High-gain and large-aperture antennas with fixed beams are required to achieve high S/N ratio for point-to-point high-speed data-communication systems in the millimeter-wave band. Furthermore, beam-scanning antennas are attractive to cover wide angle with high gain for applications of high-speed data-communication systems and high-resolution sensing systems. High-gain pencil-beam antennas are used for mechanical beam-scanning antennas. Although high antenna efficiency can be obtained by using dielectric lens antennas or reflector antennas (Kitamori et al., 2000, Menzel et al., 2002), it is difficult to realize very thin planar structure because they essentially need focal spatial length. By using printed antennas such as microstrip antennas, the RF module with integrated antennas can be quite low profile and low cost. Array antennas possess a high design flexibility of radiation pattern. However, microstrip array antennas are not suitable for high-gain applications because large feeding-loss of microstrip line is a significant problem when the size of the antenna aperture is large. They are applied to digital beam forming (DBF) systems since they consist of several sub-arrays, each of which has small aperture and requires relatively lower gain (Tokoro, 1996, Asano, 2000, Iizuka et al., 2003).
Slotted waveguide planar array antennas are free from feeding loss and can be applied to both high-gain antennas and relatively lower-gain antennas for sub-arrays in beam-scanning antennas. Waveguide antennas are more effective especially in high-gain applications than low-gain since a waveguide has the advantage of both low feeding loss and compact size in the millimeter-wave band even though the size of the aperture is large (Sakakibara et al., 1996). However, the production cost of waveguide antennas is generally very high because they usually consist of metal block with complicated three-dimensional structures. In order to reduce the production cost without losing a high efficiency capability, we propose a novel simple structure for slotted waveguide planar antennas, which is suitable to be manufactured by metal injection molding (Sakakibara et al., 2001).
We have developed two types of planar antenna; microstrip antenna and waveguide antenna. It is difficult to apply either of them to all the millimeter-wave applications with different specifications since advantages of the antennas are completely different. However, most applications can be covered by both microstrip antennas and waveguide antennas. Microstrip antennas are widely used for relatively lower-gain applications of short-range wireless-systems and sub-arrays in DBF systems, not for high-gain applications. Waveguide antennas are suitable for high-gain applications over 30 dBi.
With regard to the microstrip antennas, comb-line antennas are developed in the millimeter-wave band. In the comb-line antenna, since radiating array-element is directly attached to the feeding line, feeding loss could be quite small in comparison with other ordinary patch array antennas connected via microstrip branch from feeding lines. The branch of the comb-line antenna is no longer feeding circuit but radiating element itself. Radiation from the discontinuity at the connection of the radiating element joins to the main radiation from the element. Consequently, equivalent circuit model can not be used in the design any more. Electromagnetic simulator must be used to estimate the amplitude and phase of radiation from the elements accurately. Traveling-wave excitation is assumed in the design of comb-line antennas. Reflection waves from the elements in the feeding line degrade the performance of the antenna. When all the radiating elements are excited in phase for broadside beam, reflection waves are also in-phase and return loss grows at the feeding point. Furthermore, reflection waves from elements re-radiate from other elements. Radiation pattern also degrades since it is not taken into account in the traveling-wave design. Therefore, reflection-canceling slit structure is proposed to reduce reflection from the radiating element. Feasibility of the proposed structure is confirmed in the experiment (Hayashi et al, 2008).
On the other hand in the design of conventional shunt and series slotted waveguide array antennas for vertical and horizontal polarization, radiation slots are spaced by approximately a half guided wavelength for in-phase excitation. Interleave offset and orientation from waveguide center axis are necessary to direct the main beam toward the broadside direction (Volakis, 2007). Since the spacing is less than a wavelength in free space, grating lobes do not appear in any directions. For bidirectional communication systems in general, two orthogonal polarizations are used to avoid interference between the two signals. In the case of automotive radar systems, 45-degrees diagonal polarization is used so that the antenna does not receive the signals transmitted from the cars running toward the opposite direction (Fujimura 1995). However, in the design of the slotted waveguide array antenna with arbitrarily linear polarization such as 45-degrees diagonal polarization for the automotive radar systems, slot spacing is one guided wavelength which is larger than a wavelength in free space. All the slots are located at the waveguide center with an identical orientation in parallel unlike conventional shunt and series slotted waveguide array. Consequently, grating lobes appear in the radiation pattern. Antenna gain is degraded significantly and ghost image could be detected in the radar system toward the grating-lobe direction. In order to suppress the grating lobes, dielectric material is usually filled in the waveguide (Sakakibara et al. 1994, Park et al. 2005). However, it would cause higher cost and gain degradation due to dielectric loss in the millimeter-wave band.
