Design of a full-band polariser used in WR-22 standard waveguide for satellite communications

Transcription

1 Design of a full-band polariser used in WR-22 standard waveguide for satellite communications Soon-mi Hwang, Kwan-hun Lee Reliability & Failure Analysis Center, Korea Electronics Technology Institute, Sungnam-Si, Republic of Korea asfara@keti.re.kr Published in The Journal of Engineering; Received on 12th August 2014; Accepted on 12th August 2014 Abstract: This paper studies design of a full-band waveguide polariser using iris for satellite communications. Operation theory and design parameters of a full-band polariser are introduced, and a systematic design method has been proposed. The performance of the polariser is analysed using the well-known commercial electromagnetic simulation software high-frequency structure simulator. Using the proposed method, a full-band polariser operating in WR-22 waveguide band (33 50 GHz) is designed, fabricated and tested. Measurements of the fabricated polariser show that the phase difference is <10 as a reference point by 90, the axial ratio is <1.3 db, insertion loss is <0.1 db and return loss is >25 db in the required overall band. 1 Introduction Radio waves used in satellite communication are usually circularly polarised to avoid the Faraday rotation caused by the ionosphere [1]. To generate a linearly polarised wave in reflector antenna feeds, a waveguide-type polarisation transformer, commonly called polariser, is generally employed [2, 3]. Of many different types of polariser, iris polariser in the square waveguide finds widespread applications because of the simplicity in structure, design and fabrication. Moreover, recently multi-band antenna feeds for satellite communication are used as the number of frequency bands increases [4, 5]. Iris polariser has such desirable properties as wide or multi-band operation, compactness and simple interfacing requirements [6, 7]. The design of a square waveguide iris polariser boils down to finding an optimum set of irises placed at a proper interval that gives low axial ratio and small reflection coefficient over a required frequency range. Although many papers have been published on the subject of an iris polariser, the design of an iris polariser operating over the full bandwidth of a rectangular waveguide has not been presented yet. This paper studies design of a full-band waveguide polariser using iris for satellite communications. Operation theory and design parameters of a full-band polariser are introduced, and a systematic design method has been proposed. The performance of the polariser is analysed using the well-known commercial electromagnetic simulation software high-frequency structure simulator (HFSS). Using the proposed method, full-band polariser operating in WR-22 waveguide band (33 50 GHz) is designed, fabricated and tested. phase difference (Δθ) is expressed as follows Du = b l bl (3) If B/ > 0, the phase difference is positive and the iris acts as a capacitor. If B/ < 0, the phase difference is negative and the iris acts as an inductor. The iris acts as a shunt capacitor in the Fig. 1 Structure of the iris polariser 2 Operation theory The iris polariser consists of a number of thin metallic fins arranged on two opposite walls of a square waveguide. A pair of fins placed on two opposite walls is called an iris. Fig. 1 illustrates the structure of an iris polariser. When the iris of the same interval is arranged inside the waveguides, the iris is represented by Fig. 2 b l = cos 1 [ cos bl B/ sin bl] (1) Y 0 = [ ]1/2 1 B2 Y B cot bl Between the empty waveguide and the waveguide with iris, the (2) Fig. 2 Periodically loaded transmission lines and its equivalent circuit a Inside iris b Equivalent circuit (β, β : phase propagation constants, l: length, B/ : normalised iris susceptance and, Y 0: admittance) [8]

