Patent Publication Number: US-11660900-B2

Title: Electromagnetic wave generator, ink dryer, and ink jet printer

Description:
The present application is a continuation of U.S. application Ser. No. 16/913,542, filed Jun. 26, 2020, which is based on, and claims priority from, JP Application No. 2019-121931, filed Jun. 28, 2019, the disclosures of which are hereby incorporated by reference here in their entireties. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an electromagnetic wave generator, an ink dryer, and an ink jet printer. 
     2. Related Art 
     Various types of inkjet recording devices have been developed. For example, a technology for printing on a medium to which ink is unlikely to permeate, such as a film or a metal sheet, has been developed. When the ink is fixed on a medium that does not easily absorb the ink, the ink droplets are allowed to flow on the medium for a while after the ink is attached to the medium, and color mixing between dots or image bleeding is likely to occur. As one of the measures to suppress such a phenomenon, it is conceivable to dry the ink in as short a time as possible after the ink is attached to the medium. 
     As a method of drying the ink, for example, it is conceivable to apply a heated transport roller to the back surface of a medium to dry a film of ink droplets attached to the surface by heat conduction. However, the energy consumed is very large, and it takes time for the heat to be conducted, which is not always the best method. Further, as another method, in a drying device described in JP-A-2017-165000, an attempt has been made to dry ink by applying an AC electric field to the medium and dielectrically heating the attached ink. 
     However, in the device described in JP-A-2017-165000, a grounded conductor rod and conductor rods for applying a high-frequency voltage to both ends thereof are arranged in parallel at a predetermined interval, so that a high-frequency radiation device such as a loop antenna is provided. Such a radiation device radiates an electromagnetic wave in a relatively wide range due to the characteristics of the antenna. Therefore, it is considered that large power is radiated in addition to the power supplied to the ink film to be heated by the radiating device, and that the energy efficiency is low and the radiated electromagnetic wave needs to be shielded. Further, since an electromagnetic wave is uniformly radiated to a printing pattern, in which an area where ink does not exist and an area where ink exists, present intricately, energy efficiency deteriorates. 
     SUMMARY 
     An electromagnetic wave generator according to an aspect of the present disclosure includes: an electromagnetic wave generation section generating an electromagnetic wave; a high-frequency voltage generation section generating a voltage applied to the electromagnetic wave generation section; and a transmission line electrically coupling the electromagnetic wave generation section and the high-frequency voltage generation section to each other, in which the electromagnetic wave generation section includes a first electrode, a second electrode, a first conductor that electrically couples the first electrode and the transmission line to each other, and a second conductor that electrically couples the second electrode and the transmission line to each other, one of the first electrode or the second electrode is a reference potential electrode to which a reference potential is applied and the other is a high-frequency electrode to which a high-frequency voltage is applied, a minimum separation distance between the first electrode and the second electrode is 1/10 or less of a wavelength of an output electromagnetic wave, a minimum separation distance between the first conductor and the second conductor is 1/10 or less of a wavelength of an output electromagnetic wave, and the first conductor further includes a coil, and the coil is closer to the first electrode than the transmission line. 
     The electromagnetic wave generator according to the aspect may have a structure in which one of the first electrode and the second electrode is disposed so as to surround the other of the first electrode and the second electrode in plan view. 
     In the electromagnetic wave generator according to the aspect, the reference potential electrode may continuously surround a periphery of the high-frequency electrode, the high-frequency electrode may be coupled to an inner conductor of a coaxial cable, and the electromagnetic wave generator may have a structure in which the reference potential electrode and an outer conductor of the coaxial cable are coupled via a continuous planar conductor. 
     An ink dryer according to another aspect of the present disclosure includes the electromagnetic wave generator according to any of the above aspects that heats a thin ink film, in which the first electrode and the second electrode have a flat plate shape and are disposed in parallel to the thin ink film. 
     An ink dryer according to another aspect of the generator according to any of the above aspects heats a thin ink film, in which the first electrode and the second electrode extend with respect to the normal line direction of the thin ink film. 
     The ink dryer according to any of the aspects may further include a conductor plate, in which the conductor plate may be disposed in parallel to the thin ink film at a side opposite to the first electrode and the second electrode. 
     An ink jet printer according to another aspect of the present disclosure includes: the ink dryer according to any of the above aspects; a carriage reciprocating in a width direction of a recording medium; and an ink jet head discharging ink, in which the ink dryer and the ink jet head are mounted on the carriage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram showing the vicinity of an electrode of an electromagnetic wave generator according to an embodiment. 
         FIG.  2    is an equivalent circuit diagram of the electromagnetic wave generator according to the embodiment. 
         FIG.  3    shows an electric field density distribution when a coil is disposed near an electrode according to the embodiment. 
         FIG.  4    shows an electric field density distribution when a coil is disposed in a distant place of an electrode according to the embodiment. 
         FIG.  5    is a schematic diagram showing the vicinity of an electrode of the electromagnetic wave generator according to the embodiment. 
         FIG.  6    is a schematic diagram showing the vicinity of an electrode of the electromagnetic wave generator according to the embodiment. 
         FIG.  7    is a schematic diagram of a disposition of a first electrode and a second electrode of an ink dryer with respect to a thin ink film as viewed from the side. 
         FIG.  8    is a schematic diagram showing an aspect in which a thin ink film is disposed between parallel plate electrodes. 
         FIG.  9    is a schematic diagram showing an aspect in which a thin ink film is disposed between the parallel plate electrodes. 
         FIG.  10    shows an example of an equivalent circuit when a thin ink film is disposed between the parallel plate electrodes. 
         FIG.  11    is a schematic diagram of the vicinity of electrodes and a disposition of a conductor plate of the ink dryer according to the embodiment, as viewed from the side. 
