JPH0569324B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a wideband antenna that provides a wideband antenna element with unidirectional antenna radiation characteristics.

(Prior Art) Conventionally, as this type of wideband antenna, for example, the one shown in FIG. 5 is known.

In FIG. 5, reference numeral 1 indicates an antenna electrode of a broadband antenna, for example, an antenna electrode of a two-striped planar Archimedean spiral antenna.

In addition to the above shapes, the antenna electrode 1 may be a multi-strand log spiral antenna, a log periodic antenna, or a wideband antenna having a self-complementary structure.

Reference numeral 2 denotes a cavity, which is used to make the directional characteristics of the broadband antenna formed by the antenna electrode 1 unidirectional, and the inner wall surface of the cavity 1 is a good electrical conductor.

The function of this cavity 2 is that the radiation characteristic of the antenna electrode 1 itself is a bidirectional characteristic that radiates to both sides of the antenna electrode surface. , which makes the antenna directivity unidirectional.

For example, in Fig. 5, the wave traveling into cavity 1 is almost completely reflected because there is almost no loss within the cavity, resulting in an antenna directivity characteristic that is unidirectional in the direction shown by arrow 3. It will be done.

By the way, it is widely known that the radiation characteristics of a broadband antenna equipped with a cavity are determined by the interference between the direct wave radiated directly into free space from the antenna electrode 1 and the indirect wave re-radiated from the cavity 2. It is being In other words, at frequencies where the depth of cavity 2 is an odd multiple of 1/4 wavelength of the antenna excitation wavelength, direct waves and indirect waves constructively interfere with each other in the direction in front of the antenna as shown by arrow 3. It generates a radiation pattern with a narrow beam width and a peak in the direction. Furthermore, at frequencies where the cavity depth is an even multiple of a quarter wavelength, direct waves and indirect waves interfere to cancel each other out, producing a so-called conical pattern with a gain peak in a direction other than the front direction of the antenna.

(Problems to be Solved by the Invention) However, in the case of a conventional wideband antenna equipped with such a cavity, especially a wideband antenna equipped with a cavity 2 that serves as a lossless reflector as shown in FIG. As mentioned above, since strong interference between direct waves and indirect waves occurs, the antenna radiation pattern exhibits strong frequency dependence, and is suitable for applications where the shape of the radiation pattern must be kept approximately constant over a wide band. There was a problem that I couldn't use it.

This problem is caused by the existence of the cavity itself, and therefore cannot be solved no matter how wide-band characteristics the antenna element exhibits. In reality, with the structure shown in FIG. 5, the limit for maintaining a substantially constant radiation pattern is one octave.

On the other hand, fill cavity 2 with a radio wave absorber,
Another possible method is to absorb all the energy traveling inside the cavity and eliminate indirect waves.

However, in this case, although the radiation pattern can be maintained approximately in the same shape over several octaves, another problem occurs in that the antenna gain is significantly reduced due to absorption.

(Means for Solving the Problems) The present invention has been made in view of the above-mentioned problems in the prior art. The purpose is to provide a broadband antenna that can maintain a pattern.

In order to achieve this objective, the present invention includes:
In a wideband antenna that provides unidirectional antenna radiation characteristics through the combination of wideband antenna element 1 and cavity 2, there is a radio wave absorber on the bottom surface of cavity 2 that exhibits resonance absorption characteristics at a frequency lower than the 1/2 wavelength resonance frequency of cavity 2. body 4
It is loaded with.

(Function) According to the configuration of the present invention, the radio wave absorber loaded on the inner bottom surface of the cavity prevents 1/2 wavelength resonance of the cavity in which the beam width becomes a so-called conical pattern due to interference between direct waves and indirect waves. Since a large absorption characteristic is obtained at the point, extreme beam broadening is suppressed, and on the other hand, the beam width of the antenna is minimized due to interference between direct waves and indirect waves, which is 1/4 of the cavity.
Since a certain degree of absorption characteristics can be obtained at the wavelength resonance point, the beam width can be prevented from becoming too narrow, thereby making it possible to obtain a radiation pattern with little change in beam width over a wide frequency band.

(Embodiment) FIG. 1 is an explanatory diagram showing an embodiment of the present invention partially cut away.

First, to explain the structure, numeral 1 is an antenna electrode, and as shown in FIG.
In addition, as the antenna electrode, a multi-filament log spiral antenna, a log periodic antenna, and an antenna having a self-complementary structure are used.

2 is a cavity provided in combination with the antenna electrode 1, the inner wall of the cavity 2 is a good conductor, and in the present invention, a magnetic field absorbing type resonance absorber 4 is provided as a radio wave absorber on the bottom surface of the cavity 2. Loading.

Here, the resonance absorption frequency o of the magnetic field absorption type resonance absorber 4 is set to be a slightly lower frequency than the 1/2 wavelength resonance frequency of the cavity 2.

Next, the operation of the embodiment shown in FIG. 1 will be explained.

First, the absorption characteristics of the magnetic field absorbing type resonance absorber 4 loaded on the bottom surface of the cavity 2 are as shown in FIG. That is, although maximum absorption is exhibited at the resonance point o, absorption does not completely disappear at points other than the resonance point.

