диафрагмированные волноводные фильтры / affa37f7-c0b6-482b-b268-e38c5c54fc9d

Abstract—This letter proposes a codesigned substrate integrated waveguide (SIW) filtering slot antenna with a controllable radiation null in gain response. The antenna is composed of a singlelayered SIW cavity, four metalized posts, a transverse slot, and a vertical SMA connector. Four metalized posts are introduced to split the cavity into two TE1 1 0 -mode resonators. The transverse slot is utilized not only to generate radiation, but also to further split one TE1 1 0 -mode resonator into two half-mode resonators.

The bandpass filtering performance is achieved by the couplings between three resonators and the slot. One radiation null has been introduced for selectivity enhancement, and it will occur at the frequency where the slot is placed at about quarter-wavelength from the shorted wall of the cavity. So by changing the position of the slot, frequency of radiation null can be easily controlled, even from one side to another side of the passband. For demonstration, one antenna with a lower out-of-band radiation null and another one with an upper out-of-band radiation null have been designed, fabricated, and measured. Experimental results show that the antennas have the merits of high efficiency, high selectivity, and easily controllable radiation null.

Index Terms—Cavity resonators, filtering antenna, slot antennas, substrate integrated waveguide (SIW).

I. INTRODUCTION

ODERN WIRELESS communications demand the RF Mfront-end system to be compact, lightweight, low-cost, high-efficiency, and multifunctional. In most of the RF front ends, bandpass filter and antenna as two key components are usually designed separately and connected by a 50 or 75 Ω transmission line, which not only increases the volume but also may degrade in-band performance due to the mismatch and extra insertion loss caused by the interconnections. Recently, a concept of filtering antenna has been proposed by integrating the bandpass filter and the antenna into a single component with filtering and radiating functions simultaneously to reduce the

Manuscript received December 5, 2017; accepted December 20, 2017. Date of publication December 27, 2017; date of current version February 5, 2018. This work was supported by the Fundamental Research Funds for the Central Universities under Grant ZYGX2016J006. (Corresponding author: Chang Jiang You.)

P. K. Li, C. J. You, H. F. Yu, X. Li, and Y. W. Yang are with the School of Communication and Information Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China (e-mail: lpkuestc@163.com; cjyou@uestc.edu.cn; yuhf@uestc.edu.cn; toki_chris_lee@ 163.com; yuanwangyang@uestc.edu.cn).

J. H. Deng is with the School of Information and Software Engineering, University of Electronic Science Technology of China, Chengdu 611731, China (e-mail: Jianhua.deng@uestc.edu.cn).

Digital Object Identifier 10.1109/LAWP.2017.2787541

size and loss [1]–[5]. On the other hand, substrate integrated waveguide (SIW) technology has been applied to design highperformance filters and antennas due to its low insertion loss and radiation loss [6], [7]. Some filtering antennas based on an SIW technology have been reported. In [8], the antenna is planar and designed by cascading resonators and radiator. In [9]–[11], Three-dimensional configurations by placing the resonators under the radiator are applied. However, they are not strictly codesigned as they all need extra radiator to cascade the filtering circuit.

Frequency-controlling technology on passband has been widely investigated in antenna designs [12]–[14]. Moreover, to improve the selectivity of a filtering antenna, radiation null (transmission zero) is always introduced. Asymmetric responses allow us to increase selectivity of one out-of-band, without increasing the order of filtering antenna. In some scenarios, high selectivity of the lower or upper out-of-band is demanded. Thus, frequency-controlling technology on radiation null (transmission zero) is proposed to satisfy these diverse requirements. It has been studied in filter designs [15]–[18], but there is no reported discussion on the filtering antenna about radiation null (transmission zero) controlling.

In this letter, a novel codesigned SIW filtering slot antenna with a controllable radiation null in gain response is investigated for the first time. It has very simple structure, composed of a single-layered SIW cavity, four metalized posts, a transverse slot, and a vertical SMA connector. By introducing a nonresonant slot and the metalized posts, the cavity is split into one TE110 -mode resonator and two half-mode resonators. Low insertion loss and high total efficiency can be obtained due to the codesigned compact structure. A radiation null is achieved, and its frequency can be controlled by changing the slot offset. Therefore, the skirt selectivity of one sideband can be highly enhanced as the radiation null can be moved closely to the passband. The antenna mechanism is presented in detail, and two prototypes with a radiation null at either side of the passband, respectively, are designed, fabricated, and measured to validate the idea.

II. ANTENNA CONFIGURATION AND MECHANISM

The configuration of the proposed filtering antenna is illustrated in Fig. 1. It consists of an SIW cavity, a transverse slot, four metalized posts along the middle line of the cavity, and an SMA connector as feed, designed on an F4B-2 substrate with a thickness of 6 mm, a relative dielectric constant of 2.485, and a loss tangent of 0.0018.