We have proposed a narrow-wall slotted hollow waveguide planar antenna for arbitrarily linear polarization (Yamamoto et al., 2004). Here, we developed two different slotted waveguide array antennas with 45-degrees diagonal linear polarization. One is quite high gain (over 30 dBi) two-dimensional planar array antenna (Mizutani et al. 2007) and the other one is a relatively lower gain (around 20 dBi) antenna which can be used for a sub-array in beam-scanning antennas (Sakakibara et al. 2008). Microstrip comb-line antenna is also developed for lower-gain applications of the sub-array. Both waveguide antennas consist of the same waveguides with radiating slots designed by traveling-wave excitation. The number of the radiating waveguide and the structures of the feeding circuits are different in the two antennas. Traveling-wave excitation is common technique in the designs of the slotted waveguide array antennas and the microstrip comb-line antenna. Array fed by traveling-wave excitation suffer beam shift from frequency deviation, which causes narrow frequency bandwidth. However, in the case of narrow band application, traveling-wave excitation is quite effective to achieve high antenna efficiency.
2. Antenna configurations
Three different planar antennas are developed in the millimeter-wave band. Configurations of the antenna systems are shown in Fig. 1. Figure 1(a) shows a high-gain slotted waveguide antenna which has only one feeding port. Feeding network is included in the antenna. Second one in Fig. 1(b) is also a slotted waveguide antenna. However, the antenna system consists of some sub-arrays, each one of which has its own feeding port. Feeding network could be DBF systems or RF power divider with phase shifters for beam scanning. The waveguide antennas can be replaced by microstrip comb-line antenna for sub-arrays as shown in Fig. 1(c).
2.1. Slotted waveguide array antenna
We developed a design technology of slotted waveguide array antennas for arbitrarily linear polarization without growing grating lobes of two-dimensional array. Here, we designed an antenna with 45-degrees diagonal polarization to apply to automotive radar systems. Novel ideas to suppress grating lobes are supplied in the proposed structure of the two slot antennas in Fig. 1(a) and (b), which is still simple in order to reduce production cost. The configurations of the proposed antennas are shown in Fig. 2(a) and (b). All the radiating slots are cut at the center of the narrow wall of the radiating waveguides and are inclined by identically 45 degrees from the guide axis (
A slot is cut on the narrow wall of the radiating waveguide. The spacing of radiating waveguides in
In order to improve the return loss characteristic of the array, previously mentioned conventional slotted waveguide arrays are often designed to have some degrees beam-tilting. However, in this case, it becomes the cause to generate grating lobes or to enhance their levels because the visible region of array factor changes. In terms of the proposed antenna, the post in the waveguide is designed to cancel the reflections from the slot and from the post by optimization of their spacing. Therefore, it is not necessary to use the beam-tilting technique because the reflection from each element has already been small due to the effect of the post. Furthermore, we set an open ended cavity around each slot. Since the cavity shades the radiation toward the low elevation angle, the grating lobe level is reduced effectively. Thus, we can suppress the grating lobes in the diagonal direction.
The proposed structure has a following additional advantage for low loss. The antenna is assembled from two parts, upper and lower plates to compose a waveguide structure. Since radiating slots are cut on the narrow wall of the waveguides, cut plane of the waveguide is
2.2. Microstrip comb-line antenna
A microstrip comb-line antenna is composed of several rectangular radiating elements that are directly attached to a straight feeding line printed on a dielectric substrate (Teflon-compatible Fluorocarbon resin film, thickness
A radiation pattern with broadside beam is often used in many applications. However, when all the radiating elements are designed to excite in phase, all the reflections are also in phase at the feeding point, thus significantly degrading the overall reflection characteristic of the array. In the conventional design with beam tilting by a few degrees, reflections are canceled at the feed point due to the distributed reflection phases of the radiating elements. This means that the design flexibility of beam direction is limited by the reflection characteristics.
To solve this problem, we propose a reflection-canceling slit structure as shown in Fig. 4. A rectangular slit is cut on the feeding line near the radiating element. A reflection from each radiating element is canceled with the reflection from the slit. As the reflection from a pair of radiating element and slit is suppressed in each element, a zero-degree broadside array can be designed without increasing the return loss of the array. Because the sizes of all the radiating elements are different for the required aperture distribution, the slit dimensions and spacing of slit from the radiating element are optimized for each radiating element. Simple design procedure is required in the array design.
3. Design of linear array with traveling-wave excitation
Both waveguide antenna and microstrip antenna are designed in common procedure based on the traveling-wave excitation. Reflection wave is neglected in the design since reflection-canceling post and slit are used for the waveguide antenna and the comb-line antenna, respectively. Simple and straight-forward design procedure is expected in traveling-wave excitation. Here, design procedure based on traveling-wave excitation of the waveguide antenna is presented in this section.
