Patent Publication Number: US-2006017607-A1

Title: Amplitude modulator, selector switch, high frequency transmitting/receiving apparatus including the same, and radar apparatus, and radar apparatus-mounting vehicle and radar apparatus-mounting small ship

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an amplitude modulator for use in a millimeter wave integrated circuit, a millimeter wave radar module or the like, and more specifically, to an amplitude modulator in which modulator characteristics can be tuned with a variable resistor which is provided in a bias supply circuit of a high frequency modulation element which is a component of the amplitude modulator, and to a high frequency transmission/receiving apparatus having the amplitude modulator.  
      Furthermore, the present invention relates to a selector switch for use in a millimeter wave integrated circuit, a millimeter radar module or the like, and more specifically, relates to a selector switch whose transmission characteristics can be tuned by a variable resistor which is provided in a bias supply circuit of a PIN diode which is a component of the selector switch and a high frequency transmitting/receiving apparatus having the selector switch.  
      Furthermore, the present invention also relates to a radar apparatus having the high frequency transmitting/receiving apparatus, a vehicle equipped with the radar apparatus, and a small ship equipped with the radar apparatus.  
      2. Description of the Related Art  
      Examples of related-art amplitude modulators are those disclosed in “NRD guide high speed ASK modulator using Schottky barrier diode” by Futoshi Kuroki, Kengo Ikeda, and Tsutomu Yoneyama, in Proceedings of the General Conference of Electronic Information Communications Society, 1997, Incorporated Association Electronic Information Communications Society, published on Mar. 6, 1997, Vol.1, C-2-65, p.120; Japanese Unexamined Patent Publication JP-A 10-270944 (1998); and U.S. Pat. No. 6,034,574. For example, in the amplitude modulators disclosed in JP-A 10-270944 and U.S. Pat. No. 6,034,574, a detecting diode is provided at an end portion of a high frequency transmission line that transmits high frequency signals, and a signal source for controlling a bias, a high frequency choke and a resistor are connected to this detecting diode. In such an amplitude modulator, it is known that as a high frequency transmission line, a nonradiative dielectric waveguide (hereinafter, referred to as “NRD guide”) is preferably used.  
      The basic configuration of this nonradiative dielectric waveguide is, as shown in a partially cut perspective view in  FIG. 18 , such that a dielectric waveguide  53  whose cross section on one virtual plane that is perpendicular to the waveguide extending direction is quadrangular such as rectangular is provided between parallel plate conductors  51  and  52  that are arranged in parallel with a predetermined space a. If this predetermined space a is a ≦λ/2 with respect to the wavelength λ of a high frequency signal, noise does not enter the dielectric waveguide  53  from the outside, and a high frequency is not radiated to the outside so that high frequency signals are propagated through the dielectric waveguide  53  efficiently. The wavelength λ of the high frequency signal is a wavelength in the air (free space) at a frequency used.  
      In the amplitude modulator having such a configuration, the detecting diode operates as a high frequency modulating element for modulating high frequency signals, and also serves to detect the high frequency signals. For example, as adetecting diode that performs such a function with respect to high frequency signals in the millimeter wave band, in general, Schottky barrier diodes are known. Furthermore, in such a configuration, a resistor functions as consuming means for consuming the detected output of the detecting diode.  
      Moreover, there have been proposed high frequency transmitting/receiving apparatuses designed to operate in combination with such an amplitude modulator as a high frequency circuit element, which are expected to find applications in a millimeter wave radar module, a millimeter wave wireless radio communications apparatus, or the like. For example, such a high frequency transmitting/receiving apparatus is disclosed in Japanese Unexamined Patent Publication JP-A2000-258525. The high frequency transmitting/receiving apparatus disclosed in JP-A 2000-258525 is of the type that adopts a pulse modulation scheme.  
       FIG. 19  is a schematic block circuit diagram showing the conventional high frequency transmitting/receiving apparatus that adopts the pulse modulation scheme. For example, the high frequency transmitting/receiving apparatus is composed of: a high frequency oscillator  61  for generatinga high-frequency signal; abranching device  62  connected relatively to the output end of the high frequency oscillator  61 , for branching the high frequency signal so that the branched high frequency signal components may be outputted to one output end  62   b  and the other output end  62   c  thereof, respectively; an amplitude modulator  63  connected relatively to the one output end  62   b  of the branching device  62 , for modulating part of the high frequency signal so as to put it out as a high-frequency signal intended for transmission; a circulator  64  having a first terminal  64   a , a second terminal  64 b, and a third terminal  64   c , of which the first terminal  64   a  is connected with the output end  63   a  of the amplitude modulator  63 , wherein a high frequency signal inputted from the first terminal  64   a  is outputted to the second terminal  64   b , and a high-frequency signal inputted from the second terminal  64   b  is outputted to the third terminal  64   c ; a transmitting/receiving antenna  65  connected to the second terminal  64   b  of the circulator  64 ; and a mixer  66  connected between the other output end  62   c  of the branching device  62  and the third terminal  64   c  of the circulator  64 , for mixing the high frequency signal outputted to the other output end  62   c  of the branching device  62  as a local signal L 0  and a high frequency signal received by the transmitting/receiving antenna  65  so as to generate an intermediate frequency signal.  
      It has been known that, in such a conventional high frequency transmitting/receiving apparatus, a nonradiative dielectric waveguide is suitable for use as a high frequency transmission line for providing connection among the high frequency circuit elements and transmitting high frequency signals (refer to JP-A 2000-258525, for example).  
