Patent Publication Number: US-6335699-B1

Title: Radome

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a radome for protecting a radar antenna, for example. 
     2. Description of the Related Art 
     Generally, when a radar antenna is mounted to an aircraft, the antenna is placed inside a radome. When a radar antenna is mounted on a ship or on the ground, the antenna is also covered by a radome to protect against wind and subsequently smooth rotation of the antenna and to prevent reduction of electrical performance of the antenna due to adhesion of raindrops. 
     This kind of assembly is described in detail in “Redoomu ni tsuite” (“Radome-Antenna Housing”) by Takashi KITSUREGAWA, Mitsubishi Denki Gijutsu Hohkoku (Mitsubishi Electric Technical Reports), Vol. 29, No. 7, pp. 73-79, 1955. 
     FIGS. 12 and 13 are a perspective and a cross section, respectively, schematically showing a conventional radar assembly employing a radome. 
     In FIGS. 12 and 13, a radome  1  is called a half-wavelength plate radome, and is composed of a dielectric plate. A radar antenna  2  functioning as a radar device is disposed inside the radome  1 . Reinforced plastics such as Fiber Reinforced Plastics (FRPs), polypropylene, or engineering plastics such as ABS resin, are used in the radome  1 . 
     In consideration of the relative permittivity and dielectric dissipation factor of the dielectric material, this radome  1  is designed to permit passage of radio waves having a frequency used by the radar antenna  2  with minimal loss, in other words, reflection by the dielectric plate composing the radome  1  is reduced. 
     If we let λ 0  be the free space wavelength of the working radio wave, let ∈ r  be the relative permittivity of the dielectric material used, and let θ in  be the angle of incidence of radio waves relative to the radome, then the thickness d of the dielectric plate composing the radome  1  is represented by Expression (1) below. 
       d= ( Nλ   0 )/{2(∈ r −sin 2 θ in ) ½ }  (1) 
     Moreover, N is a natural number, called the radome order. 
     Now, by making the radome  1  (dielectric plate) a thickness d which satisfies Expression (1), reflection by the radome  1  (dielectric plate) is reduced, permitting passage of radio waves having the frequency used by the radar antenna  2  with minimal loss. 
     The relationship between the radio wave frequency f, its free space wavelength λ, and the speed of light c is given by Expression (2). 
     
       
           λ=c/f   (2) 
       
     
     Because a conventional radome  1  is constructed in the above manner, radio waves having a frequency which permits passage with minimal loss are constricted to radio waves having the working frequency of the radar antenna  2 . Thus, one problem has been that when the radar antenna  2  is not being used, external radio waves having the same frequency as the working frequency of the radar antenna  2  also pass through with minimal loss, interfering with the radar antenna  2  and giving rise to malfunctions. 
     SUMMARY OF THE INVENTION 
     The present invention aims to solve the above problems and an object of the present invention is to provide a radome enabling interference in a radar device due to external radio waves to be reduced by enabling passage of radio waves having a frequency used by the radar device to be controlled and by preventing penetration by external radio waves having the same frequency as the radio waves used by the radar device when the radar device is not being used. 
     In order to achieve the above object, according to one aspect of the present invention, there is provided a radome which has a dielectric layer whose relative permittivity is changed by the application of an electric field, and an electric field applying means for applying the electric field to the dielectric layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective schematically showing a radar assembly employing a radome according to Embodiment 1 of the present invention; 
     FIG. 2 is cross section schematically showing a radar assembly employing a radome according to Embodiment 1 of the present invention; 
     FIG. 3 is a perspective schematically showing a radar assembly employing a radome according to Embodiment 3 of the present invention; 
     FIG. 4 is a perspective schematically showing a radar assembly employing a radome according to Embodiment 4 of the present invention; 
     FIG. 5 is a cross section schematically showing a radar assembly employing a radome according to Embodiment 4 of the present invention; 
     FIG. 6 is a perspective schematically showing a radar assembly employing a radome according to Embodiment 5 of the present invention; 
     FIG. 7 is a cross section schematically showing a radar assembly employing a radome according to Embodiment 5 of the present invention; 
     FIG. 8 is a partially-cutaway perspective schematically showing a radar assembly employing a radome according to Embodiment 6 of the present invention; 
     FIG. 9 is a cross section schematically showing a radar assembly employing a radome according to Embodiment 6 of the present invention; 
     FIG. 10 is a perspective schematically showing a radar assembly employing a radome according to Embodiment 7 of the present invention; 
     FIG. 11 is a cross section schematically showing a radar assembly employing a radome according to Embodiment 7 of the present invention; 
     FIG. 12 is a perspective schematically showing a radar assembly employing a conventional radome; and 
     FIG. 13 is a cross section schematically showing a radar assembly employing a conventional radome. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will now be explained with reference to the drawings. 
