A Frequency Reconfigurable Compact Planar Inverted-F Antenna for Portable Devices

In this paper, a low-proﬁle, compact size, inexpensive, and easily integrable frequency reconﬁgurable antenna system is proposed. The proposed antenna consists of an inverted-F shape antenna, capacitors, and switching PIN diodes. The designed antenna element is fabricated on easy available and less expensive FR-4 substrate ( ε r � 4.4, tan δ � 0.02). The switching diodes are incorporated within the radiating structure of the antenna design, and by changing the diﬀerent states of PIN diodes, frequency reconﬁgurable response is achieved. While adjusting the diﬀerent states of the diodes, the antenna resonates between 0.841 GHz and 2.12GHz and covers six diﬀerent frequency bands. The proposed system has compact size of 44 × 14 × 3 . 2 mm 3 . The gain of the antenna is between 1.89 and 2.12dBi. The measurement results shows the good agreement with simulated results for diﬀerent key performance parameters. Additionally, the proposed antenna shows omni-directional far-ﬁeld characteristics for various


Introduction
Recently, there is a high demand of low-profile, efficient, compact, high-speed, planar, inexpensive, and easily integrable devices [1]. ese devices have various applications in different roles within modern telecommunication industry. Antenna is an inevitable part of advances communication systems and plays an increasingly important role in modern electronic devices. ey are mostly used for applications requiring constant functional properties, such as fixed polarization, beam-width, far-field characteristics, and so on. However, antenna systems with such attributes (constant functional properties) may increase the complexity of the circuitry and decrease the efficiency. Moreover, integration of multiple radios and the antenna systems within a single device may degrade the performance of the system as well. erefore, it is believed that to support the challenging characteristics, for instance, high data rates, high bandwidths, and low latency, of modern wireless communication systems, there is a requirement of multi-functional, low-cost, compact, and easy to fabricate and integrate antenna systems which must address these challenges and provide additional services without increasing the complexity and size of the circuitry. In addition, enhancement in devices, components, and subsystems will create new challenges. To address these demands, it will be necessary and reasonable to design a flexible, compact, inexpensive, easily controllable, and adjustable antenna system.
One possible solution will be to use reconfigurable antennas. ese antennas offer effective and promising solutions to various different challenges and also help to improve the efficiency of the system. In addition, they are compact in size, low-cost, and easily integrable. It is worthy to mention that the reconfigurable behavior is achieved by changing the resonant frequency, polarization, and radiation characteristics of the system. In literature, different techniques are used to design the reconfigurable antennas using PIN diode [2][3][4], RF-MEMS [5], varactor diode [6,7], different smart materials [8], and optical switches [9]. e main purpose of this work is to design a frequency reconfigurable antenna that must be compact, simple, and easy to fabricate and integrate. To meet the goal of the project, two PIN diodes are used to obtain the frequency reconfigurable behavior. By using ON/OFF operation of diodes, the length of the radiating element is varied, and as a result, the antenna resonates at different frequencies. Before presenting this work, a detailed literature review is conducted to provide readers a brief summary of the existing works.

