A New Approach to Suppress the Effect of Machining Error for Waveguide Septum Circular Polarizer at 230 GHz Band in Radio Astronomy
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A new stepped septum-type waveguide circular polarizer (SST-CP) was developed to operate in the 230 GHz band for radio astronomy, especially submillimeter-band VLBI observations. For previously reported SST-CP models, the 230 GHz band is too high to achieve the design characteristics in manufactured devices because of unexpected machining errors. To realize a functional SST-CP that can operate in the submillimeter band, a new method was developed, in which the division surface is shifted from the top step of the septum to the second step from the top, and we simulated the expected machining error. The SST-CP using this method can compensate for specified machining errors and suppress serious deterioration. To verify the proposed method, several test pieces were manufactured, and their characteristics were measured using a VNA. These results indicated that the insertion losses were approximately 0.75 dB, and the input return losses and the crosstalk of the left- and right-hand circular polarization were greater than 20 dB at 220–245 GHz on 300 K. Moreover, a 230 GHz SST-CP was developed by the proposed method and installed in a 1.85-m radio telescope receiver systems, and then had used for scientific observations during one observation season without any problems. These achievements demonstrate the successful development of a 230 GHz SST-CP for radio astronomical observations. Furthermore, the proposed method can be applicable for observations in higher frequency bands, such as 345 GHz.
KeywordsWaveguide Circular polarizer Radio astronomy Radio telescope Receiver VLBI
In radio astronomy, very-long-baseline interferometry (VLBI) observations, which can achieve extremely fine angular resolution through the interference of several radio telescopes placed at very far from each other, are very important in most cases to observe extremely small astral bodies, such as active galactic nuclei (AGNs) and black hole shadows. To observe such bodies, radio astronomical observers have recently come to expect the realization of submillimeter VLBI observations at frequencies of 230 GHz or more through the cooperation of international astronomical organizations.
For VLBI observation, it is necessary to conform the received polarization direction for all VLBI telescopes; thus, left- or right-hand circular polarization (LHCP or RHCP) is generally used. However, most radio receivers have the sensitive for only either linear polarization (H-pol or V-pol) and thus need somewhat that convert the inputted circularly polarized wave to linear polarization.
Using the waveguide circular polarizer (WG-CP) as shown in Fig. 1c is the simplest method, so that the various of WG-CP models have been developed [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. The models in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] are the WG-CP without stepped septum, and [11, 12, 13, 14, 15, 16, 17, 18] are the stepped septum-type WG-CPs, which are hereafter simply referred to as SST-CP. Although most of the well-known WG-CPs have less fractional bandwidth (∼20%) than using quarter wavelength plate and OMT, the WG-CP works as a single component with lower insertion loss, thus it is easy to install into receivers. This is especially true for SST-CPs, so that SST-CPs are greatly advantageous for specified band observations, such as VLBI observations [12, 13].
Most of the commonly used SST-CP design has a very thin septum for its waveguide [12, 13, 14, 15, 16, 17, 18]; for example, a H-band WG-CPs has a 1.5-mm septum for its square shaped input waveguide with dimensions of 28.5 mm × 28.5 mm. Thus, such SST-CPs cannot be simply scaled down without a loss of reliability, and new smaller models must be designed to have a very thick septum.
- (2)Even if models with a thick septum are successfully designed, the manufactured test pieces may have several very small hollow spaces on top of the septum, as shown in Fig. 2a. These spaces cause unexpected higher-mode resonance, ultimately greatly deteriorating the characteristics of the SST-CP.
This paper introduces an improved SST-CP model, which has a septum with a very large thickness of 0.20 mm for a square input waveguide with dimensions of 0.82 mm × 0.82 mm and suppresses the deterioration caused by the gap spacing on the septum. In this new model, the division surface was shifted from the top of septum to one lower step, as shown in Fig. 2b. The model was then optimized using numerical simulations with various shapes and dimensions of the gap spacing and determined the expected acceptable deterioration to suppress unexpected resonances. Using the proposed model, we developed a SST-CP that can be operated at frequency bands over 230 GHz with a practical machining performance. In section 2, we described the general mechanism of SST-CPs. The details of the designing are described in sections 3 and 4, the measurement results are in section 5, and the astronomical observation application is described in section 6.
2 General Mechanism of Septum-Type Waveguide Circular Polarizer
The SST-CP is a well-known waveguide circuit, and various models have been proposed. Models with four steps in the septum and square waveguides are the most commonly used [11, 12, 13, 14, 15, 16, 17, 18]. Their mechanism of separating RHCP and LHCP input waves is briefly described in the following .
The radio waves input from the feed horn are first polarized orthogonally and then transferred to the septum in the two fundamental propagation modes TE10 (H-pol) and TE01 (V-pol). The septum is a step-like metal wall located in the center of the square waveguide, dividing it into two equal sections.
