JP2585268B2 - Reflector antenna - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a reflector antenna provided with a primary radiator which is excited by a predetermined excitation weight.

(Prior Art) In a multi-beam antenna, it is important to use a limited frequency band as effectively as possible. The first means is a method of lowering the directivity side lobe of the antenna and using the same frequency and the same polarization between different beams, and the second means is the cross polarization component of the antenna (reverse polarization). Component) and orthogonal polarization (or forward and reverse circular polarization) is used.

In the reflector type multi-beam antenna, the method that is used to effectively use the frequency by the first method is as follows.
It is to cluster the primary radiators. The cluster means that the primary radiator is composed of a plurality of antenna elements. The directivity of each antenna element is given an appropriate weight (weighting), and the directivity is synthesized. Can be realized. There are several ways to determine the weight at this time,
Hereinafter, representative ones will be described.

As a conventional method of determining an excitation weight, a method of paying attention to a current distribution on an aperture surface of an antenna is generally used. It is a well-known fact that when the edge level of the current distribution on the aperture plane is low, a low directivity of side lobes can be obtained. In this case, the excitation weight of each antenna element constituting the primary radiator may be set. This method has a low side lobe over all angular regions except the main lobe, and is effective when there are several beams using the same frequency in a wide range. However, when considering a multi-beam antenna that illuminates a narrow and narrow area like Japan, few beams use the same frequency, and an area that requires a low sidelobe is limited. in this case,
When the side lobe is reduced by the above method, the side lobe is suppressed even in an unnecessary area, which may lead to a decrease in gain, which is not an efficient method.

On the other hand, a method has been considered in which attention is paid to the far field via a reflecting mirror, direct directional synthesis is performed, and the side lobe is reduced only in a necessary angle region. One of them is IEICE Technical Report A ・ P85-114
(February 1986), Suzuki et al., "Methodology of Multi-beam Antennas for Satellite Broadcasting by 22GHz Band" are listed. This method applies the directivity synthesis method originally considered for the array antenna to determine the excitation weight of the primary radiator of the reflector type multi-beam antenna, and restricts the main lobe gain so as not to lower it. This is a method in which, after imposing conditions, the excitation weight is determined so that the power level of the interference wave input virtually from the side lobe direction is minimized. This method is effective for a multi-beam antenna that irradiates a small area such as Japan because sidelobe suppression in any given direction can be performed while maintaining the gain of the main lobe.

Next, a second method for making effective use of frequency using orthogonal polarization
The means will be described. In general, a reflector antenna mounted on a satellite uses an offset type reflector so as not to block radiated radio waves.In this case, for example, a cross-polarization component such as a cross-polarization canceling Cassegrain antenna is used. By using a mirror system with a small amount of generation, directivity of low cross polarization can be realized and orthogonal polarization can be shared. However, when a polarization grid plate or a frequency selection plate is incorporated, cross polarization characteristics may be degraded, and it may be difficult to reuse frequencies by sharing polarization.

By the way, it is obvious that the frequency can be used most effectively by realizing the above two means simultaneously. In this case, a method is conceivable in which a cross-polarization canceling mirror surface system is used, a cluster type primary radiator is used, and the excitation weight is determined by the above-described method focusing on the far field. However, in this case, there is a restriction that the mirror surface of the cross-polarization canceling system must be used, which may limit the degree of freedom in design, or may cause the cross-polarization characteristic by a polarization grid or the like as described above. There are many problems such as the possibility of deterioration of the performance.

(Problems to be Solved by the Invention) As described above, realizing low side lobe and low cross polarization simultaneously in the reflector antenna has many problems in the conventional method. The present invention provides a directivity that effectively achieves low side lobes and low cross polarization simultaneously without causing a large decrease in efficiency and without restricting the mirror surface system. An object of the present invention is to provide a reflector antenna that can be realized by setting an excitation distribution by a predetermined formula.

[Configuration of the invention]

(Means for Solving the Problems) A reflector antenna according to the present invention for achieving the above object has a reflector, a primary radiator including a plurality of antenna elements, and a power supply for exciting the antenna elements. It consists of a part. This feed unit excites each antenna element with an excitation weight vector. Is excited so as to substantially satisfy the excitation weight Wn determined by

However Wn is represented by the excitation weight of the n-th antenna element.