         FIG.  12    is a schematic diagram of amain part of an ink jet printer according to the embodiment. 
         FIG.  13    shows s a simulation result of heating of a thin ink film. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     An embodiment of the present disclosure will be described below. The embodiment described below describes an example of the present disclosure. The present disclosure is not limited to the following embodiment at all, and includes various modifications implemented without departing from the spirit of the present disclosure. Note that not all of the configurations described below are essential configurations of the present disclosure. 
     1. Electromagnetic Wave Generator 
     The electromagnetic wave generator of the present embodiment includes an electromagnetic wave generation section that generates an electromagnetic wave, a high-frequency voltage generation section that generates a voltage applied to the electromagnetic wave generation section, and a transmission line for electrically coupling the electromagnetic wave generation section and the high-frequency voltage generation section to each other. The electromagnetic wave generation section includes a first electrode, a second electrode, a first conductor for electrically coupling the first electrode and the transmission line to each other, and a second conductor for electrically coupling the second electrode and the transmission line to each other. Further, the first conductor includes a coil, and the coil is provided at a position closer to the first electrode than the transmission line. 
     Therefore, the electromagnetic wave generator of the present embodiment includes at least a first electrode, a second electrode, and a coil.  FIG.  1    is a schematic diagram showing the vicinity of the electrode of the electromagnetic wave generator  10  according to an embodiment.  FIG.  2    is an equivalent circuit diagram of the electromagnetic wave generator  10 . The electromagnetic wave generator  10  includes an electromagnetic wave generation section including a first electrode  1 , a second electrode  2 , and a coil  3 , a coaxial cable  4  as a transmission line, and a high-frequency source as a high-frequency voltage generation section. 
     The heating energy efficiency of the ink film differs greatly depending on the position where the coil is inserted in series, even when the coil has the same inductance. Therefore, it is desirable to install the coil as close to the electrode as possible. Regarding the coil  3 , the coil  3  may be omitted by giving the electrode itself an inductance by, for example, forming the first electrode or the second electrode in a meander shape. 
     1.1. Electrode 
     The electromagnetic wave generator  10  includes a first electrode  1  and a second electrode  2 . The first electrode  1  and the second electrode  2  have conductivity. A reference potential is applied to one of the first electrode  1  and the second electrode  2 . A high-frequency voltage is applied to the other of the first electrode  1  and the second electrode  2 . The method of selecting the first electrode  1  and the second electrode  2  can be any methods. The reference potential is applied to one of the two electrodes, and a high-frequency voltage is applied to the other. In this specification, an electrode to which a reference potential is applied may be referred to as a “reference potential electrode”, and an electrode to which a high-frequency voltage is applied may be referred to as a “high-frequency electrode”. 
     The reference potential is a constant potential serving as a reference for a high-frequency voltage, and may be, for example, a ground potential. Further, when the high-frequency voltage generation circuit that generates a high-frequency voltage to be input to the electromagnetic wave generator  10  is a differential circuit, there is no distinction between the first electrode  1  and the second electrode  2 . When the frequency of the high-frequency is  1  MHz or more, there is a heating effect. Further, the dielectric loss tangent becomes maximum near the frequency of 20 GHz, and the heating efficiency also becomes maximum. In particular, when heating water such as ink, the bandwidth is desirably 2.0 GHz or more and 3.0 GHz or less, and from a legal viewpoint, a 2.4 GHz bandwidth, which is one of the ISM bandwidth, is desirable, for example, 2.44 GHz or more and 2.45 GHz or less. Further, the higher the high-frequency voltage, the greater the amount of heat supplied to the ink. However, since a high-frequency voltage is transmitted to the electromagnetic wave generator  10  through a transmission line of 50Ω normally, the high-frequency voltage applied to the electromagnetic wave generator  10  is represented as “high-frequency power=V{circumflex over ( )}2/R=V{circumflex over ( )}2/50”. Furthermore, to suppress the amount of heat generated by the internal resistance of the electromagnetic wave generator  10 , the power per electromagnetic wave generator  10  is set to about 10 W, and it is desirable to use a plurality of electromagnetic wave generators  10  to ensure the power required for drying the ink. Further, the ink is heated by dielectric heating due to an electric field generated between the first electrode  1  and the second electrode  2 . The electric field at this time has a value of about 1×10{circumflex over ( )}6 V/m. Further, the ink is heated by dielectric heating due to an electric field generated between the first electrode  1  and the second electrode  2 . At this time, the electric field between the first electrode and the second electrode has a value of about 1×10{circumflex over ( )}6 V/m by the effect of the coil  3  or the distance between the electrodes. 
     The application of the high-frequency voltage means that the central portion of a surface of the first electrode  1  or the second electrode  2  opposite to a surface facing the ink is set to a feeding point, and the power of the above described high-frequency voltage is supplied to this feeding point. Incidentally, as shown in  FIG.  6   , which will be described later, a coating portion of the coaxial cable may be connected to the electrode with a metal surface. 
     In the illustrated example, the first electrode  1  and the second electrode  2  have a flat plate shape. The plane shape of the first electrode  1  and the second electrode  2  can be any shapes, and may be, for example, a square, a rectangle, a circle, or a combination of these shapes. In the illustrated example, the first electrode  1  and the second electrode  2  have a substantially square shape in plan view. The plane size of the first electrode  1  and the second electrode  2  is 0.01 cm 2  or more and 100.0 cm 2  or less, desirably 0.1 cm 2  or more and 10.0 cm 2  or less, more desirably 0.5 cm 2  or more and 2.0 cm 2  or less, and further desirably 0.5 cm 2  or more and 1.0 cm 2  or less on one electrode, as an area in plan view. The areas of the first electrode  1  and the second electrode  2  in plan view may be the same or different. The plan view refers to a state viewed from the z direction in  FIG.  1   . 
