Substrate Integrated Waveguide Filtenna With Two Controllable Radiation Nulls

In this paper, a novel substrate integrated waveguide (SIW) filtenna with two controllable radiation nulls is proposed, which has the advantage of good radiation characteristics. Radiation nulls are generated by electric and magnetic mixed coupling structure and fundamental mode, respectively. And a quasi-elliptic characteristic can be obtained at the boresight gain direction. By changing the length of perturbation slot and the strength of electric coupling and magnetic coupling, the position of radiation nulls can be controlled independetly. Compared to conventional filtenna, the proposed SIW filtenna has no extra filtering circuit to increase the dimension and complexity. Two resonant modes in the passband can be excited through loading electric and magnetic mixed coupling structure in one SIW cavity. The measured results are in good agreement with simulated results. The good frequency selectivity and out-of–band rejection are achieved. The proposed antenna exhibits a maximum realized gain of 5.8 dBi at the operating bandwidth. The measured impedance bandwidth is from 9.44 GHz to 9.68 GHz and the cross-polarization levels are lower than -20 dB in both planes.


I. INTRODUCTION
With the development of substrate integrated waveguide (SIW) technology owning some advantages such as low profile, easy integration and so on, SIW has been studied by lots of experts in microwave and millimeter wave communication systems [1], [2]. SIW has good radiation performance and gain characteristics, and is easily integrated to planar circuit [3]. Therefore, SIW is widely used to design antennas, filters, and other RF components [4]- [6].
Filtenna is a good method to significantly reduce the size of the RF front-end of communication systems. A lot of filtennas using SIW technology have been investigated and proposed [7]- [15]. [7] proposed a filter antenna using a common cavity cascaded to radiation element. A single-layered SIW filtenna with parasitic patch was proposed in [8]. Parasitic The associate editor coordinating the review of this manuscript and approving it for publication was Santi C. Pavone. patch and two pairs shoring posts are utilized for generating two radiation nulls. A low-profile diplexing SIW antenna based on common radiating cavity with filtering response was proposed [9]. Millimeter-wave filtering antenna using SIW filtering power divider was reported [10]. However, the designs in [7]- [10] make antennas bulky with precious filtering circuit footprints. [11] proposed a third-order SIW filtering antenna, cross coupling is introduced to generate a controllable radiation null and filtering response is achieved in gain curve. [12] proposed a single-layered SIW filtering antenna, a radiation null is obtained by placing the slot in the position where E-filed is null. By changing the size of radiation slot, the position of radiation null can be controlled. A SIW filtering antenna was presented with one controllable radiation null using source-load coupling [13]. However, in previous work, only controllable one radiation null is generated in the either upper or lower stopband. In [14], two slots are etched on the top and bottom surfaces of SIW VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ cavity, respectively, not only achieving bidirectional radiation performance but also generating two radiation nulls. [15] proposed a dual-layer SIW cavity filtering antenna. Three modes (i.e. TE 210 mode, TE 010 mode and TE 110 mode) are excited to achieve a radiation band and two radiation nulls. However, the controllable radiation null is not realized in [14] and [15]. In this letter, a substrate integrated waveguide filtenna with two controllable radiation nulls is proposed. The two resonant modes (i.e. a hybrid mode [16] and TE 120 mode) are excited in a single passband. In addition, two radiation nulls can be obtained, generating a quasi-elliptic filtering response in gain curve. By using electric and magnetic mixed coupling structure, a radiation null can be introduced. Another radiation null is obtained by fundamental mode resonanting in the SIW cavity. Two radiation nulls can be controlled by changing the strength of electric and magnetic mixed coupling and the length of perturbation slot, respectively. Measured results verify these characteristics. The proposed filtenna presents a high selectivity at the frequency bands can be easily adjusted to accomplish practical applications, for instance, in satellite communications and radar systems.

II. DESIGN OF THE ANTENNA
The configuration of proposed SIW filtenna is shown in Fig.1. It is printed on the Rogers RT/Duroid 5880 substrate with 0.5 mm. The relative dielectric constant and loss tangent are 2.2 and 0.0015, respectively. It consists of a SIW cavity and electric and magnetic mixed coupling structure, a transverse slot as radiation element is located in the center of resonant cavity. The antenna is excited by a 50 stripline. In order to avoid energy leakage from side wall, the diameter of shorting-vias (d) and distance of shorting-vias (s) are designed, maintaining the conditions d/s > 0.5 and d/λ 0 < 0.1. λ 0 is the wave-length of the operating frequency in the freespace. And the dimension of SIW cavity can be obtained by (1) [17].  where w is the width of SIW cavity, w e is the width of equivalent rectangular waveguide.

A. EFFECT OF LOADING ELECTRIC AND MAGNETIC MIXED COUPLING STRUCTURE
In order to excite resonant mode and radiation null successfully, the electric and magnetic mixed coupling structure is introduced. As shown in Fig.2, three coupling structures are given: (a) original coupling structure; (b) the stepped impedance coupling structure; (c) the proposed coupling structure. The corresponding reflection coefficient and input impedance are shown Fig.3 and Fig.4, respectively. Loading structure-I, only one resonant mode is excited. In order to excite two resonant modes, structure-II is used. Impedance matching can be realized by using step-impedance transmission-line, and then the second resonant mode can be excited successfully. However, in the area A, using multistep-impedance transmission-line does not realize better impedance match. In order to reduce complexity of designing and get better impedance match, structure-III is proposed. In the area B, slot-stub Var_l is introduced.
The length of step-impedance transmission-line by varying slot-stub Var_l improves the matching characteristics.

