JP2003224414A - Millimeter wave planar antenna - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To protect an antenna from collision with obstacles such as wind and rain and pebbles accompanying the traveling of an automobile or the like, it is essential to install a radome. Although the thickness of the radome is smaller than the wavelength in the low frequency band, the thickness is about the wavelength in the millimeter wave band, and the influence is large. When this radome is installed in front of the antenna, the radio wave radiated from the antenna is attenuated by the radome. In addition, since partial reflection causes multiple reflections, radiation efficiency is reduced, excitation distribution is disturbed, and side lobes are increased, cross polarization is increased, and the like. As a result, there is a problem that an error occurs in discrimination between vehicles and that accurate measurement of an inter-vehicle distance or the like becomes impossible. SOLUTION: The millimeter wave planar antenna according to the present invention comprises:
A function to adjust the distance between the antenna and the radome is provided.
The radome thickness is an integral multiple of about a half wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a millimeter wave band planar antenna which is mounted on the front surface of an automobile and used for safety control of the automobile such as measuring the distance between the preceding automobile and the like.

[0002]

2. Description of the Related Art There are parabolic antennas, lens antennas, etc. as millimeter wave antennas, but a planar antenna is preferable in view of mountability. As an example of using a slot antenna as a millimeter-wave band planar antenna, for example, there is "Consideration of 60 GHz band radial line slot antenna" shown in Yamamoto et al. As a method using a microstrip antenna, there is, for example, Kitao et al. “Triplate antenna provided with a polarization grid” shown in B-60, General Meeting of the Institute of Electronics, Information and Communication Engineers. Since this type uses a triplate line, it is possible to suppress the radiation from the line, and a foam substrate having a low dielectric constant can be used, so that the loss can be reduced. However, because of the triplate structure, there is a problem that it is difficult to reduce the side lobe because the excitation amplitude phase is disturbed with the propagation of the parallel plate mode propagating in the space between the two ground conductors.

There has been reported an antenna of the type in which a microstrip line is used as a planar antenna which can be made low in side lobe and is suitable for mass production, and the microstrip antenna is configured on the same plane as the line. For example, F.Lalezari
and CDMassey, "mm-WaveMicrostrip Antenna", Micr
owave Journal, pp. 87-96 1987. FIG. 4 shows an example thereof. In the figure, 1
Is a ground conductor, 2 is a dielectric substrate, and 3 is a radiating conductor, and a microstrip antenna 4 is constituted by these. 5
Is a strip conductor, and the ground conductor 1 and the dielectric substrate 2 form a microstrip line 6. Reference numeral 7 is a power feeding connector, which is coaxial. Reference numeral 8 is a coaxial inner conductor.

Next, the operation will be described. Here, consider the case of a transmitting antenna. Converting from a coaxial line to a microstrip line which is an unbalanced line through a coaxial power feeding connector. As a matter of course, as a power feeding method, a waveguide can be used in addition to the coaxial connector, but since a V-band connector has been developed in recent years and the power feeding configuration is simple, an example is shown here. The electric wave mode-converted from the power supply connector to the microstrip line is distributed with a desired amplitude and phase by a distribution circuit composed of the microstrip line, and is supplied to each microstrip antenna formed on the coplanar plane. The feeding is excited by the electric field by directly connecting the end of the microstrip line to the end of the microstrip antenna. It is well known that a desired radiation pattern can be obtained by controlling the number of elements of the microstrip antenna and the amplitude and phase of feeding of the microstrip antenna.
The microstrip antenna causes a resonance phenomenon with the ground conductor 1 by setting the length of the radiation conductor 3 to about half a wavelength,
Radio waves are emitted into the space.

As a vehicle-mounted radar for collision prevention for forward monitoring, millimeter waves can be used in all weathers than a system using ultrasonic waves and lasers, and their development is greatly expected. It is a radar that detects a vehicle in front of you, but must not interfere with oncoming vehicles. Therefore, a method using 45 ° polarized waves has been proposed. As a matter of course, the antenna is required to have a small cross polarization. As a method of reducing cross polarization, there is a method of forming a slit in the main polarization direction of a microstrip antenna. An example of this is "Improvement of cross polarization characteristics of offset feed type microstrip antenna" by Yoshikawa et al. By providing a slit in the direction of the main polarization and further shorting the center of the microstrip antenna, high-order modes are suppressed and cross polarization is reduced.

[0006]

Thus, the antenna using the microstrip antenna is a copper-clad dielectric substrate 1
Since it can be configured with one sheet, a very simple configuration is possible.
Since the microstrip antenna and the microstrip line are coplanar, the microstrip antenna and the microstrip line can be easily manufactured by one etching process, are excellent in mass productivity, and can be manufactured at low cost.

However, considering the case of mounting on the front surface of a vehicle, it is necessary to install a radome in order to protect the antenna from collision of wind and rain, small stones, etc. due to running of the vehicle. The thickness of the radome is smaller than the wavelength in the low frequency band, but it is about the wavelength in the millimeter wave band, and its influence is large. When this radome is installed in front of the antenna, the radio waves emitted from the antenna are attenuated by the radome. In addition, since a part of the light is reflected to cause multiple reflection, the radiation efficiency is reduced and the excitation distribution is disturbed.
It affects side lobes and cross polarization. As a result, there is a problem in that the vehicle-to-vehicle discrimination is erroneous, and it becomes impossible to accurately measure the inter-vehicle distance.

Therefore, it is an object of the present invention to obtain antenna characteristics of high gain, low side lobes and low cross polarization even when a radome is attached.

