3-Bit Frequency-Reconfiguration Design:The influence of the bypassed part on the electrical length of the antenna is assumed to be negligible, and the current is uniformly distributed in the bending line. Switches D1, D2, and D3 are assigned to the centers of meanders 1, 2, and 3, with different positions in the x-axis direction. To achieve binary frequency reconfiguration, the length of the bypassed transmission line S1, S2, and S3 are proposed to satisfy: S1>S2>S3. By operating the three switches as shown in Table I, eight independent antenna working states can be obtained, with ”1” representing ”on” and ”0” representing ”off” (i.e., achieving the 3-bit frequency reconfiguration).
Fig. 2 depicts the simulated surface current distribution at various resonant frequencies. When one of the switches is turned on, a portion of the corresponding meander is bypassed, resulting in a new structure that can be equivalent to an open stub, as shown in the figure. As a result, the current path on the meander line is reconfigured, as indicated by the red curves, avoiding the bypassed part and flowing directly through the p-i-n diode. Therefore, the equivalent electrical length of the meander is reduced. When the switch is turned off, the related part of the meander is incorporated into the meander line, increasing the antenna’s electric length.
In Fig. 2(a), the current density of the bypassed part of meander three is significantly reduced by turning on D3, reducing the corresponding electrical length of the antenna. In addition, the resonant frequency shifts to a higher band compared with the condition when D3 is turned off.
When either of the switches, as shown in Figs. 2(b) and 2(c), are turned on, the longer the shortened current path, the shorter the equivalent electrical length of the meander line, and the higher the working frequency of the antenna are obtained.
Finally, in Fig. 2(d), the highest working frequency is attained at state 111 because the shortest current path on the meander line and the smallest equivalent electrical length of the antenna, which resulted from all three switches being turned on.
The electrical length obviously decreases sequentially from state 000 to state 111, resulting in a gradual increase in the resonant frequency of the antenna. Therefore, the frequency range of the roughly reconfigurable can be first determined based on fH and fL, followed by the resonance frequencies of the eight different operating states. Owing to miniaturization, the range of reconfigurable frequencies is limited by the size of the antenna. Thus, the operating frequency bands of each state must be optimized to ensure that they are clear and evenly distributed within a finite frequency range and minimize overlap as much as possible. All the simulation and parameter optimization of the antenna proposed in this study are realized by computer simulation technology (CST) software.
The antenna proposed in this study is designed to operate in the frequency range of 1.04–1.51 GHz with a bandwidth from 80 MHz to 150 MHz. In the practical design of the reconfigurable antenna, the intermediate dielectric layer employs Rogers RO4003C substrate, with a relative dielectric constant of 3.38 and a thickness of 0.813 mm. In addition, the RF PIN diodes from Skyworks with a PIN of SMP1331-079LF are used [19], and the diode is equivalent to a resistance of 1.7 Ω and a capacitance of 0.18 pF at the on and off states in simulation, respectively.
The optimized parameters obtained from the simulation are: L0 = 5 mm, L1 = 6 mm, L2= 12.4 mm, L3 = 2.8 mm, L4 = 6 mm, L5 = 4.4 mm, L6 = 8.6 mm, W0 = 1.8 mm, W1 =1.6 mm, W2 = 0.3 mm, S1 = 10.3 mm, S2 = 6.2 mm, and S3 = 9.8 mm. The values of the bias inductors In1, In2, In3, In4, and In5 are 180 nH. The values of the DC blocking capacitors C1, C2, C3, C4, and C5 are 100 pF.
Table I: Conditions of diodes in different states.