B. RESONANT MODE
TE 120 mode is excited to generate one resonance successfully. The H-field distribution of TE 120 mode is given in Fig.5(a). The H-field on the both sides of the slot is opposite, and there is transverse field across the slot to generate radiation into the space. Loading electric and magnetic mixed coupling structure weaken the strength of TE 120 , which leads to unequal magnitude of TE 120 on the two sides of slot. In addition, fundamental mode and TE 120 mode interact with each other, which generates a hybrid mode. The H-filed distribution of the hybrid mode is shown in Fig.5(b). It can be seen that dominant H-filed is concentrated on the right half cavity of radiation slot. The H-filed of TE 120 mode and fundamental mode cancel out each other to make H-field weak in the left half cavity of slot to generate a hybrid mode. Different magnitude of H-filed at two sides of slot can generate radiation through the slot.

C. RADIATION NULLS
Two radiation nulls locating on both sides of passband are generated due to electric and magnetic mixed coupling and fundamental mode. The radiation null (RN 1) at lower stopband is introduced by the resonance of fundamental mode. The slot is etched in the center of the resonating cavity. When fundamental mode is resonating, the current in the both sides of the slot is equal magnitude and in opposite. There is no existing across current in the slot and no broadside radiation is generated. Since the frequency of fundamental mode is determined by the dimension of cavity, in order to realize the controllability of RN 1, perturbation slot L_arm is introduced. By changing the length  of perturbation slot, the current path of fundamental mode is changed, which influences the position of RN 1, as is shown in Fig.6. As L_arm increases, the current path of fundamental mode keeps increasing, which makes the position of RN 1 shift toward lower frequency.
Another radiation null (RN 2) at upper stopband is generated through using mixed electric and magnetic coupling structure. The mixed coupling structure is composed of electric coupling structure and magnetic coupling structure. The equivalent circuit model of mixed electric and magnetic coupling is given in Fig.7 [18], [19]. As shown in Fig.7, mixed coupling coefficient can be expressed (2), where E C represents electric coupling coefficient, M C represents magnetic coupling coefficient [20]. E C and M C represent the strength of electric coupling and magnetic coupling, respectively, and decide the frequency of radiation null. Therefore, the frequency of radiation null is extracted by ω RN = 1/ √ L m C m , simply. In the area B, mixed coupling structure and cavity directly contact at parameter L g . Mixed coupling structure and cavity share the same current path, therefore the magnetic coupling is main coupling here. In the area A, coupling structure and cavity are not directly in contact, and there exists gap structure. Therefore, electric coupling is dominating here. The position of radiation null can be controlled by changing the strength of electric and magnetic mixed coupling.  We and Var_l are key parameters to adjust the strength of magnetic coupling. The length of mixed coupling structure Le is the key parameter to adjust the strength of electric coupling. As are shown in Fig.8 and Fig.9, with parameters We and Le changing, the strength of mixed electric and magnetic coupling is changed, leading to the change of the position of RN 2. However, varying parameters We and Le, the surface current of path of resonances is changed, resulting in varying frequency of resonances. Parameters We and Le cannot realize RN 2 controllability, independently. As is shown in Fig.10, changing Var_l only affects the strength of mixed electric and magnetic coupling. Var_l is located on the bottom of feed structure, varying Var_l does not affect surface current of path of resonances. Thus, with the varying of Var_l, the position of RN 2 can be changed independently. The size of mixed coupling structure is optimized, and RN 2 is obtained, locating on the right side of passband. As shown in Fig.11, with the parameter Var_l increasing, the strength of magnetic coupling decreases and the position of RN 2 shifts toward lower frequency.

D. DESIGN GUIDELINES
Based on the above analysis, a design guideline is recommended as follows.
1) Designing the dimensions of SIW cavity with a proper feedline according to the desired center frequency.
2) Determining the frequencies of the required radiation nulls outside the operating band.
3) Etching perturbation slot and electric and magnetic coupling structure in the SIW cavity, and then optimizing their dimensions to make radiation null at suitable positions. 4) Fine tune parameters and optimize the design to realize a filtering performance.

III. MEASUREMENTS RESULTS
The photograph of antenna fabricated is shown in Fig.12. The slot and mixed coupling structure are etched on the top surface of SIW cavity fabricated by printed circuit board technology. The simulated and measured reflection coefficient and realized gain are plotted in Fig.12. From the Fig.12, it can be seen that an excellent agreement between measured results   with simulated one. Two resonances are excited, leading to a flat passband. The measured operating bandwidth is from 9.44 GHz to 9.68 GHz with reflection coefficient lower than −10 dB. The obtained maximum of realized gain is 5.8 dBi. As expected, two radiation nulls are generated in the upper and lower stopband, respectively. An excellent quasi-elliptic filtering response is obtained. However, there exists some deviation in the plot, due to fabrication tolerances.
The radiation patterns of the proposed antenna are shown in Fig.13 and Fig.14. The E-plane (y-z plane) and H-plane (x-z plane) measured patterns are compared with simulated results. Cross-polarization levels are lower than −20 dB in both planes. A comparison with previous works is tabulated in Table 1. It can be found the proposed antenna has advantage in generation and controllability of radiation nulls. The proposed filtenna has no extra filtering circuit, which increases designing complexity, and keeps an excellent in-band antenna performance and filtering performance.

IV. CONCLUSION
A substrate integrated waveguide filtenna with two controllable radiation nulls is presented, designed and fabricated. Two resonant mode are excited through electric and magnetic mixed coupling structure to produce a flat frequency response. The maximum realized gain can reach 5.8 dBi. Exciting fundamental mode and loading electric and magnetic mixed coupling structure can generate two radiation nulls to realize a quasi-elliptic filtering response. The measured results also verify the correctness of the proposed antenna. Radiation patterns show a great unidirectional radiation characteristic and low cross-polarization levels.