диафрагмированные волноводные фильтры / 704706f5-2c47-434f-aae0-6f214faab07f

Abstract—In this paper, a printed planar filtering antenna composed of the stepped impedance dipole (SID), stepped impedance resonator (SIR) and low-pass filter (LPF) is presented. The SIR is used as a parasitic element in proximity to the SID radiator to improve its upper band-edge selectivity and to widen its operation bandwidth. Our extensive study is at first conducted to exhibit that the upper harmonic band of radiation can be tremendously pushed upward by appropriately replacing the main dipole radiator and the parasitic resonant element with the SID and SIR. Next, a stepped-impedance LPF is employed to further effectively suppress the high-order harmonic band of radiation as quantitatively demonstrated. The measured reflection coefficient has achieved a wide fractional bandwidth up to 27.5%, covering a frequency range of 2.5 - 3.3 GHz, with |S11| < 10 dB. In particular, its first harmonic band for radiation has been significantly extended from 7.2 GHz beyond 14.0 GHz as confirmed in simulation and experiment.

Index Terms—Filtering antenna, upper harmonic band, stepped impedance resonator (SIR), stepped-impedance dipole (SID), and low-pass filter (LPF).

I. INTRODUCTION

As the rapid development of wireless communication technology, radio-frequency (RF)/microwave circuits and components are highly demanded towards compactness in size. Development of a variety of multifunctional circuits turns out to be a good method to reduce their overall size and it has become a hot research topic now. A filtering antenna is in general formed by the integration of the antenna and filter,

which has the functions of both components.

In recent years, extensive research has been conducted to explore various filtering antennas. A co-design approach was presented in [1] to integrate a filter with its respective antenna. In [ 2]–[4], a few antennas with good frequency selectivity were reportedly realized by including and synthesizing a bandpass filter (BPF). In [5], a filtering antenna with the feature of high selectivity was achieved by combination of substrate integrated waveguide and cavity-backed slot antenna. By introducing the shorting pins and parasitic elements, the antennas with high

Manuscript received at 12 October. 2015. This work is supported by the Program for New Century Excellent Talents in University (NCET-13-0214), and the Fundamental Research Funds for the Central University (2014ZZ0029).

G.H. Sun, S.W. Wong, and Q.X. Chu are with the School of Electronic and Information Engineering, South China University of Technology, Guangzhou City, Guangdong Province, 510640, PRC. (Corresponding author's e-mail: eewsw@scut.edu.cn, wongsaiwai@ieee.org).

L. Zhu is with the Department of Electrical and Computer Engineering, University of Macau, Faculty of Science and Technology, Macau SAR, China.

selectivity were constituted in the absence of filter [6-7]. These reported filtering antennas exhibited good filtering

selectivity, but they unfortunately suffer from the high-order harmonic band for parasitic radiation. As such, the radiation in this high-order harmonic band would consume additional power and cause signal interference with other circuits or systems. The primary motivate of this work is not only to improve the frequency selectivity feature but also to effectively suppress the harmful radiation in the upper high-order harmonic band of a resonator-type antenna.

In fact, the stepped-impedance resonator (SIR) has been widely used in designing of microstrip-line BPFs with good harmonic suppression and wide operating bandwidth [8-10]. By appropriately selecting the impedance ratio of highand low-impedance segments in the SIR, the first spurious harmonic band could be tremendously pushed up, so as to create an extremely wide upper stopband. This fascinating property could be also applied in the design of antennas.

In this paper, the main dipole radiator is reformed as a steppedimpedance dipole (SID) and it is further placed in proximity to a parasitic SIR. Thus the first harmonic band of this antenna is pushed up to high frequency a great extent. This harmonic band is then fully suppressed by an integrated low-pass filter (LPF). As a result, a filtering antenna with good suppression of upper harmonic band is constituted. As compared with the BPF, the LPF integrated with an antenna has electrically small size, thus making the overall size of the resultant filtering antenna become more and more compact.