Fig. 1. Configuration of the proposed antenna. (a) Top view. (b) Side view.

Fig. 4. Surface current distributions. (a) On broad wall of SIW with end shorted. (b) On broad wall for Structure III at resonant frequency f1 , f2 , f3 and the frequency of radiation null fn u ll as marked in Fig. 3.

Fig. 2. Configuration of the SIW slot structures. (a) Transverse slot in SIW with end shorted. (b) Transverse slot in an SIW cavity. (c) Transverse slot in an SIW cavity and four metalized posts along the middle line.

Fig. 3. Simulated results for Structures I, II, and III with the same ys of 13.2 mm. (a) Realized gains. (b) Reflection coefficient S1 1 . In the simulation, a = 1.6, s = 3.2, ls = 28.7, ws = 4, d = 4.6, xt = 9.2, W = 49, and L = 51 (unit: mm).

The proposed antenna is originated from a nonresonant transverse slot etched on the broad wall of the SIW with end shorted, and then designed by constructing an SIW cavity fed by an SMA connector and introducing four metalized posts along the middle line of the cavity, as illustrated in Fig. 2. It should be noticed that the slot is nonresonant because its length is larger than half a guide wavelength at work band. The realized gains, and reflection coefficient for Structures I, II, and III with same parameters are investigated by simulation and given in Fig. 3. It can be found in Fig. 3(a) that Structure III has better sideband suppression compared with Structures I and II. This is because the order of resonances increases, as shown in Fig. 3(b). Actually, in Structure I, there is no obvious in-band resonance as the slot is nonresonant at work band. In Structure II, the antenna is mainly resonating at TE120 mode of the whole cavity as discussed in [6]. In Structure III, four metalized posts are utilized to divide the TE120 -mode resonator into two TE110 -mode resonators. The slot can further split one TE110 resonator into two half-TE110 -mode resonators [18]. Thus, one TE110 -mode resonator labeled as R1 and two half-mode resonators labeled as

R2 and R3 in Fig. 2(c), respectively, are realized. What is more, couplings between resonators are introduced by the metalized posts and slot. Consequently, a three-order bandpass filteringlike radiation gain response is achieved. It should be mentioned that the slot can not only split the cavity into two half-mode resonators, but also can generate radiation. This merged design is distinctive from the method using cascade connection between resonators and radiator. A radiation null is observed in Fig. 3 for all the three structures and occurs at the same frequency. This phenomenon has not been reported in the SIW slot antenna designs. It will be explained in detail here and utilized in the following designs.

The surface current distribution on broad wall of SIW with end shorted, which is excited by TE10 -mode excitation, is simulated and shown in Fig. 4(a). If the slot is placed in region 1, the current is nonzero, thus radiation will be generated. If the slot is placed at point B, the current is zero and radiation will be suppressed. Fig. 4(b) gives the surface current distributions for Structure III at resonant frequencies f1 , f2 , f3 , and the frequency of radiation null fnull as marked in Fig. 3. It can be found that, at all in-band resonant frequencies f1 , f2 , f3 , the current at the slot is nonzero, while at fnull , the current at the slot is zero. It can be deduced that the radiation null occurs at the frequency where the slot is placed at the position about a quarter-wavelength from the shorted wall of the cavity where IB = 0. So, neglecting the slot effect on phase constant β, the frequency of the radiation null can be roughly estimated at the condition (1), which is as follows:

where β(f) represents the phase constant at frequency f, ys denotes the distance between the slot and the SIW end wall posts, εr is the relative dielectric constant of the substrate, c is the light speed in free space, and w is effective width, which can be approached using the formula in [17].

The effect of the slot offset ys on realized gains is investigated for the proposed antenna (Structure III), as shown in Fig. 5. It can be found that the radiation null shifts upward with the decreasing of ys, and it can be located at the lower or the upper side of the passband, even at the middle of the passband. To verify the above-mentioned analysis and demonstrate the relation between the frequency of radiation null fnull and the slot offset ys, fnull in the three structures with different ys are obtained by simulation. The formulas (1) and (2) are also solved to give the

Fig. 6. Calculated and simulated frequency of radiation null fn u ll with different value of ys in three structures.

calculated fnull . They are plotted in Fig. 6 for comparison. It can be observed that the calculated fnull agrees well with the simulated ones, and the simulated fnull of the three structures are in accordance with each other. Other parameters such as the cavity length L, the posts space xt, slot length ls, and width ws have also been studied but not shown here for brevity. It is found that they have little effect on fnull compared to ys. The little effect is caused by the nonzero width of the slot and the unequal current strength at its two sides. A conclusion can be drawn that fnull mainly depends on the slot offset ys, and the resonances caused by the cavities make little effect on it.