Related Work
In literature, the existing works include various different feeding networks and impedance matching circuits incorporated with the reconfigurable antenna that not only increases the complexity of the system but also leads to large size and low efficiency. A T-shaped feeding strip with T-shaped parasitic elements system is used to cover low bands (GSM 850/900) and high bands (GSM 1800/1900/UMTS 2100/LTE 2300/2500 bands) in [10]. However, it fails to represent the behavior of the PIN diode using copper strip, and computed and experimental results showed greater than −10 dB impedance matching for the lower bands. In [11], a frequency reconfigurable antenna for multi-band applications was presented. e proposed antenna design is implemented on an FR-4 substrate layer, and two PIN diodes were used to cover three operating modes, i.e., IFA, monopole, and loop mode. In another work [12], a frequency reconfigurable PIFA antenna using defected ground structure (DGS) was proposed [12]. ree PIN diodes were used, and their positions were optimized using genetic algorithm (GA) to obtain three different operating frequencies, namely, 2.1 GHz, 2.4 GHz, and 3.5 GHz, but it suffers from narrow bandwidth and large size. In [13], a microstrip patch antenna with slots in the ground plane was designed to cover frequencies between 2.2 GHz and 6 GHz. e authors used SPICE model of PIN diode in CST software to analyse the real impact of voltage on the performance of the antenna. is antenna resonated at ten different frequencies but suffers from the large size.
A cedar-shaped reconfigurable antenna to cover WiMAX, Bluetooth, GPS, and WLAN band was reported in [14]. To achieve reconfigurable behavior, a combination of PIN diode and three pairs of varactor diodes were used. e antenna became lossy due to large number of varactor diodes and had large antenna size. In another work [15], a coupled-fed loop antenna to cover octa-band for LTE smartphone was proposed. Low modes were achieved with the combination of loop mode of 0.5λ and chip capacitor, and the antenna was operating at the high band by using the combination of 1λ, 1.5λ, and 2λ modes with 0.5λ mode of coupling loop. However, this antenna showed −6 dB reflection coefficient for lower modes.
A compact frequency reconfigurable PIFA antenna based on nested slots was designed in [16]. In the first case, a multi-band antenna was designed without any RF switch with large antenna area. In the second case, two switching diodes were used to achieve the different frequencies with 60% reduction in the antenna size. e antenna resonated between 0.77 GHz and 3.55 GHz frequency band, but it showed −6 dB reflection coefficient for all desirable frequencies. In [17], an E-shaped wearable dipole for IoT applications was proposed. A detailed study was conducted for different bending conditions. However, the proposed antenna design suffers from large size constraint. A CPW-fed multi-band frequency reconfigurable antenna for wireless applications was presented in [18]. Four PIN diodes were used to obtain frequency reconfigurable behavior for 2 GHz to 10 GHz frequency range. It is stated that the biasing line was used for PIN diodes, capacitor, and inductor. However, copper strip was used as a lumped switch instead of real PIN diodes. In [19], an antenna with differential reconfigurable capability was designed to cover sub-6 GHz 5G and WLAN application. e antenna structure consists of two dipoles, PIN diodes, mode switching system, and feeding structure.
e antenna measurement results have good agreement with simulated results, but the size of the antenna is large. In a similar study [20], a low-profile simple monopole antenna to operate in five modes, covering 9 frequencies, was proposed. e presented antenna consists of simple L-shaped monopole with truncated metallic ground plane. is is done to achieve high gain and high efficiency, but the proposed antenna is large in size. A frequency reconfigurable MIMO antenna for LTE and 5G applications was presented in [21]. e proposed MIMO antenna system consists of two meandered radiation arms connected with a 50 Ω feedline.
e system resonates at 2.4 GHz and 3.5 GHz with an interelement isolation better than 12 dB. is system also suffers from large size constraint.
In summary, a detailed literature review was conducted, and it was found that most of the works suffered from large size, complexity, and narrow bandwidth, and this is the motivation behind this work. erefore, it is reasonable to propose an antenna system that can address these issues and provide a smart solution.

