TEM-TE10 conversion efficiency in a frequency bandwidth superior to 42 % - fig. 3. The ultimate part of the mode converter is composed of a standard multi-steps transition from rectangular to a circular waveguide, - fig 4a, that allows a TE10-TE11 conversion in a frequency bandwidth more than 50%.
(b)
Figure 1: (a) Radiation pattern of the TM01 circular waveguide mode. (b) Mode converter internal view and electric field patterns in various waveguide sections.
Figure 2: Multi-steps circular waveguide to coaxial line transition
2 Design discussions The fact that in a circular waveguide the TM01 mode has a cutoff frequency higher than the fundamental TE11 mode make impossible the direct conversion without any reflection in the signal to be transmitted. The mechanical alternative proposed in this work allows initially the conversion of the TM01 mode of the circular waveguide to the TEM coaxial transmission line mode, which has a cutoff frequency equal to zero. The intrinsic separation of the mode of the coaxial line might be then used in the mutual isolation between the modes of the output and the input. This philosophy divides the design into two parts: the first part transforms the TM01 circular mode towards the TEM coaxial mode and the second one transforms the TEM mode to the TE11 circular mode. Unfortunately, the conversion TEM-TE11 cannot be obtained in wide band, due to the proximity between the cutoff frequencies of the two modes, TM01 and TE11 circular waveguide modes. Therefore, the second part was realized using an intermediate mode TE10 of rectangular waveguide and therefore it was necessary to design an axial intermediate TEM-TE10 converter that was feeding a later TE10-TE11 conversion. The fig.1a shows a 3D internal view of the whole device of the mode converter TM01-TE11 together with the configuration of the electrical field in the different sections. Taking into account that the TM 01 mode comes on the left part of the structure, the
Figure 3: Coaxial to rectangular waveguide transition through a Septum formed by three steps
Figure 4: Multi-steps rectangular to circular waveguide transition To better clarifying, we are going to propose a more detailed description of the intermediate modes that intervene in structure showed in fig. 1b. Table 1 shows the cutoff frequencies of all the modes that exist in the different sections of mode converter. The column A shows the cutoff frequencies of all the modes that can be excited in the circular waveguide of the input up to the first mode beyond the desired frequency band. It can be observed that other five unwanted modes can exist in this waveguide.
Consequently, trying to convert directly to the TE 11 mode will excite necessarily also the modes TE21, TM11 and TE31. These undesired modes previously mentioned
absorb a quantity of energy and obviously deteriorating the performance of the device. Nevertheless, we can avoid all these modes converting the TM01 mode to the TEM mode. The next higher order mode that would be excited by these means is the TM02 mode, which has a cutoff frequency of 2.5 that of the TM01 mode. The region of the TEM mode begins, in the converter, in the same diameter as the waveguide A, fitting towards the smallest diameter of the waveguide B. This smaller diameter is necessary to remove the possible propagation of the TE11 coaxial transmission line mode of a frequency beyond the band of interest, for what it acts as a shock of the TE11 mode when it is excited in the output line of the waveguide. The isolation is fundamentally determined by the quotient of the superior edge of the operation bandwidth and the cutoff frequency of the TE11 mode, and is proportional to the length divided by the diameter of the section. In these conditions, the energy propagates along the central conductor of the hybrid rectangular waveguide (transition across the Septum). While the central conductor is in the axis, the TEM mode exists. Similarly, in the interspase between the septum end of the rectangular waveguide, the TE11coaxial mode together with the TE10 mode of the open rectangular waveguide C are excited. This TE11 mode cannot propagate again inwards of the coaxial section with a smaller diameter and is caught between the small coaxial and the Septum. The choice of the appropriate dimensions in this region provides a transition with a wide frequency band between the incident TEM mode and the transmitted TE10 mode of rectangular waveguide. The most problematic mode is the TM11 rectangular waveguide mode, which corresponds with the TM01 circular waveguide mode. The TM11 mode is excited by the Septum but it keeps on being evanescent in the rectangular waveguide, therefore, only the TE10 mode exists in the end of the rectangular waveguide. A rectangular - to - circular standard transition (waveguide D) through three rectangular sections allows the TE10-TE11 conversion in a frequency bandwidth superior to 42 % and avoids the excitation of higher order modes. The new proposed alternative of the mode converter can be used also to convert the TM01 mode of the circular waveguide to the coaxial TEM mode since they are very useful for many sources of high power microwave applications [6]. It even can be used as a mode launcher of a circular TM01 mode from the TE10 mode of the rectangular waveguide as it can be seen in fig. 5. Cutoff frequencies:
A TE11: 6.27GHZ TM01: 8.19GHZ TE21: 10.4GHZ TM11: 13.05GHZ TE01: 13.05GHZ TE31: 14.31GHZ TM21: 17.01GHZ D TE11:9.16GHZ TM01:11.96GHZ TE21:15.19GHZ TM11:19.06GHZ TE01 :19.06GHZ
B TEM:0.0GHZ
C TE10:7.88GHZ TE20:15.77GHZ TE01:15.77GHZ TM11:17.64GHZ
Table 1: Mode distributions along the different waveguide sections.