A configuration of a post-loaded waveguide slot with open ended cavity is shown in Fig. 3. A slot element with post is designed at 76.5 GHz. The slot is cut on the waveguide narrow wall and is inclined by 45 degrees from the guide axis. The slot spacing becomes one guided wavelength which is larger than a wavelength in free space. The guided wavelength of the TE10 mode in the hollow waveguide is given by
where λ0 is a wavelength in free space and
Radiation is controlled by both the slot length
An open ended cavity is set on each slot. Since the cavity shades the radiation from the slots to the low elevation angle, the grating lobe level is reduced. Figure 7 shows calculated element radiation pattern and array factors in
from the broadside. It is observed that the grating lobe level of antenna without cavities is −22 dB, which is suppressed to −36 dB by using cavity. However, radiation pattern near broadside is almost the same and independent on the cavity. No mutual coupling between slots are taken into account in the design because the mutual coupling is very small due to the element radiation pattern of cavity. Simple design procedure can be applied. The effect of the circular cavity to the slot impedance and coupling is shown in Fig. 5. High-Q resonance characteristic of cavity structure is observed, that is, maximum coupling power is larger when cavity is installed, on the other hand, coupling power from slot with cavity is smaller than without cavity in lower frequency than resonance. Since optimum parameters for minimum
A 13-element array is designed at 76.5 GHz. Thirteen radiating slots are arranged on one radiating waveguide, which corresponds to a linear array antenna. A terminated element composed of a post-loaded slot and a short circuit is used at the termination of each radiating waveguide. All the remaining power at the termination radiates from the element and also contributes antenna performance. Reflections from all the elements are suppressed by the function of post-loaded slot. So, design procedure for traveling-wave excitation is implemented (Sakakibara, 1994). Thirteen slot elements are arrayed and numbered from the feed point to the termination. Required coupling from slots are assigned for Taylor distribution on the aperture to be a sidelobe level lower than −20 dB. Incidence
The previously mentioned parameters
4. Design of feeding circuits
The developed linear arrays are arranged to compose two-dimensional planar array. Required feeding circuits depend on the transmission lines and the number of the linear arrays. Waveguide 24-way and two-way power dividers are developed to feed the waveguide antennas. Microstrip-to-waveguide transition is also developed to feed the microstrip comb-line antenna from waveguide.
4.1. Waveguide feeding 24-way power divider
In the development of the two-dimensional planar waveguide antennas, a single-layer 24-way power divider composed of E-plane T-junctions feeding narrow-wall slotted waveguide planar array are designed as is shown in Fig. 2(a). It is composed of one feeding waveguide and 24 radiating waveguides slotted on the narrow walls. The antenna input port is located at the center of the feeding waveguide. All the radiating waveguides are fed from the feeding waveguide. The radiating waveguides are connected on the broad wall of the feeding waveguide, which forms a series of E-plane T-junctions shown in Fig. 10(a) (Mizutani et al. 2005). The broad-wall width of the feeding waveguide is determined so that the guided wavelength of feeding waveguide corresponds just twice the narrow-wall width of the radiating waveguide including the wall thickness between the radiating waveguides since adjacent waveguides are fed in an alternating 180 degrees out of phase. A coupling window is opened at each junction. Coupling to the radiating waveguide is controlled by the window width
terminated E-bends, shown in Fig. 10(b), in order to make all the remaining power contribute to the antenna performance. The size of the post is designed for matching and the width
In order to feed radiating waveguides in alternating 180 degrees out of phase, we designed the E-plane T-junctions. The broad-wall width
A 24-way power divider is designed at 76.5 GHz. Field amplitude and phase distributions of the twenty four output ports are shown in Fig. 12. The simulated and measured results almost agree well with the design having the error smaller than 1 dB in amplitude and 5 degrees in phase. Simulated frequency dependency of reflection of the input T-junction with and without all the twenty four input ports is shown in Fig. 13. Resonant frequency is observed at the design frequency 76.5 GHz. Bandwidth of the reflection below −20 dB is approximately 8 GHz.
