1. A Wideband Bandpass Filter by Integrating a
Section of High Pass HMSIW with a
Microstrip Lowpass Filter
Fan Fan He, Ke Wu, Fellow, IEEE, Wei Hong, Senior Member, IEEE
State Key Laboratory ofMillimeter Waves
School ofInformation Science and Engineering, Southeast University
Nanjing, 210096, P. R. China
E-mail: ffheem-field.org, ke.wu.;eee.org, weihongseu.edu.cn
Abstract-This paper presents a wideband bandpass filter
based on the highpass or cutoff characteristic of the half mode
substrate integrated waveguide (HMSIW) and the lowpass
characteristic of a microstrip lowpass filter. The HMSIW can be
seen as a highpass filter because it is inherent sharp cutoff in
lower frequency. A section of HMSIW and a microstrip lowpass
filter can be cascaded to form a wide-band filter. A prototype is
designed and implement with the passband from 7.1-16.6GHz,
74% bandwidth with low insertion loss and a flat good group
delay in pass band are achieved.
I. INTRODUCTION
Recently, the substrate integrated waveguide (SIW)
technology which can be integrated in the dielectric substrate
with low insertion loss, low radiation loss and high Q has been
developed widely. Also, the SIW can be fabricated by many
processes including the standard PCB, the LTCC and the Thin-
Film process. On the basis of the SIW, many passive and
active devices such as filters, couplers, antennas and mixers
using theses processes were proposed and realized [1]-[5].
Meanwhile, in order to reduce the size ofthe SIW devices, a
novel technology named half mode substrate integrated
waveguide (HMSIW) was proposed [6]. As shown in Fig.1, the
HIMSIW is realized by cutting the SIW along the center plane
where can be considered as an equivalent magnetic wall when
the SIW is used with a dominated mode. The HIMSIW can
keep the original performance of SIW with nearly a 50%
reduction in size. In addition, the HMSIW has the wider
dominant mode range than the SIW due to the former
intrinsically can not support the TE(2m)n modes as well as TM
modes. Some devices based on the HMSIW were realized with
good performances [7]-[9].
The SIW and HMSIW are highpass transmission line
because they have the inherent cutoff frequency as the metal
rectangular waveguide. The inherent sharp cutoff behavior in
low side can be used to develop some SIW or HMSIW devices
such as filters [3][9]. In [3], the highpass characteristic was
used to develop a super wideband bandpass filter (BPF) by
combining EBG structures into the SIW, where EBG and SIW
provide the bandstop and highpass behaviors, respectively.
In this paper, we take the advantage of the highpass
characteristic ofthe HMSIW to simply realize a super
NA'W'
Figure 1. Dominant field distribution in HMSIW and SIW.
wideband BPF by cascading a section of HIMSIW and a
microstrip lowpass filter (LPF). The HMSIW and microstrip
LPF decide the lower side and upper side of the amplitude
response of the wideband BPF, respectively. Firstly, the width
ofthe HIMSIW is determined for the cutoff frequency. In other
words, the lower side of the frequency response of the
wideband BPF is determined by the width of the HMSIW.
Secondly, the microstrip LPF is designed to determine the
upper side of the frequency response. Finally, the wide-band
BPF is developed, simulated and measured.
II. WIDEBAND BPF DESIGN
Fig.2 shows the schematics of the forming process of the
wideband BPF consists of the LPF and highpass filter (HPF).
The stop-band frequency ofthe LPF and IHPF are the upper
Lowpass filter + Highpass filter
(m1cr iip L[P (VimILW
_I
Wide-band
bandpass filter
*~~~~~~~~~~~~~~~~~~~~~~~~~~~...... ........
Figure 2. schematics ofthe forming process ofthe wide-band bandpass filter
side and lower side stop-band frequency of the wideband BPF,
respectively. Meanwhile, the group delay of the designed filter
978-1-4244-1886-2/08/$25.00 C2008 IEEE. GSMM2008 Proceeding

2. is the sum of those of the LPF and HIPF. In this paper, the
stepped-impedance Butterworth LPF is selected because it has
a good performance with the group delay. The filter is
simulated and designed with the full-wave CAD software CST,
and fabricated on Rogers Duroid 5880 substrate with dielectric
constant of2.2 and a thickness of0.5mm.