2 transmission line for the mode with its electric field normal to the iris. When the electric field is parallel to the iris, the iris can be represented as a series inductor [9]. In the former case, the capacitive loading increases the phase propagation constant, whereas in the latter case the phase propagation constant is decreased. When a linearly polarised wave with its electric field at 45 to the iris is incident on the input plane of the iris polariser, it is converted to a circularly polarised wave after propagating through the polariser. In other words, the electric field applied in 45 angle direction can be split into the vertical mode (TE x 10 ) and the horizontal mode. After passing through a polariser, the horizontal mode phase becomes smaller by 90 than the vertical mode phase, so that a right-handed circular polarisation signal is generated. If the circuit has been fully matched, voltage standing wave ratio and phase difference are as follows [ VSWR MAX. = Y ] 2 0 or [ ] 2 (4.1) Du = 180 2bl (4.2) 3 Design parameters The design parameters of the iris polariser are the square waveguide size, the iris thickness, the iris interval, the iris number and the iris height. Fig. 3 illustrates the design parameters of the iris polariser. Square waveguide size (a) is determined so that all channel signals are passed and the higher-mode signal is isolated. Waveguide size is determined by using the two equations below a = ( )l m (5.1) 2a = l c = c (f f c = (0.7:1.2)f c ) (5.2) c where f c is a centre frequency and λ m is a wavelength of centre frequency. Iris thickness (t) is one element to control the phase characteristic of the polariser. Since phase difference is increased with iris thickness, a thin iris is useful for phase control. Generally, we use the below value t = ( )l m (6) Iris interval (e) brings increase of phase difference when there are many irises. Generally iris interval is twice the iris thickness e 2t (7) Iris height (d) is a parameter which gives big influence at phase change and reflect characteristic. The phase difference increases when iris height is high. Generally, we use below range value [10] d = ( )l m (8) In the case of iris number (N), generally we use In the case of full band, if it is possible we select iris number to be Δφ = 90 in the required all frequency range. 4 Design method The focus of the design is to develop design procedures for a polariser having an excellent performance in the entire operating frequency range of a standard rectangular waveguide. The design of a full-band square waveguide iris polariser starts with the choice of the waveguide size. For a full-band operation, the waveguide size is made larger than that of the standard rectangular waveguide. Next, spacing between irises, iris thickness, iris height and the number of irises are optimised to satisfy axial ratio specifications. The iris height is properly tapered to realise a low reflection. Guidelines for selecting dimensional parameters of a full-band square waveguide iris polariser are derived by analysing the performance of the polariser against changes in polariser dimensions. If investigated through a commercial software, HFSS, the design process of a waveguide iris polariser used in the overall band of the WR-22 standard waveguide (33 50 GHz) is as follows. 4.1 Determine the size of square waveguide Determine the size of waveguide using (5). After setting f c to 23.1 GHz, which is 0.7 times of 33 GHz, the lowest frequency of operational frequency band, the output value of a becomes about 6.49 mm. The reason to set f c to the value of 0.7 times of the lowest frequency of operational frequency band is that this point is less affected by attenuation, which gets more serious as it is approaching closer to cutoff frequency, and to reduce the waveguide length, which is lengthened towards infinite as it is approaching closer to cutoff frequency. Vary the size of waveguide to more or less 6.49 mm, which is obtained theoretically. At that time, if higher mode is not occurring in overall band, check that the phase difference between these two modes is close to 90 and find the most appropriate size of waveguide. This process is conducted through computer simulation. Select the thinnest value within the possible limit, because the thinner the iris thickness the shorter the time of computer simulation and the size of computer memory. 4.2 Determine the iris thickness Determine the iris thickness within the range suggested in (6). The thinner the iris thickness, the easier it will be to adjust overall reflection characteristic and phase characteristic when the number of irises is increasing. 4.3 Determine the iris height Determine the number of irises as 5, because it should be more than 5 in order to give a taper in iris. Set the space between irises with two times the iris thickness and adjust the iris height so as to equalise the phase difference of transmission coefficients in the minimum frequency and the maximum frequency of the operational frequency band [11]. Determine the iris height using (8). Fig. 3 Design parameters of the iris polariser 4.4 Adjust the space between irises Make a fine tuning on the space between irises so as to equalise the phase difference of transmission coefficients in the minimum frequency and the maximum frequency of the operational frequency band.