II. GEOMETRICAL STRUCTURE OF FILTERING ANTENNA

The geometry of the proposed filtering antenna is depicted in Fig.1. The entire antenna is implemented on a dielectric substrate with permittivity 2.55 and thickness of 0.8 mm. The antenna, composed of the SID and SIR, is printed on the top surface of this substrate. Herein, the LPF is designed by using the parallel strip line and it is then used to feed the SID radiator without installing any extra balun. The top strip conductor of the LPF output port directly connects the left arm of the SID while its bottom strip conductor is connected with the right arm of this SID through a metal via hole. The filtering antenna is excited by an elliptically tapered microstrip line from the 50port. All the physical dimensions of this filtering antenna are numerically determined and they are tabulated in Table I.

(b)

Fig. 1. Configuration of proposed filtering antenna. (a) 3-D view; (b) Top view.

TABLE I

GEOMETRICAL PARAMETERS OF THE PROPOSED FILTERING ANTENNA

Fig. 4. Simulated input impedances of the printed dipole without and with parasitic element (Lp=33 mm).

III.DESIGN PROCESS AND ANALYSIS OF FILTERING ANTENNA

A.Parasitic Element in Proximity to a Radiated Dipole

As well known, the traditional printed dipole suffers from a narrow impedance bandwidth and poor selectivity in the upper band. To circumvent these problems, a parasitic element is introduced in proximity to the main dipole radiator, as shown in Fig.2. The simulated reflection coefficients and the total radiated powers with and without parasitic element are plotted in Fig.3, exhibiting emergence of an additional resonant mode in the operating band of radiation. When Lp is set to 33 mm, the impedance bandwidth can be largely widened from 11% to 27%. Moreover, this introduced parasitic element can create a

Fig.4 depicts the input impedance of an dipole antenna with and without a parasitic element, and it is utilized to explain how these improved performances can be achieved. The real and imaginary part of input impedance of the printed dipole without the parasitic element rapidly increase around 2.6 GHz. Looking at the dipole with a parasitic element (Lp=33 mm), the imaginary part of the input impedance has a small variation around zero from 2.5 to 3.3 GHz, provide a hint on realization of wideband impedance matching. Its respective real part begins to decrease at 2.9 GHz and drops to zero to achieve full reflection at 3.7 GHz, where radiation null emerges. As a consequence, both of expected good filtering selectivity and wide impedance bandwidth can be simultaneously achieved by slightly lengthening the size of the original dipole antenna.

B. Formulation and Analysis of an antenna with SID and SIR

As shown in Fig.5, the uniform dipole has a harmonic band at 7.6 GHz for radiation and the uniform impedance resonator (UIR) causes a harmonic radiation at 9.8 GHz. As usual, these unwanted harmonics for radiation are not considered in the design of an antenna itself and the task for suppression of them is completed by introduced an additional filter. As an attractive feature, a filtering antenna can be designed to fully address this problematic issue, i.e., realization of harmonic suppression in advance, in order to avoid the redundant work in filter design. As such, in our design, the uniform dipole radiator and UIR resonator are replaced by the SID and SIR, respectively.

Fig.5 exhibits that the upper harmonic band can be really improved as the SID and SIR are introduced. When Wp2 increases from 0.5 to 3.5 mm, the harmonic at 9.8 GHz is moved up to 11.6 GHz. When Wd2 is raised from 0.75 to 6.75 mm, the harmonic at 7.2 GHz is further pushed up to 10.1 GHz. Meanwhile, the values of Wd2 +Ld2 and Wp2 +Lp2 are kept as the same in order to maintain the fundamental resonant frequency.

As well studied in [8], the overall length of the constituted filtering antenna is decreased to 17 mm, i.e., 26% reduction, if the SIR and SID with appropriate impedance ratios are utilized. Due to upward movement of the harmonic band, the cut-off frequency of the LPF implemented in our proposed filtering antenna is shifted to higher frequency. Thus, the LPF becomes smaller in size and the upper-stopband becomes wider in bandwidth. In addition, the introduction of SIR and SID has no effect on the radiation null as will be shown later on.