     It is desirable that the first electrode  1  and the second electrode  2  are disposed so as not to overlap with each other in plan view. In the illustrated example, the first electrode  1  and the second electrode are disposed in parallel on the same plane. With such a disposition, a predetermined electromagnetic wave can be generated efficiently. The shapes and dispositions of the first electrode  1  and the second electrode  2  will be further described later. The details of the generated electromagnetic waves will be described later. 
     The first electrode  1  and the second electrode  2  are formed of a conductor. Examples of the conductor include metals, alloys, and conductive oxides. The first electrode  1  and the second electrode  2  may be the same material or different materials. The first electrode  1  and the second electrode  2  may be appropriately formed by selecting the thickness or strength so that the first electrode  1  and the second electrode  2  can be self-supporting, or can be formed on a surface of a substrate or the like made of a material (not shown) having a low dielectric loss tangent that transmits electromagnetic waves when it is difficult to maintain the strength of the first electrode  1  and the second electrode  2 . 
     Each of the first electrode  1  and the second electrode  2  are electrically coupled to a coaxial cable  4  coupled to the high-frequency source via an inner conductor  4   a  and an outer conductor  4   b , as schematically shown in  FIG.  1   . The inner conductor  4   a  and the outer conductor  4   b  are disposed on a surface opposite to the surface facing the thin ink film of the first electrode  1  and the second electrode  2 . In other words, the first electrode  1  and the second electrode  2  are disposed closer to the thin ink film than the inner conductor  4   a  and the outer conductor  4   b . 
     1.2. Electrode Interval 
     The minimum separation distance d between the first electrode  1  and the second electrode  2  is 1/10 or less of the wavelength of the electromagnetic wave output from the electromagnetic wave generator  10 . For example, when the frequency of the electromagnetic wave output from the electromagnetic wave generator  10  is2.45 GHz, the wavelength of the high-frequency is substantially 12.2 cm, and in this case, the minimum separation distance between the first electrode  1  and the second electrode  2  is substantially 1.22 cm or less. In the example in  FIG.  1   , the coil  3  is provided on the inner conductor  4   a . The distance between the coil  3  and the first electrode  1  in the transmission line of the inner conductor  4   a  is desirably shorter than the distance between the coil  3  and the coaxial cable  4 . Normally, the coil  3  is coupled only to the first electrode, but can be coupled only to the second electrode or to both the first electrode and the second electrode. 
     By setting the minimum separation distance d between the first electrode  1  and the second electrode  2  to be 1/10 or less of the wavelength of the output electromagnetic wave, most of the electromagnetic waves can be attenuated near the first electrode  1  and the second electrode  2 . Thereby, the intensity of the electromagnetic wave reaching the distant place from the first electrode  1  and the second electrode  2  can be reduced. 
     That is, the electromagnetic wave radiated from the electromagnetic wave generator  10  is very strong near the first electrode  1  and the second electrode  2  and very weak far from the first electrode  1  and the second electrode  2 . In this specification, an electromagnetic field generated by the electromagnetic wave generator  10  near the first electrode  1  and the second electrode  2  may be referred to as a “near electromagnetic field”. Further, in this specification, an electromagnetic field generated by a general antenna (antenna) for transmitting electromagnetic waves to a distant place may be referred to as a “distant electromagnetic field”. Note that the boundary between the near and far distances is a position separated from the electromagnetic wave generator  10  by substantially ⅙ of the wavelength of the generated electromagnetic wave. 
     The electromagnetic wave generator  10  is used for applications such as televisions and mobile phones, and is not an electromagnetic wave generator that transmits electromagnetic waves at intervals of m units. Instead, the electromagnetic wave generator  10  is an electromagnetic wave generator in which during the transmission of the distance of ⅙ of the wavelength of the generated electromagnetic wave, the electric field density of the electromagnetic wave is attenuated to 30% or less of the electric field density between the first electrode  1  and the second electrode  2 . That is, the electromagnetic wave generator  10  is not suitable for a communication. Furthermore, since the electromagnetic wave generated by the electromagnetic wave generator  10  has a high attenuation rate, the range of the electric field is suppressed. Therefore, unnecessary radiation hardly occurs in an area farther from the device than the distance of substantially the wavelength of the generated electromagnetic wave. Therefore, it is unnecessary or easy to comply with regulations by the Radio Law and the like, and even when compliance is required, it is possible to reduce the scattering of electromagnetic waves around the electromagnetic wave generator by a simple electromagnetic wave shield or the like. Such properties of the electromagnetic wave generator  10  are caused by the small size of the electrodes, the short distance between the electrodes, the difficulty of resonance, and the like. 
     In other words, the electromagnetic wave generator  10  of the present embodiment is not a device for generating a distant electromagnetic field such as a dipole antenna, but is equivalent to a slot antenna where the negative/positive is inverted with respect to the dipole antenna and the slot width is made sufficiently small with respect to the wavelength to make it difficult to generate distant electromagnetic fields. The present structure only generates an electric field like a capacitor, and this electric field does not generate a magnetic field as a secondary matter. Therefore, a so-called distant electromagnetic field in which an electric field and a magnetic field are generated in a chain and an electromagnetic wave is transmitted to a distant place is not generated. 
     1.3. Coil 
     The electromagnetic wave generator  10  includes a coil  3 , and the coil  3  is coupled to the first electrode  1  or the second electrode  2  in series. The first electrode  1  or the second electrode  2  is coupled to a path to which a high-frequency voltage is applied via the coil  3 . 
     The coil  3  is installed for three purposes: matching, increasing an electric field generated between electrodes, and enhancing by adding an electric field generated by a coil to an electric field generated between electrodes. 
