Planar Hexagonal Antenna with Dual Reconfigurable Notched Bands for Wireless Communication Devices

In this paper, a planar hexagonal antenna with dual tunable notched band using varactor diodes is presented. The designed antenna operates in the frequency range of 2 GHz to 8 GHz and is loaded by a complementary split ring resonator (CSRR) to achieve the notch-band characteristics. The CSRR produced two stop-bands at the frequencies 3 GHz and 6.8 GHz. In order to obtain the reconfigurability, one varactor diode was used on each ring of the CSRR. The variation of the DC bias of the diodes produced double notched bands yielding a tunable coverage in the 0.6 GHz and 1.6 GHz ranges. The continuous agility and the wide tuning range of the notched bands are the major advantages of this structure. The antenna prototype was manufactured and a good agreement has been achieved between the measured and simulated results. The proposed antenna can be a good candidate for wireless applications that cover the UMTS, the Wi-Fi and the WiMAX bands.


Introduction
During the last two decades, the wideband (WB) technology has drawn great attention to the RF design optimization in terms of complexity, cost, and power consumption. This technology is being intensively studied for many wireless communication systems and applications such as LTE, ISM, GSM, Wi-Fi, WLAN and WiMAX [1].
To overcome the possible interferences and distortions of electromagnetic signals, numerous efforts [2], [3] have been made to design and optimize wideband antennas with specific notched band characteristics. In [4], [5], the authors used the Coplanar Waveguide (CPW) technique to miniaturize wideband (WB) antennas while improving their operation bandwidth. Later, a new compact defected ground structure (DGS) was used for a microstrip line [6] in order to design a compact low pass filter (LPF). In [7], Electromagnetic Band Gap (EBG) was used to minimize the mutual coupling of microstrip antennas. Split ring resonators (SRR) [8,9,10] and complimentary split ring resonators (CSRR) [11], [12] have been a subject of a growing interest in recent years. In fact, they have been used to improve the out-of-band rejection of the antennas as described in [13][14][15][16].
Thanks to the growing demand for smart antennas, tuning the notched bands has become a suitable solution. In fact, a reconfigurable frequency structure is appropriate for many wireless applications. Frequency agility is obtained by using several switching technologies such as PIN diodes [17,18,19], MEMS (Micro-Electro-Mechanical-Systems) switches [20], [21], and varactor diodes [22].
In this paper, a planar broadband antenna with tunable dual stop-bands has been successfully designed. The main importance of the proposed structure is its high selectivity and its wide tuning range of notched bands.
The first section of this paper is dedicated to introduce the geometry of the broadband antenna and the CSRR structure. The second section is reserved for simulations and measurement results. By introducing a CSRR to the designed antenna, two rejected bands at S-band (3 GHz) and C-band (6.8 GHz) are obtained. Then, the reconfigurability of the double notched bands is obtained by the integration of one varactor diode on each ring of the resonator to decrease the complexity of the bias network.

Planar Hexagonal Antenna Design
In this work, the design procedure is performed in several steps. First, the structure of the patch antenna is printed on a 0.76 mm thick Rogers substrate (RO4350B) with a dielectric constant of 3.66 and a loss tangent of 0.035. The width W and length L of the antenna, calculated from equations cited in [23], are 30 mm and 40 mm, respectively. A hexagonal patch is placed in a circular aperture with radius of 14 mm etched on the CPW ground plane. The back side of the substrate is without metalization. The monopole antenna is connected to an SMA connector and is excited by a 50 Ω coplanar waveguide (CPW) feed-line. A complementary split ring resonator (CSRR) is placed in front of the feed-line as shown in Fig. 1(a).
The expression of the resonant frequency of the proposed antenna f r is as proposed in [24], and inspired from that of the resonant frequency of circular patch antennas for different TM nm modes such that where c is the speed of light, m and n are the radial and angular modes, respectively, and a e is the effective antenna radius such that Here, s is the hexagon's side length. For the TM 11 resonant frequency, X 11 is found to be 1.841 as given by [24].