Proposed Antenna Design
e design, working principle, switching method, and parametric studies of the designed antenna are discussed in this section. e basic design methodology of the designed antenna has been presented previously [22]. e proposed system consists of F and I-shaped radiating elements. is system is etched on a double-sided widely available FR-4 substrate (ε r � 4.4, tan δ � 0.02), as shown in Figure 1. e ground plane is on the other side of the board, and the thickness of the substrate and the metal is 3.2 mm and 0.035 mm, respectively. Multiple shorting vias are used to connect the ground plane and the radiating element. Further, a ground plane is also used to improve impedance matching. A 50 Ω SMA connector is used to feed the system. Note that the shorting vias, ground plane, and the radiating elements are made up of copper with a conductivity of 5.87 × 10 7 S/m. e dimensional parameters of the designed antenna are depicted in Table 1. e complete structure of the proposed system is depicted in Figure 1. From Figure 1(b), it can be seen that the system has PIN diodes, D 1 and D 2 , capacitors, C 1 and C 2 , and a ground plane. It is worthy to mention that the different states of PIN diodes are used to achieve the required response at desired frequencies. Note that the diodes are located on the radiation elements, i.e., D 1 is inserted between the long radiation strip (L 1 ) and L shaped strip (L 2 ), and both are connected with the ground plane by vias. Similarly, D 2 is placed between the strips (L 8 and L 9 ) and the cathode of D 2 and then connects to shorting vias. By using different combinations of ON/OFF states of the PIN diodes, the proposed antenna system operates at four different states, where three states provide a closed path for the current to flow within the system. In state 1, D 1 is ON and D 2 is OFF; therefore, the total length (L 1 + L 2 + L 5 + L 6 + L 7 ) of the antenna becomes 62.45 mm, and it covers GSM 850/900 frequency bands. In state 2, D 1 is OFF and D 2 is ON; therefore, the total length (L 1 + L 5 + L 6 + L 7 + L 8 + L 9 ) of the antenna becomes 52.7 mm and the antenna operates in loop mode covering UMTS 2100, GLONASS 1616, PCS 1900, and DCS 1800 frequency bands. Finally, in state 3, when both diodes are OFF, the total length (L 1 + L 5 + L 6 + L 7 ) of the antenna becomes 49.8 mm and antenna resonates to cover GSM 900 and UMTS 2100 bands.

Switching Techniques.
In modern portable devices, switches play an increasingly important role to provide an extra degree of freedom to shift between different modes of operations. erefore, in this work, PIN diodes are used as a switch. PIN diodes have unique advantages, for example, fast switching speed, better isolation, low power handling, excellent repeatability, and long life as compared to the other RF switches, such as MEMS and varactor diode. PIN diode behaves as a currentcontrolled resistor within microwave frequency range. In this work, SMP1345 PIN diode from Skyworks has been used. is diode is suitable for 10 MHz to 6 GHz frequency range applications. e working principle is as follows.
When a PIN diode is forward biased, it acts like a conventional PN-junction diode which allows current in one direction and blocks in the other direction. e circuit equivalent models for forward-biased and reverse-biased conditions and biasing circuit are shown in Figure 2. It is worth to mention that, when the diode is in ON state, it can be represented as a series combination of a resistor of 5 Ω and an inductor of 0.7 nH. Note that the value of the resistor is very small that allows current to pass through the radiating element, thus resulting in change of the resonant frequency. In other scenario, when the diode is in OFF state, the equivalent circuit model consists of a parallel combination of a resistor of 5 kΩ and a capacitor of 0.2 pF in series with an inductor of 0.7 nH. A high-value resistor restricts the current flow through the radiation element. For biasing circuit, a 100 pF capacitor, C 1 , is used to block DC component between D 1 and the probe feed. For similar reasons, another capacitor C 2 is used between D 2 and I-shaped radiation element, and a biasing voltage of 0.89 V and 0 V is applied to PIN diodes for ON and OFF states, respectively.

Parametric Studies.
To understand the behavior and impact of different parameters of the antenna, a parametric study was conducted to optimize and achieve desirable response of the system.