     Embodiment 1 
     FIGS. 1 and 2 are a perspective and a cross section, respectively, schematically showing a radar assembly employing a radome according to Embodiment 1 of the present invention. 
     In FIGS. 1 and 2, the radome  10  includes: a pair of glass plates  11  disposed with a predetermined spacing relative to each other; a liquid crystal layer  12  functioning as a dielectric layer composed of low-molecular-weight liquid crystals sealed hermetically between the pair of glass plates  11 ; and control electrode layers  13  composed of metal electrodes each formed in a frame shape and disposed on an upper and a lower surface of the pair of glass plates  11 , respectively. In use, this radome is disposed so as to cover a radar antenna  2  functioning as a radar device. Here, an electric field applying means is composed of a power source  9  and the control electrode layers  13 . 
     In this radome  10 , voltage is applied between the pair of control electrode layers  13  by the power source  9 , and the permittivity of the liquid crystal layer  12  changes when an electric field arises between the control electrode layers  13 . Here, the state in which voltage is being applied between the control electrode layers  13  and an electric field is present in the control electrode layers  13  is called the “controlled state” of the liquid crystal layer, and the state in which voltage is not being applied between the control electrode layers  13  and an electric field is not present in the control electrode layers  13  is called the “non-controlled state” of the liquid crystal layer. Let ∈ rco  be the relative permittivity of the liquid crystal layer in the controlled state, and let ∈ rnc  be the relative permittivity of the liquid crystal layer in the non-controlled state. Let f 0  be the radio wave frequency used in the radar antenna  2 , and λ 0  be the free space wavelength thereof. 
     The liquid crystal layer  12  of the radome  10  is selected to have a thickness d which satisfies the above Expression (1) in the controlled state, that is, when ∈ r =∈ rco . In other words, in the controlled state of the liquid crystal layer  12 , reflection by the radome  10  of radio waves having a frequency f 0  is reduced, permitting passage of radio waves having the frequency used by the radar antenna  2  with minimal loss. Moreover, the relative permittivity of the liquid crystal layer  12  is controlled by the magnitude of the applied electric field and by the liquid crystal material. 
     In a radome  10  constructed in this manner, when the radar antenna  2  is being used, voltage is applied between the control electrode layers  13  using the power source  9 , and the liquid crystal layer is in the controlled state. At that time, the relative permittivity of the liquid crystal layer  12  is ∈ rco , and radio waves having the working frequency of the radar antenna  2  can pass through the region of the liquid crystal layer surrounded by the control electrode layers  13  of the radome  10  with minimal loss. Thus, the radar antenna  2  can transmit and receive signals without hindrance. 
     On the other hand, when the radar antenna  2  is not being used, voltage application between the control electrode layers  13  is terminated, and the liquid crystal layer is in the non-controlled state. At that time, the relative permittivity of the liquid crystal layer  12  is ∈ rcn , and radio waves having the working frequency of the radar antenna  2  cannot pass through the region of the liquid crystal layer surrounded by the control electrode layers  13  of the radome  10 . Thus, even if external radio waves having the same frequency as the working frequency arrive, the external radio waves are blocked by the radome  10  and prevented from reaching the radar antenna  2 . Consequently, interference in the radar antenna  2  due to the arrival of external radio waves is reduced, enabling the occurrence of malfunctions to be suppressed. 
     In this manner, according to Embodiment 1, because the liquid crystal layer  12  functioning as a dielectric layer is held between the pair of glass plates  11 , and the control electrode layers  13  are disposed on an upper and a lower surface of the pair of glass plates  11 , respectively, the permittivity of the liquid crystal layer  12  can be changed by applying a voltage between the control electrode layers  13 . Thus, if the thickness and relative permittivity of the liquid crystal layer  12  are selected to permit passage of radio waves having the working frequency of the radar antenna  2  when the liquid crystal layer is in the controlled state, then by synchronizing the controlled state of the liquid crystal layer with the operation of the radar antenna  2 , radio waves having the working frequency can pass through the radome  10  with minimal loss and the radar antenna  2  can transmit and receive signals without hindrance when the radar antenna  2  is being used, and penetration by external radio waves having the same frequency as the working frequency can be blocked when the radar antenna  2  is not being used, enabling interference in the radar antenna  2  due to external radio waves to be reduced. 