[0009]

The millimeter-wave planar antenna according to the present invention has a function of adjusting the distance between the antenna and the radome, and the radome thickness is an integral multiple of about half a wavelength.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1 FIG. 1 is a schematic configuration diagram for explaining the present invention. In the figure, 1
Reference numeral 8 is a radome, and 19 is a space between the antenna and the radome. A radome is needed to protect the antenna. Various materials are used for the radome depending on the application. When cloth or plastic is used or strength is required, a relatively strong material such as FRP or ceramic may be used. Considering an antenna mounted on the front surface of an automobile, in particular, in order to protect the antenna surface from collisions such as wind and rain and small stones, it is necessary to have a certain thickness to provide strength. In the low frequency band, the thickness of the radome is smaller than the wavelength, so the influence on the electrical characteristics is small. However, in the millimeter wave band, the radome thickness becomes about the wavelength, and its influence cannot be ignored.

When the radome is placed on the entire surface of the antenna,
The radio waves radiated from the antenna partially pass, but the rest are reflected by the radome. This reflected radio wave is further reflected by the antenna surface, so that a standing wave stands between the antenna surface and the radome. Therefore, the active impedance of the microstrip antenna changes due to this standing wave. Since the deterioration of the active impedance leads to the decrease of the gain, it is necessary to reduce the reflection of the radome as much as possible. However, as described above, the thickness of the radome is about the wavelength in the millimeter wave band, and no matter how low a dielectric constant material is used, reflection at the radome cannot be avoided. Therefore,
As long as it is a standing wave, its node and antinode exist, so it is possible to use this to avoid degradation of the active impedance. This can be easily adjusted by changing the distance between the antenna and the radome. In fact, it is difficult to adjust the active impedance by the antenna alone with millimeter waves, and it is extremely important for practical use that it can be easily adjusted by changing the distance of the radome.

FIG. 2 shows measured values of gain and return loss when the distance between the radome and the antenna is changed. The radome thickness was about half a wavelength. The radome was made of low-dielectric-constant polypropylene with low reflection. The gain when there is no radome is used as a reference, and the gain when the interval is changed is shown relatively. By setting the distance to 20 mm, the gain is increased by 0.2 dB as compared with the case without the radome. On the other hand, when the distance is 24 mm, on the contrary, it is decreased by -0.7 dB, and when the distance is 4 mm, it is changed by 0.9 dB. It can be seen that the standing wave changes and the gain changes by changing the distance. The change in return loss is relatively small, but this is the measured value seen from the feed point of the array antenna, and the loss in the feed line is large, and is the value synthesized by the array. Since the relationship between the upper and lower gains and the return loss is the same, it can be seen that the active impedance changes with the radome distance.

FIG. 3 is a schematic configuration diagram showing the first embodiment of the present invention. In the figure, 20 is a height adjusting function. Even if the distance between the antenna and the radome is set optimally, it is expected that the radome will deform irregularly or change over time depending on the environmental conditions of use. Therefore, by providing a function for finely adjusting the distance, even if the radome is deformed, it can be easily adjusted. This adjustment function may be a simple one, as long as the height can be adjusted to about 1 mm.

According to this embodiment, by providing the mechanism capable of adjusting the distance to the radome, fine adjustment can be performed even when the radome material is changed or deformed.
This has the effect of always increasing the gain. Further, by using a polypropylene material for the radome, there is an effect that a radome with little loss and excellent mass productivity can be obtained. Further, by making the radome thickness an integral multiple of about half a wavelength, it is possible to reduce multiple reflections, and it is possible to reduce gain variation even if the distance is slightly shifted.

[0015]

According to the present invention, since the mechanism for adjusting the distance from the radome is provided, fine adjustment can be performed even when the radome material is changed or deformed.
The gain can always be increased, and by using polypropylene material for the radome, a radome with less loss and excellent mass productivity can be obtained, and by making the radome thickness an integral multiple of about half a wavelength, multiple reflection can be reduced. Therefore, there is an effect that the fluctuation of the gain can be reduced even if the interval is slightly deviated.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram for illustrating the principle of a millimeter wave planar antenna according to a first embodiment of the present invention.

FIG. 2 is a diagram showing measured values of antenna gain and return loss for illustrating the principle of the millimeter wave planar antenna according to the first embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of a millimeter wave planar antenna according to the first embodiment of the present invention.

FIG. 4 is a schematic configuration diagram showing an example of a conventional antenna.

[Explanation of symbols]

1 ground conductor, 2 dielectric substrate, 3 radiating conductor, 4 microstrip antenna, 5 strip conductor, 6 microstrip line, 7 coaxial feed connector, 8
Inner conductor, 18 radome, 19 distance between antenna and radome, 20 height adjustment function.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Teruo Furuya             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. (72) Inventor Junichiro Fujiwara             2-6-2 Otemachi 2-chome, Chiyoda-ku, Tokyo             Ryoden Engineering Co., Ltd. F-term (reference) 5J021 AA05 AA09 AA11 AB06 CA03                       HA10                 5J045 AA05 AA13 AA26 DA10 EA07                       HA03 JA04 KA02 LA03 NA01                 5J046 AA03 AA13 AB13 RA05 RA07                       RA11

Claims (3)

[Claims] 1. A ground conductor, a dielectric plate provided on the ground conductor, a feed line provided on the dielectric plate, and a radiation conductor provided on the same plane as the feed line and fed by the feed line. A millimeter-wave planar antenna comprising a microstrip antenna and a plurality of the microstrip antennas arranged in a plane to form an array antenna, and the radiation conductor and a radome arranged with a space therebetween. A millimeter-wave planar antenna characterized in that its thickness is approximately an integral multiple of half a wavelength. 2. The millimeter wave planar antenna according to claim 1, further comprising a mechanism for adjusting a distance between the array antenna and the radome. 3. The millimeter wave planar antenna according to claim 1, wherein a polypropylene material is used as the radome.