In the present invention in which the magnetic field absorption type resonance absorber 4 shown in FIG. 2 is loaded on the bottom surface of the cavity 2, the resonance absorption frequency of the magnetic field absorption type resonance absorber 4 is
By shifting o to a slightly lower side than the 1/2 wavelength resonance point of the cavity 2, the relationship shown in FIG. 3 is set, for example.

In Figure 3, from the viewpoint of antenna broadband operation, the beam pattern width at _1/4 of the 1/4 wavelength resonance point of cavity 2 is very narrow compared to the case of a single antenna element, so In order to reduce the effect of narrowing the beam width, the cavity 2 is reduced to 1/2 by the magnetic field absorption type resonant absorber 4.
It is desirable that some amount of absorption exists even at the four-wavelength resonance point _1/4 , as at point A.

On the other hand, the beam pattern width becomes extremely wide and becomes a conical pattern.
At the two-wavelength resonance point _1/2 , it is desirable that the amount of absorption by the magnetic field absorption type resonance absorber 4 is infinite, but in practical terms, even if it deviates from the maximum value as shown at point B, -
If it is about 15dB, the radiation pattern will not become a conical pattern.

FIG. 4 is a characteristic diagram showing the beam width versus frequency for the case where a radio wave absorber is provided in the cavity and the case where it is not provided.

In FIG. 5, if the cavity does not have a radio wave absorber as in the conventional case, the broken line 50
As shown in , the beam width becomes extremely wide at the 1/2 wavelength resonance point _1/2 , and the beam width becomes narrow at the 1/4 wavelength resonance point _1/4 . On the other hand, in the present invention, as shown in the characteristic shown by the solid line 52, the 1/2 wavelength resonance point
At _1/2 , the beam width is suppressed due to the large amount of absorption by the magnetic field absorption type resonance absorber 4, and even at the 1/4 wavelength resonance point _1/4 , the absorption amount by the magnetic field absorption type resonance absorber 4 is suppressed. Since the amount is a certain amount, the beam width will be widened, and as a result, the beam width will be approximately constant, and the usable frequency band will be significantly expanded.

Further, in the present invention, the magnetic field absorption type resonance absorber 4 is used in the above embodiment as the radio wave absorber provided in the cavity 2, and this is for the following reason.

Two types of radio wave absorbers can be considered: an electric field absorption type resonance absorber that absorbs an electric field component and a magnetic field absorption type resonance absorber that absorbs a magnetic field component.

Here, the latter type of magnetic field absorption type resonant absorber can be obtained with a thickness that is sufficiently small compared to the wavelength, but the former type of electric field absorption type resonant absorber requires a material that is considerably thicker than the wavelength. .

Therefore, when a thick electro-absorption type resonant absorber is provided on the bottom of the cavity 2, the surface of the absorber and the antenna electrode become close, and the absorber causes loss to the current on the antenna electrode, causing the antenna to This will reduce the gain.

Therefore, in the present invention, the magnetic field absorption type resonant absorber 4 is used as the radio wave absorber as shown in the above embodiment, which is desirable in terms of obtaining an antenna with high gain.

(Effects of the Invention) As described above, according to the present invention, in a wideband antenna that provides unidirectional antenna radiation characteristics by the combination of the wideband antenna element 1 and the cavity 2, the cavity 2 is provided on the bottom surface of the cavity 2. Because it is loaded with a radio wave absorber 4 that exhibits resonance absorption characteristics at a frequency lower than the 1/2 wavelength resonance frequency, it is a unidirectional antenna that can maintain a substantially constant antenna radiation pattern over a wide frequency band and has little change in radiation pattern. Radiation characteristics can be obtained.

In addition, since a magnetic field absorption type resonant absorber with a thickness sufficiently smaller than the resonant wavelength is used as the radio wave absorber, the radiation pattern has characteristics with little change over a wide frequency band without reducing the antenna gain. can be obtained.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention in a partially cutaway manner, FIG. 2 is a graph diagram showing the absorption characteristics of the radio wave absorber in the embodiment of FIG. 1, and FIG. A graph showing the relationship between the resonance point of the absorber and the 1/2 wavelength and 1/4 wavelength resonance points of the cavity in the embodiment shown in the figure. Graph diagram shown in comparison with example, 5th
The figure is an explanatory diagram showing a conventional example. 1: antenna electrode, 2: cavity, 4: magnetic field absorption type resonance absorber (radio wave absorber).

Claims (1)

[Scope of Claims] 1. In a wideband antenna that provides unidirectional antenna radiation characteristics by a combination of a wideband antenna element 1 and a cavity 2, the bottom surface of the cavity 2 is provided with an antenna at a frequency lower than the 1/2 wavelength resonant frequency of the cavity 2. A wideband antenna characterized by being loaded with a radio wave absorber 4 exhibiting resonance absorption characteristics. 2 Magnetic absorption type resonance absorber 4 as radio wave absorber 4
2. A wideband antenna according to claim 1, characterized in that the broadband antenna uses: