Antenna device and array antenna device

A sealed case (6) includes a first electrode (4) and a second electrode (5). The maximum size of each of these electrodes and a distance between them are equal to or smaller than one tenth the wavelength of a signal of interest. The sealed case (6) is configured such that the internal gas becomes a plasma state. The second electrode (5) is connected to a first conductor (1), and the first electrode (4) is connected to a second conductor (2) disposed to be perpendicular to the first conductor (1).

TECHNICAL FIELD

The present invention relates to an antenna device loaded with a variable capacitive element using plasma and whose radiation pattern is variable, an antenna device whose operation frequency is variable, and an array antenna device in which such antenna devices are used.

BACKGROUND ART

Variable capacitive diodes (also referred to as, for example, varactor (Variable Reactor) diodes or varicap (Variable Capacitor)) are often used for switching of a radiation pattern of an antenna (radiation directivities, or simply, directivities) or its operation frequency (the frequency at which the antenna works). As such an antenna, for example, an antenna is known that includes a non-excitation element (also referred to as a passive element or a parasitic element) in the vicinity of a driven element (which is directly connected with a feeder path) to control the directivities by changing the value of the reverse bias applied to the variable capacitive diode loaded on the non-excitation element.

A technique is also known in which a variable capacitive diode is used for a matching circuit of a monopole antenna on a ground plate and the value of the reverse bias applied to the variable capacitive diode is changed to change the matching frequency between the antenna and the feeder path, namely, to change the operation frequency of the antenna (see Patent Literature 1, for example).

CITATION LIST

Japanese Unexamined Patent Application Publication No. 2002-232313

SUMMARY OF INVENTION

Technical Problem

In the antenna device described in Patent Literature 1, if the power of a high frequency (RF) wave used in radar or communication is low, the RF voltage superposing the DC reverse bias is low, resulting in desired operation without difficulties. Therefore, this type of antenna device is often used as a receiver antenna. However, if the RF power treated by the antenna device is higher, the RF voltage superposing the DC reverse bias is also higher, and thus, the RF voltage becomes too high for the variable capacitive element to operate normally. As a result, the variable capacitive element in the antenna device cannot operate as desired so that it is difficult to perform switching of the directivity of an antenna or a matching circuit in the antenna using the variable capacitive diode.

The present invention has been made in view of the above problem, and an object of the present invention is to provide an antenna device that can surely switch the directivity or operation frequency.

Solution to Problem

The antenna device according to the present invention includes: a first conductor; a second conductor disposed to be perpendicular to the first conductor; a sealed case comprising a first electrode and a second electrode, the maximum size of each of the first and second electrodes and a distance between the first and second electrodes being equal to or smaller than one tenth the wavelength of a signal of interest, the sealed case containing rare gas; and a power source applying variable voltage to the first and second electrodes to ionize the rare gas in the sealed case into a plasma state. The first electrode is connected to the second conductor, and the second electrode is connected to the first conductor.

Advantageous Effects of Invention

The antenna device according to the present invention uses a variable capacitive element using plasma as an element of a variable matching circuit. Due to such a configuration, switching of the operation frequency can be surely performed.

DESCRIPTION OF EMBODIMENTS

Some embodiments of the present invention will be described in more detail for explaining the present invention with reference to the accompanying drawings.

An antenna structure loaded with a variable capacitive element using plasma will be described. The relative permittivity εrof collisionless and low-temperature plasma is represented by the following Formula (1):

where nerepresents the electron density, methe mass of an electron, e the charge of the electron, ε0the vacuum permittivity, and ω the angular frequency of the electromagnetic wave. Each of these parameters other than the electron density neis a constant. InFIG. 1, Formula (1) is represented by a graph, where the horizontal axis indicates the frequency of a radio wave and the vertical axis indicates the relative permittivity of plasma. By changing the voltage and current applied to electrodes, the plasma frequency fp=ωp/2π can be changed, and as a result, the relative permittivity εrof plasma can be dynamically controlled. On the other hand, the capacitance C of a capacitor is represented by:

where S represents the area of each of the two conductive plates used as electrodes, and d the distance between the two conductive plates (the distance between the electrodes).