     Embodiment 2 
     In Embodiment 1, the liquid crystal layer  12  of the radome  10  is selected to have a thickness d satisfying Expression (1) above in the controlled state, that is, when ∈ r =∈ rco , but in Embodiment 2, the liquid crystal layer  12  of the radome  10  is selected to have a thickness d satisfying Expression (1) above in the non-controlled state, that is, when ∈ r =∈ rnc . 
     In Embodiment 2, by making the non-controlled state of the liquid crystal layer  12  when the radar antenna  2  is being used, radio waves having the working frequency of the radar antenna  2  can pass through the region of the liquid crystal layer  12  surrounded by the control electrode layers  13  of the radome  10  with minimal loss. Thus, the radar antenna  2  can transmit and receive signals without hindrance. 
     On the other hand, by applying voltage between the control electrode layers  13  and making the controlled state of the liquid crystal layer when the radar antenna  2  is not being used, radio waves having the working frequency of the radar antenna  2  cannot pass through the region of the liquid crystal layer surrounded by the control electrode layers  13  of the radome  10 . Thus, even if external radio waves having the same frequency as the working frequency arrive, the external radio waves are blocked by the radome  10  and prevented from reaching the radar antenna  2 . 
     Consequently, the same effects are achieved in Embodiment 2 as in Embodiment 1 above. 
     Moreover, in Embodiments 1 and 2 above, the relative permittivity and thickness of the liquid crystal layer  12  are selected to prevent passage of external radio waves having the same frequency as the working frequency of the radar antenna  2 , but in uses requiring the reduction of interference in the radar antenna  2  relative to external radio waves having a specific frequency other than the working frequency of the radar antenna  2 , the relative permittivity and thickness of the liquid crystal layer  12  may also be selected to reduce the penetration of external radio waves having that specific frequency. 
     Embodiment 3 
     As shown in FIG. 3, in Embodiment 3, the control electrode layers  13  of a radome  10 A are formed in a grid shape on two surfaces of the pair of glass plates  11 . Moreover, the rest of the construction is the same as in Embodiment 1 above. 
     In Embodiment 3, because the control electrode layers  13  are formed in a grid shape, radio waves having polarity at right angles to a longitudinal direction of the grid can pass through the control electrode layers  13 , achieving the same effects as in Embodiment 1. 
     Embodiment 4 
     FIGS. 4 and 5 are a perspective and a cross section, respectively, schematically showing a radar assembly employing a radome according to Embodiment 4 of the present invention. 
     In FIGS. 4 and 5, a radome  10 B includes two liquid crystal layers  12  stacked in a thickness direction. One of the liquid crystal layers  12  is selected to have a thickness and relative permittivity satisfying Expression (1) above relative to radio waves having a frequency f 1  in the controlled state, and the other liquid crystal layer  12  is selected to have a thickness and relative permittivity satisfying Expression (3) below relative to radio waves having a frequency f 2  in the controlled state. As described below, f 1  and f 2  are chosen to be frequencies close to f 0  so that superposed penetration characteristics are not lost. Moreover, the rest of the construction is the same as in Embodiment 1 above. 
     Now, when radio waves of free space wavelength λ 0  arrive at a dielectric layer of relative permittivity ∈ r  at an angle of incidence θ in , the thickness d of the dielectric layer minimizing reflection of those radio waves is calculated by Expression (1) above. When radio waves of free space wavelength λ 0  arrive at a dielectric layer of relative permittivity ∈ r  at an angle of incidence θ in , the thickness d of the dielectric layer maximizing reflection of those radio waves is calculated by Expression (3) below. 
     
       
           d= ( Nλ   0 )/{4(∈ r −sin 2 θ in ) ½ }  (3) 
       
     
     Moreover, N is an odd number. 
     In this radome  10 B, at one of the liquid crystal layers  12 , reflection of radio waves having the frequency f 1  which is slightly offset from the frequency f 0  of the radio waves used by the radar antenna  2 , that is, reflection of radio waves of a free space wavelength λ 1  is reduced in the controlled state, and radio waves having the frequency f 1  can pass through with minimal loss. On the other hand, at the other liquid crystal layer  12 , reflection of radio waves having the frequency f 2  which is slightly offset from the frequency f 0  of the radio waves used by the radar antenna  2 , that is, reflection of radio waves of a free space wavelength λ 2  is increased in the controlled state, and radio waves having the frequency f 2  cannot pass through. 
