Low-Proﬁle Frequency Reconﬁgurable Antenna for Heterogeneous Wireless Systems

: A low-proﬁle (0.21 λ g × 0.35 λ g × 0.02 λ g ) and a simply-structured frequency-switchable antenna with eight frequency choices is presented in this paper. The radiating structure (monopole) is printed on a 1.6-mm thicker, commercially-available substrate of FR-4 ( (cid:101) r = 4.4, tan δ = 0.020). it uses three PIN diodes in the designated places to shift the resonant bands of the antenna. The antenna operates at four different modes depending on the ON and OFF states of the PIN diodes. While in each mode, the antenna covers two unique frequencies (Mode 1 = 1.8 and 3.29 GHz, Mode 2 = 2.23 and 3.9 GHz, Mode 3 = 2.4 and 4.55 GHz, and Mode 4 = 2.78 and 5.54 GHz). The performance results show that the proposed antenna scheme explores signiﬁcant gain ( > 1.5 dBi in all modes) and reasonable efﬁciency ( > 82% in all modes) for each mode. Using a high-frequency structure simulator (HFSS), the switchable antenna is designed and optimized. The fabricated model along with the PIN diode and biasing network is tested experimentally to validate the simulation results. The proposed antenna may also be combined in compact and heterogeneous radio frequency (RF) front-ends because of its small geometry and efﬁcient utilization of the frequency spectrum.


Introduction
Recent advancements in wireless communication systems need transceivers to be efficiently utilized with heterogeneous systems such as wireless local area network (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), Long-Term Evolution (LTE), Fourth-Generation (4G), LTE Advanced Pro (4.5G), Fifth-Generation (5G), and many more [1]. One of the important components of the transceiver is antenna. The antenna being an important component of the transceiver occupies a major portion of the system [2,3]. The size increases further by incorporating any tuning mechanism in it. Since, the antenna required for modern transceivers must be compact and frequency switchable to meet the standard of the transceivers. In this regards, many approaches have been used to reduce the size of the antenna and to make it tunable for many useful applications.
The antenna which is capable of switching its resonant band according to the user requirement, is known as a frequency-reconfigurable antenna. Various techniques have been adopted to change the surface current distribution of the antenna, which produces a highly-reconfigurable antennas. In [4,5], the lumped element between the monopole and the parasitic patch is used to switch the antenna from a single band to a dual band. A compact-sized frequency-reconfigurable antenna for mobile handsets is reported in [6]. The reported antenna in [6] covers Long-Term Evolution (LTE) LTE700, LTE2300, LTE2500, Global System for Mobile (GSM) GSM850, GSM900, GSM1800, GSM1900, and Universal Mobile Telecommunications System (UMTS) (1920-2170 MHz) bands for different switching conditions. A minimally-sized hexagonally-shaped reconfigurable slot antenna for WLAN is reported in [7]. The reported antenna covers dual bands (2.4 and 5.2 GHz) and a single band (2.4 GHz) for the PIN diode in the ON and OFF states, respectively. Two different types of frequency-reconfigurable antennas are reported in [8] for multiband applications. The reported antenna covers three bands: the Wireless Fidelity (WiFi) band, the Worldwide Interoperability for Microwave Access (WiMAX) band, and the WLAN band, for different biasing conditions of PIN diodes. Furthermore, a frequency-reconfigurable implantable antenna for the Medical Implant Communications Services (MICS: 402-405 MHz) and Industrial, Scientific, and Medical (ISM: 902-928 MHz) band is examined in [9]. Micro-electromechanical switches (MEMSs) are used in [10] to reconfigure the operating band of the antenna. However, the reported antenna cannot be efficiently used with any planner and compact components due to its large dimensions and 3D geometry. Following that, a polarization and frequency-reconfigurable antenna is reported in [11]. The optical switches are inserted between the main monopole and the parasitic patch in [12] to achieve different frequency responses with a wider bandwidth and high gain at the cost of a large antenna size. In [13], the PIN diode on the multi-layer antenna, control the patch and feeding lengths, generating two unique frequencies in the WLAN and 5G bands. In [14], two varactor diodes are connected with an F-shaped feed to reconfigure the operating band continuously in a wideband filtering patch antenna. Moreover, the hexa-band frequency-reconfigurable antenna for multi-standard wireless communication systems using PIN diodes is investigated in [15][16][17]. A fluidic channel-based frequency-reconfigurable monopole antenna is proposed in [18], which operates in three modes depending on the filled channels.
In the challenging world of communication, the size and performance of the reconfigurable antenna are important factors to be considered before using it in the RF front-end. So far, many antennas have been designed for reconfiguration of the frequency bands. However, their large dimensions and limited operational bands limit its applications in modern RF front-ends where limited space is available for the antennas. Therefore, the proposed antenna is designed with lower dimensions (0.21λ g ×0.35λ g ×0.02λ g ) and multiple frequency bands with high gain and efficiency for each operating band. The organization of the rest of the paper is as follows: Section 2 presents the antenna design equations and theoretical analysis and simulation results for each mode. Fabricated results and discussions, as well as a comparison with the state-of-the-art reconfigurable antennas are presented in Section 3.