And The matrix component Rnm of Here, i is a number (i = 1,... I) representing a sampling point within a range that restricts the radiation field intensity, θi is the sampled direction, and gn (θ) is the reflection of the nth antenna element. The directivity radiated through the mirror, αi is a proportionality coefficient, * represents a complex conjugate, and j is a number (j = 1,... J) representing a sampling point within a range requiring sidelobe suppression. Where θ′j is the sampled direction, βj is a proportionality coefficient, and k is a number (k) representing a sampling point within a range where reverse polarization suppression is required.
= 1,... K), θ′k is the sampled direction, and hn (θ) is the reverse polarization radiated through the reflector of the nth antenna element (hn (hn for linear polarization) θ), the directivity of hn (θ) and gn (θ) is the inverse of the polarization in the case of circular polarization and circular polarization), γk is a proportional coefficient, and Pnm is a constant.

Also, And bi is a complex constraint value in the direction θi that limits the radiation field intensity.

Note that the suffix T represents the complex conjugate transpose of the matrix, and -1 represents the reverse example.

(Operation) In the reflector antenna of the present invention, a plurality of antenna elements constituting the primary radiator are excited by a weight vector. By providing a power feeding unit that excites the excitation weight Wn substantially determined by the above, sidelobe suppression in an arbitrary predetermined direction and cross polarization suppression in an arbitrary predetermined direction can be simultaneously performed, and the degree of the suppression Can be set freely.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a feed system of a primary radiator that represents the principle of the present invention. Here, the horns 1 to 9 serving as antenna elements are each provided with an excitation weight W ₁ by a weighting device.
To take W _9, radio waves input to each horn are combined by the combiner 10 is weighted. Here, the purpose of weighting will be described with a simple example.

Now, as shown in FIG. 3, an antenna composed of an offset parabolic reflector 51 and a primary radiator 52, for example, directs a beam from the geosynchronous orbit of the earth to the ground to areas 21 to 24 as shown in FIG. Consider the case of radiation.
Here, the same frequency is placed in every other area (for example, area 21).
And 24 use the same polarization and the same frequency), and further consider the frequency reuse of using orthogonal polarization within the same area.
The characteristic of the antenna required to realize this is that the radiation directivity is, for example, for a beam radiating to the area 21, the side lobe is sufficiently large in the area of the area 24, and in the area of the area 21, It is required that the cross polarization component be small. Weighting is performed to obtain the directivity of such characteristics, and a method of determining an excitation weight (weighting) according to the present invention will be described below.

When an excitation weight that achieves directivity with low side lobes and low cross polarization in an arbitrary angle region is obtained, the directivity synthesized by this excitation weight will be a decrease in the gain of the main beam. there is a possibility. Therefore,
In order to avoid this, it is necessary to keep the gain in the main beam constant.

Now, it is assumed that the transmission (or reception) level always becomes bi in the main beam direction θi. At this time, the excitation weight is Will be satisfied. Here, gn (θi) is the directivity of the nth horn in the θi direction, N is the number of horns, and I is the number of directions restricting the transmission level.

Now Using a vector representation of It is expressed. (T indicates a complex conjugate transpose.) The excitation weight Wn imposes the constraint condition as shown in Expression (5), and then combines the directivity with low side lobe and low cross polarization. The method at that time is based on the following concept.

Now, virtually, the main lobe direction θi (i = 1...)
A desired wave having power of αi arrives from I), an interference wave having the same polarization as the desired number having power of βj from the side lobe direction θ′j (i = 1... J), and a direction θ ″ k (k = 1 ...
From K), it is assumed that a desired wave having a power of γk and an interference wave which is a reverse polarization respectively arrive, and noise Pnn is generated in the n-th horn. At this time, the output power Pout of the entire antenna is expressed as follows.

Pout = W | ^T RxxW | where Now, assuming that the desired wave, the interference wave of the positive polarization, the interference wave of the reverse polarization, and the noise are not correlated with each other.
The element Rnm of Rxx is It is represented as Here, gn (θ) is the directivity of the n-th horn for the positive polarization, hn (θ) is the directivity of the n-th horn for the reverse polarization (cross polarization), and the subscript * indicates the complex conjugate. Awarasu.