     Role of Coil ( 1 ): Matching 
     Generally, a voltage applied to an antenna is transmitted to the antenna via a coaxial cable (for example, a characteristic impedance of 50Ω). It is desirable that the impedance of the antenna matches the impedance of the high-frequency voltage generation circuit or the impedance of the coaxial cable transmitted from the circuit to the antenna. By matching or approaching the impedance of the antenna to the impedance of a cable or the like, the energy transmission efficiency is improved. Conversely, when a high-frequency voltage of a sine wave is input to the antenna and the impedance of the high-frequency voltage generation circuit does not match the impedance of the antenna, signal reflection occurs at a discontinuous place of impedance, and it is difficult to input a signal to the antenna. Therefore, at the coupling place between the antenna and the coaxial cable where impedance discontinuity is likely to occur, a matching circuit constituted by a coil and a capacitor is inserted, the impedance of the antenna is adjusted, and the energy transmission efficiency improvement is performed between the inner conductor of the coaxial cable and the electrode of the antenna, or between the outer conductor and the electrode of the antenna. The coaxial cable is normally 50Ω, and the matching circuit is adjusted so that the antenna also has 50Ω. If the coaxial cable has an imaginary impedance, the antenna is adjusted to an imaginary impedance conjugate to the imaginary impedance. Such a coil is called a so-called matching coil. 
     Role of Coil ( 2 ): Increasing Electric Field Density Between Electrodes 
       FIG.  2    is an equivalent circuit of the ink dryer. The electromagnetic wave generation circuit A corresponds to the electromagnetic wave generator  10 . The capacitor C of the electromagnetic wave generation circuit A corresponds to the first electrode  1  and the second electrode  2 , and the resistance R of the electromagnetic wave generation circuit A corresponds to the radiation resistance of the radiated electromagnetic wave. The high-frequency source corresponds to the high-frequency voltage generation circuit B, and the resistance R of the high-frequency voltage generation circuit B is an internal resistance of the high-frequency voltage source. The coil L inserted between the high-frequency voltage generation circuit B and the electromagnetic wave generation circuit A corresponds to the coil  3  coupled to the first electrode  1  or the second electrode  2  in series. 
     As described above, since the electromagnetic wave generating circuit A includes the capacitor C, a specific resonance frequency can be obtained by coupling the coil L so as to be in series with the capacitor C. Further, when the inductance of the coil L is increased and the capacitance of the capacitor C is reduced as much as possible, the transmission efficiency is improved. The inductance of the coil L and the capacity of the capacitor C are appropriately designed. 
     The radiation resistance is smaller (for example, substantially 7Ω) than the impedance of the coaxial cable (for example, 50Ω), and the capacity of the capacitor C apparently formed by the first electrode  1  and the second electrode  2  is, for example, substantially 0.5 pF. 
     In the electromagnetic wave generator  10  including a first electrode  1  and a second electrode  2  each having a plane shape of 5 mm×5 mm square and a minimum separation distance of 5 mm, and a 10 nH coil L coupled to the second electrode  2  in series, when a voltage of 1V is generated from the high-frequency voltage generation circuit B as shown in  FIG.  2   , it is known from a simulation that the voltage applied to the antenna terminal (the voltage applied between the point on the L side of C and GND) is substantially 2 V. The resistance R indicates the radiation resistance of the antenna. Further, the higher the inductance of the coil, the higher the voltage applied to the antenna. By thus inserting a coil in series between the antenna constituted by the first electrode  1  and the second electrode  2  and the coaxial cable, the voltage between the antenna electrodes can be increased. Thereby, the electric field between the first electrode  1  and the second electrode  2  becomes stronger. The stronger the electric field applied to the ink, the more efficiently the ink is heated. 
     Role of Coil ( 3 ): Adding an Electric Field Generated by a Coil to an Electric Field Generated Between Electrodes to Enhance the Electric Field 
     The coil  3  is typically configured as a winding of a long electric wire of metal such as copper, which has an internal resistance as well as an inductance component. For example, when the inductance component is substantially 30 nH, the internal resistance is normally substantially 3Ω. Due to the inductance and the internal resistance, a potential difference is generated at both ends of the coil, and an electric field is generated at a place where the potential difference exists.  FIG.  3    shows the results of a simulation of the electric field density distribution when the coil  3  is disposed in contact with the first electrode, and  FIG.  4    shows the results of simulation of the electric field density distribution when the coil  3  is separated from the first electrode by substantially 4 mm. The electric field density in  FIGS.  3  and  4    represent a higher value as the color approaches black to white. When a coil is installed in the immediate vicinity of the first electrode  1  as shown in  FIG.  3   , all the voltage increased by the coil  3  is applied to the first electrode, and a strong electric field is generated near the first electrode  1 . Furthermore, when the direction of the electric field of the coil  3  and the direction of the electric field generated between the first electrode  1  and the second electrode  2  match, the electric field generated in the coil  3  overlaps with the electric field generated between the first electrode and the second electrode, thereby the electric field near the first electrode  1  is made stronger. In contrast to this, when the coil  3  in  FIG.  4    is separated from the first electrode, the increased voltage by the coil  3  is applied to the conductor  32  and the first electrode  1 , and the electric field cannot be concentrated near the first electrode  1  where a strong electric field is required. At the same time, a strong unnecessary electric field is generated around the coil  3  distant from the first electrode  1  which does not require a strong electric field. In the structure shown in  FIG.  3    and the structure shown in  FIG.  4   , in this example, the heating efficiency of the thin ink film T is 70% in the former case and substantially 8% in the latter case, thereby big difference occurs, and it is more effective to dispose the coil  3  as close as possible to the first electrode  1 . For this purpose, it is possible to make the shape of the first electrode, for example, a meander shape to make the first electrode itself a coil, and omit the coil  3 . 