      Furthermore, one example of a conventional radar apparatus provided with such a high frequency transmitting/receiving apparatus and a vehicle equipped with the radar apparatus is disclosed in Japanese Unexamined Patent Publication JP-A 2003-35768.  
      However, in the conventional amplitude modulators as disclosed in JP-A 10-270944 (1998) and U.S. Pat. No. 6,034,574, an adjusting (tuning) mechanism for tuning the transmission characteristics of high frequency signals that are transmitted through the amplitude modulator, which are modulator characteristics, after incorporating the amplitude modulator as a high frequency circuit element into a module or other elements as described above is not provided, so that it is difficult to tune the modulator characteristics in the state in which the amplitude modulator is incorporated into a module or the like. Therefore, for example, it is difficult to keep the output of the amplitude modulator constant with respect to variations in the characteristics of the high frequency circuit element.  
      In addition, in the conventional high frequency transmitting/receiving apparatus having such an amplitude modulator, because of tuning inaccuracy or instability in the amplitude modulator, it is impossible to ensure a uniform output of high frequency signals for transmission. This gives rise to a problem of difficulty in attaining excellent characteristics with stability.  
      Furthermore, in the case where the high frequency oscillator  61  and the amplitude modulator  63  or the high frequency oscillator  61  and the mixer  66  are connected selectively, using a RF (radio frequency) switch, instead of the branching device  62 , it is difficult to tune the transmission characteristics of the RF switch, and in a conventional high frequency transmitting/receiving apparatus provided with such a RF switch, because of tuning inaccuracy or instability in the RF switch, it is impossible to ensure a uniform output of high frequency signals for transmission. This gives rise to a problem of difficulty in attaining excellent characteristics with stability.  
      In a radar apparatus using such a high frequency transmitting/receiving apparatus, erroneous detection tends to occur because the output of the high frequency signals for transmission is not stable, and therefore detection of an object to be detected is delayed.  
      Further, in the vehicle or small ship equipped with such a radar apparatus, an objected to be detected is detected by the radar apparatus. In response to the detected information, the vehicle or small ship takes proper action such as collision avoidance and braking. However, because of the delay of target detection, an abrupt action is caused in the vehicle or small ship after the detection operation.  
     SUMMARY OF THE INVENTION  
      In the light of the above, objects of the invention are to provide an amplitude modulator capable of tuning the modulator characteristics in a simple manner by a bias supply circuit of a high frequency modulating element which is a component of the amplitude modulator, and to provide a high performance high frequency transmitting/receiving apparatus capable of stabilizing high frequency signals for transmission at a predetermined output intensity with a simple configuration by being provided with such an amplitude modulator.  
      Furthermore, other objects of the invention are to provide a selector switch capable of tuning the transmission characteristics of the selector switch in a simple manner by a bias supply circuit of a PIN diode which is a component of the selector switch, and to provide a high performance high frequency transmitting/receiving apparatus capable of stabilizing high frequency signals for transmission with a predetermined output intensity with a simple configuration by being provided with such a selector switch.  
      In addition, further another object of the invention is to provide a radar apparatus having the high performance high frequency transmitting/receiving apparatus, a vehicle equipped with the radar apparatus, and a small ship equipped with the radar apparatus.  
      The invention provides an amplitude modulator comprising:  
      two high frequency transmission lines for transmitting high frequency signals;  
      a high frequency modulating element that is provided between the high frequency transmission lines and that modulates a high frequency signal input from one of the high frequency transmission lines and outputs the high frequency signal to the other of the high frequency transmission lines; and  
      a bias supply circuit that is connected to the high frequency modulating element and supplies a bias voltage to the high frequency modulating element,  
      wherein the bias supply circuit includes a variable resistor for adjusting a bias current flowing through the high frequency modulating element.  
      According to the invention, in the amplitude modulator, between two high frequency transmission lines for transmitting high frequency signals is provided a high frequency modulating element that modulates a high frequency signal input from one of the high frequency transmission lines and outputs the high frequency signal to the other of the high frequency transmission lines, and a bias supply circuit that is connected to the high frequency modulating element includes a variable resistor for adjusting a bias current flowing through the high frequency modulating element. Therefore, the variable resistor sets the bias current to an appropriate value with respect to the high frequency modulating element and operates so as to adjust the transmission characteristics of the high frequency signal transmitted through the amplitude modulator, which are modulator characteristics, so that an amplitude modulator can be obtained in which the modulator characteristic can be tuned with the variable resistor in a simple manner even after the amplitude modulator is incorporated into a module or the like.  
      In the invention, it is preferable that the variable resistor is constituted by a trimmable chip resistor.  
      According to the invention, when the variable resistor is constituted by a trimmable chip resistor, the resistance value that has been set can be held reliably, even if the ambient conditions such as vibration are added after the resistance value has been adjusted because the trimmable chip resistor has no variable portion. Therefore, an amplitude modulator whose modulator characteristics are stabilized can be obtained.  
      In the invention, it is preferable that the variable resistor is constituted by a trimmer potentiometer.  
      According to the invention, when the variable resistor is constituted by a trimmer potentiometer, the modulator characteristics can be further stabilized for the following reason. Since the trimmer potentiometer serves to set the resistance value dynamically in accordance with a control signal that is externally input, tuning can be performed so that desired modulator characteristics can be obtained, in spite of changes in environment conditions such as ambient temperature or the like or temporal changes in the characteristics of the high frequency modulating element.  
      In the invention, it is preferable that the high frequency modulating element is constituted by a PIN diode.  