     In a radome  10 B constructed in this manner, when the radar antenna  2  is being used, voltage is applied between the control electrode layers  13  using the power source  9 , and the two liquid crystal layers  12  are in the controlled state. At that time, one of the liquid crystal layers  12  is in a state in which radio waves having the frequency f 1  can pass through with minimal loss, and the other liquid crystal layer  12  is in a state in which radio waves having the frequency f 2  cannot pass through. Thus, the radio wave penetration characteristics of the radome  10 B are the superposed radio wave penetration characteristics of the two liquid crystal layers  12 , and only an extremely narrow range of wavelengths centered on the free space wavelength λ 0  can pass through. Consequently, radio waves having the working frequency of the radar antenna  2  can pass through the region of the liquid crystal layers  12  surrounded by the control electrode layers  13  of the radome  10 B with minimal loss, and the radar antenna  2  can transmit and receive signals without hindrance. 
     On the other hand, when the radar antenna  2  is not being used, voltage application between the control electrode layers  13  is terminated, and the two liquid crystal layers  12  are in the non-controlled state. At that time, both liquid crystal layers  12  are in a state in which radio waves having the working frequency of the radar antenna  2  cannot pass through the region of the liquid crystal layer surrounded by the control electrode layers  13  of the radome  10 B. Thus, even if external radio waves having the same frequency as the working frequency arrive, the external radio waves are blocked by the radome  10 B and prevented from reaching the radar antenna  2 . Consequently, interference in the radar antenna  2  due to the arrival of external radio waves is reduced, enabling the occurrence of malfunctions to be suppressed. 
     In this manner, the same effects can be achieved in Embodiment 4 as in Embodiment 1 above. 
     Furthermore, in Embodiment 4, because the two liquid crystal layers  12  are stacked in the thickness direction, by selecting the thickness and relative permittivity of one of the liquid crystal layers  12  in the controlled state so that radio waves having the frequency f 1  can pass through with minimal loss and selecting the thickness and relative permittivity of the other liquid crystal layer  12  in the controlled state so that radio waves having the frequency f 2  cannot pass through, radio wave penetration characteristics having a sharp peak centered on the frequency f 0  can be achieved. Thus, when the radar antenna  2  is being used, passage of external radio waves in the vicinity of the frequency f 0  used by the radar antenna  2  can also be reduced, enabling interference in the radar antenna  2  due to external radio waves to be suppressed. 
     By sharing the control electrode layer  13  disposed between the liquid crystal layers  12 , the control electrode layers  13  can be reduced to three layers. 
     Moreover, in Embodiment 4 above, the thickness and relative permittivity of one of the liquid crystal layers  12  in the controlled state are selected so that radio waves having the frequency f 1  can pass through with minimal loss, and the thickness and relative permittivity of the other liquid crystal layer  12  in the controlled state are selected so that radio waves having the frequency f 2  cannot pass through. However, the thickness and relative permittivity of one of the liquid crystal layers  12  in the non-controlled state may be selected so that radio waves having the frequency f 1  can pass through with minimal loss, the thickness and relative permittivity of the other liquid crystal layer  12  in the non-controlled state being selected so that radio waves having the frequency f 2  cannot pass through. Or, the thickness and relative permittivity of one of the liquid crystal layers  12  in the controlled state may be selected so that radio waves having the frequency f 1  can pass through with minimal loss, the thickness and relative permittivity of the other liquid crystal layer  12  in the non-controlled state being selected so that radio waves having the frequency f 2  cannot pass through. 
     Furthermore, in Embodiment 4 above, two liquid crystal layers  12  are stacked in the thickness direction, but the stacked liquid crystal layers  12  are not limited to two layers, and there may be three or more layers. 
     Embodiment 5 
     Because a radome  10 C according to Embodiment 5 employs a radar antenna  2  composed of separate transmit and receive antennas, two liquid crystal layers  12  are disposed on a plane so as to be positioned above the transmit antenna and the receive antenna, respectively, and two sets of control electrode layers  13  and power sources  9  are disposed to enable electric fields to be applied independently to the two liquid crystal layers  12  as shown in FIGS. 6 and 7. Moreover, the rest of the construction is the same as in Embodiment 1 above. 
     In Embodiment 5, the relative permittivity and thickness of the two liquid crystal layers  12  are selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the controlled state. 
