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.