IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
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Reflectarray Element Using Cut-Ring Patch Coupled to Delay Line B. D. Nguyen, Kien T. Pham, Van-Su Tran, Linh Mai, and N. Yonemoto
Abstract—In this letter, new phase-shifter cells for reflectarray applications using the cut-ring patch coupled to the delay line through an annular slot are presented. The new configuration allows the phase shift to be obtained by adjusting the length of a single or two different delay lines. Various prototypes of the elements have been fabricated and measured in X-band using the waveguide simulator technique. A good agreement between the measurement and simulation for both reflection phase and magnitude is achieved. Experimental results indicate good characteristics of the reflectarray element with a wide range of reflection phase and good linearity of the phase curve. Index Terms—Phase shifter, reflectarray antennas, reflectarray element.
I. INTRODUCTION
M
ICROSTRIP reflectarray antennas have been under investigation over the last two decades. Their advantages over the parabolic reflectors are well known for light weight, low cost, and planar structure. Furthermore, they can allow electronic beam steering by integrating with active devices. The design principle is based on the capability of phase adjustment of reflecting elements to compensate for the different phase delays caused by the different path lengths from the illuminating feed antenna. Therefore, the critical point in the reflectarray design is to obtain a reflection phase range over 360 . Several structures of reflectarray element have been proposed to control the reflection phase such as: fixed-size patches with variable-length stubs [1], patches with variable size [2], or variable rotation angle of patches [3]. However, a common disadvantage of microstrip reflectarray antennas is their narrow bandwidth. The performance of a reflectarray antenna depends on the sensitivity of the reflection phase to the frequency. A smoother phase variation can provide a broader bandwidth. To ameliorate the bandwidth performance, multilayer structures have been used. A stack-patch element proposed in [4] can broaden the reflectarray bandwidth. However, this kind of reflectarray element can only be used for the fixed-beam reflectarray antennas.
Manuscript received June 17, 2014; revised August 05, 2014; accepted November 02, 2014. Date of publication November 20, 2014; date of current version February 06, 2015. This work was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant No. 102.99-2012.38. B. D. Nguyen, K. T. Pham, V.-S. Tran, and L. Mai are with the School of Electrical Engineering, International University-VNU, Ho Chi Minh City, Vietnam (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). N. Yonemoto is with the Avionics Department, Electronic Navigation Research Institute, Tokyo 182-0012, Japan (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2372316
The reflectarray element based on aperture-coupled patch is a good solution to provide a very wide bandwidth [5] and a wide range of the phase delay [6]. The geometry of the elements is based on a square patch coupled to the delay line through a rectangular slot. A considerable advantage of this design is that the delay line to vary the phase is placed on the nonradiation surface that is opposite to the reflecting surface. The two surfaces are separated with each other by a common ground plane. Therefore, the delay line does not interfere with the radiation pattern. Furthermore, various kinds of active devices can be integrated into the aperture-coupled element to implement electronically reconfigurable reflectarrays such as varactors [7], [8], p-i-n diodes [9], or RF microelectromechanical systems (MEMS) [10], [11]. In [7] and [8], a continuous phase shift over a range of about 320 has been obtained with a single varactor diode for each reflectarray element. This configuration has demonstrated a good beam-scanning capability. In [9]–[11], p-i-n diodes and RF-MEMS switches were integrated into the delay line to control the phase shift by changing the length of the delay line through ON/OFF states. These active elements showed a wideband characteristic the same as passive elements did, thanks to the capability of producing the true-time delay. Another advantage of digital approach is the capability to overcome the sensitivity of the phase to the dc control voltage. Nevertheless, it is clear that the use of switching devices does not allow a continuous phase adjustment, which leads to a degradation of reflectarray gain due to the phase errors. The increase in numbers of switching devices to increase the number of phase states can minimize the degradation. However, in [9]–[11], the phase shift performed by varying the length of only one delay line shows a drawback when more switching devices are added. Because these devices are arranged in series on a delay line, increasing the number of devices will lead to an increase in loss due to the accumulation of the insertion loss of the devices, as described in [11]. With the same number of phase states, the losses can be improved if the number of switching devices in ON state at the same moment is reduced. In this letter, we propose two multilayer reflectarray elements based on a cut-ring patch coupled to a delay line. The phase shift can be obtained by varying the length of one delay line or two delay lines. Both types of elements can provide a wide bandwidth response, a large range, and good linearity of the phase shift. For the case of electronically beam-switching reflectarrays, the element with two delay lines can provide more room for active devices as well as reduce the number of devices arranged in series than that of the element with single delay line. The elements have been designed to operate in the X-band. Several prototypes of the elements with different lengths of the delay line have been fabricated and measured to validate the design.
