JPS6231844B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for broadband low-loss demultiplexing of electromagnetic waves using a frequency-selective surface. In particular, it relates to a demultiplexing method suitable for implementation in microwave or millimeter wave shared antenna circuits.

[Prior art and its problems] With the increasing demand for satellite communications, a system has been developed in which four frequencies, two frequencies each for transmission and reception, are carried out using one shared antenna. In order to realize a four-frequency shared antenna, a duplexer is required to separate the existing transmission and reception band from the newly added transmission and reception band. As a demultiplexer that satisfies this purpose, there is a method that uses a conventional waveguide demultiplexer, but when the two frequency bands to be separated are far apart, the higher frequency band is This is not preferable because it falls into a higher mode region.
Therefore, a method of demultiplexing an electromagnetic wave propagating in free space by arranging a frequency-selective surface whose transmission and reflection characteristics differ depending on the frequency is being considered. Frequency-selective surfaces used in satellite communications are desired to have low loss over a wide band.

A conventional method of this type will be explained using an example of a frequency selective surface in which square windows are periodically bored in a metal plate. FIG. 1 is a structural diagram of a conventional duplexer, in which 1 is a metal square grid. FIG. 2 is a side view showing the path of electromagnetic waves. When an incident wave 2 enters such a grating, a part of it is reflected and becomes a reflected wave 3,
The rest is transmitted and becomes transmitted wave 4. At this time, the ratio of transmitted energy to incident energy (transmittance) is generally as shown in FIG.

That is, in a relatively low frequency region, this grating apparently acts as an inductance, and the transmittance is theoretically 1 at a resonance frequency of ₁ . In a higher frequency region, higher-order modes occur, and the resonance frequency ₂ of each mode appears.

One of the conventional methods is to utilize the low frequency region of this characteristic. At low frequencies,
The equivalent circuit of one grid 1 as shown in FIG. 4a can be represented by an inductance as shown in FIG. 4b. Therefore, the characteristics shown in FIG. 4c are obtained. A known method is to arrange several such gratings at appropriate intervals and perform demultiplexing by utilizing resonance caused by interaction between the gratings. Figure 5a
5 is an example in which two gratings 1 are arranged at an interval l, and FIG. 5b shows an equivalent circuit diagram, and FIG. 5c shows a characteristic diagram. In this method, two gratings are used in a relatively low frequency region.
It is possible to create two resonance points. In this case, since the slope of the resonance curve is sharp, a larger number of elements is required to obtain a wide band pass characteristic. Moreover, in this case, there is a drawback that loss due to conductor loss etc. increases.

A second example of a conventional method is a method that takes advantage of the inherent resonance phenomenon of the grating itself. The resonance points ₁ , ₂ , etc. shown in FIG. 3 mentioned above are the resonance points, and the method is to select a pass band around these resonance points and use it as a duplexer. This method has several drawbacks.

The first drawback is that the resonance point cannot be freely selected. That is, by changing the ratio of the grating period to the wavelength, the first resonance point f ₁ can be changed, but the ratio of each resonance point of the higher order mode to f ₁ is f ₂ /f ₁ , f ₃ /f ₁ ... cannot be changed significantly.

A second drawback is that there is a region between each resonance point where the transmittance is significantly reduced. This limits the bandwidth.

The third drawback is that the frequency band above the resonance point f ₁ becomes a high-order mode region. As shown in FIG. 6, the higher-order modes of the grating include a normal transmitted wave 4 and a diffracted wave 5 having a propagation direction different from that of the reflected wave 3.
This is called the grating lobe. It is known that the resonance points f ₁ , f ₂ . . . occur in correspondence with the generation of grating lobes, and the resonance of the grating is always accompanied by the generation of grating lobes. These grating lobes cause radio waves to propagate in directions other than the desired direction, resulting in loss. This is why the resonance points f ₂ and f ₃ in FIG. 3 are not completely transparent. Furthermore, since the radio waves propagate in directions other than the normal direction, noise from the grating lobe direction is picked up in the reception band, making it unsuitable for satellite communications that require low noise characteristics.

The fourth drawback is that even if a plurality of such gratings are used in a stacked manner, it cannot be expected that the band will be widened. This is because there is a point between each resonance point where the transmittance is extremely low, so the band is limited, and because it is a high-order mode region, the characteristics are expected to be complex.

The fifth drawback is that it is difficult to analyze the equivalent circuit in the higher-order mode region, which complicates the design.

In addition, a proposal has been made to provide a cross-shaped metal in a square lattice to give an equivalent L-C resonance characteristic, and to use the region below the unique resonance point as the operating band (Reference BSTJVOL.54No.2P.263 ―283
“Resonant―Grid Quasi―Optical
Diplexers”).