International Journal of Antennas and Propagation
First, the width (g) is varied from 1 mm to 2 mm with an increase of 0.5 mm while other parameters are kept constant. Here, three different scenarios are considered, first, D 1 is ON and D 2 is OFF, second, D 1 is OFF and D 2 is ON, and third, both diodes are OFF. Considering Figure 3, a few conclusions can be drawn as follows: Similarly, the variations in the reflection coefficient are studied by changing the length (L 1 ) while keeping other parameters same. Like the previous study, here again three different cases are studied and the length is varied between 27.6 mm and 37.6 mm with an increase of 5 mm. From Figure 4, the following observations can be made:

Results and Discussion
Different key performance parameters of the proposed antenna design are discussed in detail in this section. e fabricated prototype of the proposed antenna design is shown in Figure 5. To drive PIN diode, power is supplied using connecting wires and these connecting wires are connected to the back plane to avoid the disturbance within the radiation pattern.
e PIN diode consumes 10 mA current for the forward-biased voltage of 0.89 V. For the reverse-biased condition, the voltage provided is assumed to be zero. e measured and simulated reflection coefficient comparison for three different combinations of diodes states is shown in Figure 6. When both diodes are OFF as shown in Figure 6(a), the antenna operates in dual-band mode at 1.94 GHz to 2.12 GHz covering UMTS 2100 and GSM 900/ 950 bands. On the other hand, when D 1 is ON and D 2 is OFF, the frequency is shifted towards the lower band, resonating at 0.841 GHz to 0.964 GHz covering GSM 850/ 900 bands, as shown in Figure 6(b). When D 1 is OFF and D 2 is ON, it is observed that the antenna covers DCS 1800, PCS 1900, and UMTS 2100 bands, as shown in Figure 6(c). e measured results are slightly shifted as compared to the simulated results, and this is because of the parasitic impedance due to PIN diodes, dielectric constant, loss tangent, and solder mask effects. It is worthy to mention that overall the results are in good agreement. e current distribution of the proposed antenna design is explained in Figure 7 to better understand the working principle at the fundamental level. ese current distributions are shown for three different combinations of the diode states at 0.85/0.9 GHz, 1.6 GHz, 1.8 GHz, 1.9 GHz, and 2.1 GHz. When D 1 is ON and D 2 is OFF, the current flows in F-shaped radiated element through feed, and also, it is almost the quarter wavelength for GSM 850/900 bands, as shown in Figure 7(a). On the other hand, when D 1 is OFF and D 2 is ON, the surface current flows in I-shaped and long-strip element from feed, as shown in Figure 7(b). Note that, in this case, four different frequencies are covered (1.54 GHz to 2.13 GHz). Finally, when both diodes are OFF, the surface current distribution is illustrated in Figure 7(c). In this case, the current mainly flows in long-strip and I-shaped element. e simulated and measured far-field characteristics are shown in Figure 8. e proposed antenna shows dipole-like omni-directional patterns. In Figures 8(a) and 8(b), the radiation characteristics for E and H planes are shown when both diodes are OFF. In Figures 8(c) and 8(d), the far fields are illustrated when D 1 is OFF and D 2 is ON. e radiation patterns for the both planes when D 1 is ON and D 2 is OFF are shown in Figures 8(e) and 8(f ).

International Journal of Antennas and Propagation
To further demonstrate the contribution of this work, a detailed comparison of the proposed work with available literature for different performance and geometrical parameters is illustrated in Table 2. Table 2 shows the comparison in terms of antenna size, substrate, resonant frequencies, and the number of switches.

Conclusion
In this work, a frequency reconfigurable antenna for GSM 850/ 900 and UMTS 2100 frequency bands is proposed. e proposed antenna system consists of a radiating element, vias, and two PIN diodes. is system is designed on an FR-4 substrate, and  International Journal of Antennas and Propagation PIN diodes are used to achieve reconfigurable response. e design, working principle, and effects of different parameters are discussed. To verify the simulated results, a prototype is fabricated, and it is found that the experimental and the measured results are in good agreement. Different key performance parameters, such as reflection coefficient, far-field radiation patterns, and current distribution, are presented. Based on results, it is believed that the requirements of several wireless standards are met. e main feature of the proposed antenna system is that it is highly compact in size, while demonstrating a performance comparable with existing designs.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.