Thus, by disposing a plasma medium between the two conductive plates (electrodes) and changing the voltage and current applied between the electrodes, it is possible to change the electrostatic capacitance between the electrodes. However, if the size of the conductive plates (electrodes) is not small enough relative to the radio wavelength used for communication or radar, for example, half of the radio wavelength, resonance phenomenon occurs in the frequency of the radio waves, thus the conductive plates no longer operate as a capacitor. It should be noted that, in this explanation, the size of an electrode refers to the size of the maximum length part of the electrode plate regardless of its shape and will be hereinafter referred to as the maximum size of the electrode.

FIG. 2is a block diagram illustrating an antenna device according to Embodiment 1.

The antenna device according to Embodiment 1 includes a first conductor1, a second conductor2, an input/output terminal3, a first electrode4, a second electrode5, a sealed case6, a high-voltage power source7, and a transceiver8. The first conductor1is a ground plate as an antenna device. The second conductor2is an antenna radiation conductor disposed to be perpendicular to the first conductor1and functions as a driven element. The input/output terminal3functions as a terminal for supplying radio waves used in radar or communication to the first conductor1and the second conductor2during a transmission operation, and functions as a terminal for outputting signals received by the first conductor1and the second conductor2to the outside during a reception operation. The first electrode4and the second electrode5are disposed to face each other in the sealed case6and are formed such that the interval between them and the maximum size of each of them are equal to or smaller than one tenth the wavelength of the radio wave to be used. In the sealed case6, rare gas which is easily ionized, for example, helium, neon, or argon is contained. The high-voltage power source7applies high voltage to the first electrode4and the second electrode5and ionizes the gas contained in the sealed case6into a plasma state. In the drawing, the high-voltage power source7is represented as an AC power source. Instead, a DC power source may be used. The transceiver8is connected between the input/output terminal3and the second electrode5. The transceiver8is a device for transmitting a signal during the transmission operation of the antenna device and receiving a signal through the input/output terminal3during the reception operation of the antenna device.

The operation of the antenna device according to Embodiment 1 will now be explained.

During the transmission operation, the radio wave supplied from the transceiver8through the input/output terminal3is radiated into the air from the second conductor2. Since the second electrode5is connected to the first conductor being a ground plate through an appropriate conductor, the first electrode4and the second electrode5are connected to the input/output terminal3in parallel and thus operate as a capacitor. In order to operate as a capacitor, the maximum size of the first electrode4and the second electrode5must be small enough relative to the radio wavelength used in radar or communication. The interval (distance) between the first electrode4and the second electrode5must also be small enough relative to the radio wavelength. The size and interval are preferably equal to or smaller than one tenth the wavelength. The sealed case6contains rare gas which is easily ionized, and high voltage of equal to or higher than several kilovolts is applied between the first electrode4and the second electrode5by the high-voltage power source7. The gas contained in the sealed case6can be thereby ionized to be in the plasma state. As described before, the electrostatic capacitance of a capacitor is proportional to the permittivity of the medium between the electrodes, and thus the permittivity of the plasma can be controlled by the applied voltage between the electrodes. Hence, according to the above configuration, an antenna with variable operation frequency can be obtained.

In a reception operation, a signal at the operation frequency determined in accordance with the voltage applied between the first electrode4and the second electrode5from the high-voltage power source7is received from the second conductor2and is received by the transceiver8through the input/output terminal3.

In this manner, in Embodiment 1, since a variable capacitive element using plasma is adopted, an operation desired as an antenna with variable operation frequency can be achieved. Namely, the total voltage V applied to the variable capacitive element is the sum of the RF voltage (Vrf) used in communication or radar and the other externally applied voltage (V0): V=V0+Vrf. Thus, if V0>>Vrf, the variation in V is very small, and an operation desired as a variable frequency antenna can be thereby achieved. On the other hand, when a conventional variable capacitive diode is used, because of the relation V0<<Vrf, the variation in V is large so that a desired operation cannot be achieved. Thus, the antenna device according to the present embodiment can provide a solution to such problems.