LPF behaves as a transmission line in the low passband, and it hardly affects the antenna performance in the operating band of radiation since it is applied to suppress the harmonic band at around 10.7 GHz. As illustrated in Fig.6, the antenna with LPF has much better harmonic rejection than the antenna without LPF.

Fig. 6 Reflection coefficient of the dipole antennas without and with LPF

IV. SIMULATED AND MEASURED RESULTS

To validate the predicted performance of this proposed filtering antenna, a prototype antenna was fabricated and tested. Fig.6 shows the topand bottom-view photographs of the fabricated antenna. Simulated results, inclusive of return losses, radiation patterns and gains, were numerically derived by using the commercial CST studio software. Measurement on the fabricated antenna is carried out by using the Agilent N5230A Network Analyzer in anechoic chamber.

Fig. 5. Shift of upper harmonic band as the parasitic UIR and main dipole are reformed as SIR and SID.

C. Integration of a filtering Antenna with LPF

In the normal case, the antenna itself is fed by a quarter wavelength. Instead, a seven-order stepped-impedance LPF is utilized instead of a feeding line of the antenna. The length of the LPF is found as nearly 26 mm, and it is approximately equal to a quarter wavelength at center frequency of the operating band. Thus, this LPF brings out no extra length in the entire antenna, but it can effectively suppress the high-order harmonic band as demanded in realization of a wide upper stopband.

Fig.6 depicts the simulated |S12| of this designed LPF, and demonstrates that its cut-off frequency appears at 5.5 GHz. The

Fig. 7 Photography of the fabricated filtering antenna prototype. (a) top-view;

(b)bottom-view;

A.Return Loss and Radiation Gain

Fig.8 plots the measured and simulated reflection coefficient and gains of the designed filtering antenna. The result shows good agreement with each other especially in the operating band of radiation. As predicted in simulation, the measured impedance bandwidth of the filtering antenna has achieved 27.5 % covering a frequency range from 2.5 to 3.3 GHz under |S11| < -10 dB. From Fig.8, we can clearly observe the emergence of a radiation null at 4.5 GHz, thus proving that the introduced parasitic SIR in proximity to the main SID radiator

has a great capacity in effectively improving the upper band-edge selectivity. The radiation gain in the operating band is found as 2.5 dBi since no metallic reflector or ground plane is installed in this prototype antenna, but it exceeds 10.3 dB with respect to the maximum gain in the upper stopband of 4-14 GHz.

element creates a radiation null to improve the upper band-edge selectivity, and the resultant antenna has successfully extended the upper-stopband beyond 14.0 GHz. The designed filtering antenna has achieved the measured impedance bandwidth of 27.5%, and its radiation gain in the desired operating has exceeded that in the upper stopband by 10.3 dB. It is our belief that several advantages of compact, simple structure, wide upperstopband and simple feeding geometry make the proposed filtering antenna very useful in a variety of applications.

B. Radiation Pattern

Fig.9 describes the simulated and measured radiation patterns in the E-plane (XoZ-plane) and H-plane (YoZ-plane) at the center frequency of 2.9 GHz. The coand cross-polarization directions are intuitively oriented along the X- and Y-direction, respectively. The LPF have no effect on radiation patterns because it is vertically placed with the antenna. The parasitic element slightly enhanced the gain in y-direction and weaken the gain in z-direction. The whole radiation pattern in the operating frequency is the almost same as that of a conventional dipole antenna.

TABLE

COMPARISON WITH PRIOR FILTERING ANTENNAS

Table compares the performances of the proposed antenna with several prior works. Our presented antenna herein has been exhibited to have better performance than prior works in term of size and harmonic suppression. Moreover, up to authors’ knowledge it is the first time that the harmonic suppression is considered in the design of filtering antenna.

V. CONCLUSION

In this paper, a novel printed filtering antenna with good upper harmonic suppression is presented. The parasitic SIR

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