     1.4. Variation of Disposition and Structure of Electrode 
     The electromagnetic wave generator may have a structure in which one of the first electrode  1  and the second electrode  2  is disposed so as to surround the other, as the electromagnetic wave generator  12  shown in  FIG.  5   .  FIG.  5    is a schematic diagram showing the vicinity of the electrodes of the electromagnetic wave generator  12 . The electromagnetic wave generator  12  has a structure in which the second electrode  2  is disposed so as to surround the first electrode  1 . 
     The first electrode  1  of the electromagnetic wave generator  12  has a square shape in plan view. In the electromagnetic wave generator  12 , the second electrode  2  is disposed in a hollow square shape so that the second electrode  2  surrounds the first electrode  1  in plan view. Although not shown, the first electrode  1  may have a circular shape in plan view, the second electrode  2  may have an annular shape in plan view, or a hexagonal outer periphery. The plane or spatial disposition of the first electrode  1  and the second electrode  2 , and the coil  3  are the same as those of the above-described electromagnetic wave generator  10 , and thus the description will be simplified. 
     In the electromagnetic wave generator  12 , a high-frequency potential and a reference potential are fed to the rectangular first electrode  1  disposed at the center in plan view and a hollow rectangular shape (frame shape) second electrode  2  surrounding the first electrode  1 , respectively. The coil  3  is inserted between the first electrode  1  and the inner conductor  4   a  of the coaxial cable  4 , and it is important that the coil  3  is positioned as close to the first electrode  1  as possible. 
     In the electromagnetic wave generator  12 , when the second electrode  2  is a hollow rectangular shape in plan view, the length of one side of the outer periphery is, for example, 0.1 cm or more and 10.0 cm or less, desirably 0.3 cm or more and 5.0 cm or less, and more desirably 0.4 cm or more and 1.0 cm or less. Further, in this case, the width of the second electrode  2  in plan view is 1.0 mm or more and 2.0 mm or less, desirably 1.4 mm or more and 1.6 mm or less, and more desirably substantially 1.5 mm. 
     In the electromagnetic wave generator  12 , the minimum separation distance d between the first electrode  1  and the second electrode  2  is 1/10 or less of the wavelength of the electromagnetic wave output from the electromagnetic wave generator  12 . 
     As the electromagnetic wave generator  14  shown in  FIG.  6   , the electromagnetic wave generator may have a structure in which one electrode continuously surrounds the other electrode, the other electrode is coupled to the inner conductor of the coaxial cable, one electrode and an outer conductor of a coaxial cable are coupled via a continuous conductor.  FIG.  6    is a schematic diagram showing the vicinity of the electrodes of the electromagnetic wave generator  14 . In the electromagnetic wave generator  14 , the inner conductor  4   a  of the coaxial cable  4  is coupled to the first electrode  1  via the columnar conductor  32 , and the outer conductor  4   b  of the coaxial cable  4  is coupled to the second electrode  2  via the continuous conductor  30  surrounding the conductor  32 . 
     The plane shape and disposition of the first electrode  1  and the second electrode  2  of the electromagnetic wave generator  14  are the same as those of the electromagnetic wave generator  12 . 
     In the electromagnetic wave generator  14 , the minimum separation distance d between the first electrode  1  and the second electrode  2  is 1/10 or less of the wavelength of the electromagnetic wave output from the electromagnetic wave generator  12 . 
     Although not shown, in the electromagnetic wave generator  14 , the conductor  30  maybe integral with the second electrode  2 . In this case, the conductor  30  becomes the second electrode  2 . Similarly, the first electrode  1  of the electromagnetic wave generator  14  may be integrated with the columnar conductor  32 . In this case, the conductor  32  becomes the first electrode  1 . Further, the coupling may be made without the inner conductor  4   a  of the coaxial cable  4  and the conductor  32 . In this case, the inner conductor  4   a  becomes the first electrode  1 . 
     In the electromagnetic wave generator  14 , when it is a structure that the second electrode  2  is set as a reference potential electrode, the first electrode  1  is set as a high-frequency electrode, the high-frequency electrode is coupled to the inner conductor  4   a  of the coaxial cable  4 , and the reference potential electrode and the outer conductor  4   b  of the coaxial cable  4  are coupled via a continuous conductor, the electromagnetic wave generator  14  becomes a structure similar to a coaxial cable. Therefore, the manufacturing becomes easier. Further, in the electromagnetic wave generator  14 , the heating efficiency of the thin ink film described later is improved. 
     Furthermore, in the electromagnetic wave generator  14 , when it is a structure that the second electrode  2  is set as a reference potential electrode, the first electrode  1  is set as a high-frequency electrode, the high-frequency electrode is coupled to the inner conductor  4   a  of the coaxial cable  4 , and the reference potential electrode and the outer conductor  4   b  of the coaxial cable  4  are coupled via a continuous conductor, a shield effect by the reference potential electrode is obtained and the electromagnetic wave is less likely to leak outside the reference potential electrode. With such a structure, a transmission mode is formed near the electrode, so that a target object (for example, a thin ink film described later) can be sufficiently irradiated with the electromagnetic wave even when an interval from the target object to be irradiated with the electromagnetic wave is large. That is, with such a structure, it is possible to make the electromagnetic waves generated from the device have directivity and to extend a reaching distance of the nearby electromagnetic field. 
     In the electromagnetic wave generator  14 , it has been found that the width w of the second electrode  2  in plan view affects the heating efficiency of the thin ink film described later. The width w of the second electrode  2  in plan view is 1.0 mm or more and 2.0 mm or less, desirably 1.4 mm or more and 1.6 mm or less, and more desirably substantially 1.5 mm from the viewpoint of increasing the heating efficiency. Further, it has been found that the plane shape of the first electrode  1  also affects the heating efficiency. A rectangular shape (not shown) increases the heating efficiency as compared with a square shape as shown in the figure, for example, when the rectangular shape is 0.5 mm×5.0 mm, the heating efficiency is further improved. 