As describe above, the antenna device according to Embodiment 1 includes: a first conductor; a second conductor disposed to be perpendicular to the first conductor; a sealed case comprising a first electrode and a second electrode, the maximum size of each of the first and second electrodes and a distance between the first and second electrodes being equal to or smaller than one tenth the wavelength of a signal of interest, the sealed case containing rare gas; and a power source applying variable voltage to the first and second electrodes to ionize the rare gas in the sealed case into a plasma state. The first electrode is connected to the second conductor, and the second electrode is connected to the first conductor. As a result, the operation frequency can be surely switched.

In Embodiment 2, a high-voltage breaker is provided between the second conductor2and the first electrode4. The high-voltage breaker becomes electrically open at the frequency applied by the high-voltage power source7.

FIG. 3is a block diagram illustrating an antenna device according to Embodiment 2, in which the antenna device further includes a high-voltage breaker9in addition to the configuration of Embodiment 1 illustrated inFIG. 2. Other components in this configuration are the same as those inFIG. 2so that they are denoted by the same reference numerals and detailed descriptions thereof are omitted. The high-voltage breaker9is provided between the first electrode4and the second conductor2, and a capacitor can be used if the high-voltage power source7supplies a direct current. Since the impedance of the capacitor is 1/(jωC), a capacitance C at which the capacitor is supposed to be substantially short-circuited at the frequency of the radio wave used in radar or communication may be selected. Alternatively, the value of the capacitor may be selected such that the high-voltage breaker9is also used as a matching circuit for the antenna.

If the high-voltage power source7supplies an alternating current, several methods can be employed. If the ratio of the transmission frequency to the frequency of the high-voltage power source7is more than several tens, by using a capacitor having an appropriate capacitance value as the high-voltage breaker9, it is possible for the high-voltage breaker9to be electrically open substantially at the frequency of the high-voltage power source7and electrically short-circuited substantially at the transmission frequency. On the other hand, if the ratio of the transmission frequency to the frequency of the high-voltage power source7is less than several tens, an LC parallel resonance circuit whose resonance frequency is the frequency of the high-voltage power source7may be used as the high-voltage breaker9.

The antenna device configured in such a manner in Embodiment 2 can prevent the voltage applied by the high-voltage power source7from being applied to the second conductor2. Namely, if the high-voltage breaker9does not exist between the first electrode4and the second conductor2, the high-voltage from the high-voltage power source7is applied to the second conductor2. Thus, the high-voltage is undesirably applied to the transceiver8through the input/output terminal3. In such a case, there arises a problem, for example, that the operation of the transceiver8may be obstructed, or the transceiver8may be damaged. On the contrary, in Embodiment 2, the voltage from the high-voltage power source7can be blocked at the high-voltage breaker9and the above problem can be solved.

As described above, the antenna device according to Embodiment 2 includes a high-voltage breaker between the second conductor and the first electrode. The high-voltage breaker becomes electrically open at the frequency applied by the power source so that decreasing of the performance as the antenna device can be prevented while the effects same to those of Embodiment 1 can also be achieved.

Further, according to the antenna device of Embodiment 2, since a capacitor is used as the high-voltage breaker, the high-voltage breaker can be manufactured at low cost.

Moreover, according to the antenna device of Embodiment 2, since an LC parallel resonance circuit is used as the high-voltage breaker, even when the ratio of the transmission frequency to the frequency of the power source is small, the high-voltage breaker can be configured.

In Embodiment 3, in addition to the configuration of Embodiment 2, a high-frequency breaker10is provided. The high-frequency breaker10is disposed between the high-voltage power source7and the first electrode4and blocks a signal having a transmission frequency received through the input/output terminal3.

