Window Hairpin Structure Based Microstrip BPF with

Symmetrical T-Feeder Coupling Line Resonators

Ram Krishna Maharjan and Nam-Young Kim

RFIC Lab, Department of Electronics Engineering, Kwangwoon University

Nowon-Gu, Seoul 139-701, Republic of Korea.

Email: rkmahajn@gmail.com

Abstract – In this paper, a typical structure of planar window hairpin bandpass filter

(BPF) with symmetric T-shaped feed line coupling resonators is introduced. By varying

the length along the X-axis and width along with the Y-axis of the window of the

hairpin resonator configuration, the correspondingly changes in the resonant frequency

and useable bandwidth are observed. Using these methods, frequency tuning can be

easily achieved by adjusting the hairpin window dimensions of the structure. Placement

of symmetric T-shaped feeder lines are employed for maintaining 50 Ω input/output

impedance matching. The measurement results show that the return loss (S11) for the

filter is less than 23.0 dB, and the insertion loss (S21) is less than 0.81 dB at a resonant

frequency of 5.13 GHz. The S-parameter responses of the fabricated filter nearly match

the electromagnetic (EM) simulated results; therefore, the practical application of the

proposed filter is expected to be feasible.

Index Terms – Hairpin resonator, bandpass filter, feeder coupling resonator, coupling

effects, wireless local area network (WLAN).

1. Introduction

In the past, people believed that Wi-Fi was a means for users to stay connected to the

internet at home or in their offices simply to check emails, or browse the web. Today,

that trend is rapidly changing. Wi-Fi is now being used not only for those applications,

but for content consumption such as streaming music and videos as well. Many users

demand entertainment through the live broadcasting of FM radio, TV programs, and

wireless multimedia, and they desire high-definition (HD) video content through Wi-Fi

networks on TVs, laptops, and their mobile devices such as smart phones and tablets.

However, the previous Wi-Fi standards such as IEEE 802.11 a/b/g/n operate in the 2.4

GHz band, with IEEE 802.11n optionally supporting the 5 GHz band around [1-2].

WLAN connections operating at 5 GHz are used around the home, and offices to

eliminate the use of cables when sharing printers, scanners, and high-speed internet

connections. The main advantage of 5 GHz WLAN connections is that they are simple

to set up and require only one access point connected directly to the internet through a

Wi-Fi router. Wireless networking through the 5 GHz band is a currently emerging

technology that is becoming popular over the long-existing 2.4 GHz technology with

which we are all familiar. The 5 GHz wireless networking band has higher output power

limits and better non-line-of-site (NLOS) scatter capabilities, which increases the

penetrative effect through buildings relative to that of the 2.4 GHz band [3-4]. Currently,

increasing consumers’ demand for wireless multimedia and higher throughput is

required. Even, the IEEE 802.11n and the HIPERLAN/2 standards have been designed

and finalized to accommodate this demand by providing transmission data rates of up to

150 Mbps in the 5 GHz wireless networking band. However, HD video content, high-

quality TV programming and video content in general presents a challenge for existing

Wi-Fi 802.11n based networks, as they may suffer from interference in the 2.4 GHz

band. Hence, the next-generation Wi-Fi standard IEEE 802.11ac that has, recently been

introduced which promises to deliver multiple HD video streams simultaneously, it can

also reach maximum throughputs well above one gigabit per second [4-5].

Here, we present a typical BPF based on window hairpin resonator with T-coupling

feeders for rejecting the spurious passband and achieving an acceptable transmission

coefficient. This research work can be applied for standards IEEE 802.11ac. Several

studies [6-10] have performed on different band planar filters for many applications

using various types of hairpin structures; however, some configurations of them still

occupy a large circuit areas and relatively not so good transmission and reflection

coefficients. These issues have been very crucial in the development of planar

microwave filters.

2. Window Hairpin BPF Design

Figure 1 shows a schematic design layout of the compact microstrip hairpin BPF used

in our design analysis. It was designed, simulated and fabricated on a Teflon substrate

with a relative dielectric constant, εr of 2.52, a thickness of 0.504 mm, and a loss tangent

of 0.0017. T-shaped feeder lines behave as coupling resonators, which are mounted

beside the window hairpin configuration used to transform the desired passband by

maintaining 50 Ω impedance matching with input/output devices. Some research studies

[11-13] have applied asymmetric feed lines for coupling and matching input to output

devices. Here, we used symmetric T-shaped feeder line resonators combined with the

window hairpin resonator to give a selective resonance frequency with good spurious

band suppression and a stop-band response as well. In the analysis, as the length along

the X-axis or width along the Y-axis was increased or decreased in the hairpin window

configuration, the resonant frequency, as well as the useable bandwidth, were varied.

Therefore, consequent changes in the insertion loss and the return loss were observed.

In this work, we present a combination of three resonators that produce a single

resonance response of approximately 5.13 GHz with two similar T-shaped feed lines.

Furthermore, the proposed BPF is able to reject the spurious band and achieve an

acceptable transmission coefficient. The major finding is that the compact geometrical

areas of window hairpin configuration for the designed filter was measured to be

approximately 0.04 The simulated results are used to predict desired features and

optimize the proposed BPF while taken design process.

3. EM Simulation and Analysis

By analyzing the predicted results plotted in Fig. 2 to Fig. 7, it was observed that the

change in window width is much more sensitive and effective than the change in

window length. Therefore, internal window width, Y, was plotted at S-parameters, S21

and S11 versus frequency. By varying Y, in increments of 0.2 mm, more remarkable

results for the insertion loss (S21) and return loss (S11) parameters were obtained relative

to those corresponding to the variation in W. The plot S21 versus frequency, f with

respect to the change in Y in the hairpin structure is clearly plotted in Fig. 2. Similarly, Y

plot in the S11 with frequency is illustrated in the Fig. 3. Figures 4 and 5 present the

deviations of the S21 and S11 parameters with respect to given frequencies as changing

the value of given L. Furthermore, it is also concluded that the required resonant

frequency, f0, and useable frequency bandwidth, BW, can also be controlled by varying

the external geometrical area of the window hairpin structure. Figures 6 and 7 are used

for plotting the S21 and S11 versus f, with variations of external window width, W.

