Mechanically Reconfigurable, Dual-Band Slot Dipole Antennas

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 7, JULY 2015

[14] P. F. Sammartino, C. J. Backer, and H. D. Griffiths, “Frequency diverse MIMO techniques for radar,” Trans. Aerosp. Electron. Syst., vol. 49, no. 1, pp. 201–222, Jan. 2013. [15] W. Q. Wang, H. S. Shao, and J. Cai, “Range-angle-dependent beamforming by frequency diverse array antenna,” Int. J. Antennas Propag., vol. 2012, pp. 1–10, Jan. 2012. [16] W. Q. Wang, “Range-angle dependent transmit beam pattern synthesis for linear frequency diverse arrays,” IEEE Trans. Antennas Propag., vol. 61, no. 8, pp. 4073–4081, Aug. 2013.

Mechanically Reconfigurable, Dual-Band Slot Dipole Antennas Ibrahim T. Nassar, Harvey Tsang, Dane Bardroff, Craig P. Lusk, and Thomas M. Weller

Abstract—Mechanically reconfigurable dual-band slot dipole antennas with wide tuning ranges are developed in this communication. The dualband operation is achieved by inserting parallel slots with different lengths and the tuning is realized by incorporating a dual rack and pinion mechanism to slide parasitic patches to vary the slot lengths and the frequency of operation of each band. To demonstrate the proposed approach, two frequency-tunable antennas are prototyped. The first design uses two motors to tune the frequency band ratio between 1 and 2.6. The second design uses one motor to maintain a constant frequency band ratio of 2 over the tuning range of 2–3 GHz for the lower band and 4–6 GHz for the upper band. Both designs show minimal variation in gain, polarization, and radiation pattern over the tuning range. Index Terms—Dual-band antenna, frequency tunable, reconfigurable antenna, slot antenna.

I. I NTRODUCTION Frequency reconfigurable and multiband antennas are attractive candidates for modern wireless systems including satellite and mobile communications, as well as radar [1]. These antennas have the potential to satisfy the increasing demand for multifunctionality, compact size, and low cost. In comparison to the use of a single wideband antenna, these antennas provide frequency selectivity that is useful for minimizing interference and jamming effects and reduce the complexity and size of the receiver front end [2]. In addition, they provide compact alternatives to multiantenna installations. The challenge with the design such antennas, however, is achieving operation at different frequency bands with consistent radiation characteristics and without degrading the impedance match BW. Manuscript received June 19, 2014; revised March 06, 2015; accepted April 03, 2015. Date of publication April 17, 2015; date of current version July 02, 2015. This work was supported by the National Science Foundation under Grant ECS-0925929 and Grant CMMI-1053956. I. T. Nassar was with the University of South Florida, Tampa, FL 33620 USA. He is now with ANSYS Inc, Irvine, CA 92602 USA (e-mail: [email protected]). H. Tsang is with the University of Texas El Paso, El Paso, TX 79902 USA (e-mail: [email protected]). D. Bardroff, C. P. Lusk and T. M. Weller are with the University of South Florida, Tampa, FL 33620 USA (e-mail: [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2015.2423699

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Slot dipole antennas are widely used for the purpose of achieving multiband operation (e.g., [3], [4]). One of the common techniques is to exploit the first higher order mode. Unlike the fundamental mode which has a broadside bidirectional radiation pattern, the natural radiation pattern of the first harmonic mode has a null in the broadside direction, although different methods can be introduced to modify the radiation pattern to be similar to the fundamental mode patterns. For example, in [5], coupling slots are shown to be an effective solution while in [6]–[8] the problem is addressed by using slots of different lengths. Achieving frequency band ratios that are greater than 2 or 3 along with consistent radiation characteristics remains a challenging problem. An alternative approach to multiband operation is to employ a frequency tunable antenna. In the literature, tunable antennas are realized by incorporating microelectro-mechanical systems (MEMS), tunable materials, and different solid state devices such as varactors and PIN diodes [9], [10], reactive FET components [11], and shunt switches [12]. With these approaches, the tuning range is limited in part due to the added loss and nonlinearity induced upon the RF signal, and the radiation properties are not preserved over the entire tuning range. In addition to the limited tunability, the use of nonlinear devices may produce undesired signals through the production of harmonic and intermodulation products [13]. To avoid those limitations and improve the tunability range, low cost mechanical reconfiguration methods can be implemented [14]. The drawbacks of these approaches are the slow speed and reduced reliability, compared to the use of electronic devices. In this communication, a new method for the design of tunable dualband slot antennas is proposed. The dual-band performance is achieved by inserting two slot pairs with different lengths, similar to the method presented in [8], and tunability is accomplished by employing a rack and pinion mechanism to slide parasitic patches over the antenna to vary the slot lengths and thus the frequency of operation of each band. Two tunable antennas are presented herein. The first design uses two separate motors to slide parasitic patches over each slot pair independently. Using this approach, a frequency band ratio between 1 and 2.6 is obtained. The second design incorporates one motor to slide the parasitic patches uniformly over both of the slot pairs, maintaining a fixed frequency band ratio. In this demonstration the ratio is fixed at 2 to cover the 2–3 GHz range for the lower band and 4–6 GHz for the upper band, or 40% tunability for each band. Over the tuning range the polarization remains linear without degradation in the co-pol to cross-pol ratio and the gain and radiation patterns are consistent. To reduce the cost and weight of the antennas, three-dimensional (3-D) printing technology is employed. In the following sections, the design, testing, and analysis of both of the dual-band antennas are presented. This communication is organized as follows. Section II illustrates the primary antenna structure. Section III describes the antenna with the arbitrary frequency ratio and Section IV presents the second design with the frequency ratio of two. Section V presents the mechanical testing of the antenna integrated with a commercial stepper motor. II. A NTENNA S TRUCTURE Fig. 1 illustrates the dual-band slot dipole antenna, the primary radiating structure used in the proposed design. Similar to the design presented in [8], the dual-band operation is accomplished by reducing the primary slot antenna length (L) and placing an additional thin slot pair in a symmetrical manner along the primary slot axis. The lower band frequency is determined by the length of the longer thin