Antenna Design Methodology
The geometry (top view, bottom view, and side view) of the proposed frequency-switchable monopole antenna is illustrated in Figure 1. The proposed frequency-switchable monopole antenna was printed on a 1.6-mm thicker, commercially-available substrate of FR-4 ( r = 4.4, and tanδ = 0.020). The main radiating part of the antenna consisted of a hook-shaped monopole with three parasitic patches connected to it through three PIN diodes. A gap of 1 mm was kept between the parasitic patches to install the PIN diodes. The overall geometry of the proposed antenna was 0.21λ g × 0.35λ g × 0.02λ g . The effective length of the monopole was calculated using transmission model theory [19]. The effective length of the monopole was calculated using Equation (1). and: where c is the speed of light in a vacuum, λ g is the guided wavelength, e f f is the effective dielectric constant, w is the width of the substrate, and h is the thickness of the substrate.

Frequency Reconfigurability
The frequency reconfigurability in the antenna is obtained by adjusting the ON and OFF states of the PIN diodes used between the corresponding parasitic patches. The proposed antenna operates at four different modes. The states of the PIN diodes at each mode are summarized in Table 1

Switching Technique
In this arrangement, the PIN diode acts as a variable resistor in the RF frequency range. The PIN diode Model HPND-4005 (640 µm × 220 µm) was used for frequency reconfiguration of the antenna. The equivalent circuit model of the PIN diode in the ON and OFF state is shown in Figure 1d [20]. The equivalent circuit model of the PIN diode is the series connection of the inductor (L = 0.15 nH) and low resistance resistor (R = 4.7 Ω) for the ON state of the PIN diode. The equivalent circuit model for OFF state of the diode was modeled as a parallel combination of the capacitor (C = 0.017 pF) and high-resistance resistor (R = 7 kΩ) in series connection with an inductor (L = 0.15 nH). For simplicity, the reconfigurability of the antenna was studied only in terms of the resistance, based on the concept that the PIN diode acts as an open circuit for a high resistor value and as a closed circuit for a lower value of the resistor.

Results and Discussions
The fabricated prototype of the antenna is shown in Figure 1e. Our simulation results are compared with the results obtained from the fabricated prototype. PIN diode Model HPND-4005 having dimensions of 640 µm × 220 µm was used for frequency reconfigurability in the antenna model. The proposed diode offered fewer insertion losses (0.4 dB) due to its small geometry and a small capacitance value (C = 0.017 pF). Conductive epoxy was used to mount the diodes on the antenna surface. Figure 1d shows the biasing circuitry of the antenna. It is illustrated from the biasing circuit that the PIN diode was excited through a current-limiting resistor (R = 47 Ω) and radio frequency (RF) chock (L = 68 nH).