     In each of the electromagnetic wave generators  12  and  14 , the minimum separation distance d between the first electrode  1  and the second electrode  2  is 1/10 or less of the wavelength of the output electromagnetic wave, and since the coil  3  is coupled to the second electrode  2  in series, an electromagnetic field can be efficiently generated near the device. 
     1.5. High-Frequency Source 
     The electromagnetic wave generator according to the present embodiment includes a high-frequency source. The high-frequency source includes the high-frequency voltage generation circuit B described above. The high-frequency source generates a high-frequency voltage applied to the first electrode  1  and the second electrode  2 . The high-frequency source includes, for example, a quartz crystal oscillator, a phase locked loop (PLL) circuit, and a power amplifier. The high-frequency power generated by the high-frequency source is supplied to the first electrode  1  and the second electrode  2  via, for example, a coaxial cable. 
     The basic peripheral circuit configuration of the electromagnetic wave generator of the present embodiment is such that a high-frequency signal generated by a PLL is amplified by a power amplifier and fed to the first electrode  1  and the second electrode  2 . When a large number of sets of the first electrode  1  and the second electrode  2  is used, for example, one power amplifier may be used for one set of the first electrode  1  and the second electrode  2 , and electromagnetic waves may be individually generated by dividing the output of the PLL and transmitting the output to the power amplifier. Further, a plurality of power amplifiers may be used, and in such a case, the amplification factor of each power amplifier can be individually controlled more easily. 
     2. Ink Dryer 
     The electromagnetic wave generator of the above embodiment can be used as an ink dryer. The ink dryer is the above-described electromagnetic wave generator, in which the first electrode and the second electrode  2  are disposed in parallel with respect to the thin ink film, and by applying a high-frequency voltage, the thin ink film can be heated very efficiently. 
       FIG.  7    is a schematic diagram of a disposition of the first electrode  1  and the second electrode  2  of the ink dryer  10  of the present embodiment with respect to the thin ink film T as viewed from the side. Since the ink dryer  10  is the same as the above-described electromagnetic wave generator  10 , the same reference numerals as in the above description are assigned and the duplicated description is omitted. 
     2.1. Thin Ink Film 
     The thin ink film dried by the ink dryer  10  may be a thin film obtained by attaching ink to a sheet such as paper or a film, a thin film obtained by attaching ink to a surface of a molded body having a three-dimensional shape or the like. The method for attaching the ink is not particularly limited, but may be an ink jet method, a spray method, a coating method using a brush, or the like. In the illustrated example, a thin ink film T formed by attaching ink on one side of a recording medium M using the ink jet method is illustrated. 
     The thickness of the thin ink film T is, for example, 0.01 μm or more and 100.0 μm or less, desirably 1.0 μm or more and 10.0 μm or less. Various components may be contained in the ink, and examples of components to be dried by the ink dryer  10  include water and an organic solvent. When the frequency of the electromagnetic wave radiated by the ink dryer  10  is from substantially 1 MHz to 30 GHz, since water can be efficiently heated and dried, the ink desirably contains water. As a frequency actually used, 2.45 GHz used in a microwave oven has a clear legal standard and is easy to use. 
     When the thin ink film T is irradiated with an electromagnetic wave, the water in the ink is heated. The main principle of the heating is frictional heat due to the vibration of water molecules due to dielectric heating and/or Joule heat due to eddy current generated in the water. When the ink is an ink having a high ion concentration, such as dye ink, conductivity is generated, so that the effect of heating by Joule heat increases. In the ink dryer  10  of the present embodiment, since a vibration electric field is easily applied in parallel to the thin ink film T, when the ink is water-based, both heating principles can be used. 
     2.2. Heating Mechanism 
     It is known that when electromagnetic waves (3 GHz) are incident on the surface of the water, although it depends on the temperature, the depth reached by the electromagnetic wave is substantially 1.2 cm at 20° C. This depth is called the skin depth. As described above, the thickness of the thin ink film is extremely thin as compared with the penetration depth of the electromagnetic wave. Therefore, it can be assumed that when the thin ink film is irradiated with the electromagnetic wave perpendicularly, almost all electromagnetic waves penetrate, and water in the thin ink film can hardly be heated, or even when it can be heated, the efficiency becomes very poor. 
     According to a preliminary experiment conducted by the inventor, it has been found that even when a heating operation is performed with a sheet having the ink attached thereto in a microwave oven (microwave oven), the ink can hardly be heated. It is considered that the reason is that, the power, among the power of the electromagnetic waves with which the thin ink film is irradiated, that turns into heat inside the ink is very low by the electromagnetic wave penetrating the ink thin film. 
     As described above, the electromagnetic wave generator of the present embodiment generates a near electromagnetic field. That is, by disposing the thin ink film to the ink dryer at an appropriate distance, it is possible to irradiate in a narrow range around the thin ink film with the electromagnetic waves with concentration. Since the electromagnetic wave generated from the ink dryer of the present embodiment presents only in a nearby narrow space and has a very weak distant electromagnetic field, energy is less dissipated, and by appropriately disposing the thin ink film in the area where electromagnetic waves present, the thin ink film can be heated very efficiently. 
     The mechanism of heating the thin ink film T by the ink dryer  10  will be described below.  FIGS.  8  and  9    are schematic diagrams showing a mode in which the thin ink film T is disposed between the parallel plate electrodes E.  FIG.  10    is an example of an equivalent circuit when the thin ink film T is disposed between the parallel plate electrodes E. 