Remarkable changes in the resonant frequency and the bandwidth can be observed as a

result of small variations on the order of 0.2 mm of Y, L, and W depicted in Figs. 2 to 7.

By the plots in the illustrated Figs., it also shows that the best result each and among all

four parameters was Y=12.2 mm, L=9.4 mm, and W=14.5 mm, which were selected for

the design of the proposed BPF as an optimization in the design work.

The current distribution conditions at the resonance frequency and other adjacent

frequencies were simulated and are illustrated in Fig. 8. The current distributions in the

proposed BPF using a full-wave Sonnet EM simulator [14] were simulated to determine

and distinguish the resonance and non-resonance states at specific frequencies. The

current distributions at four different frequencies, (a) 4.2 GHz, (b) 4.7 GHz, (c) 5.2 GHz

and (d) 5.7 GHz, were observed. Simulations were conducted in 500 MHz steps as

frequency spacing with the resonant frequency and among all four cases. The results

also show the current density of the proposed bandpass filter at the significant states

around the resonance condition. Figure 8(c) shows the essential state of almost equal

and quite good current distribution with high current density at the resonant frequency

compared with the adjacent frequencies lying 0.5 GHz about the resonant frequency. At

resonance, the reactive components are equal in magnitude and opposite in polarity;

thus, they cancel each other out and leave only the resistance, resulting in the maximum

current flow relative to the currents observed in the other bands of frequencies.

4. Fabrication and Measurement Result

The proposed hairpin filter was characterized and simulated to achieve a resonance

frequency of 5.2 GHz. After fabrication and measurement, the window hairpin BPF was

observed to resonate at approximately 5.13 GHz. After simulation and optimization, the

geometric dimensions (Fig. 1) of the proposed hairpin BPF were determined to be: X=

2.2 mm, Y= 12.2 mm, L= 9.4 mm, W= 14.5 mm, S1= 1.15 mm, S2= 0.9 mm, S3= 1.15

mm, S4= 1.15 mm, S5= 2.0 mm, g1= 0.15 mm, g2= 0.25 mm, g3= 0.25 mm, d1= 2.0 mm,

and d2= 7.8 mm, where d1 is the port reference which is especially for connections

between the connectors and the fabricated device. The geometrical area of the middle

window hairpin configuration (L×W) of the filter was 0.16λg × 0.25λg, and the overall

dimensions of the fabricated filter were 0.28λg ×0.48λg, where λg is the wavelength of

the guided operating frequency to be resonated (FBW) for the filter was measured about

10.34% at 3 dB bandwidth. The spurious suppression was measured less than -25 dB at

around 2.0 GHz and 8.0 GHz side-by frequencies. In the measurement, the resonant

frequency was almost 70 MHz shifted from the originally predicted position. The

reasons of frequency shift can be due to dielectric substrate loss, the limitation of the

accuracy in physical dimension and lack of accurate connection while connectors

soldered with the fabricated real device [15].

5. Conclusion

Miniaturized window hairpin structures with symmetric T-feeder coupled line resonator

based microstrip bandpass filters were designed, fabricated, and measured. The

experimental results are quite similar to the predicted results. The measured resonant

frequency was about 5.13 GHz for the proposed filter. The geometrical dimensions of

the window hairpin structure, excluding the feeder lines, were measured to be

approximately 0.16λg × 0.25λg. A window hairpin resonator design with symmetric T-

feeder line resonators was introduced as an alternative to the miniaturization of the filter

structure. Therefore, the combination of two types of hairpin resonators can appreciably

minimize the filter size. The proposed filter design can be applied in Wi-Fi network

system based on the IEEE 802.11ac standard. It is also possible to fabricate similar

patterns using MMIC design technology to further reduce the physical dimensions for

even higher-frequency applications.

Acknowledgment

This research was supported by the National Research Foundation of Korea (NRF) and

a Grant from the Korean government (MEST) (No. 2012-0009224), and IPD Project No.

2012R1A1A2004366. This work was also supported by a Research Grant from

Kwangwoon University in 2013.

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Figures:

Figure 1. Schematic design layout of the window hairpin filter.

Figure 2. Insertion loss, S21 responses of the filter with varying inside window width, Y

of 11.8, 12.0, 12.2 and 12.4 (mm).

Figure 3. Return loss, S11 responses of the designed filter with varying inside window

width, Y of 11.8, 12.0, 12.2 and 12.4 (mm).

Figure 4. Insertion loss, S21 responses of the designed filter with varying outside

window length, L of 9.2, 9.4, 9.6 and 9.8 (mm).

.

Figure 5. Return loss, S11 responses of the proposed filter with varying outside

window length, L of 9.2, 9.4, 9.6 and 9.8 (mm).

Figure 6. Insertion loss, S21 responses of the proposed filter with varying outside

window width, W of 14.3, 14.5, 14.7 and 14.9 (mm).

Figure 7. Return loss, S11 responses of the designed filter with varying outside window

width, W of 14.3, 14.5, 14.7 and 14.9 (mm).

Figure 8. Current density in the proposed BPF at (a) 4.2 GHz, (b) 4.7 GHz, (c) 5.2

GHz and (d) 5.7 GHz frequencies.

Figure 9. Photograph of the fabricated window hairpin BPF.

Figure 10. Simulated and measured S-parameter responses of the proposed filter.