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
Fig. 1. Layer structure of the element with a single delay line. (a) Expanded view. (b) Side view and top view.
II. REFLECTARRAY ELEMENT CONFIGURATION A. Element With Single Delay Line The reflectarray element is shown in Fig. 1. The design is based on a double-layer substrate that is RT/Duroid 5880 and dielectric loss tangent substrate with permittivity . The element consists of a fixed-sized cut-ring patch printed on a top dielectric substrate, an annular coupling slot on the common ground, and a variable-length microstrip line attached to circular patch arranged on the bottom of the second substrate. The cut-ring patch is used as a radiating element that is optimized to resonate near the frequency of 11.8 GHz. The operation of the element is that when incident wave arrives at the cut-ring patch, it will be coupled to the line through an annular slot. Because the delay line is open-ended, the wave will be reflected and will be recoupled to the cut-ring patch. Therefore, the phase delay is proportional to the line length. In this configuration, the variation of the reflection phase is controlled by the length of the delay line. The length of the delay line ( ) is defined by summing three segments , and . The element is designed for linear polarization where the incident wave polarization must be perpendicular to the direction from the center to the cut-edge of the patch, i.e., E-field is along the -axis, as depicted in Fig. 1. The annular slot dimensions are optimized to maximize the coupling between the line and the cut-ring patch at the working frequency. The circular patch attached to the line can be used to tune the matching impedance between the line and the cut-ring patch in order to obtain a good linearity of the phase curve. A thick substrate is also used for the upper substrate to achieve a wide bandwidth. The element is optimized to work in the X-band with the center frequency of 11.8 GHz. The optimized parameters of the element are reported in Table I. Numerical simulations were carried out by using ANSYS HFSS software. The infinite periodic structure is used by placing electric and magnetic walls around a unit cell. It means that the simulated elementary cell is placed in an infinite array containing identical and equally spaced elements. The infinite array approach allows giving adequate results by taking into account the mutual coupling between neighboring elements. This approach is described in [12]. The simulated reflection phase and magnitude for a normal incident wave are shown in Fig. 2 for the 11.4–12.4-GHz band. In order to increase the length of the delay line, the line is bent
Fig. 2. Simulated magnitude and phase of reflected wave versus delay line length. TABLE I DIMENSIONS OF THE ELEMENT WITH TWO DELAY LINES
and stretched. It can be observed that the reflection phase covers a range of around 900 for a variation of the length of 25 mm. Reflection phase curves at different frequencies are almost parallel, and they show a smooth behavior. The magnitude curves show the local minima due to the resonance of the delay line at specific lengths. The resonance of the line causes an increase in the refection loss as well as in the nonlinearity of the phase curve. Compared to the single-layer reflectarray element, the reflection loss in this case is higher. This is due to the back-radiation generated by the annular slot and the line. This loss can be ameliorated by adding a ground plane underneath the delay line with a distance of . Switching devices such as p-i-n diodes or RF-MEMS switches can be integrated into the line in order to implement electronically beam-switching reflectarrays. With a variation of the line length of 25 mm, a phase range of three cycles is obtained. A large phase range provides more options for the location of active devices. When switching devices are placed in series on a delay line, of devices will produce phase states. However, reflection loss is proportional to the number of switching devices due to the insertion loss of the device. Indeed, the loss caused by a device in the ON state is twice as its insertion loss because the coupling wave travels twice on the delay line. B. Element With Two Delay Lines Geometry of the element with two delay lines is shown in Fig. 3. For this element, two delay lines are connected to a common circle patch. The common circular patch is connected to the inner circle of the annular slot through a shorting pin. This
NGUYEN et al.: REFLECTARRAY ELEMENT USING CUT-RING PATCH COUPLED TO DELAY LINE
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Fig. 3. Layer structure of the element with two delay lines. TABLE II DIMENSIONS OF THE ELEMENT WITH A SINGLE DELAY LINE