Figure 5: TM01 mode launcher from TE10 rectangular waveguide mode 3 Results and yield analyses A device prototype was designed, fabricated and tested for Ku-band by using milling technique based in alumagal-6850. As it can be observe in the fig.6, the return losses are in the order of 33dB in both modes TM01 and TE11 giving arise to a conversion efficacy of 99.9 % in a frequency bandwidth superior to 46%. In order to control the robustness of the design and, consequently, the mechanical tolerances, a yield analysis has been carried out where a tolerance of ±0.05mm has been allowed all the geometry dimensions of the device. As it can be seen in the fig.7, the design is strongly robust to the mechanical tolerances. With the same manner, we have obtained very good results for the TE10-TM01 mode launcher as shown in the fig.8. The return loss performances are in the order of 35dB, exhibiting a transmission in a fraccional bandwidth more than 46 %.
Figure 6: Scattering parameter in Ku-band.
Figure 7: Yield analyses using a mechanical tolerance of 0.05mm
Figure 8: Scattering parameters in Ku-band. 4 Conclusions A TM01-TE11 circular waveguide mode converter has been designed, machined and tested, using as intermediate structure coaxial lines and rectangular waveguide, covering a frequency bandwidth > 46 %, with a return and insertion losses in the order of 35dB and 0.05dB, respectively. A yield analysis has been carried out allowing a mechanical tolerances that validate the compact and robustness of the design in the Ku-band. Moreover, a mode launcher of the TM01 circular waveguide mode from the TE10 rectangular waveguide mode has been studied in the extended Ku-band. Acknowledgments The authors like to thank Spanish MICINN and AECID through their codes TEC-2008-06684-C0301, CSD2008-68. The authors wish also to thank the staff of the Spanish Agency for International Development Cooperation (AECID) as well as to Indra Espacio for his useful suggestions. References [1] M. J. Thumm and M. S. Ayza, “Design of short high-power TM01-TE11 mode converters in
highly overmoded corrugatedwaveguide,” IEEE Trans. Microwave Theory Tech., vol. 39, no. 2, pp.301–309, Feb. 1991. [2] S.W. Yang and H. F. Li, “Optimization of novel high-power millimeterwave TM01-TE11 mode converters,” IEEE Trans Microwave Theory Tech., vol. 45, no. 4, pp. 552–554, Apr. 1997. [3] R.W. Lemke, S. E. Calico, and M. C. Clark,“Investigation of a load-limited magnetically insulated transmission line oscillator (MILO),” IEEE Tans. Plasma Sci., vol. 25, no. 2, pp. 364–374, Apr. 1997. [4] R.L.Einsenhart “A novel wideband TM01-TE11 mode converter”IEEE MTT-sint. symp.Dig., pp. 249-252, 1998. [5] S. B. Sakrabarty, V. K. Singh, S. B. Sharma, “TM01 mode transducer using circular and rectangular waveguides”, International Journal of RF and Microwave Computer-Aided Engineering, Vol. 20, No. 3, Mai 2010. [6] C. W. Yuan, Q. x. Liu, H.H.Zhong, B.L.Qian “A novel TEM-TE11 mode converter” IEEE Microwave and wireless components letters, Vol. 15, No. 8, August 2005. [7] B. M. Lee, S. H. Lee, S.Kwon, Y. J. Yoon, J. H. So,”The design of x-band non-constant serpentine TM01-TE11 mode converter with short length” IEEE 2005. [8] B.M.Lee,S.H.Lee,S.Kwon,Y.J.Yoon,J.H.So ”xband TM01-TE11 mode converter with short length for high power” Ieee Electronic Letters,vol.40.No.18,pp1126-1128.sep 2004. [9] M. Thumm, A. Jacobs, and M. S. Ayza, ”design of short high-power TM01- TE11 mode converters in highly overmoded corrugated waveguide,” IEEE Tran. Microwave theory tech, vol. 39,pp.301-309, Feb.1991.