     As shown in  FIG.  8   , when the thin ink film T is provided between the parallel plate electrodes E in parallel with the electrodes, even when a high-frequency voltage is applied to the parallel plate electrode E, the energy absorbed by the thin ink film T is very small. However, as shown in  FIG.  9   , when the thin ink film T is provided between the parallel plate electrodes E and perpendicular to the electrodes, the thin ink film T is heated very efficiently. Even with a thin ink film having the same volume and the same thickness, the heating efficiency can be increased  100  times by changing the direction of the thin ink film surface from horizontal to vertical with respect to the electrode. 
       FIG.  10    shows an equivalent circuit in the disposition shown in  FIG.  9   . As shown in  FIG.  10   , when the thin ink film T is provided between the parallel plate electrodes E and perpendicular to the electrodes, it is considered that this is equivalent to a circuit in which a capacitor CW where a space between the electrodes is filled with water and a capacitor CA where a space between the electrodes is filled with air are coupled in parallel. When a high-frequency voltage is applied in this circuit, the current and the electric field concentrate on the capacitor CW because the capacity of the capacitor CW filled with water between the electrode plates is larger. When the thin ink film T is made parallel to the direction of the electric field, the effect of improving the efficiency by increasing the length in the direction of the electric field by the parallel plate electrode E and the effect of concentrating the electric field are obtained, and the thin ink film can be heated very efficiently. 
     When the electric field is applied in parallel to the thin ink film T, the heating efficiency of the thin ink film T is improved. Therefore, it is desirable that the direction of the electric field is as parallel as possible to the thin ink film T, and in the ink dryer  10  of the present embodiment, the first electrode  1  and the second electrode  2  having a structure capable of applying such an electric field are adopted. Further, as the electric field of the electromagnetic wave with which the thin ink film T is irradiated increases, the amount of heat generated by the thin ink film T increases. Since the electric field increases as the potential difference between the electrodes increases, the amount of heat generated can be increased by increasing the potential difference by the coil  3  as described above. The coil  3  has an effect of impedance matching in addition to the effect of increasing the potential difference. Further, since the coil  3  itself generates an electric field, the coil  3  is disposed near the first electrode  1  or the second electrode  2 , and the electric field generated by the coil  3  is added to the electric field generated between the electrodes to enhance the electric field and improve the heating efficiency. 
     2.3. Disposition of Electrode 
     The first electrode  1  and the second electrode  2  may be disposed perpendicular to the thin ink film T. For example, in the above-described electromagnetic wave generator  14 , when the conductor  32  and the first electrode  1  are integrally formed and the conductor  30  and the second electrode  2  are integrally formed, the first electrode  1  becomes a columnar electrode, the second electrode  2  becomes a cylindrical electrode, and the extending direction becomes a direction of a normal line of the thin ink film T. In this case, when the electromagnetic wave generator  14  is installed so as to face the thin ink film T, the first electrode  1  and the second electrode  2  are disposed with respect to the thin ink film T in a posture in which the extending direction extends in a direction perpendicular to the surface where the thin ink film T spreads. Even with such a disposition, the thin ink film T can be efficiently heated. 
     2.4. Conductor Plate 
     The ink dryer of the present embodiment may include a conductor plate.  FIG.  11    is a schematic diagram of the vicinity of the electrodes of the ink dryer  16  provided with the conductor plate  5  and the disposition of the conductor plate as viewed from the side. The conductor plate  5  is disposed in parallel to the thin ink film T at a side opposite to the first electrode  1  and the second electrode  2 . The conductor plate  5  is disposed at a position overlapping the first electrode  1  and the second electrode  2  in plan view. The thickness and plane size of the conductor plate  5  are not particularly limited. 
     The conductor plate  5  has conductivity. The conductor plate  5  is disposed to face the first electrode  1  and the second electrode  2  via the thin ink film T, and thus it is possible to suppress a change in impedance of the ink dryer  16  due to the thin ink film T. The above-described ink dryer  10  having no conductor plate  5  transmits energy to the thin ink film T very efficiently, and this may cause the thin ink film T to be electrically coupled to such an extent that it can be considered as a part of the ink dryer  10 . In such a case, the impedance of the ink dryer  10  changes depending on the thickness, volume, conductivity, and the like of the thin ink film T. 
     The ink dryer  16  can suppress such a change in impedance by disposing the conductor plate  5 . Further, by disposing the conductor plate  5 , the energy may be transmitted to the thin ink film T more efficiently. 
     Regarding the conductor plate  5 , for example, when the ink dryer  16  is provided in an ink jet printer, the platen can be formed of a conductive material and set as the conductor plate  5 . 
     2.5. Operation Effect 
     According to the ink dryer of the present embodiment, the heating efficiency, that is, the ratio of the power, among the high-frequency power input to the antenna, used for increasing the temperature of the ink can be increased to  80 % or more. According to the ink dryer of the present embodiment, the generated electromagnetic waves can be present only in a very limited area around the thin ink film. Thereby, the heating efficiency of the thin ink film is very good. 
     Since the ink dryer of the present embodiment uses a small electromagnetic wave generator having a minimum separation distance of 1/10 or less of the wavelength of the electromagnetic wave, the ink dryer can be used with saving the power and a simple shield can be used even when it becomes necessary to suppress the scattering of electromagnetic waves. Further, since the power is saved, a circuit for generating a high-frequency voltage can be downsized. 
     Since the ink dryer of the present embodiment utilizes the near electromagnetic field, it is possible to suppress the propagation of the energy to an object such as a sheet on which the thin ink film is attached. Therefore, for example, even when the sheet is made of a material that is affected by the temperature, the sheet is not easily heated, so that the deterioration of the sheet can be suppressed. 
