s Fig. 2 Photograph of the designed bandpass filter.
0
– 10
; S11; , ; S21; (dB)
– 20
– 30
– 40
– 50
1.5 1.7 1.9 2.1 2.3
FREQUENCY (GHz)
2.5 2.7 2.9
s Fig. 3 Measured performance of the
proposed filter.
line,
6 a meander spur-line provides
more slow-wave effect and occupies
a smaller circuit area.
7 Without coupling gaps in the I/O ports, a low insertion loss performance is obtained.
In order to perturb two degenerate
modes, the two slot lengths are not
equal and the two feed lines are orthogonal.
The side length of the patch is designed to be 20 mm and the feed lines
are connected to a
50 V microstrip line.
The slot’s length and
width are indicated
by S and g, respectively. The difference in length between the two slots
is DS. The physical
dimensions of the
meander spur-line
are described by b,
c, d, g and m. The
proposed filter is
fabricated on a substrate with a relative
permittivity εr = 4. 5
and a thickness of
0.8 mm. The center
operating frequency
is designed to be 2.3
GHz. The physical
parameters are cho-
sen as follows: S = 20 mm, g = 0.3 mm,
b = 2 mm, c = 0.4 mm, m = 1.3 mm,
d = 1.4 mm and DS = 0.2 mm. A size
reduction of approximately 36 percent
is obtained at the same resonant frequency, compared to a conventional
dual-mode bandpass filter.
6 A photograph of the designed dual-mode
bandpass filter is shown in Figure 2.
b
4
1.5
RESULTS AND DISCUSSION
Figure 3 shows the measured
transmission performances of the designed dual-mode BPF; the measurement was obtained using a HP8722
network analyzer. A center frequency
of 2.3 GHz and a 3 dB bandwidth of
23 percent are measured. Note that
the two degenerate modes are located
at 2.27 and 2.33 GHz, respectively.
There are two transmission zeros on
both sides of the passband, providing
a sharp rejection and selectivity. They
are - 49. 4 and - 37. 8 dB at the frequencies of 1.85 and 2.58 GHz, respectively. Furthermore, the insertion loss is
better than -1.5 dB from 2.22 to 2.40
GHz and the return loss is better than
- 25 dB from 2.25 to 2.35 GHz. The
minimum insertion loss is -0.9 dB at
2.26 GHz including SMA connector
loss.
The simulated current distribution at the resonant frequency is
shown in Figure 4, which is obtained by a commercial full-wave
EM simulator, Ansoft HFSS. One
port is driven by current in orthogo-
s Fig. 4 Simulated current distribution at
the resonant frequency; (a) phase = 0° and
(b) phase = 90°.
0
S = 17mm S = 19mm S = 21mm
– 10
; S21 ; (dB)
– 20
– 30
– 40
– 50
– 60
1.5
1.9 2.3
FREQUENCY (GHz)
2.7 3.0
s Fig. 5 Simulated insertion loss for different slot lengths.
nal phase while the other port is terminated in a 50 V microstrip line.
Two degenerate modes, related to
mode TM010 and TM100, can be observed. Note that the high current
density distributions (red area) are