     When the radar antenna  2  is transmitting, an electric field is applied to the liquid crystal layer  12  positioned above the transmit antenna of the radar antenna  2 , but an electric field is not applied to the liquid crystal layer  12  positioned above the receive antenna. Thus, because external radio waves having the working frequency are reflected by the liquid crystal layer  12  positioned above the receive antenna and are prevented from reaching the receive antenna, interference in the receive antenna due to external radio waves is suppressed. 
     On the other hand, when the radar antenna  2  is receiving, an electric field is applied to the liquid crystal layer  12  positioned above the receive antenna but an electric field is not applied to the liquid crystal layer  12  positioned above the transmit antenna. Thus, because external radio waves having the working frequency are reflected by the liquid crystal layer  12  positioned above the transmit antenna and are prevented from reaching the transmit antenna, interference in the transmit antenna due to external radio waves is suppressed. 
     In this manner, according to Embodiment 5, the penetration of radio waves passing through each of the liquid crystal layers  12  positioned above the transmit and receive antennas can be controlled independently. In other words, penetration by external radio waves through the liquid crystal layer  12  above the receive antenna is reduced when the radar antenna  2  is transmitting, and penetration by external radio waves through the liquid crystal layer  12  above the transmit antenna is reduced when the radar antenna  2  is receiving, enabling interference in the radar antenna  2  due to external radio waves to be suppressed. 
     Moreover, in Embodiment 5 above, the two liquid crystal layers  12  are disposed on the same plane, but it is not necessary for the two liquid crystal layers  12  to disposed in the same plane as each other, and the same effects can be achieved if the two liquid crystal layers  12  are disposed on different planes. 
     Furthermore, in Embodiment 5 above, two liquid crystal layers  12  are disposed on a plane, but three or more two liquid crystal layers  12  may also be disposed on a plane. In that case, penetration of radio waves can be independently controlled at three or more positions in the plane. 
     In Embodiment 5 above, the two liquid crystal layers  12  control penetration by radio waves having the same frequency, but the two liquid crystal layers  12  may also control penetration of radio waves having different frequencies. In that case, if the two liquid crystal layers  12  are disposed above two radar antennas  2  each having different working frequencies and the penetration of radio waves having the working frequency of each antenna is controlled, it becomes possible to suppress interference due to external radio waves in the two radar antennas  2 . 
     Furthermore, in Embodiment 5 above, the relative permittivity and thickness of the two liquid crystal layers  12  are selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the controlled state. However, the relative permittivity and thickness of the two liquid crystal layers  12  may also be selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the non-controlled state. Furthermore, the relative permittivity and thickness of one the liquid crystal layers  12  may also be selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the controlled state, the relative permittivity and thickness of the other liquid crystal layer  12  being selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the non-controlled state. 
     Embodiment 6 
     In a radome  10 D according to Embodiment 6, the liquid crystal layer  12  is arranged in a matrix shape as shown in FIGS. 8 and 9. Moreover, the rest of the construction is the same as in Embodiment 1 above. 
     Because the relative permittivity and thickness of the liquid crystal layer  12  are selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the controlled state, the same effects can be achieved by this radome  10 D as in Embodiment 1 above. 
     Furthermore, because the liquid crystal layer  12  in this radome  10 D is arranged in a matrix shape, the liquid crystal layer  12  functions as a polarizer. In other words, by selecting the thickness of the liquid crystal layer  12  and the width and period of the matrix appropriately, a polarity changing function can be added to the radome  10 D, enabling further reduction of interference acting on the radar antenna  2 . 
     Moreover, in Embodiment 6 above, the liquid crystal layer  12  is arranged in a matrix shape, but the liquid crystal layer may also be arranged in a grid shape. In that case, by selecting the thickness of the liquid crystal layer  12  and the width and period of the grid appropriately, a polarity changing function can be added to the radome, achieving the same effect. 
     Furthermore, in Embodiment 6 above, the relative permittivity and thickness of the liquid crystal layer  12  are selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the controlled state, but these may also be selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the non-controlled state. 
     Embodiment 7 
     In Embodiment 1 above, low-molecular-weight liquid crystals are used in the dielectric layer, but in Embodiment 7, liquid crystalline polymers (LCPs) are used in the dielectric layer. 