The circularly polarized waves are composed of two orthogonal linearly polarized waves of equal amplitude with a 90° phase difference. Thus, the combined electrical field direction is rotated clockwise or counterclockwise. For radio astronomical equipment, the clockwise and counter-clockwise waves observed by receivers are referred to as RHCP and LHCP, respectively.
When a circularly polarized wave is input into a SST-CP, and if its V-pol component lagged from H-pol component by 90°, an additional 90° delay for V-pol would be caused at the left side of septum, so that the input waves are canceled by virtue of being antiphase with the H-pol component. However, at the right side of septum the lagged V-pol would be proceeded by 90° in contrast, and then doubled as a result of the coherence with H-pol component. Therefore, the input waves of V-pol lagged should be output from the right side only, and in contrast, the proceeded V-pol waves should be output from the left side only.
In contrast to the higher band, the frequency band lower than the TE11 resonance is known to be a very stable band. Generally, the conventional SST-CPs are designed to be used only this band, so that their fractional bandwidth are limited to ∼20%. The proposed 230 GHz SST-CP was also designed in this stable band, because VLBI observation requires a very stable receiver.
3 Design of 230 GHz Septum-Type Waveguide Circular Polarizer
Thus, an alternative model called the mirror model was also simulated, as shown in Fig. 5b. This model is composed of the single model and a mirror duplicate connected directly at the square input port. With this model, the insertion losses of circular polarization and X-pol were optimized. The insertion losses of S 31 from port 1 to port 3 and S 42 from port 2 to port 4 can be treated as square the circularly polarization insertion loss of single model. This is because at one of the SST-CPs in the mirror model, the input wave from the output port of the single model is converted to a predefined circularly polarized wave and the other SST-CP divides the generated circular polarization. Additionally, the transmission levels S 41 from port 1 to port 4 and S 32 from port 2 to port 3 represent twice the X-pol of single model, because there is double the possibility of inducing X-pol in each septum section. Thus, the X-pol of the single model is almost –3 dB (half) from S 41, S 32. These characteristics can also be measured using a vector network analyzer (VNA) ; thus, the simulated design can be verified with very high accuracy. This is an important advantage in the adaptation of the proposed design method.
According to these results, the designed model has a return loss of greater than 20 dB on every port at 218–247 GHz (S 11, S 22, and S 33), and the output port isolation S 23 is greater than 37 dB, also the X-pol S 41 is greater than 20 dB at 213–247 GHz. Additionally, there is significant performance degradation near 249 GHz, which is the effect of TE11 mode resonance, as explained by Fig. 4. This indicates that the described performance under 247 GHz is in the stable band of SST-CP. Furthermore, because of this very good design performance, even if an unexpected deterioration is caused by manufacturing error, the manufactured SST-CP could remain functional.
4 Suppressing of the Deterioration by Expected Manufacturing Errors
The manufacturing errors and also assembling alignment errors often lead the very serious deterioration of waveguide circuit characteristics, and the WG-CP at higher band such as 230 GHz is one of the most sensitive case. We had studied what prevents the 230 GHz WG-CP from practical using with several test pieces, then we found that the space on the top of septum might be a major suspect. In the almost past models, the waveguide circuit is divided into two pieces (the body and cover part) on the top surface of septum. On such structure, the spaces around top of septum due to the defective touch of two pieces are inevitable problem for very precise components.
Manufacturing and assembling alignment errors often lead to the very serious deterioration of the waveguide circuit characteristics, and the operation of SST-CPs in higher frequency bands, such as the 230 GHz band, is one of the most sensitive cases. The features that are most likely to prevent the practical use of the 230 GHz SST-CP were determined using several test pieces, and it was found that the space above the septum is a major factor. In most previously developed models, the waveguide circuit is divided into two parts, the body and the cover, on the top surface of the septum. In such a structure, the formation of spaces around the top of the septum by the unintended contact of the two pieces is an inevitable problem for very precise components.
As shown in Fig. 9a, there were large deteriorations in the simulation, especially at 220–245 GHz, and return losses were degraded worse than 20 dB for sz > 4 μm. The deterioration with varying sy showed a periodic pattern with a period of approximately 0.6 mm, as shown by the black and red lines in Fig. 9b; this periodicity may depend on the wavelength. Additionally, when sy was relatively low, the deterioration was larger than when sy was high. The results of X-pol shown in Fig. 10b indicate that the X-pol return losses show the same tendencies; however, no clear periodic pattern was observed. Furthermore, the TE11 mode resonance frequency was fixed, but its magnitude varied somewhat with varying sy.