      According to the invention, when the high frequency modulating element is constituted by a PIN diode, an amplitude modulator can be obtained in which even if the intensity of the high frequency signal that is input is changed, the modulator characteristics can be relatively stable for the following reason. Since the PIN diode has no detecting function with respect to high frequency signals, for example, signals in the millimeter wave band, when modulating such a high frequency signal, even if the intensity of the high frequency signal that is input is changed, the bias current flowing through the PIN diode is not changed thereby, so that the transmission characteristics of the high frequency signal transmitted through the PIN diode can be stabilized.  
      The invention provides a selector switch comprising:  
      an input side high frequency transmission line having an input terminal:  
      two output side high frequency transmission lines each having an output terminal;  
      PIN diodes, one of which is provided between the input side high frequency transmission line and one of the output side high frequency transmission lines, and another of which is provided between the input side high frequency transmission line and the other of the output side high frequency transmission lines; and  
      a bias supply circuit provided so as to individually correspond to each of the PIN diodes, for supplying a bias voltage to the PIN diodes,  
      wherein the bias supply circuit includes a variable resistor for adjusting a bias current flowing through the PIN diodes.  
      According to the invention, in a selector switch, when a high frequency signal is supplied to an input terminal of an input side high frequency transmission line, the high frequency signal is supplied to each PIN diode. A bias supply circuit that is provided so as to correspond to the PIN diode and that can apply a bias voltage is individually connected to the PIN diode, and the bias supply circuit includes a variable circuit for adjusting a bias current flowing through the PIN diodes. Therefore, the variable resistor sets the bias current to an appropriate value with respect to the PIN diode and operates so as to adjust the transmission characteristics of the high frequency signal transmitted through the PIN diode, so that a selector switch can be obtained in which the modulator characteristic can be tuned with the variable resistor in a simple manner even after the selector switch is incorporated into a module or the like.  
      Furthermore, since the PIN diode is used, the transmission characteristics of the selector switch advantageously can be relatively stable, even if the intensity of the high frequency signal that is input is changed for the following reason. Since the PIN diode has no detecting function with respect to high frequency signals, for example, signals in the millimeter wave band, even if, when transmitting such a high frequency signal in the state in which the switch is on, in other words, when transmitting such a high frequency signal in a state in which a bias current is supplied to the PIN diode, the intensity of the high frequency signal that is input is changed, the bias current flowing through the PIN diode is not changed, so that the transmission characteristics of the high frequency signal transmitted through the PIN diode can be stabilized.  
      In the invention, it is preferable that the variable resistor is constituted by a trimmable chip resistor.  
      According to the invention, the variable resistor is constituted by a trimmable chip resistor. The resistance value that has been set can be held reliably, even if the ambient conditions such as vibration are added after the resistance value has been adjusted, because the trimmable chip resistor has no variable portion. Therefore, an amplitude modulator whose modulator characteristics are stabilized can be obtained.  
      The invention provides a high frequency transmitting/receiving apparatus comprising:  
      a high frequency oscillator for generating a high frequency signal;  
      a branching device having two output portions and connected to the high frequency oscillator, for branching the high frequency signal supplied from the high frequency oscillator and outputting signals from one of the two output portions and the other of the two output portions;  
      the amplitude modulator mentioned above in, which the one of the high frequency transmission lines is connected to the one output portion of the branching device, for modulating a high frequency signal branched to the one output portion and outputting a high frequency signal for transmission from the other high frequency transmission line;  
      a signal divider having a first terminal, a second terminal and a third terminal, the other of the high frequency transmission lines of the amplitude modulator being connected to the first terminal, the high frequency signal for transmission input from the first terminal being output from the second terminal, the high frequency signal input from the second terminal being output from the third terminal;  
      an antenna for transmission/reception that is connected to the second terminal; and  
      a mixer that is connected between the other: output portion of the branching device and the third terminal, for mixing the high frequency signal that is branched and output from the other output portion and a high frequency signal received at the antenna for transmission/reception and outputting an intermediate frequency signal.  
      According to the invention, in a high frequency transmitting/receiving apparatus, a high frequency signal oscillated from a high frequency oscillator is supplied to a branching device and branched at the branching device, and the branched high frequency signals are output from one output portion and another output portion of the branching device. The high frequency signal output from the one output portion is supplied to the one of the high frequency transmission lines of the amplitude modulator andmodulated, and supplied to a first terminal of a signal divider as a high frequency signal for transmission. The signal divider outputs the high frequency signal for transmission input to the first terminal from the second terminal, and the high frequency signal for transmission is radiated from an antenna for transmission/reception that is connected to the second terminal as a ratio wave. The high frequency signal received by the antenna for transmission/reception is supplied to the second terminal, and the signal divider outputs the high frequency signal supplied to the second terminal from the third terminal. The signal divider can divide the high frequency signal for transmission supplied to the antenna for transmission/reception and the high frequency signal received by the antenna for transmission/reception. The high frequency signal output from the third terminal is supplied to a mixer, and, at the same time, to the mixer, a high frequency signal is supplied from the branching device, so that the mixer mixes the high frequency signal received by the antenna for transmission/reception and the high frequency signal that is oscillated from the high frequency oscillator but not yet modulated by the modulator and outputs an intermediate frequency signal. In such a high frequency transmitting/receiving apparatus, the amplitude modulator serves to tune the modulator characteristics in accordance with the characteristics of the high frequency modulating element or its mounting state, resulting in a high performance high frequency transmitting/receiving apparatus in which good transmission output can be obtained stably. Furthermore, when the amplitude modulator includes a trimmable chip resistor or a trimmer potentiometer, the trimmable chip resistor or the trimmer potentiometer can hold a predetermined resistance value stably even under an environment in which vibration or temperature changes are extreme, and therefore, even in such an environment, good modulator characteristics can be maintained. Therefore, a high frequency transmitting/receiving apparatus with constantly stable performance can be obtained.  