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Fig. 1. Configuration of the dual-band antenna structure.

Fig. 3. Dual-band slot antenna measured radiation patterns; H-plane (left) and E-plane (right). θ = 0o is the broadside direction (top surface of the antenna).

TABLE I D UAL -BAND S LOT A NTENNA D IMENSIONS IN mm

Fig. 2. Measured and simulated S11 of the dual-band slot antenna.

slot (LL ) and the length of the primary slot (2 × (LL + L) ≈ λ/2), and the upper band frequency is mainly determined by the length of the upper slot (2 × (LU + L) ≈ λ/2). The slot lengths are optimized to have the lower and upper bands occur at 2 and 4 GHz, respectively. The antenna is fed with a coplanar waveguide (CPW). The widths W and Ws are optimized using ANSYS HFSS v14 to tune the impedance match. The size of the ground plane and the length of the feed line have minimal impact on the antenna performance, and they are optimized to improve the impedance match and have enough room to place the linkage. Table I lists the antenna dimensions. The antenna is fabricated with 3-D printing technology by depositing Dupont CB-028 silver ink (conductivity of ∼2.1 × 105 S/m) on a 100-mils thick Acrylonitrile Butadiene Styrene (ABS) substrate, relative dielectric constant of ∼2.4 and loss tangent of ∼0.0038. The silver is printed using an nScrypt direct print additive manufacturing tool and the substrate is fabricated with the fused deposition modeling (FDM) process. The weight of the antenna is measured to be 16 g. Fig. 2 shows the S11 response of the antenna. As seen, the measured and simulated data are in good agreement. The 10-dB return loss (RL) bandwidth (BW) of the lower and upper bands is ∼17% and ∼8.25%, respectively. The resonance that occurs at 5.3 GHz can be removed using a harmonic trap, as described in [15]. Fig. 3 illustrates the measured E- and H-plane radiation patterns at both of the frequency bands. The antenna exhibits broadside bi-directional radiation with a maximum measured broadside gain of 4.5 and 6.8 dBi at the lower and the upper bands, respectively. The gain is lower at the lower band due to the smaller electrical size of the antenna. The antenna is linearly polarized with a measured co-to-cross pol ratio greater than 25 dB in both of the bands.

Fig. 4. CAD illustration of the reconfigurable dual-band antenna with arbitrary frequency ratio; top view (top) and side view (bottom).

Fig. 5. Photograph of the dual-band slot antenna with arbitrary frequency ratio.

III. A NTENNA W ITH T UNABLE F REQUENCY BAND R ATIO Fig. 4 illustrates the first prototype antenna with the linkage attached. The linkage comprises a pinion and dual racks which convert rotational motion into linear motion and is actuated by a miniature stepper motor from the bottom side. The sliding motion of the racks is guided by two supports mounted onto the top surface of the substrate. These supports allow the racks to slide horizontally with minimal resistance yet inhibit rotation and vertical translation. The nature of the dual rack and pinion mechanism allows for the horizontal translation of the two racks in opposite directions. A photograph of the first prototype is shown in Fig. 5. The linkage is also printed of ABS material to minimize the weight, and thus the required power to operate it.


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Fig. 6. Simulated S11 of the dual-band antenna with arbitrary frequency ratio for different XL and XU = 0 mm (left) and different XU and XL = 0 mm (right).

Fig. 8. Measured (dashed) and simulated (solid) S11 for XL = 0 mm and different XU values.

Fig. 7. Measured (dashed) and simulated (solid) S11 for XU = 0 mm and different XL values.