3 4.5 Make the phase difference of 90 Multiply the number of irises by 90 /Δφ and make Δφ = 90. For example, if Δφ = 15, 90 /15 = 6, then the total number of iris becomes 5 6 = 30. When calculating the axial ratio, TE x =[A x, φ x ], TE y =[A y, φ y ], it is determined by (9) according to the sizes and phases of the two modes A x = A y (9.1) f x f y = 90 + D (9.2) where Δ is a value to satisfy axial ratio 1.5 db and its value is Change the iris height into the taper to improve the reflection characteristic Taper is used to improve the reflection characteristic. The kinds of tapers are linear taper, sine taper and exponential taper. Among these, the exponential taper makes good return loss characteristic [12]. Since the reflection characteristic in a waveguide is largely impacted by the height of the front iris, the purpose of including a taper is also to reduce the height of the front iris. If the overall iris volume is not changed in that moment, there should be theologically no change in the transmission phase difference. To enhance the reflection characteristic, the lower the height of the iris is the better. However, it should be normally more than 0.2 mm in consideration of machining. Fig. 4 illustrates the vertical structure of the polariser according to the number of irises linear taper: d(z) = d 1 + d o d 1 z (10.1) L [ sine taper: d(z) = d 1 + (d o d 1 ) 1 C ] z + C sin 2 pz L 2L (10.2) C = taper coefficient, 0, C, 1 [ ( exponential taper:d(z) = d 1 exp ln d ) ] o z d 1 L (10.3) 4.7 Make the transmission phase difference to exactly 90 Giving a taper in the iris height actually brings a slight change to the transmission phase difference. Theoretically speaking, giving a taper should not bring any change in the total iris volume inside the waveguide, but the design frequency is so high that a change in phase difference occurs even by a minute change in volume because of an extremely small value change in the design dimension. As the last step, make a fine tuning on the iris height and interval so as to make the transmission phase difference exactly 90 in the required band. 5 Design and measurement result Using the proposed method, a full-band polariser operating in the WR-22 waveguide band (33 50 GHz) is designed, fabricated and tested. The design of the polariser is analysed using the well-known commercial electromagnetic simulation software HFSS. The number of irises in finally designed iris polariser is 23 and the waveguide size is 7.5 mm, the iris thickness is 0.5 mm, the iris interval is Fig. 4 Vertical structure of the polariser according to the number of iris a Even number (2M) b Odd number (2M +1) Fig. 5 Design dimensions of the WR-22 standard waveguide polariser a External b Inside (unit: millimetres)

4 Fig. 6 Electric-field distribution of the polariser a TE 10 mode b TE 01 mode Fig. 7 Design result of the transition a Position of the transition b Design dimensions (unit: millimetres) Fig. 8 Simulation result of the transition a Return loss b Insertion loss 1.0 mm and the total length of the polariser is 43.5 mm. For the iris height, an exponential taper is used. Fig. 5 illustrates the design dimensions and Fig. 6 illustrates the electric-field distribution. Since the port of the polariser is not a standard size, the transition device is required to connect polariser and standard waveguide for measurements. The transition is designed to have low loss over a required frequency range. Fig. 7 illustrates the design result and Fig. 8 illustrates the simulation result. The designed polariser was produced with milling processing. Since it was impossible to make an all-in-one processing, it was divided into four parts for production. Two transitions were produced for measurement. Fig. 9 illustrates the fabricated polariser and transition. Fig. 9 Fabricated the WR-22 standard waveguide polariser and transition

5 as the REFLECT, waveguide of quarter times length of the centre wavelength as the LINE, and the THRU was used connecting directly to each other waveguide. Fig. 10 illustrates measurement structure of the polariser and Fig. 11 illustrates the results of the design and measurement of the WR-22 standard waveguide polariser. In the case of simulation results, the return loss is <30 db and the insertion loss is close to zero in overall band. The transmission phase difference is (87 97 ), and the axial ratio is within 1.1 db in the required overall band. The result of measurement, the return loss is <25 db and the insertion loss is an excellent characteristic that was <0.1 db in the required overall band. The transmission phase difference is ( ) and axial ratio is within 1.3 db in the required overall band. Fig. 10 Measurement structure of the polariser Two-port network analyser was used to measure characterisation of the polariser. For the accurate measurements, the calibration of the network analyser was done at the end (P1 and P2 ) of polariser using TRL(THRU/REFLECT/LINE) method. The TRL calibration method is widely used in two-port microwave circuit measurement [13]. The TRL calibration, a full two-port calibration method using three standards: THRU, REFLECT and LINE, is often employed [14]. In this paper, we used short load 6 Conclusion This paper studies design of a full-band waveguide polariser using iris for satellite communications. Operation theory and design parameters of a full-band polariser are introduced, and a systematic design method has been proposed. Using the proposed method, the full-band polariser operating in the WR-22 waveguide band is designed, fabricated and tested. Measurements of the fabricated polariser shows that the phase difference is <10 as a reference point by 90, the axial ratio is <1.3 db, the insertion loss is <0.1 db and the return loss is >25 db in the required overall band. Agreements between simulated and measured performances are good. The Fig. 11 Results of the design and measurement of the WR-22 standard waveguide polariser a Return loss b Insertion loss c Phase difference d Axial ratio

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