      The invention provides a high frequency transmitting/receiving apparatus comprising:  
      a high frequency oscillator for generating a high frequency signal;  
      a branching device having two output portions and connected to the high frequency oscillator, for branching the high frequency signal supplied from the high frequency oscillator and outputting signals from one of the two output portions and the other of the two output portions;  
      the amplitude modulator mentioned above in which the one of the high frequency transmission lines is connected to the one output portion of the branching device, for modulating a high frequency signal branched to the one output portion and outputting a high frequency signal for transmission from the other high frequency, transmission line;  
      an isolator having an input terminal and an output terminal, for, when supplied with a high frequency signal for transmission at the input terminal from the amplitude modulator, outputting the high frequency signal for transmission from the output terminal;  
      an antenna for transmission that is connected to the output terminal;  
      an antenna for reception; and  
      a mixer that is connected to the other output portion of the branching device and the antenna for reception, for mixing the high frequency signal that is branched and output from the other output portion and a high frequency signal received at the antenna for reception and outputting an intermediate frequency signal.  
      According to the invention, in a high frequency transmitting/receiving apparatus, a high frequency signal oscillated from a high frequency oscillator is supplied to a branching device and branched at the branching device, and the branched high frequency signals are output from one output portion and another output portion of the branching device. The high frequency signal output from the one output portion is supplied to the one of the high frequency transmission lines of the amplitude modulator and modulated, and supplied to an input terminal of an isolator as a high frequency signal for transmission. The isolator transmits the high frequency signal for transmission supplied to its input terminal and outputs the high frequency signal for transmission from the output terminal, and the high frequency signal for transmission is radiated from an antenna for transmission that is connected to the output terminal as a ratio wave. The high frequency signal received by the antenna for reception is supplied to a mixer, and, at the same time, to the mixer, a high frequency signal is supplied from the branching device, so that the mixer mixes the high frequency signal received by the antenna for reception and the high frequency signal that is oscillated from the high frequency oscillator but not yet modulated by the modulator and outputs an intermediate frequency signal. In such a high frequency transmitting/receiving apparatus using separate antennas, one of which is for transmission and another for reception, the amplitude modulator serves to tune the modulator characteristics in accordance with the characteristics of the high frequency modulating element or its mounting state, resulting in a high performance high frequency transmitting/receiving apparatus in which good transmission output can be obtained stably. Furthermore, when the amplitude modulator includes a trimmable chip resistor or a trimmer potentiometer, the tritmable chip resistor or the trimmer potentiometer can hold a predetermined resistance value stably even under an environment in which vibration or temperature changes are extreme, and therefore, even in such an environment, good modulator characteristics can be maintained. Therefore, a high frequency transmitting/receiving apparatus with constantly stable performance can be obtained.  
      The invention provides a high frequency transmitting/receiving apparatus comprising:  
      a high frequency oscillator for generating a high frequency signal;  
      the selector switch mentioned above, whose input terminal is connected to the high frequency oscillator, for selectively outputting the high frequency signal supplied from the high frequency oscillator, from the one and the other output side high frequency transmission line;  
      a signal divider having a first terminal, a second terminal and a third terminal, an output terminal of the one of the output side high frequency signal transmission lines of the selector switch being connected to the first terminal, the high frequency signal input from the first terminal being output from the second terminal, the high frequency signal input from the second terminal being output from the third terminal;  
      an antenna for transmission/reception that is connected to the second terminal; and  
      a mixer that is connected to an output terminal of the other of the output side high frequency transmission lines of the selector switch and the third terminal, for mixing the high frequency signal that is output from the output terminal of the other of the output side high frequency transmission lines and a high frequency signal received at the antenna for transmission/reception and outputting an intermediate frequency signal.  
      According to the invention, in a high frequency transmitting/receiving apparatus, a high frequency signal oscillated from a high frequency oscillator is supplied to an input terminal of a selector switch. The selector switch outputs the high frequency signal supplied from the high frequency oscillator by selectively switching the output terminal of the one of the output side high frequency transmission lines and the output terminal of the other of the output side high frequency transmission lines. The high frequency signal output from the output terminal of the one of the output side high frequency transmission lines is supplied to a first terminal of a signal divider as a high frequency signal for transmission. The signal divider outputs the high frequency signal for transmission input to the first terminal from the second terminal, and the high frequency signal for transmission is radiated from an antenna for transmission/reception that is connected to the second terminal as a ratio wave. The high frequency signal received by the antenna for transmission/reception is supplied to the second terminal, and the signal divider outputs the high frequency signal supplied to the second terminal from the third terminal. The signal divider can divide the high frequency signal for transmission supplied to the antenna for transmission/reception and the high frequency signal received by the antenna for transmission/reception. The high frequency signal output from the third terminal is supplied to a mixer, and, at the same time, to the mixer, a high frequency signal output from the output terminal of the other of the output side high frequency transmission lines of the selector switch is supplied as a local signal and the mixer mixes the high frequency signal received by the antenna for transmission/reception and the high frequency signal that is oscillated from the high frequency oscillator and outputs an intermediate frequency signal. In such a high frequency transmitting/receiving apparatus, the selector switch serves to tune the transmission characteristics of the selector switch in accordance with the characteristics of the PIN diode as a switching element or its mounting state, resulting in a high performance high frequency transmitting/receiving apparatus in which good transmission output can be obtained stably. Furthermore, when the selector switch includes a trimmable chip resistor, the trimmable chip resistor can hold a predetermined resistance value stably even under an environment in which vibration or temperature changes are extreme, and therefore, even in such an environment, good modulator characteristics can be maintained. Therefore, a high frequency transmitting/receiving apparatus with constantly stable performance can be obtained.  