On the bottom side of the racks, parasitic patches are attached to vary the slot lengths as the racks slide. XL represents the inward compression of the longer slot pair and XU represents the decrease in the shorter slot pair. Fig. 6 (left) shows the simulated S11 for XU = 0 mm and different XL values. As seen, varying XL changes the lower band frequency without affecting the upper band. As XL changes from 0 to 18 mm the lower band frequency increases from 2 to 4 GHz, which occurs when LL and LU are equal. Fig. 6 (right) shows the simulated S11 performance for XL = 0 mm and different XU values. As XU changes from 0 to 10 mm, the lower band frequency does not change and the upper band frequency increases from 4 to 5.25 GHz. The upper frequency can increase further if XL increases along with XU . By varying XL and XU independently the frequency band ratio can vary between 1 and 2.6. Figs. 7 and 8 compare the measured and simulated S11 for different XL and XU values. As seen, good agreement between the measured and simulated data is achieved. Fig. 9 (left) illustrates the measured E- and H-plane radiation patterns for the case of XL = 12 mm and XU = 0 mm at 3 GHz and Fig. 9 (right) shows the measured patterns for XL = 0 mm and XU = 5 mm at 5 GHz. As seen, the patterns are similar to those shown in Fig. 3. As XL varies from 0 to 18 mm, the simulated gain varies between 4.5 and 6 dBi over the lower band range and as XU varies between 0 and 10 mm the gain varies between 7.2 and 5.3 dBi over the upper band range. For the cases shown in Fig. 9, the measured gain was found to be in good agreement with the simulated data. The measured co-pol to cross-pol ratio also remains greater than 22 dB as XL and XU are varied.

Fig. 9. Measured E-and H-plane for XL = 12 mm and XU = 0 mm at 3 GHz (left) and for XL = 0 mm and XU = 5 mm at 5 GHz (right).

IV. A NTENNA W ITH A F REQUENCY BAND R ATIO OF 2 Using the same reconfiguration mechanism, a tunable dual band antenna with a fixed frequency band ratio can be developed. This design approach can be accomplished using a single actuator and motor to have the parasitic patches simultaneously move together. In this demonstration, the ratio is fixed to a value of 2 to realize a tunable harmonic antenna. These antennas can be used for harmonic radar applications [16], [17] to minimize the radar unit size and reduce the weight. While these antennas have bi-directional patterns that may not be desired for high power radar applications, several approaches can be employed to realize unidirectional radiation including backing the antenna with a cavity [18], an artificial magnetic conducting reflector [19], or a closely-spaced ground plane [20]–[22]. These approaches, however, may degrade the BW of each band and the tuning range. Fig. 10 illustrates the second design geometry. Compared to the previous design, the primary slot is meandered and LL and LU are optimized to be 27.5 and 10.25 mm, respectively, to maintain a frequency ratio of 2 and good impedance match at both of the bands as X1 varies. Fig. 11 shows the simulated S11 for different X1 values. As X1 changes between 0 and 10 mm, the lower band frequency changes uniformly between 2 and 3 GHz and the upper band frequency changes between 4 and 6 GHz. While the S11 minimum of the second band does not occur at exactly twice the frequency of the first band over the entire range, the 10-dB RL BW of the second band does contain the second harmonic frequency. Fig. 12 compares the simulated and measured S11 for different X1 values. As seen, good agreement is found at the lower band, while

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Fig. 10. Configuration of the dual-band slot antenna with frequency ratio of 2 and one motor. Top view (top) and backside view (bottom).

Fig. 12. Comparison of the simulated (solid) and measured (dashed) S11 of the second design for different X1 values.

Fig. 11. Simulated S11 of the dual-band antenna with a frequency ratio of 2 for different X1 values.

there is a considerable deviation at the upper band. The deviation can be attributed to the high sensitivity to the alignment of the parasitic patches, contact between the parasitic patches and the primary conductive plane, and symmetry of the slot length compression during the rack movement. These issues arose with this design because of fabrication and assembly errors. The measured radiation patterns of the second design for the cases of X1 = 0 and 8 mm are illustrated in Figs. 13 and 14, respectively. As seen, the patterns are consistent with those shown in Fig. 3, while there is distortion that stems primarily from the minor asymmetry of the slot lengths which occurs due to printing errors and the nonuniform motion of the linkage. As X1 changes from 0 to 10 mm, the simulated peak broadside gain varies between 4.5 and 6 dBi over the lower band range, and varies between 6.2 and 5.4 dBi at the upper band range. The gain was measured at 2 and 4 GHz and found to be 4 and 6.8 dBi, respectively. The co-pol to cross-pol ratio is also measured and found to be similar to the previous design.

V. M ECHANICAL T ESTING The presented linkage and mechanism can be actuated by numerous stepper motors with varying size, step angle, accuracy, speed, and power consumption. Standard stepper motors can have a step angle as small as 1◦ /step, which is equivalent to 0.1 mm horizontal movement and a corresponding shift in the antenna resonant frequency of