     As shown in FIGS. 10 and 11, a radome  10 E according to Embodiment 7 includes: a liquid crystal layer  20  composed of liquid crystalline polymers; control electrode layers  13  formed in a frame shape on two surfaces of the liquid crystal layer  20 ; and a power source  9  for applying an electric field to the liquid crystal layer  20  by means of the control electrode layers  13 . The material of the liquid crystal layer  20  is selected such that the relative permittivity of the liquid crystal layer  20  in the controlled state is ∈ rco  and the relative permittivity of the liquid crystal layer  20  in the non-controlled state is ∈ rnc , and the thickness of the liquid crystal layer  20  is selected to satisfy Expression (1) above when in the controlled state (∈ r =∈ rco ). In other words, when the liquid crystal layer  20  is in the controlled state, reflection of radio waves with a free space wavelength λ 0  is reduced in the radome  10 E, permitting radio waves having the frequency used in the radar antenna  2  to pass through with minimal loss. 
     Because the relative permittivity and thickness of the liquid crystal layer  20  are selected so that radio waves having the working frequency of the radar antenna  2  can pass through with minimal loss in the controlled state, the same effects can be achieved by this radome  10 E as in Embodiment 1 above. 
     Furthermore, because the liquid crystal layer  20  in this radome  10 E is composed of liquid crystalline polymers, glass plates  11  are not required, thereby increasing design freedom, reducing the number of component parts, improving productivity, and enabling costs to be lowered compared to Embodiment 1. 
     Now, the liquid crystal layer  20  in Embodiment 7 above replaces the liquid crystal layer  12  in the radome of Embodiment 1, but naturally the same effects can be achieved by applying the liquid crystal layer  20  to the radomes of any of Embodiments 2 to 6. 
     Moreover, each of the above embodiments has been explained using a radar antenna  2  as an example of a radar device, but the radar device is not limited to a radar antenna and may be any transceiver device. 
     In each of the above embodiments, metal electrodes such as copper are used for the control electrode layers  13 , but the control electrode layers  13  are not limited to metal electrodes and may be any conducting material such as tin oxide (SnO 2 ) or indium oxide (In 2 O 3 ), for example. 
     Furthermore, in each of the above embodiments, metal electrodes which reflect and absorb radio waves are used for the control electrode layers  13  and it is necessary to form the control electrode layers  13  into frame or grid shapes to ensure a penetration zone for radio waves, but if a material which does not reflect or absorb radio waves is used, the control electrode layer can be formed over an entire surface of the glass plates  11  or the liquid crystal layer  20 . In that case, because the electric field can be applied uniformly to the liquid crystal layers  12  or  20 , the penetration of radio waves can be made uniform over the entire region of the liquid crystal layers  12  or  20 . 
     In each of the above embodiments, radomes  10  to  10 E are formed in a flat plate shape, but the radomes  10  to  10 E are not limited a flat plate shape and may also be formed in a curved shape appropriate to the mounted position of the radome. 
     Furthermore, in each of Embodiments 1 to 6, the liquid crystal layer  12  is held between a pair of glass plates  11 , but the same effects can be achieved if plastic plate or plastic film is used instead of glass plate  11 . 
     The present invention is constructed in the above manner and exhibits the effects described below. 
     According to one aspect of the present invention, there is provided a radome which has a dielectric layer whose relative permittivity is changed by the application of an electric field, and an electric field applying means for applying the electric field to the dielectric layer, enabling penetration by radio waves obtained from the free space wavelength of the radio waves used with the dielectric layer to be changed by controlling the application of an electric field and changing the relative permittivity of the dielectric layer, thereby providing a radome enabling interference due to external radio waves having a frequency the same as the working frequency of a radar device to be reduced when the radar device is not being used. 
     The dielectric layer may also include a liquid crystal layer, enabling the relative permittivity of the dielectric layer to be easily changed by controlling application of the electric field. 
     A number of liquid crystal layers may also be stacked in a thickness direction, enabling the radio wave penetration to be precisely controlled. 
     A number of liquid crystal layers may also be disposed on a plane, thereby dividing the zone of radio wave penetration and enabling the radio wave penetration of each zone division to be controlled separately. 
     The liquid crystal layer may also be constructed in a grid shape or in a matrix shape, adding a polarity changing function to the radome and enabling interference in the radar device to be further suppressed. 
     The thickness and relative permittivity of the dielectric layer may also be set such that radio waves having a specific frequency pass through when the electric field is being applied, enabling interference due to external radio waves having a frequency the same as the working frequency of the radar device to be reduced when the dielectric layer is in a noncontrolled state. 
     The thickness and relative permittivity of the dielectric layer may also be set such that radio waves having a specific frequency pass through when the electric field is not being applied, enabling interference due to external radio waves having a frequency the same as the working frequency of the radar device to be reduced when the dielectric layer is in a controlled state.