      The invention provide a high frequency transmitting/receiving apparatus comprising:  
      a high frequency oscillator for generating a high frequency signal;  
      the selector switch mentioned above, whose input terminal is connected to the high frequency oscillator, for selectively outputting the high frequency signal supplied from the high frequency oscillator, from the one and the other output side high frequency transmission line;  
      an antenna for transmission that is connected to an output-terminal of the one of the high frequency transmission lines;  
      an antenna for reception; and  
      a mixer that is connected to an output terminal of the other of the output side high frequency transmission lines of the selector switch and the antenna for reception, for mixing the high frequency signal that is output from the output terminal of the other of the output side high frequency transmission lines and a high frequency signal received at the antenna for reception and outputting an intermediate frequency signal.  
      According to the invention, in a high frequency transmitting/receiving apparatus, a high frequency signal oscillated from a high frequency oscillator is supplied to an input terminal of a selector switch. The selector switch outputs the high frequency signal supplied from the high frequency oscillator by selectively switching the output terminal of the one of the output side high frequency transmission lines and the output terminal of the other of the output side high frequency transmission lines. The high frequency signal output from the output terminal of the one of the output side high frequency transmission lines is supplied to an antenna for transmission as a high frequency signal for transmission, and is radiated from an antenna for transmission/reception as a ratio wave. The high frequency signal received by the antenna for reception is supplied to a mixer, and, at the same time, to the mixer, a high frequency signal output from another output portion of the selector switch is supplied as a local signal so that the mixer mixes the high frequency signal received by the antenna for reception and the high frequency signal that is oscillated from the high frequency oscillator and outputs an intermediate frequency signal. Also in such a high frequency transmitting/receiving apparatus using separate antennas, one of which is for transmission and another for reception, the selector switch serves to tune the transmission characteristics of the selector switch in accordance with the characteristics of the PIN diode as a switching element or its mounting state, resulting in a high performance high frequency transmitting/receiving apparatus in which good transmission output can be obtained stably. Furthermore, when the selector switch includes a trimmable chip resistor, the trimmable chip resistor can hold a predetermined resistance value stably even under an environment in which vibration or temperature changes are extreme, and therefore, even in such an environment, good characteristics of the selector switch can be maintained. Therefore, a high frequency transmitting/receiving apparatus with constantly stable performance can be obtained.  
      The invention provides a radar apparatus comprising:  
      the high frequency transmitting/receiving apparatus mentioned above; and  
      a distance information detecting device for processing the intermediate frequency signal output from the high frequency transmitting/receiving apparatus and detecting information on a distance up to an object to be detected.  
      According to the invention, a radar apparatus comprises the high frequency transmitting/receiving apparatus mentioned above, and distance information detecting device for processing the intermediate frequency signal output from the high frequency transmitting/receiving apparatus and detecting information on a distance up to an object to be detected. Therefore, the high frequency transmitting/receiving apparatus transmits high frequency signal for transmission stably at good transmission output, so that a radar apparatus that can detect an object to be detected fast and reliably and an object to be detected that is near or far away fast and reliably. Furthermore, with the high frequency transmitting/receiving apparatus of the invention that provides stable performance even in an environment in which vibration, temperature changes or the like are extreme, a radar apparatus that operates even under such extreme conditions can be obtained.  
      The invention provides a radar apparatus-mounting vehicle comprising the radar apparatus mentioned above, which is used to detect an object to be detected.  
      According to the invention, a radar apparatus-mounting vehicle comprises the radar apparatus mentioned above, which is used to detect an object to be detected. Since the radar apparatus can detect fast and reliably another vehicle or an obstacle, which is an object to be detected, appropriate control of the vehicle and appropriate warning to the driver can be performed without causing the vehicle to perform a sudden action, for example, to avoid the obstacle.  
      The invention provides a radar apparatus-mounting small ship comprising the radar apparatus mentioned above, which is used to detect an object to be detected.  