However, this method creates a complex-shaped grid within a square grid to provide L-C resonance characteristics.
Adjustment of the resonance point and anti-resonance point requires changing the shape of the lattice pattern, which has the disadvantage of complicating the work.

The present invention solves these drawbacks, and aims to provide a method for performing broadband low-loss demultiplexing with a simple configuration.

[Means for solving problems]

The present invention combines a plurality of frequency-selective surface plates formed in a periodic structure of a square lattice using metal, dielectric material, or a combination of metal and dielectric material, and transmits or reflects electromagnetic waves. In the wave method, at least three of the frequency-selective surface plates are arranged in parallel, and the Q value of the resonance characteristic determined by the area of the window of the frequency-selective surface plate is adapted to the passing frequency region. The problem of the surface plate is set at the point where the resonance point due to interaction occurs in a region lower than the resonance point unique to each frequency-selective surface plate, and the frequency range where the frequency-selective surface plate can be equivalently regarded as an inductance is set. It is characterized in that a passing frequency region is defined in a frequency region including the resonance point that is high and lower than the natural resonance frequency of the frequency-selective surface plate.

[Effect]

A square grid is configured to have LC resonance characteristics in the passband, and the value of Q determined by the window ratio is determined to match the passband.

The distance 1 between each frequency-selective face plate is determined so that a resonance point due to interaction is generated in a region lower than the natural resonance frequency ₁ of the frequency-selective face plate.

A frequency range higher than the range where the plurality of frequency-selective surface plates can be equivalently regarded as inductance and lower than the natural resonant frequency can be used as the pass frequency range.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

The principle of the present invention will be explained by taking as an example a case where a frequency selective surface is formed by forming a square lattice on a thin metal plate. First, consider a square grid as a combination of a parallel grid in the vertical direction and a flat grid in the horizontal direction. That is,
It is assumed that the square lattice shown in FIG. 9a is constructed by combining the parallel lattice of FIG. 7a and the parallel lattice of FIG. 8a. When the polarization plane E is parallel to the parallel grating as shown in Figure 7a, the equivalent circuit is as shown in Figure 7b,
It can be expressed as an inductance L. Also the 8th
When the plane of polarization E is perpendicular to the parallel grating as shown in FIG. 8a, the equivalent circuit is represented by the capacitance C as shown in FIG. 8b. Therefore, the equivalent circuit of the square lattice is represented by an LC resonant circuit as shown in FIG. 9b. However, as mentioned above, the frequency band above the resonance point _f1 is a higher-order mode region, so the equivalent circuit cannot be expressed simply. Furthermore, in a low frequency region, the effect of the capacitance C described above is reduced, so that only the inductance L remains. FIG. 9C is a characteristic diagram of this region.

In the method of the present invention, the lattice is equivalent circuit-wise, L-C
One feature is that the passband is set in a region that can be expressed by a resonant circuit.

FIG. 10 is a structural diagram of a duplexer implementing the method of the present invention. Three square grids 6 are arranged in parallel. Let the intervals be l ₁ and l ₂ , respectively. This equivalent circuit can be expressed as shown in FIG. If the unique resonance point of each lattice is set equally at f ₁ , then
Even when a three-element configuration is used, f ₁ is the point of complete transmission.
Furthermore, in order to avoid the higher-order mode region, f ₁ is set slightly above the upper limit of the passband used.
If the resonance sharpness Q of each LC resonant circuit is Q ₁ , Q ₂ , Q ₃ , then if Q ₁ , Q ₂ , Q ₃ and element spacing l ₁ , l ₂ are appropriately selected, each element In addition to the unique resonance point f ₁ , the resonance point due to the interaction of the lattice is the first
As shown in 7 in Figure 2, two locations (in the case of three sheets) can be made. In this case, the Q of each element and the element should be adjusted so that these two resonance points do not enter the higher-order mode region, and so that these resonance points are realized at frequencies lower than f ₁ and can cover the normal band. Decide on the interval. In this way, the characteristics shown in FIG. 12 are realized.

Note that the Q of each lattice is as shown in FIG.
It is determined by the window size ratio a/d _x . Further, the resonance point f ₁ is determined by the ratio d _x /λ of the period of the grating to the wavelength λ. Therefore, by appropriately selecting a and d _x , the grid can be set to arbitrary f ₁ and Q.

Note that the present invention can be practiced by forming a square lattice on a metal thin film on a dielectric plate, and the number of lattices in combination is not limited to three, but may be more than three. Furthermore, there is no need for air between each plate, and a dielectric material may be used, or a metal plate may have a thickness and a dielectric material is mounted in the window portion.

Application examples of the present invention are shown in FIGS. 13 to 17.

FIG. 13 shows an example in which the frequency selective surface 11 is formed into a curved shape by the method of the present invention and used as a curved mirror of a beam wave guide. 12
1 is a curved reflector, and 13 is an electromagnetic horn.