CSRR Geometry
The CSRR is an artificially produced structure frequently exploited for antenna design and often used for the design of antennas with rejected bands [25], [26] to improve performances by reducing size and enhancing bandwidth, gain, and radiation patterns. Figure 2(a) shows the geometry of the proposed CSRR, which is made of two coupled slot rings etched on the upper conductor plane of the substrate and separated by a gap and a split at opposite sides of each ring. The lengths of the outer and inner rings, L 1 and L 2 , are 5.5 mm and 3.5 mm, respectively. The common width for the two rings is w = 0.5 mm. The space between the rings g is 0.5 mm and the opening gap s is 0.7 mm. The simulation of this structure is carried out using the CST Microwave Studio  software. To simulate and acquire desired results, boundary conditions and excitation sources need to be identified. The simulated transmission coefficient magnitude |S 21 | of the proposed CSRR is illustrated in Fig. 3. As it can be observed, two resonant frequencies appeared at 3.5 GHz and 7 GHz.

Simulations Results
In this work, the simulation results of the designed antenna were investigated by exploiting the CST Microwave Studio software. Figure 4 presents the reflection coefficient magnitudes of the designed antenna with and without the CSRR resonator. It is observed that the designed antenna without CSRR can produce wideband impedance matching from 2 to 8 GHz with a reflection coefficient magnitude |S 11 | ≤ -10 dB. Whereas the insertion of the CSRR on the designed antenna structure provides the appearance of two notched bands, each ring of the resonator controls a single notched band independently. The first rejected frequency band is spread around 3 GHz and is controlled by the external ring while the second one is spread around 6.8 GHz and is controlled by the internal ring. This makes the resonator suitable for monitoring two notched frequency bands simultaneously or independently.

Simulation Results of the Designed Antenna Structure with and without CSRR
A parametric study of different positions of the CSRR on the antenna along the y-axis was performed and it proved that the optimal position is obtained when the CSRR is near the feed-line with y 1 = 4.7 mm, where the unit cell illustrates the highest rejection band |S 11 | ≥ -2 and -4 dB, respectively. Moreover, the integration of the CSRR in the structure reduces around 20% of the antenna size since its resonant frequency decreased from 5.8 GHz to 4.8 GHz.  The obtained maximum gain of the antenna over the frequency range of [2 to 8 GHz] with and without the CSRR is presented in Fig. 5. It should be noted that the increase of frequency leads to the increase of the antenna gain without the CSRR. By adding the CSRR to the antenna, the gain is improved except for the two rejection zones where the gain decreased to -2 dBi.
The three-dimensional radiation patterns of the designed antenna with and without the CSRR at the frequency of 5 GHz are presented in Fig. 6. It can be observed that the realized gain increased from 4 dBi without CSRR to 5.13 dBi with CSRR. The obtained results show also that, the antenna has an omnidirectional radiation pattern in the (x, y) plane. Therefore, it can be concluded that the insertion of the CSRR in the antenna structure improves the behavior of the antenna without disturbing its radiation features.
To manifest the performance of the designed antenna, the total efficiency with and without the CSRR is simulated and drawn in Fig. 7. For both cases, the antenna has a good radiation efficiency of up to 90%, apart from the notched frequency bands where the efficiency decreased to 20%.  Figure 8 shows the final designed antenna layout. The reconfigurability is obtained by loading the SKYWORKS varactor diode (SMV1430-040LF) on each ring of the CSRR [27].

Simulation of the Designed Antenna Structure using Varactor Diodes
The equivalent circuit of the diode is shown in Fig. 9. The capacitance of the used diodes can be controlled from 0.3 to 1.29 pF by varying the reverse bias voltage. In addition, two metalized vias are used to connect the diodes with two insulated square patches laid on the back side of the structure. These patches with the appropriate dimension (5 × 5 × 0.035 mm 3 ) lead to the highest rejection band (|S 11 | ≥ -3 dB). Then, the DC voltage is connected via a CC45T47K240G5 inductor to isolate the RF signal and a 20 kΩ resistor to protect the diodes [21]. During the simulation, the varactor is treated as a lumped element by considering a resistance R in series with a capacitance C. The values are extracted from the datasheet of the component.
To load the diodes in the appropriate positions, the surface current distribution in the antenna is studied as shown in Fig. 10. Indeed, at the rejected frequency 3 GHz, the current is concentrated in the outer ring, while at 6.8 GHz (the second reject frequency), the current is concentrated in the inner ring. However, in both cases the surface current distribution is concentrated near slots.  The proposed antenna has been simulated for equal capacitance values C = C 1 = C 2 . Figure 11 shows the magnitude of the obtained reflection coefficient |S 11 | for different values of C in [0.3, 0.5, 0.8, and 1.29] pF. As shown in the figure, increasing C causes the notched bands to shift towards lower frequencies. The first band shifted from 2.8 to 2.2 GHz, leading to a tuning range of 0.6 GHz, while the second band is shifted from 5 to 3.4 GHz leading to a tuning range of 1.6 GHz. This makes the antenna a good candidate for multimode and multi standard applications such as UMTS, Wi-Fi and WiMAX.  Figure 12 illustrates the total efficiency of the antenna for several values of C. An efficiency as high as 80% has been observed in the pass-band frequencies, while a low efficiency, not exceeding 40%, has been observed at the rejection zones that change position with the value of C.
Since each diode controls one rejection zone, varying C 1 and C 2 separately allows to control the two rejection zones separately and hence offers more flexibility and adds more reconfigurability to the proposed design. Figure 13