FIG. 4is a block diagram illustrating an antenna device of Embodiment 3 in which the high-frequency breaker10is added to the configuration of Embodiment 2 shown inFIG. 3. The high-frequency breaker10is disposed between the high-voltage power source7and a connection node of the first electrode4and the high-voltage breaker9. The high-frequency breaker10is configured using, for example, an LC parallel resonance circuit whose resonance frequency is the transmission frequency of a signal from the transceiver8which is input through the input/output terminal3. Other components in this configuration are the same as those of Embodiment 2 illustrated inFIG. 3so that they are denoted by the same reference numerals and the descriptions thereof are omitted.

The antenna device configured in this manner in Embodiment 3 can block a transmission frequency signal input through the input/output terminal3and applied to the high-voltage power source7. Namely, the current of the radio wave supplied from the transceiver8may flow into the high-voltage power source7through the input/output terminal3, resulting in deterioration in antenna characteristics. However, in Embodiment 3, since the high-frequency breaker10can block such a current flow, the influence on the voltage applied from the high-voltage power source7to the first electrode4and the second electrode5can be eliminated.

In the above Embodiment 3, the high-frequency breaker10is added to the configuration of Embodiment 2. Alternatively, the high-frequency breaker10may be added to the configuration of Embodiment 1. In other words, only the high-frequency breaker10may be added to the configuration of Embodiment 1.

As described above, the antenna device of Embodiment 3 includes a high-frequency breaker between a power source and a first electrode. The high-frequency breaker blocks transmission frequency signals applied to a first conductor and a second conductor. As a result, in addition to the effects of Embodiment 1, the deterioration in performance as an antenna device can be prevented.

Further, the antenna device of Embodiment 3 includes a high-voltage breaker disposed between the second conductor and the first electrode and being electrically open at the frequency applied by the power source; and the high-frequency breaker disposed between the power and the first electrode and blocking a transmission frequency signal to be supplied to the first and second conductors. As a result, in addition to the effects of Embodiment 1, the deterioration in performance as an antenna device can be prevented.

Moreover, in the antenna device of Embodiment 3, an LC parallel resonance circuit is used as the high-frequency breaker. As a result, the high-frequency breaker can be configured at low cost.

In the above embodiments, a plasma variable capacitive element is used as an element in a variable matching circuit, and the operation frequency of an antenna (the impedance matching frequency of the antenna and the feeder path) is variable. In the present embodiment, the plasma variable capacitive element is used to switch the radiation directivity of the antenna.

FIG. 5is a block diagram illustrating an antenna device of Embodiment 4. The structural differences of Embodiment 4 from the embodiments described before are that the antenna device does not include the conductor connecting the first electrode4and the second conductor2, and the first electrode4is connected with a third conductor11. The third conductor11is a non-excitation element. Other components in this configuration are the same as those of Embodiment 1 illustrated inFIG. 2and denoted by the same reference numerals, and they are not described in detail.

In the antenna device according to Embodiment 4, by appropriately selecting the interval between the second conductor2being a driven element and the third conductor11being a non-excitation element and appropriately switching the voltage applied from the high-voltage power source7to the first electrode4and the second electrode5, the direction of the radio wave based on the transmission signal supplied from the input/output terminal3and radiated into the air can be controlled.FIGS. 6A to 6Cillustrate, for example, the calculation results of variation in the radiation directivity by means of numerical electromagnetic field analysis method in the FDTD method in the case where the transmission frequency is 100 MHz, and the value of the capacitance formed by the first electrode4and the second electrode5and the plasma in the sealed case6is switched between 80 pF and 20 pF.FIG. 6Ais a schematic perspective view of an antenna device.FIG. 6Billustrates the radiation pattern at C=80 pF (Z=−j20Ω).FIG. 6Cillustrates the radiation pattern at C=20 pF (Z=−j80Ω). As is apparent from the respective radiation patterns inFIGS. 6B and 6C, it can be understood that the directivity can be largely changed even when the change rate of the capacitance is about 1:4.

As described before, since the value of capacitance is proportional to the relative permittivity of the plasma in the sealed case6, the radiation directivity of the antenna illustrated inFIG. 5can be changed by changing the applied voltage by the high-voltage power source7. It should be noted that the interval (distance) between the second conductor2and the third conductor11may be any value as long as the value is in a range where the conductors are electromagnetically coupled to each other and the radiation directivity can be changed. The illustrated example indicates the case where the interval is λ/4. Normally, the distance is equal to or less than half of the wavelength of the radio wave radiated into the air.