Therefore, it was concluded that the thickness of the space above the septum has a large influence on the deterioration of manufactured SST-CPs, perhaps more than any other factor. Additionally, a new method of manufacturing SST-CPs is required to suppress this deterioration. This is because it is too hard to realize a nearly zero sz while controlling sy such that it falls on the desired point in the periodical deterioration curve. It is very difficult to achieve a reasonable and stable SST-CP with previously developed models.
Next, it was determined why spaces above the septum lead to such serious problems. One hypothesis is that the space above the septum acts as an additional step in the septum. When designing the septum, it is necessary to determine the number of steps. However, when the unintended space above the septum acts as an additional step, the designed septum characteristics, such as the impedance, differ from the actual characteristics of the manufactured septum, especially when the space length is low (e.g., sy = 0.3 mm). Additionally, the electric field becomes centered around the top of the septum, increasing the magnitude of this effect.
A comparison of these results clearly indicates that the shifted space model suppresses deterioration and is especially resilient against increasing space thickness sz2. Although the return loss deterioration obtain when varying sy2 was not stable and showed a periodic pattern, the shifted space model was very effective in stabilizing the X-pol; as shown in Fig. 13b, the X-pol remained over 30 dB at 220–245 GHz for all considered sy2 values. Additionally, in contrast to the behavior in the stable band, the TE11 mode resonance effect was not changed at all.
5 Manufacturing and Measurement
These results indicate that the performance of the SST-CP manufactured with the proposed method would not be deteriorated so much even if the gap spacing reaches the maximum allowable gap spacing of the conventional model. Furthermore, the fact of the TE11 resonance frequency corresponds to that in the design indicates that the manufacturing of the 230 GHz SST-CP was achieved with very good overall accuracy. These achievements demonstrate the successful realization of a 230 GHz SST-CP and indicate that the proposed method in this paper can be applicable for the higher frequency bands.
6 Installation of 230 GHz Septum-Type Waveguide Circular Polarizer in 1.85-m Telescope
The developed 230 GHz SST-CP was installed in the 1.85-m telescope of Osaka Prefecture University (OPU). This millimeter/submillimeter Cassegrain Nasmyth reflector radio telescope with a main reflector of 1.85 m in diameter was developed in a previous study by a radio astronomy group of OPU. It was installed in the Nobeyama Radio Observatory of the National Astronomical Observatory of Japan (NAOJ/NRO) in 2007 and has been used to simultaneously observe the J = 2–1 spectral lines of three CO isotopes by using the sideband separation receiver [21, 22, 23]. Thus, this telescope is a reasonable environment to demonstrate the performance of the proposed 230 GHz SST-CP receiver.
Based on these achievements, the proposed shifted space submillimeter SST-CP has been or would be installed into several other telescope receivers, including the NANTEN2 Observatory 4-m telescope of Nagoya University in the 110 GHz band, and the Green Land Telescope (GLT) of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), which is the dedicated telescope for submillimeter VLBI observations of black hole shadows .
In this study, a SST-CP that is operational in the 230 GHz band for use in radio astronomy, especially submillimeter-band VLBI observations, was developed. This device separates the input dual circularly polarized waves into LHCP and RHCP outputs at 230 GHz, a frequency band too high for previously proposed models to achieve their design characteristics because of machining errors.
A novel method involving shifting the division surface from the top step of the septum to the second step from the top was shown to be able to compensate for the expected machining errors and suppress serious deterioration. The manufactured test pieces were measured using a VNA. The results indicate that there is a space of approximately 12 μm in height above the second step of the septum; however, the measured characteristics remained sufficient for radio astronomical observations. Moreover, the 230 GHz SST-CP was installed in the OPU 1.85-m radio telescope and used for actual scientific observations during one observation season.
The above achievements demonstrate the successful development and realization of a reasonable 230 GHz SST-CP and the successful practical use on radio astronomical line spectra observation. Furthermore, from the fact that the GLT of ASIAA has adopted the proposed SST-CP for submillimeter VLBI observation receiver, submillimeter-band VLBI observation at 230 GHz has been made possible with the proposed 230 GHz SST-CP.
The authors would like to thank Satoshi Ochiai and Akifumi Kasamatsu of the National Institute of Information and Communications Technology (NICT) and Kenichi Kikuchi of the National Astronomical Observatory of Japan (NAOJ) for measuring the WG-CP with a VNA at NICT. We are also very grateful to Masanori Ishino of Kawashima Manufacturing Co., Ltd. (KMCO) for conducting the high-precision machining of the SST-CP.
Additionally, we would like to thank all of those on the 1.85-m telescope team for installing and operating the developed FSF receiver to verify its performance. Further thanks go to Shinichiro Asayama for supporting to design and analyze WG-CP. This work was financially supported by JSPS KAKENHI (Grant Nos. 22244014 and 26247026), and the Toray Science Foundation.
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