     3. Ink Jet Printer 
     The ink jet printer of the present embodiment includes the above-described ink dryer, a carriage that reciprocates a recording medium in the width direction, and an ink jet head that discharges ink, and the ink dryer and the ink jet head are mounted on the carriage.  FIG.  12    is a schematic diagram of a main part of the ink jet printer  200  of the present embodiment.  FIG.  12    shows a carriage  50  and a recording medium M. The ink jet printer  200  includes an ink dryer  10  and the carriage  50 . 
     The inkjet printer  200  includes an ink jet head  60  on the carriage  50  and a plurality of ink dryers  10 . A first electrode  1 , a second electrode  2 , and a coaxial cable  4  of the ink dryer  10  are mounted on the carriage  50 . Although not shown, the ink jet printer  200  includes a high-frequency source for driving each of the ink dryers  10 . Further, although not shown, the plurality of ink dryers  10  are arranged so as to cover an area equal to or longer than the length of a nozzle row of the ink jet head  60  in a moving direction SS of the recording medium M. The ink jet printer  200  is a serial type printer, and has a mechanism for moving the recording medium M and a mechanism for performing a reciprocation operation on the carriage  50 . 
     The inkjet printer  200  forms a predetermined image on the recording medium M by repeating moving and disposing the recording medium M at a predetermined position and a plurality of times, and discharging ink from the inkjet head  60  while scanning the carriage  50  in a direction intersecting the moving direction SS of the recording medium M and attaching the ink to a predetermined position on the recording medium M with a predetermined amount, a plurality of times. 
     The ink dryer  10  is arranged in the carriage  50  on one side or both sides of the ink jet head  60  in the scanning direction MS of the carriage  50 . In the illustrated example, a plurality of ink dryers  10  are arranged on both sides of the ink jet head  60  in the scanning direction MS. With this arrangement, the ink discharged from the ink jet head  60  and attached to the recording medium M to form a thin ink film can be dried quickly in a short time after a lapse of time in accordance with a moving speed of the carriage  50  and a distance from the nozzle of the ink jet head  60  to the ink dryer  10  in the scanning direction MS. 
     In  FIG.  12   , the ink dryers  10  are arranged in four rows on both sides of the ink jet head  60  in the scanning direction MS of the carriage  50 . This is because, under the condition that 9 W of high-frequency power is input to the ink dryer  10  for drying the thin ink film 1/20 second is required, whereas the time required for the 5 mm ink dryer  10  to pass a specific coordinate at 1 m/s is 1/200 second, which is short of 1/20 second. The ink heating range of the 5 mm ink dryer  10  is set to 12.5 mm×12.5 mm, and by arranging four of ink dryers  10 , the range of 50 mm×50 mm can be heated simultaneously. Since it takes 1/20 second for the 50 mm ink dryer  10  to pass the specific coordinates, the time required for drying can be secured. 
     In  FIG.  12   , the ink dryer  10  are arranged in five rows in a direction perpendicular to the scanning direction MS of the carriage  50 . This is because the nozzle row of the ink jet head  60  has a length, and one ink dryer  10  of 5 mm×5 mm cannot cover the length. The length of the nozzle row is set to 70 mm, and the length is covered by arranging five ink dryers. 
     The ink jet printer  200  of the present embodiment is particularly effective when the recording medium M is made of a material such as a film to which the ink does not soak or hardly soaks. However, even with a recording medium M that absorbs ink such as paper, a sufficient drying effect can be obtained. 
     Although the serial type ink jet printer  200  has been described as an example, the ink dryer can be applied to a line type ink jet printer. In the case of a line type ink jet printer, an ink dryer is disposed downstream of the line type ink jet head in a direction in which the recording medium flows. 
     3. Experimental Example 
     Hereinafter, the present disclosure will be further described with reference to experimental examples, but the present disclosure is not limited to the following examples. 
     A simulation of the heating state of the thin ink film by the ink dryer having a structure of the electromagnetic wave generator  14  described above is performed.  FIG.  13    shows the results of the electromagnetic field simulation. The electromagnetic field simulation is performed using HFSS software. 
     In the electromagnetic field simulation, a second electrode, which has a cubic outer shape with a side of 5 mm, and a hollow and open lower surface, is used, and the thickness of the side surface of the second electrode is set to 0.1 mm. The first electrode is disposed at the center of the second electrode, and has a rectangular (1 mm×1 mm) plate shape in plan view. The thin ink film has a sufficiently large area and a thickness is set to 5 μm. The distance between the upper surface of the thin ink film and the lower surface of the electrode is set to 2 mm. Furthermore, a conductor plate having a sufficiently large area is disposed on the lower surface side of the thin ink film. 
     A coil having an inductance of 25 nH is coupled in series with the high-frequency electrode (first electrode  1 ) in the ink dryer. The frequency of the high-frequency voltage is set to 2.45 GHz. The feeding power is set to 1 W. 
       FIG.  13    shows a distribution of a temperature rise in the thin ink film. In  FIG.  13   , the outlines of the first electrode and the second electrode are drawn with broken lines. The part where the temperature rise is large is shown in white. Note that, although the outer peripheral part of the figure is shown in white, there is no temperature rise. As shown in  FIG.  13   , according to the ink dryer, it is found that the vicinity of the electrodes can be sufficiently heated. 
     The present disclosure is not limited to the embodiments described above, and various modifications are possible. For example, the present disclosure includes substantially the same configurations, for example, configurations having the same functions, methods, and results, or configurations having the same objects and effects, as the configurations described in the embodiments. Further, the present disclosure includes a configuration obtained by replacing non-essential portions in the configurations described in the embodiments. Further, the present disclosure includes a configuration that exhibits the same operational effects as those of the configurations described in the embodiments or a configuration capable of achieving the same objects. The present disclosure includes a configuration obtained by adding the configurations described in the embodiments to known techniques.