At this time, if the excitation weight is determined so as to minimize the output power Pout under the constraint condition of the equation (5), as a result, only the component of the desired wave remains in the output power and the interference wave component is not obtained. Will be done. Excitation weight vector that minimizes Pout Is analytically obtained as follows.

Here, -1 means an inverse matrix.

In the description so far, it has been shown that the excitation weight of the equation (9) gives an output that cancels the interference wave component while virtually leaving the desired wave component when the desired wave and the interference wave are received. In other words, the radiation directivity synthesized when each horn is excited by the excitation weight of Expression (9) is such that the gain is maintained in the direction in which the desired wave is set, and the side in which the interference wave is set. This means that a zero point is formed in the direction of the polarization component crossed with the lobe.
Applying this idea, the direction of the main lobe where you do not want to lower the gain as the desired wave direction, the direction of the side lobe suppression as the direction of the interference of the positive polarization, and the cross polarization suppression as the direction of the interference of the reverse polarization By selecting excitation directions in each of the directions for performing the above, directivity of low side lobe and low cross polarization in a predetermined angle region is synthesized.

The excitation weight determining method in the present invention is an improvement of the above-described conventional excitation weight determining method used for lowering side lobes so that cross polarization can be suppressed at the same time. The characteristic feature is that the excitation distribution of the primary radiator suppresses the cross polarization component at the same time as the side lobe. In addition, this is performed by incorporating an interference wave of the reverse polarization into the component of Rxx as shown in the equation (8). Another characteristic is that the total power is obtained by summing the values in total, and the excitation weight that minimizes the total power is obtained.

5 and 6 show an example of radiation directivity synthesized when the primary radiator is excited by the excitation weight according to the present invention. FIG. 5 shows the directivity of the forward polarization and FIG. 6 shows the directivity of the reverse polarization, and the solid line 71 indicates the directivity combined by the excitation weight of the present invention. In this example, the direction of the arrow 40 in the area 21 is used as the direction for restraining the main lobe, the direction of the arrows 41 and 42 in the area 24 using the same frequency as the area 21 is used as the direction of side lobe suppression, and the cross polarization suppression is performed. Is selected as the direction of arrows 43 and 44 in the same area 21 as the main lobe. 5 and 6, for comparison, the directivity obtained by the conventional method of determining the excitation weight based on the aperture distribution is indicated by a dotted line 72 in the far field, and only the low side lobe is suppressed. The directivity according to the method is indicated by a broken line 73, but the directivity according to the present invention is superior to these methods in that low side lobes and low cross polarization can be simultaneously realized.

As described above, by using the primary radiator in which the excitation weight is set according to the present invention, it is possible to realize radiation directivity for lowering the side lobe and lowering the cross polarization in an arbitrary predetermined direction. Also, constants αi, βj, γ in equation (8)
By setting k and Pnn, there is an advantage that the suppression amounts of the side lobes and cross polarization can be freely adjusted. This is very important in designing a multi-beam antenna.

FIG. 2 shows a configuration example of a feed system in which the above-described excitation weight is provided to each antenna element of the primary radiator.
The excitation weight is given by a complex number, which can be divided into two, excitation amplitude and excitation phase. The excitation phase is set by phase shifters 11 to 19 directly connected to the horns 1 to 9, respectively, and the excitation amplitude is set to three power dividers 31 to 34.
Is set by setting the distribution ratio. Considering the waveguide configuration here, the phase shifter can be easily realized by a method such as a metal rod insertion type that changes the wavelength inside the tube, and the power distributor can be easily realized by a directional coupling type or septum type. it can.
From the above examples, it can be seen that it is relatively easy to set the sample phase and the excitation amplitude arbitrarily.

By the way, the excitation weight vector of equation (9) Is a solution that can be obtained analytically. Is given as follows.

However Where II is a unit matrix, μ is a coefficient for controlling the convergence speed of the solution, and m is a sequential circuit.

Excitation weight vector of equation (10) The directivity synthesized by the excitation weight Wn ^{(m + 1)} determined by the following equation is obtained by gradually increasing the sidelobe suppression amount and the cross-polarization suppression amount as the number m of iterations is increased, and setting m to infinity. This corresponds to the combined directivity by the excitation weight shown in 9). Therefore, besides having the same effect as the case of the excitation weight shown in the equation (9), it is also possible to avoid performing the side lobe suppression and the cross polarization suppression more than necessary, and the directivity combination adversely affects the main lobe. Can be prevented as much as possible. In other words, by sequentially calculating the combined directivity of the excitation weights by calculation, by using the excitation weights where the level of the side lobes and cross-polarizations have become equal to or less than a predetermined value as the excitation distribution of the primary radiator, Further, it is possible to suppress the side lobes and the cross polarization more than necessary, so that the gain of the main lobe is not adversely affected.