As explained above, in Embodiment 4, since a variable capacitive element using plasma is adopted, a desired operation as an antenna with switchable radiation directivity can be achieved. In other words, similarly to the case of the operation as a matching circuit, even if the current generated by the radio wave input from the input/output terminal3leaks to the side of the high-voltage power source7, the relation V0>>Vrf is also satisfied in Embodiment 4. The variation in V is thus very small, and the influence of the voltage applied to the variable capacitive element due to the RF voltage can be reduced.

As explained above, the antenna device according to Embodiment 4 includes a first conductor; a second conductor disposed to be perpendicular to the first conductor; a third conductor disposed to be parallel to the second conductor; a sealed case including a first electrode and a second electrode, the maximum size of the first and second electrodes and the distance therebetween being set to be equal to or smaller than one tenth the wavelength of a signal of interest, the case containing rare gas; and a power source applying a voltage to the first and second electrodes to ionize the rare gas in the sealed case into a plasma state and the applied voltage being variable. The third conductor is connected with the first electrode, and the second electrode is connected with the first conductor. Thus, the directivity can be surely switched.

In Embodiment 5, a high-frequency breaker10is disposed between the high-voltage power source7and the first electrode4. The high-frequency breaker10blocks a signal of transmission frequency input through the input/output terminal3.

FIG. 7is a block diagram illustrating an antenna device according to Embodiment 5 where the antenna device further includes the high-frequency breaker10in the configuration of Embodiment 4 illustrated inFIG. 5. The high-frequency breaker10is disposed between the high-voltage power source7and the first electrode4. The high-frequency breaker10includes the LC parallel resonance circuit like Embodiment 3 where its resonance frequency is the frequency of the radio wave transmitted by the transceiver8. Other components in this configuration, which are the same as those of Embodiment 4 illustrated inFIG. 5and are denoted by the same reference numerals, are not described in detail.

In the antenna device having such a configuration, the high-frequency breaker10, which is disposed between the high-voltage power source7and the first electrode4, can block the current of radio wave from the transceiver8even if the current leaks to the high-voltage power source7through the input/output terminal3.

As described above, the antenna device according to Embodiment 5 includes a high-frequency breaker disposed between a power source and a first electrode. The high-frequency breaker blocks a signal sent to a first conductor and a second conductor at the transmission frequency, thereby preventing deterioration in antenna performance.

Two or more antenna devices described in Embodiments 1 to 5 may be arrayed at predetermined intervals to form an array antenna device using high power. Further, although in each of the above embodiments, an example of a monopole antenna where a first conductor1is used as a ground plate has been described, the present invention may be easily applied to a dipole antenna by applying the image theory (the method of mirror images) to the first conductor1being a ground plate. Moreover, in the above explanation, the second conductor2being a driven element and the third conductor11being a non-excitation element are described as linear conductors. Alternatively, the elements may be bent to decrease the heights (height reduction) or may have a linear conductor or a planar conductor parallel to the first conductor1on the top (top loading) of the elements to achieve the same effects. In addition, the same effects may also be achieved by disposing two or more non-excitation elements each loaded with a plasma variable capacitive element.

As described above, according to the array antenna device of Embodiment 6, since two or more antenna devices according to any one of Embodiments 1 to 5 are arrayed, switching of the directivity or the operation frequency can be performed surely.

It should be noted that the present invention can include any combination of embodiments, or modifications or omission of any component in the embodiments within the scope of the invention.

INDUSTRIAL APPLICABILITY

As described above, the antenna device and the array antenna device according to the present invention include a variable capacitive element using plasma as a switching means of an element of the variable matching circuit or the radiation directivity of an antenna. The variable capacitive element using plasma is suitable for use in an antenna device having variable radiation patterns or variable operation frequencies.

REFERENCE SIGNS LIST