According to the above analysis, fnull of the proposed antenna can be controlled by adjusting the slot offset, and the skirt selectivity can be enhanced by moving the radiation null close to the passband. To validate the idea, an antenna with a lower out-of-band radiation null (Ant-1) and another one with an upper out-of-band radiation null (Ant-2) are designed and optimized. The detailed dimensions of Ant-1 are L = 51, W = 49, m = 4.8, d = 2, xt = 9.1, ls = 28.7, ws = 4.2, ys = 13.2, and the dimensions of Ant-2 are L = 51, W = 49, m = 4.3, d = 2, xt = 8.5, ls = 30.6, ws = 6.9, ys = 11.4 (unit: mm).

III. RESULTS AND DISCUSSION

Two prototypes are fabricated and measured. The simulated and measured reflection coefficient S11 , realized gains and total efficiencies of Ant-1 and Ant-2 are shown in Fig. 7. It can be seen that the measured impedance bandwidth (S11 < −10 dB) of Ant-1 is given by 6.1% (4.29–4.56 GHz), agreeing reasonably with the simulated value of 7% (4.25–4.56 GHz), and that of Ant-2 is 5.4% (4.33–4.58 GHz), agreeing well with the simulated one of 5.6% (4.35–4.59 GHz).

On the gain responses, a lower null in Ant-1 and an upper null in Ant-2 are observed, and the gains are flat across the passband with ripples less than 0.3 dB, which is attributed to the nonresonant slot radiation. Ant-1 has a measured average realized gain of 6.5 dBi and maximum realized gain of 6.7 dBi, while those of Ant-2 are almost of the same value with Ant-1. Measured gain responses are in accordance with the simulated ones. Due to the controllable radiation null in gain response, the antenna can achieve high skirt selectivity in either sideband by properly moving the radiation null close to the passband. In the demonstrated design Ant-1, the left out-of-band skirt selectivity is 266 dB/GHz, and the suppression level is more than 27 dB. For Ant-2, the right out-of-band suppression level is more than 26 dB, and the skirt selectivity is 134 dB/GHz as the radiation null is little far away from the passband.

The total efficiency η of the antenna is defined as η = radiated power/incident power. The simulated total efficiencies are calculated according to the simulated results. The measured efficiencies are obtained by using a Satimo Starlab System. It can be seen in Fig. 7, the passband is remarkable as the efficiencies at out-of-band approach 0, and the measured maximum efficiency reaches 93% for Ant-1 and 94% for Ant-2, which means insertion loss of about only 0.3 dB is introduced. This is because the proposed structure is compact and almost without extra size compared to a TE120 -mode SIW slot antenna as shown in Fig. 2(b). Consequently, no extra insertion loss is introduced, and high total efficiency is achieved.

The simulated and measured radiation patterns in the center frequencies are shown in Fig. 8. In both antennas, almost symmetrical radiation patterns are observed in E/H-plane. The slight tilts at E-plane patterns are mainly caused by the asymmetrical structure and the unequal current strength at two edges of the wide slot due to the half-mode resonances. Good front-to-back ratios and cross-polarization levels of more than 20 dB can be

Fig. 8. Simulated and measured radiation patterns of the antennas in E-plane and H-plane. (a) Ant-1. (b) Ant-2.

TABLE I

COMPARISON WITH PREVIOUS REPORTED WORKS USING AN SIW SLOT

attenuation of the first out-of-band radiation null or 20 dB attenuation if no radiation null and α m i n is the 3 dB attenuation; fz and fc are their corresponding frequency.

seen in both planes. The radiation patterns are stable over the passband according to our study.

A comprehensive comparison with previous works is summarized in Table I. It can be found the bandwidths of our designs are comparable to their counterparts. The lower skirt selectivity of Ant-1 and the upper skirt selectivity of Ant-2 are much higher than the previous works. It is benefited from the merit that the proposed antenna in this letter can control the radiation null, and the radiation null can be moved very closely to the passband. Due to codesign and no extra size compared to the single TE120 -mode SIW slot antenna, our designs provide relatively higher efficiencies than the others who employ more layers and full-mode resonators so as to occupy larger lossy circuit area, which can be evaluated by size×layers.

IV. CONCLUSION

In this letter, a single-layered SIW filtering antenna with a controllable radiation null in gain response is proposed. The antenna is of no extra size compared to the traditional TE120 - mode cavity back SIW slot antenna, thus high efficiency can be

achieved as without introducing more insertion loss. A radiation null is created, and its frequency can be controlled from one side to another side of the passband by simply changing the slot offset. Two antennas with a radiation null at either side of the passband are implemented. Measurements show that total efficiency as high as 94%, good skirt selectivity of 266 dB/GHz, and out-of band suppression of more than 26 dB can be obtained. The proposed antenna is very suitable for practical integrated module with filtering response and radiation performance in wireless front-end system.

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