FIGS. 14 and 15 illustrate a planar frequency selective surface 14 according to the present invention in a beam wave.
This is an example of application to a guide. 12 is a curved reflector;
13 indicates an electromagnetic horn.

FIG. 16 shows an example in which the present invention is implemented as the sub-reflector 16 of a Cassegrain antenna to configure a frequency sharing antenna.

FIG. 17 shows an example in which a frequency-selective surface 14 in the form of a plate according to the present invention is inserted into an electromagnetic horn to construct a frequency-sharing horn.

〔Effect of the invention〕

As explained above, by setting the frequency range in which the frequency selective surface is used is higher than the range where it equivalently operates as an inductance and lower than the range where higher-order modes occur, and by combining multiple pieces of this, It is possible to configure a duplexer with low loss over a wide band. Conventional methods that utilize only the inductance region require at least two gratings to create one resonance point, but
Since the method of the present invention utilizes resonance between the gratings, in principle half the number of gratings can be used to obtain the same characteristics. Furthermore, since no higher-order mode region is used, no loss occurs due to grating lobes. Furthermore, it is easy to design using equivalent circuit analysis, and has the advantage of being able to freely select frequencies and bands.

In addition, since the L-C resonance characteristic can be obtained using only a square grid, the resonance point and anti-resonance point can be adjusted by simply adjusting the spacing between each frequency-selective surface plate, and there is no need to change the shape within each grid. This has the effect that it is easy to create a frequency-selective surface plate with a simple structure.

[Brief explanation of the drawing]

Figure 1 is a structural diagram of a square lattice. FIG. 2 is a diagram showing the relationship among incident waves, reflected waves, and transmitted waves. FIG. 3 is a diagram showing general characteristics of a square lattice. FIG. 4 is an explanatory diagram of a conventional method that utilizes a region equivalent to inductance. a is a structural diagram, b is an equivalent circuit diagram. c.
is a characteristic diagram. FIG. 5 is an explanatory diagram of a conventional method that also uses two grids. a is a structural diagram, b is an equivalent circuit diagram, and c is a characteristic diagram. Figure 6 is an explanatory diagram of the grating lobe. FIG. 7 is an explanatory diagram when the plane of polarization is parallel to the parallel grating. a is a structural diagram, b is an equivalent circuit diagram. FIG. 8 is an explanatory diagram when the plane of polarization is perpendicular to the parallel grating. a is a structural diagram, b is an equivalent circuit diagram. FIG. 9 is an explanatory diagram of a square grid. a is a structural diagram, b is an equivalent circuit diagram, and c is a characteristic diagram. FIG. 10 is an explanatory diagram of a structure in which the method of the present invention is implemented. FIG. 11 is its equivalent circuit diagram.
Figure 12 is its characteristic diagram. FIG. 13 is a block diagram showing an application example in which a curved frequency-selective surface is applied to a beam wave guide. FIG. 14 is a configuration diagram showing an application example in which a frequency selective surface is inserted into the convergent beam of a beam wave guide. FIG. 15 is a structural diagram showing an application example in which a frequency selective surface is inserted into a parallel beam of a beam wave guide. 16th
The figure is a structural diagram showing an application example in which the present invention is implemented on a sub-reflector of a Cassegrain antenna to configure a frequency sharing antenna. FIG. 17 is a structural diagram showing an example of application of the present invention to a frequency sharing horn. 1... Square grid, 2... Incident wave, 3... Reflected wave, 4
...Transmitted wave, 5...Diffraction wave, 6...Square lattice, 7...Resonance point due to interaction, 8...Dielectric material, 9...Metal plate,
10... Dielectric plate, 11... Curved frequency selective surface, 12... Curved reflector, 13... Electromagnetic horn, 14
... Planar frequency selective surface, 15 ... Main reflector, 1
6...Sub-reflector.

Claims (1)

[Claims] 1. A method of transmitting or reflecting electromagnetic waves by combining a plurality of frequency-selective surface plates formed in a periodic structure of a square lattice using metal, dielectric, or a combination of metal and dielectric. In the electromagnetic wave demultiplexing method, at least three frequency-selective surface plates are arranged in parallel, the Q value of the resonance characteristic determined by the area of the window of the frequency-selective surface plate is adapted to the passing frequency region, and the above-mentioned method is performed. The spacing between the frequency-selective surface plates is set at a point where the resonance point due to interaction occurs in a region lower than the resonance point specific to each frequency-selective surface plate, and the frequency-selective surface plate can be equivalently regarded as an inductance. higher than the frequency domain,
Moreover, a method for demultiplexing electromagnetic waves, characterized in that a passing frequency region is determined in a frequency region including the resonance point that is lower than the unique resonance frequency of the frequency-selective surface plate.