In the above, the description has been made using the horn antenna as the antenna element. However, another antenna element such as a patch antenna may be used, and the number thereof may be arbitrarily selected. In addition, the effect of the present invention is not impaired even if the number I of the direction in which the gain is determined to determine the excitation weight, the number J of the direction of sidelobe suppression, and the direction K of the cross polarization suppression are freely selected according to the situation. . In the above description, linear polarization orthogonal to normal polarization and reverse polarization has been considered. However, the same effect can be obtained by considering clockwise or counterclockwise circular polarization.

In the above description, the case where the offset parabolic reflecting mirror is used has been described, but the present invention can obtain the same effect by using another reflecting mirror.

In general, in a Cassegrain antenna or the like, the cross-polarization component generated in the mirror system is small, so that cross-polarization suppression is not particularly considered. However, as shown in FIG. When one of the polarizations has a property of passing and the other has a property of reflecting the light, the cross polarization characteristic is often deteriorated. The present invention is also effective in such a case, and the directivity gn shown in Expression (8) is used.
If the excitation weight is obtained using (θ) and hn (θ) including the influence of the polarization grid plate 60, the directivity of low sidelobe and low cross polarization is obtained as in the case of the offset parabolic reflector. Is obtained. This is because, besides the polarization grid plate 60, other structures such as a frequency selection plate (which has a property of reflecting low frequencies and passing high frequencies) are geometrically optical as shown by the hatched portions in FIG. The same can be said for the case where the radio wave exists in the area where the radio wave passes.

[Effects of the Invention] As described above in detail, according to the present invention, suppression of side lobes and suppression of cross-polarization in an arbitrary angle region without reducing the gain of the main lobe, Performance can be improved. Further, the suppression region and suppression level of the side lobe and cross polarization can be freely set.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating the principle of the present invention, FIG. 2 is a diagram illustrating an example of a feed system configuration according to an embodiment of the present invention, FIG. 3 is a diagram illustrating an offset parabolic reflector antenna,
FIG. 4 is a diagram showing the arrangement of the multi-beams, FIG. 5 is a diagram showing the combined directivity of the positive polarization, FIG. 6 is a diagram showing the combined directivity of the cross polarization, and FIG. It is a figure showing other examples. 1,2,3,4,5,6,7,8,9 ...... antenna elements, W ₁ to W ₉ ...... weighter, 10 ...... synthesizer.

Claims (3)

(57) [Claims] 1. A primary radiator comprising a reflector, N antenna elements (N is a natural number of 2 or more), and each antenna element (Note the subscript ^T is the complex conjugate transposed matrix, ^-1 is an inverse matrix) reflector antenna characterized by comprising a weighting unit for exciting the elements W _n of excitation vectors determined by. However, each matrix element R _nm is (I is a number representing a sampling point within an angle range that restricts the radiation field intensity, j is a number representing a sampling point within an angle range that requires sidelobe suppression, and k is an angle range that requires reverse polarization suppression. Α _i , β _i , and γ _i are proportional coefficients, θ _i , θ ′ _j , θ ′ _k are the angles indicating the sampled directions, and g _n (θ) is n The directivity radiated through the reflector of the n th antenna element, h _n (θ) is the directivity of the reverse polarization radiated through the reflector of the n th antenna element, * is complex conjugate, P _nm Is a constant.) (B _i is the complex constraint value in the direction θ _{i that} limits the radiation field intensity) 2. The reflector antenna according to claim 1, wherein a plurality of said primary radiators are provided to form a multi-beam. Wherein ^{W (m + 1) = W} (m) -μ (PRPW (m) + PRF) relational expression (where, P = I-C (C T C) -1 C T, F = C ( C ^T C) ^-1 B I is a unit matrix, μ is a coefficient for controlling the convergence speed of the solution, m is the number of iterations, and m is an excitation vector W ^{(m + 1).} The reflector antenna according to claim 1, wherein the antenna is determined.