диафрагмированные волноводные фильтры / aa11b764-f9f3-4cca-ba96-78143ee52fa8

Abstract − In this paper, we compare three different concepts of compact antenna arrays fed by substrate integrated waveguides (SIW). Antenna concepts differ in the type of radiators. Slots represent magnetic linear radiators, patches are electric surface radiators, and Vivaldi slots belong to travelling-wave antennas. Hence, the SIW feeders have to exploit different mechanisms of exciting antenna elements. Impedance and radiation properties of studied antenna arrays have been related to the normalized frequency. Antenna arrays have been mutually compared to show fundamental dependencies of final parameters of the designed antennas on state variables of antennas, on SIW feeder architectures and on related implementation details.

1 INTRODUCTION

Substrate integrated waveguides (SIW) have been proposed for a low-cost fabrication of waveguide structures [1]:

•SIW can be fabricated from conventional microwave substrates;

•Properties of SIW are comparable to the

parameters of conventional metallic waveguides. Thanks to SIW, active circuits, passive components, and radiating elements, can be

integrated into the same substrate.

SIW feeders are attractive to be used for the excitation of millimeter-wave antennas:

•Parasitic radiation of SIW feeders is negligible compared to microstrip transmission lines and coplanar waveguides;

•Integration of the feeder into a substrate to feed

antenna elements via apertures results in a compact architecture of the antenna array.

In the paper, we exploit SIW for an efficient feeding of three different antenna arrays:

•The array of slots represents a set of magnetic linear radiators;

•The array of patches is a system of electric surface radiators;

•The array of Vivaldi elements can be understood

as a set of travelling-wave sources.

All the three described compact arrays were designed for the operation in the Ka frequency band (see Table 1). Since central frequency of the operation of antenna arrays to be compared differs, frequency responses of antennas have been related to the normalized frequency and compared.

In Section 2, we briefly introduce the designed antenna arrays to be compared. Section 3 summarizes and compares outputs of simulations of antenna arrays. In Section 4, simulated results are experimentally verified. Section 5 concludes the paper.

Table 1: Operational frequencies of studied SIW-fed antenna arrays.

2 DESIGNED ARRAYS

In the following paragraphs, we provide details of the design of the studied SIW-fed antenna arrays.

2.1 Slot array

In [2], we described the design of the slot array for operation in the bandwidth from fmin = 26.5 GHz to fmax = 27.5 GHz. The antenna was designed adopting Elliott’s procedure. The antenna array consisted of a transition between the metallic rectangular waveguide and the SIW, the 10-way feeding divider,

65 mm × 105 mm × 12 mm including a radome. The structure of the array is depicted in Figure 1.

The array was designed for the TACONIC substrate

constant εr, rad = 2.77 and thickness hrad = 2.00 mm. The feeding network consisted of a transition from

the rectangular waveguide to SIW and a 10-way

array of radiating slots

power divider

RWG-to-SIW transition antenna input

Figure 1: Slot array fed by SIW: radome removed (top), cut view (bottom).

port 6

Figure 2: Structure of the rectangular waveguide to SIW transition and the power divider.

power divider (see Figure 2). The excitation power propagating in the rectangular waveguide was coupled to SIW through an aperture located in the output wall of the waveguide. The coupling slot was ls = 5.32 mm long and ws = 0.40 mm wide.

In the operation band, the designed antenna showed the reflection coefficient |S11| <−6 dB, the gain varied

from Gmin = 20.0 dB to Gmax = 22.9 dB, the side-lobe levels were SLL <−20 dB, and the front-to-back ratio

was F/B <−35 dB.

The antenna was designed to be used for point-to- point wireless communication.

2.2 Vivaldi array

In [3], we described the design of a Vivaldi array for operation in the bandwidth 26.5−40.0 GHz. The array was designed for a substrate of thickness h = 0.508 mm and dielectric constant εr = 2.2. The overall size of the array was 30.2 mm × 60.0 mm. The array elements were spaced 0.85 λ (at ~32 GHz) to obtain the in-phase feeding for each antenna element. The structure of the array is depicted in Figure 3.

Figure 4 shows the SIW feeding network conceived as an 8-way power divider with a wideband transition from SIW to a grounded coplanar waveguide (GCPW). The network exhibited a narrowband phase balance. Out of the band, the divider produced grating lobes due to reflections of the unbalance phase. In order to force the SIW feeding network to

Figure 3: Vivaldi array fed by SIW.

Figure 4: SIW feeding structure (top), SIW to GCPW transition (left).

divide the power properly to obtain an almost constant radiation pattern, we completed the network by tuning pins.

The impedance bandwidth of the array was 34 % (from 27 GHz to 38 GHz). The bandwidth related to the stable radiation pattern was about 10 % (from 31 GHz to 34 GHz). The developed antenna achieved a high gain G >14 dB, a narrow beam width in the E-plane θE <9.0° and sufficient side lobe suppression

Xpol <−14 dB in the entire band.

The antenna was designed for microwave imaging, detection of buried objects, non-destructive testing, and tracking.

2.3 Patch array

For a body-centric communication in ISM band 5.8 GHz, we developed a two-pole filtering antenna based on a SIW resonator, which was coupled by a rectangular aperture to a patch (see Figure 5). The filtering antenna was requested to keep a constant level of gain in the main-lobe direction over the operating band. Thanks to the stability of field distribution in the resonator over the dominant-mode frequency band, the requirement was accomplished.

In the second step, antenna elements were grouped into a 2×2 array (see Figure 6). The array was developed as a filtering antenna with the synthesized frequency response of gain.

Figure 5: Multilayer schematics of a patch element fed by a SIW resonator via an aperture.

Figure 6: Patch array fed by SIW.

The array was designed to operate at 31 GHz. The feeding network consisted of two parallel SIW bandpass filters sharing a common cavity. The last cavity of each SIW filter was loaded by an aperture exciting a patch. The second SIW cavity was also loaded by an aperture exciting a patch to support radiation. The phase shift between serial patches was tuned to avoid the main lobe deflection. The filtering array reached the gain bandwidth 7.5 % (from 29.3 GHz to 31.8 GHz). The realized gain in the pass-band was 13.2 dB. Out of the pass-band, the realized gain decreased for more than 15 dB. In the pass-band, the total main lobe deflection was 2 ° in the E plane.

3 COMPARISON OF SIMULATIONS

Simulated reflection coefficients |S11| of all antenna arrays normalized to their central frequency are compared in Figure 7. Obviously, Vivaldi array has the widest bandwidth. The other arrays have comparable bandwidths about one-third.

The simulated radiation patterns of antenna arrays at their central frequencies are depicted in Figure 8. They indicate low side lobe level which does not exceed −20 dB in E-plane except the patch array. If we design larger antenna array we could suppress the side lobes better. The front-to-back ratios are below −20 dB. In the required frequency bands, the gains of the antenna array are within the range from 13 dB to

Figure 7: Frequency responses of reflection coefficient related to normalized frequency.

Figure 8: Normalized patterns at central frequencies; E-plane (top) and H-plane (bottom).

Table 2: Simulated parameters of developed antennas.

20 dB. Table 2 summarizes the radiation parameters of the antenna arrays at operational frequencies.

Figure 9: Prototypes of the slot array and the Vivaldi array.

Acknowledgments

Research described in this paper was financially supported by the Czech Science Foundation under grant no. P102/12/1274. Research is a part of the COST Action IC1102 VISTA supported by the grant of the Czech Ministry of Education LD12012. Measurements were performed at SIX Research Center.