Reflection Coefficient
The reflection coefficient of the antenna was measured using a network analyzer for four operating modes of the antenna. The simulated and measured results for all operating modes of the antenna are compared in Figure 3. Figure

Surface Current Distribution
The surface current distribution of each mode for their respective operating bands (Mode 1: 1.8 and 3.3 GHz, Mode 2: 2.25 and 3.9 GHz, Mode 3: 2.4 and 4.5 GHz, Mode 4: 2.78 and 5.54 GHz) is plotted in Figure 4. The surface current was distributed uniquely on the antenna surface for different operating bands. It is evident from the plots that the effective length was inversely proportional to the operating frequencies.

Radiation Pattern
The antenna's radiation characteristics were analyzed using 2D and 3D radiation patterns for each resonant frequency of the four operational modes. The 2D radiation pattern of the antenna on both principal planes (φ = 0 • and φ = 90 • ) was measured in an anechoic chamber. The simulated and measured 2D radiation pattern is illustrated in Figure 6. The proposed antenna had an omni-directional radiation pattern for a lower operating band at Phi = 0 • in all four operating modes. At Phi = 90 • , the antenna had the same radiation pattern as a figure of eight for all operating modes. For the upper operating band at Phi = 0 • , the radiation pattern was omni-directional for Mode 1; however, the radiation pattern was distorted in Mode 2, Mode 3, and Mode 4. The 3D pattern of the antenna is illustrated in Figure 7.

Specific Absorption Rate Analysis
The proposed antenna covered different frequency bands such as GSM, 5G, 3G Advanced/LTE, WiFi, WLAN, ISM, and Bluetooth. The specific absorption rate (SAR) analysis was important for the proposed antenna, because most of the portable devices use these frequency bands. The SAR analysis was performed by placing the antenna on the three layers (skin, fat, and muscle) of a flat human model. The gap between the antenna and skin was kept as 4 mm. According to the International Commission of Non-Ionizing Radiation Protection (ICNIRP), the SAR value should not exceed 1.6 W/kg for one-gram and 2 W/kg for 10-gram standards [21,22]. Peak SAR values (10 grams) of 9.37, 8.99, 7.93, 5.98, 7.4, 7.11, 5.33, and 4.7 W/kg were noted for 1.8, 2.25, 2.4, 2.78, 3.3, 3.9, 4.5, and 5.54 GHz, respectively, for an input power of 1 W, as shown in Figure 8. The designed antenna exceeded the SAR limits (2 W/kg) for all operating bands using an input power of 1 W. However, many portable devices use power in the mW range [23]. Based on the above calculated SAR value, our antenna is safe if the input power is less than 213, 222, 252, 334, 270, 281, 375, and 425 mW for 1.8, 2.25, 2.4, 2.78, 3.3, 3.9, 4.5, and 5.54 GHz respectively. Hence, it is not recommended to install it directly into any portable device. An SAR reduction technique must be used before employing it in portable devices if the input power is more than the limit provided above.
An overall summary of the proposed antenna is presented in Table 2. Table 3 compares the performance of the proposed antenna with the state-of-the-art antennas. It can be noticed that the proposed antenna had smaller dimensions than the antennas presented in the literature. Furthermore, the proposed antenna used a fewer number of switches, which significantly reduce the system complexity and enhance the power efficiency. Additionally, the antenna had reasonably good gain and efficiency in all operating bands.

Conclusions
A low-profile (0.21λ g × 0.35λ g × 0.02λ g ) and a simply-structured frequency-switchable antenna with eight frequency choices was presented in this paper. The antenna operated in four different modes depending on the ON and OFF states of the PIN diodes. In each mode, the antenna covered two unique frequencies (Mode 1 = 1.8 and 3.29 GHz, Mode 2 = 2.23 and 3.9 GHz, Mode 3 = 2.4 and 4.55 GHz, and Mode 4 = 2.78 and 5.54 GHz). The proposed antenna achieved high gain and reasonable efficiency in each mode. A fabricated model was tested experimentally to validate the simulation results. The proposed antenna can have a significant impact on compact and heterogeneous RF front-ends, because of its small geometry and efficient utilization of the frequency spectrum.