A. HMSIW
Fig. 3 depicts the layout of the HMSIW with a cutoff
frequency of 7.1 GHz, where D and S are the diameter and
period of metallic vias, and WHMSIW stands for the HMSIW
width that determines its cutoff frequency. This cutoff
frequency is also the lower side's that of the wideband BPF.
The SIW-microstrip transition is used for connecting 50Q2
testing system, where W50 and Wtaper are the widths at both
ends ofthe microstrip taper, and Liaper is the length ofthe taper.
Dimensions of the HIMSIW can be expressed as D=0.4mm,
S=0.8mm, WHMsIw=7.1mm, Wso=1.5mm, WUtaper2.5mm and
Liaper114mm.
As shown in Fig.4, simulated results indicated that the
HIMSIW has a good highpass characteristic with a low
insertion loss in pass band and sharp cutoff. The return loss
over 7.1-19GHz is better than -lOdB and the insertion loss is
more than 30dB below 5GHz.
substrate
practical line impedance is 125Q2, the lowest is 20Q2; the
insertion loss is more than 30dB at 20GHz. To satisfy these
specifications, an eleven order LPF is designed as shown in
fig.. /WI, W2 and Li are the physical microstrip line widths and
lengths of low impedance and high impedance lines,
respectively. Dimensions of the LPF are WU=5.31mm,
W2=0.27mm, L1=0.08mm, L2=0.40mm, L3=0.57mm,
L4=0.84mm, L5=0.85mm and L6=1.OOmm.
Simulated results of the LPF are shown in fig. 6. It is found
that the insertion loss is less than 0.3dB and the return loss is
more than 15dB. Simulated performance ofthe LPF can satisfy
specifications proposed in advance.
Figure 5. Configuration and dimensions ofthe LPF
metallic via
-10
as
mE
.H)
Q0
Figure 3. Configuration and dimensions ofthe HMSIW.
-20
-30
-40 _
0
-10
m
2-20
-a
E -30
-40
s~~~~~~~~~~~~~i I
---Simulated S1 1
40 / I, Simulated S21 ---------- -. X
4 6 8 10 12 14 16 18 20
Frequency (GHz)
-50
Figure 4. Simulated frequency responses ofthe HMSIW
B. MicrosrlipLPF
As stated above, the stepped-impedance LPF has a
maximally flat response. The specifications of this filter are:
cutoff frequency of 16.6GHz; impedance of 50Q2; the highest
-50
5 10 15 20
Frequency (GHz)
25 30
Figure 6. Simulated frequency responses ofthe LPF
C. Wide-bandBPF
As shown in fig. 7, the wide-band BPF is formed when the
LPF and a section of HJMSIW are cascaded together.
Meanwhile, the performance of the wideband BPF is already
determined by the LPF and HMSIW. The predicated center
frequency and bandwidth are 12.85GHz and 74%, because the
cutoff frequency of the LPF and HIMSIW are 7.1GHz and
16.6GHz, respectively. The upper side and lower side stopband
frequency response should be the same as that of the LPF and
IHMSIW.
As can be seen from Fig 8, simulated and measured results
agree with the predicated results very well. The lower side
stopband is within 0-7.1GHz., while the insertion is more than
30dB below 5GHz. In the pass band, the insertion is about 1+
0.4dB and the return loss is better than lOdB. The wideband
metal

3. BPF also has a wide stop band at upper side from 16.6 to
30GHz, while the insertion loss more than 30dB within 19-
30GHz. Fig.9 shows measured and simulated group delay
which are plate in pass band.
Figure 7. Photograph ofthe proposed wideband BPF.
O0 ...........
-40
Frequency (GHz)
Figure 8. Simulated and measured frequency responses ofthe wideband BPF
1 .0
0.8
0.6U)
X 0.4
a)
= 0.20
0.0
-0.2
-0.4 L
5 6 7 8 9 10 11 12 13 14 15 16 17
Frequency (GHz)
measured. Good performances related to the insertion loss,
return loss cutoff characteristic and group delay are observed
for our fabricated samples designed from 5 to 30 GHz.
ACKNOWLEDGEMENT
This work was supported in part by NSFC under Grant
60621002 and in part by the National High-Tech Project under
Grant 2007AAOlZ2B4.
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Figure 9. Simulated and measured group delay
111. CONCLUSION
This paper provides a simple method to develop a wideband
BPF by a section of HIMSIW and a LPF being cascaded
together. This wideband BPF is simulated, fabricated and