      According to the invention, the radar apparatus-mounting small ship comprises the radar apparatus mentioned above, which is used to detect an object to be detected. Since the radar apparatus can detect fast and reliably another small ship or an obstacle, which is an object to be detected, appropriate control of the small ship and appropriate warning to the driver can be performed without causing the small ship to perform a sudden action, for example, to avoid the obstacle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein:  
       FIG. 1  is a schematic block circuit diagram showing a configuration of an amplitude modulator according to one embodiment of the invention;  
       FIGS. 2A and 2B  are schematic perspective view and plan view showing the amplitude modulator shown in  FIG. 1 , respectively;  
       FIG. 3  is a schematic plan view showing an example of a high frequency modulating portion M in the amplitude modulator shown in  FIGS. 2A and 2B ;  
       FIG. 4  is a schematic plan view showing the configuration of an amplitude modulator according to another embodiment of the invention;  
       FIG. 5  is a schematic plan view showing the configuration of a selector switch according to one embodiment of the invention;  
       FIGS. 6A and 6B  are schematic plan view and side view showing an example of a trimmable chip resistor which is a component of a bias supply circuit shown in  FIG. 1 ;  
       FIGS. 7A  to  7 E are schematic plan views showing an example of another trimming method in the trimmable chip resistor shown in the  FIGS. 6A and 6B , respectively;  
       FIG. 8  is a perspective view showing a configuration of a trimmer potentiometer;  
       FIG. 9  is a schematic block circuit diagram showing a configuration of a high frequency transmitting/receiving apparatus according to a first embodiment of the invention;  
       FIG. 1O  is a schematic plan view of the high frequency transmitting/receiving apparatus shown in  FIG. 9 ;  
       FIG. 11  is a schematic perspective view showing one example of a substrate on which a diode for a mixer featuring a nonradiative dielectric waveguide is mounted;  
       FIG. 12  is a schematic block circuit diagram showing a configuration of a high frequency transmitting/receiving apparatus according to a second embodiment of the invention;  
       FIG. 13  is as schematic plan view of the high frequency transmitting/receiving apparatus shown in  FIG. 12 ;  
       FIG. 14  is a schematic block circuit diagram showing a configuration of a high frequency transmitting/receiving apparatus according to a third embodiment of the invention;  
       FIG. 15  is a schematic block circuit diagram showing a configuration of a high frequency transmitting/receiving apparatus according to a fourth embodiment of the invention;  
       FIG. 16  is a graph showing a relationship between a bias current and a bias voltage applied to a PIN diode;  
       FIG. 17  is a graph showing a relationship between a resistance value of the trimmable chip resistor and an attenuation amount of a high frequency signal;  
       FIG. 18  is a partially cut perspective view showing a basic configuration of a nonradiative dielectric waveguide; and  
       FIG. 19  is a schematic block circuit diagram showing an example of a conventional high frequency transmitting/receiving apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERED EMBODIMENTS  
      Now referring to the drawings, preferred embodiments of the invention are described below.  
      At the outset, an amplitude modulator, a selector switch, a high-frequency transmitting/receiving apparatus having the amplitude modulator and the selector switch embodying the invention will be described in detail with reference to the accompanying drawings.  
       FIG. 1  is a schematic block circuit diagram showing a configuration of an amplitude modulator according to one embodiment of the invention.  FIGS. 2A and 2B  are schematic perspective view and plan view showing the amplitude modulator shown in  FIG. 1 , respectively.  FIG. 3  is a schematic plan view showing an example of a high frequency modulating portion in the amplitude modulator shown in  FIGS. 2A and 2B .  FIG. 4  is a schematic plan view showing the configuration of an amplitude modulator according to another embodiment of the invention.  FIG. 5  is a schematic plan view showing the configuration of a selector switch according to one embodiment of the invention.  FIGS. 6A and 6B  are schematic views showing an example of a trimmable chip resistor  4  which is a component of a bias supply circuit C in the amplitude modulator shown in  FIG. 1 .  FIG. 6A  is a plan view and  FIG. 6B  is a side view thereof.  FIGS. 7A  to  7 E are schematic plan views showing an example of another trimming method in the trimmable chip resistor  4  shown in the  FIGS. 6A and 6B , respectively.  FIG. 8  is a perspective view showing a configuration of a trimmer potentiometer  104 .  FIG. 9  is a schematic block circuit diagram showing a configuration of a high frequency transmitting/receiving apparatus  110  according to a first embodiment of the invention.  FIG. 10  is a schematic plan view of the high frequency transmitting/receiving apparatus  110  shown in  FIG. 9 .  FIG. 11  is a schematic perspective view showing one example of a substrate on which a diode for a mixer featuring a nonradiative dielectric waveguide is mounted.  FIG. 12  is a schematic block circuit diagram showing a configuration of a high frequency transmitting/receiving apparatus  120  according to a second embodiment of the invention.  FIG. 13  is a schematic plan view of the high frequency transmitting/receiving apparatus  120  shown in  FIG. 12 .  FIG. 14  is a schematic block circuit diagram showing a configuration of a high frequency transmitting/receiving apparatus  130  according to a third embodiment of the invention.  FIG. 15  is a schematic block circuit diagram showing a configuration of a high frequency transmitting/receiving apparatus  140  according to a fourth embodiment of the invention.  FIG. 16  is a graph showing the relationship between a bias current and a bias voltage applied to a PIN diode  3 .  FIG. 17  is a graph showing a relationship between a resistance value of the trimmable chip resistor  4  and a attenuation amount of a high frequency signal.  FIG. 18  is a partially cut perspective view showing a basic configuration of a nonradiative dielectric waveguide.  
      In FIGS.  1  to  7 E, reference numerals  1  and  2  denote high frequency transmission lines;  1   a  and  2   a  denote an input terminal and an output terminal;  1 ′ and  1 ″ denote dielectric waveguides for input;  2 ′ and  2 ″ denote dielectric waveguides for output;  1 ′ a  and  1 ″ a,  and  2 ′ a  and  2 ″ a denote input terminals and output terminals (these terminals are ends on one side of the dielectric waveguides  1 ′ and  2 ′);  1 ′ b  and  2 ′ b  denote end portions (these end portions are the other ends of the dielectric waveguides  1 ′ and  2 ′) ;  3  and  3 ″ denote PIN diodes;  4  and  4 ″ denote trimmable chip resistors;  4   a  denotes a dielectric substrate;  4   b  denotes a resistor layer;  4   c   1  and  3   c   2  denote electrodes;  4   d  and  3   d   1  to  3   d   4  denote trimming portions;  5  and  5 ″ denote choke inductors;  5 ′ denotes choke type bias supply line,  5 ′ a  denotes a wide line;  5 ′ b  denotes a narrow line;  5 ′ c  denotes a line conductor;  5 ′ d  denotes a conductor for connection;  5 ′ e  denotes an island-shaped conductor;  6  and  6 ″ denote signal sources;  7  denotes a substrate;  8  denotes a ferrite plate;  9  denotes a dielectric waveguide;  10  denotes a non reflective terminator;  81 ,  82  and  83  denote transmission lines for high frequencies;  81   a  denotes an input terminal; and  82   a  and  83   a  denote output terminals.  