Measurements Results
The reconfigurable wideband antenna was manufactured as shown in Fig. 14. Two varactor diodes were soldered on the realized antenna. To measure the magnitude of the reflection coefficient over the frequency range from 2 GHz to 8 GHz, a vector network analyzer was used. The gain and the radiation pattern measurements were performed in an anechoic chamber. For experimental purposes, the capacitances of the varactor diodes are considered equal: C = C 1 = C 2 .  Figure 15 presents the simulated and measured reflection coefficient magnitudes of the designed antenna. The figure reveals a small discrepancy between the measured and simulated results that can be due to the approximate boundary conditions used in simulation as well as the accuracy of the diode's model, in addition to the material defects. Based on the capacitance values, the antenna can cover UMTS, Wi-Fi, and WiMAX bands depending on the desired application. Figures 16 and 17 show the simulated and measured radiation patterns of the proposed broadband antenna in the E-plane (yoz) at some selected frequencies for C= 0.8 pF and C= 0.5 pF. A stable radiation pattern is obtained and is well maintained over the whole operating frequency bandwidth. As a conclusion, the reconfigurable CSRR removes the unwanted bands without affecting the antenna radiation performance.  From Tab. 1, the capacitance value and the applied voltage are inversely proportional. In addition, varying the capacitance values of the two diodes attached to each ring of CSRR from 0.3 to 1.29 pF leads to a variation of the notched frequency band from 2.63 to 2 GHz for the first notched band and from 5.15 to 3.55 GHz for the second notched band, respectively. The tuning range of the first rejected band is around 0.6 GHz while the second is around 1.6 GHz. These effects make the antenna advantageous in terms of dual wide tuning ranges. The simulated and measured results show a good agreement. Table 2 presents a comparative study of performances of the designed antenna with other existing reconfigurable notched antennas [3], [28,29,30]. It is observed that the designed reconfigurable notched antenna presents an important improvement in terms of radiation characteristics and simplicity of design. Furthermore, a wide tuning range of 1.6 GHz is achieved.

Conclusion
In this work, a wideband antenna with double tunable notched bands is designed. The integration of the CSRR resonator in the antenna structure provides two rejected bands at 3 GHz and 6.8 GHz. The tuning of the rejected bands is obtained using two varactor diodes in the antenna structure. The tuning ranges of the first and second notched bands are around 0.6 GHz and 1.6 GHz, respectively.
These wide tuning ranges are obtained by varying the capacitance values of the varactor diodes by sweeping their reverse bias voltage from 0 to 30 V. The developed wideband planar antenna with reconfigurable frequency notched bands, tuning range, moderate gain, and stable radiation patterns may be a potential structure for modern wireless communication systems. His main scientific interests cover investigation of planar passive elements and subsystems of microwave technology, and propagation of electromagnetic waves in periodic structures and metamaterials. He is the author or co-author of more than 250 publications in scientific journals, including international ones as well as national conferences.
Sabri BELDI was born in Bousselem, Tunisia. He received the master's degree in Electrical Engineering from the Faculty of Sciences of Tunis, Tunisia. He received his Ph.D. in Electronics in 2016 from the Faculty of Sciences of Tunis, Tunisia. He was named an Assistant Professor of Electrical Engineering in 2016. His main scientific interests cover investigation of planar passive elements and subsystems of microwave technology, propagation of electromagnetic waves in periodic structures and metamaterials. He is the author or coauthor of more than 25 publications in scientific journals, including international ones as well as national conferences.