connection is very important in order to establish the starting point (point zero) for each delay line where they are considered as two parallel lines. This allows the phase to be changed independently with respect to the length of each line. The element is optimized for working in the X-band at central frequency of 11.8 GHz. The RT/Duroid 5880 substrates are also used. A thicker substrate of 0.508 mm is used for the second substrate in order to decrease the nonlinearity of the phase curve when two lines vary with different lengths. This leads to a modification in the size of the annular slot compared to the element with single delay line. The optimized dimensions for the element with two delay lines are presented in Table II. Fig. 4 shows the phase and magnitude of the reflected wave versus the length of delay lines for the element with two symmetrical delay lines, i.e., two lines have the same length. The radius is adjusted to 2.5 mm in order to optimize the linearity of the phase curve at 11.8 GHz. It can be observed that a wide range of the reflection phase with very good linearity is obtained. The losses are better than those of the element with single delay line. Fig. 5 shows the reflection phase and magnitude characteristics for the elements with asymmetrical delay lines where the length of one line ( ) varies while the other line ( ) is fixed, in comparison to the element with symmetrical delay lines. For each length of the fixed delay line, the radius of the circular patch should be readjusted to optimize the linearity of the phase curve when the length of the other delay line varies. As can be seen, when the length of one line is fixed, the reflection phase varies proportionally with respect to the length of the other delay line. Although the phase curve is not as linear as that of the element with symmetrical delay lines, it still shows a good smooth behavior. The reflection loss also remains at low level, same as that in the symmetric case.
Fig. 4. Simulated magnitude and phase of reflected wave versus delay line length for the element with two symmetrical delay lines.
Fig. 5. Simulated magnitude and phase of the element with asymmetrical delay lines, at 11.8 GHz.
In the case of designing electronically beam-switching reflectarrays, switching devices will be placed on each line to control the phase shift through controlling the length of each delay line. This is the main advantage of the element with two delay lines over the element with a single delay line, which is to provide more room for active devices as well as to reduce the number of devices arranged in series on a line, hence reduce the losses. III. EXPERIMENTAL VALIDATION In practice, a well-known solution to validate the design of reflectarray elements is to use the waveguide simulator (WGS) technique. This approach is performed by putting a few of fabricated elements inside a waveguide. The reflection phase and magnitude are determined by measuring the reflection coefficient ( ) at the excitation port. Several prototypes with different line lengths have been built and measured to validate the characteristics of the element. Each prototype contains four identical cells, and the surface is of mm , as shown in Fig. 6. A standard X-band waveguide (WR-90) was used as a waveguide simulator. In order to insert prototypes into the waveguide, an open-end tapered waveguide of mm was
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
Comparisons between the simulation and measurement results in terms of the reflection phase and magnitude for both kinds of elements at several frequencies are shown in Figs. 7 and 8. A good agreement between these results can be observed. The measured reflection losses are slightly higher compared to those of the simulation. This can be the result of manufacturing tolerances, the manipulation setup, and material losses. IV. CONCLUSION
Fig. 6. Measurement using waveguide simulator.
Two kinds of reflectarray elements based on a cut-ring patch coupled to a delay line have been validated. Measurement results demonstrate good performance of reflectarray element with a large reflection phase range and a low loss. A smooth phase variation and wide bandwidth are also achieved. Both kinds of elements can be applied for both passive reflectarray and electronically beam switching reflectarray antennas. Switching devices such as MEMS switches and p-i-n diodes can be easily integrated into the delay line to implement an electronically reconfigurable beam reflectarray antenna. The element with two delay lines allows independent variation of the reflection phase with respect to the length of each line. This is a considerable advantage of the element based on cut-ring patch coupled to delay line through an annular slot in order to provide more room for active devices as well as to reduce the number of which arranged in series. Furthermore, the proposed reflectarray element is printed on the low-cost substrate. It can be used as a low-cost solution to replace conventional reflectors and phased array antennas. REFERENCES
Fig. 7. Comparison between simulated and measured reflection coefficient for the element with a single delay line.
Fig. 8. Comparison between simulated and measured reflection coefficient for mm; mm; the element with two asymmetrical delay lines, varies.
implemented and then connected to the WR-90 standard waveguide. Measurement of the phase and magnitude of the reflected wave was carried out by Network Analyzer.
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