      In FIGS.  9  to  15 , reference numeral  11  denotes a high frequency oscillator;  12  denotes a branching device;  13  denotes a modulator;  14  denotes a circulator constituting a signal divider;  15  denotes a transmitting/receiving antenna;  16  denotes a mixer;  17  denotes a switch;  18  denotes an isolator;  19  denotes an antenna for transmission;  20  denotes an antenna for reception;  21  and  31  denote plate conductors;  22  and  32  denote first dielectric waveguides;  23  and  33  denote second dielectric waveguides;  24  and  34  denote ferrite plates as magnetic bodies;  25  and  35  denote third dielectric waveguides;  26  and  36  denote fourth dielectric waveguides;  27  and  37  denote fifth dielectric waveguides;  28 ,  38   a  and  38   b  denote nonreflective terminators;  39  denotes a sixth dielectric waveguide;  40  denotes a substrate;  41  denotes a choke type bias supply line;  42  denotes a terminal for connection;  43  denotes an element for detecting high frequencies;  12   a  denotes an input terminal;  12   b  denotes one output terminal;  12   c  denotes another output terminal;  13   a  and  18   a  denote input terminals;  13   b  and  18   b  denote output terminals;  14   a,    24   a  and  34   a  denote first terminals;  14   b,    24   b  and  34   b  denote second terminals; and  14   c,    24   c  and  34   c  denote third terminals. Furthermore, reference numeral  71  denotes a selector switch (RF switch) ;  72  denotes a second selector switch (RF switch) constituting a signal divider;  7 l a  denotes an input terminal;  71   b  denotes one output terminal;  71   c  denotes another output terminal;  72   a  denotes an input/output terminal;  72   b  denotes an input terminal; and  72   c  denotes an output terminal. In  FIG. 18 , reference numerals  51  and  52  denote plate conductors, and  53  denotes a dielectric waveguide.  
      In  FIGS. 2A, 2B  and  4 , the plate conductors are not shown. In  FIGS. 10 and 13 , the upper plate conductors are not shown.  
      The amplitude modulator according to one embodiment of the invention has the following configuration as shown in a schematic circuit diagram in  FIG. 1 . A PIN diode  3  serving as a high frequency modulating element for modulating a high frequency signal input from the high frequency transmission line  1  on one side and outputting the modulated signal onto the output terminal  2   a  side of the high frequency transmission line  2  on the other side is provided between two high frequency transmission lines  1  and  2  for transmitting high frequency signals. A bias supply circuit C that is connected to the PIN diode  3  includes a trimmable chip resistor  4  serving as a variable resistor that adjusts a bias current flowing through the PIN diode  3 . In this configuration, a choke inductor  5  and a signal source  6  are further connected in the bias circuit C, and thus the trimmable chip resistor  4 , the choke inductor  5  and the signal source  6  are connected to the PIN diode  3 .  
      The signal source  6  supplies a bias voltage to the PIN diode  3  via the trimmable chip resistor  4  and the choke inductor  5 . The signal source  6  supplies a bias voltage to the PIN diode  3  based on a control signal for amplitude-modulating an externally supplied high frequency signal, and herein applies selectively either a constant forward direction bias voltage and a constant reverse direction bias voltage or a voltage of 0V. The cathode of the PIN diode  3  is grounded, and the anode is connected to the choke inductor  5 . More specifically, the trimmable chip resistor  4  is connected between the choke inductor  5  and the signal source  6 .  
      In the amplitude modulator shown in  FIG. 1 , the high frequency transmission lines  1  and  2  are constituted by nonradiative dielectric waveguides, and as shown in a perspective view and a plan view in  FIGS. 2A and 2B , between plate conductors (not shown) provided in parallel with a gap of ½ or less of the wavelength of a high frequency signal, dielectric waveguides  1 ′ and  2 ′ for input and output are arranged with the PIN diode  3  of a high frequency modulating portion M interposed between their opposing end portions  1 ′ b  and  2 ′ b.  In the high frequency modulating portion M, the PIN diode  3  is connected to a choke type bias supply line  5 ′ formed on a substrate  7  shown in a plan view of  FIG. 3 . The dielectric waveguide  1 ′ for input and the dielectric waveguide  2 ′ for output are provided along their extending direction. The end portion l′ b  of the dielectric waveguide  1 ′ for input on the side of the dielectric waveguide  2 ′ for output is in contact with one surface in the thickness direction of the substrate  7 , and the end portion  2 ′ b  of the dielectric waveguide  2 ′ for output on the side of the dielectric waveguide  1 ′ for input is in contact with the PIN diode  3  that is mounted on the other surface in the thickness direction of the substrate  7 .  
      The choke type bias supply line  5 ′ corresponds to the choke inductor  5 . The dielectric waveguides  1 ′ and  2 ′ correspond to the high frequency transmission lines  1  and  2  in  FIG. 1 , respectively.  