4.2. Waveguide feeding two-way power divider
In order to excite all the slots in phase with a triangular lattice arrangement in the two-waveguide antenna, two radiating waveguides are fed in 180 degrees out of phase each other. We propose the compact power divider for the feeding circuit of the sub-array as shown in Fig. 14. The feeding waveguide is connected at the junction of the two radiating waveguides from the opposite side of the slots. There is a feeding window at the boundary between the radiating waveguide and the feeding waveguide for matching. A matching post is installed at the opposite side of the feeding window. The reflection characteristic is controlled by changing the size of the feeding window
4.3. Design of microstrip-to-waveguide transition for feeding microstrip antenna
For feeding circuit of microstrip comb-line antenna from waveguide, microstrip-to-waveguide transition is developed. Ordinary microstrip-to-waveguide transitions require back-short waveguide on the substrate. In order to reduce number of parts and assembling error of the back-short waveguide, transition with planar structure is developed (Iizuka et al. 2002). Figure 16 shows a configuration of the planar microstrip-to-waveguide transition. A microstrip substrate with metal pattern is on the open-ended waveguide. Microstrip line is inserted into the ground pattern of waveguide short on the upper plane of the substrate. Electric current on the microstrip line is electromagnetically coupled to the current on the patch in the aperture at the lower plane of the substrate. Via holes surround the waveguide in the substrate to prevent leakage. Figure 17 shows
5.1. 24-waveguide antenna
A 24-waveguide planar antenna is fabricated to evaluate the antenna performance. The photograph of the antenna is shown in Fig. 18. The fabricated antenna is assembled from two parts, upper and bottom aluminum plates with groove structures to compose waveguides. Cut plane is at the center of the broad wall of the waveguide and are fixed by screws. Twenty-four waveguides with 13 slots are arranged in parallel. Consequently, aperture size of antenna is 71.5 mm (in
We discuss performance of the fabricated planar antenna in this section. Figure 19(a) shows the measured and designed radiation patterns in
The sidelobe level is −16.8 dB which is 3.2 dB higher than design. Figure 19(b) shows radiation patterns in
efficiency are achieved. The measured reflection characteristics are indicated in Fig. 22. The measured reflection level is −22.0 and −16.5 dB at 76.0 and 76.5 GHz, respectively. On the other hand, large reflection is observed at 77.0 GHz, whose level is −10 dB. It is one of the causes of gain degradation. The cause of reflection increasing at 77.0 GHz would be that the proposed slot element is narrow frequency band width as is shown in Fig. 5. All the reflections from antenna elements due to frequency shift of fabrication error would be summed up in phase at 77.0 GHz.
5.2. Two-line waveguide antenna
The designed antenna was fabricated and feasibility was confirmed by experiments. Photograph of the developed antenna is shown in Fig. 23. Two metal plates of aluminium alloy were screwed together. Cut plane is at the center of the waveguide broad wall as well as the 24-waveguide antenna shown in the previous section. Posts were located in the waveguide to increase radiation from slots and to improve reflection characteristics. The cavity was set on each slot.
Figure 24(a) shows measured and simulated radiation patterns in the plane parallel to the waveguide axis at the design frequency 76.5 GHz. Beam direction was approximately 0 degree as was the same with the broadside beam design. Sidelobe level was around −20 dB as was almost the same level with the design of Taylor distribution for −20 dB sidelobe level.
Some portion of the grating lobes were observed in 50 degrees which were about 7 dB higher than the simulation and still lower than −20 dB. Figure 24(b) shows measured and simulated radiation patterns at 76.5 GHz in the plane perpendicular to the waveguide. Almost symmetrical radiation pattern was obtained in the experiment. Sidelobe level was around −20 dB as was the same with the simulation. Figure 25 shows reflection characteristics. Since the resonant frequency corresponded to the design frequency 76.5 GHz, reflection level was lower than −20 dB at the frequency. Although the bandwidth was wider than 3 GHz for reflection lower than −10 dB, the center frequency of the bandwidth shifted by a few GHz lower than the design frequency. Figure 26 shows gain and antenna efficiency. Gain and antenna efficiency were 21.1 dBi and 51 %, respectively. They were degraded in the lower frequency band due to the return loss mentioned in Fig. 25. However, the efficiency was still relatively high compared with other millimeter-wave planar antennas.
5.3. Microstrip comb-line antenna
Microstrip comb-line antenna with two lines of 27 elements and with broadside beam is fabricated for experiments as is shown in Fig. 27. Reflection level of the fabricated antenna is −12.9dB at the design frequency 76.5 GHz as shown in Fig. 28. Measured beam direction in the plane parallel to the feeding line is −1.0 degree, and sidelobe level is −17.9 dB shown in Fig. 29(a). Symmetrical radiation pattern is obtained in the plane perpendicular to the feeding line as shown in Fig. 29(b). The measured radiation pattern almost agrees well with the array factor. High antenna efficiency 55 % with antenna gain 20.3 dBi is obtained at the design frequency 76.5 GHz. The efficiency is almost the same level with the two-waveguide antenna.
We have developed three types of millimeter-wave low-profile planar antennas; high-gain two-dimensional planar waveguide array antenna with 24 waveguides, two-line waveguide antenna and microstrip comb-line antenna which can be applied to the sub-arrays for beam-scanning antennas. Microstrip comb-line antenna is advantageous at the points of low cost and lower feeding loss compared with other microstrip array antennas. High efficiency is achieved, which is almost the same level with the waveguide when the aperture size and gain is relatively small. Waveguide antenna possesses much higher performance capability due to the low loss characteristic of waveguide feeding when the aperture size and gain is large. However, cost reduction is one of the most serious problems for mass production. Metal injection molding could be a solution for the waveguide antenna.
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