      More specifically, in the above configuration, in the choke type bias supply line  5 ′ formed on the substrate  7 , as shown in the plan view of  FIG. 3 , a wide line  5 ′ a  and a narrow line  5 ′ b  are connected alternately with a length of a λ/4 (λ is the wavelength of a high frequency signal transmitted to the dielectric waveguides  1 ′ and  2 ′) period, and a line conductor  5 ′ c  and a conductor  5 ′ d  for connection are provided in a portion where the alternate arrangement is interrupted. The substrate  7  is formed of a material having electrical insulating properties. In  FIG. 3 , for clarification, the wide line  5 ′ a,  the narrow line  5 ′ b  and the conductor  5 ′ d  for connection are gridded. The wide line  5 ′ a,  the narrow line  5 ′ b , the line conductor  5 ′ c  and the conductor  5 ′ d  for connection are formed of a maternal having a conductivity on the other surface in the thickness direction of the substrate  7  in such a manner that their centers in their width direction are matched to each other. The width direction is the direction perpendicular to the direction in which the choke type bias supply line  5 ′ extends and the thickness direction of the choke type bias supply line  5 ′. The shapes of the wide line  5 ′ a,  the narrow line  5 ′ b , the line conductor  5 ′ c  and the conductor  5 ′ d  for connection when viewed from one side of the thickness direction are rectangular. The PIN diode  3  is provided in a portion where the chock type bias supply line  5 ′ is interrupted, between the two conductors  5 ′ d  provided a part in such a manner that the PIN diode  3  is connected to the two conductor  5 ′ d  for connection. The anode of the PIN diode  3  is connected to one of the conductors  5 ′ d  for connection, and the cathode of the PIN diode  3  is connected to the other conductor  5 ′ d  for connection. The wide line  5 ′ a  , the narrow line  5 ′ b , the line conductor  5 ′ c  and the conductor  5 ′ d  for connection on one side (the left side of  FIG. 3 ) with respect to the PIN diode  3  of the choke type bias supply line  5 ′ are integrally formed, and the wide line  5 ′ a , the line conductor  5 ′ c  and the conductor  5 ′ d  for connection on the other side (the right side of  FIG. 3 ) with respect to the PIN diode  3  of the choke type bias supply line  5 ′ are integrally formed.  
      Furthermore, island-shaped conductors  5 ′ e  having a conductivity are provided in the vicinity of the conductors  5 ′ d  for connection on both sides on the substrate  7 . More specifically, the island-shape conductors  5 ′ e  are provided near to the opposing end portions of the conductors  5 ′ d  for connection, on both sides of the conductors  5 ′ d  for connection in the width direction, apart from the conductors  5 ′ d  for connection, on both sides of the PIN diode  3  in the width direction, apart from the PIN diode  3 . When positioning the high frequency modulating portion M between the end portions  1 ′ b  and  2 ′ b  of the dielectric waveguides  1 ′ and  2 ′, the PIN diode  3  is provided on the substrate  7  such that high frequency signals transmitted through the dielectric waveguides  1 ′ and  2 ′ enter the PIN diode  3  and a current flows in a direction substantially parallel to the direction of the electric field of its LSE mode, and the PIN diode  3  is connected to the conductor  5 ′ d  forconnection. The bias supply circuit C (not shown) is connected to one end (left end portion in  FIG. 3 ) of extending direction of the choke type bias supply line  5 ′ so that a bias is supplied to the PIN diode  3 , and the other end (right end portion in  FIG. 3 ) of extending direction of the choke type bias supply line  5 ′ is grounded. In order to connect the PIN diode  3  to the conductor  5 ′ d  for connection on the substrate  7 , flip chip connection or wire bonding connection may be performed.  
      When a forward direction bias voltage is applied to the PIN diode  3 , the PIN diode  3  transmits a high frequency signal, and when a reverse direction bias voltage is applied or no voltage is applied to the PIN diode  3 , the PIN diode  3  does not transmit but reflect a high frequency signal.  
      Furthermore, as shown in a plan view of  FIG. 4 , the amplitude modulator according to another embodiment of the invention includes two ferrite plates  8 , and a dielectric waveguide  1 ″ for input, a dielectric waveguide  9  for modulation, a nonreflective terminator  10  and a dielectric waveguide  2 ″ for output which are provided radially with respect to the two ferrite plates  8 , between plate conductors (not shown) arranged in parallel with a gap of ½ or less of the wavelength of a high frequency signal. The two ferrite plates  8  are arranged opposed to and apart from each other in the inner faces of the plate conductors. High frequency signals are input to the dielectric waveguide  1 ″ for input. The PIN diode  3  is provided in a leading end portion of the dielectric waveguide  9 , which is on the opposite side of the ferrite plate  8 , for modulation. The nonreflective terminator  10  is provided in the extended direction of the leading end portion of the dielectric waveguide  9  for modulation to terminate the high frequency signals that have been transmitted through the PIN diode  3 . The dielectric waveguide  2 ″ for output outputs the high frequency signals whose amplitudes have been modulated by the PIN diode  3 . The PIN diode  3  is connected to the bias circuit C (not shown) in which the trimmable chip resistor  4  for adjusting a bias current flowing through the PIN diode  3 , the choke inductor  5  and the signal source  6  are connected as the configuration shown in  FIG. 1 . The dielectric waveguide  1 ″ for input, the dielectric waveguide  9  for modulation, and dielectric waveguide  2 ″ for output extend along the plate conductor, and extend radially from the positions that are displaced by 120 degrees each around the axis perpendicular to the ferrite plane  8 .  
      More specifically, in the above-described configuration, the PIN diode  3  is connected to the conductor  5 ′ d  for connection of the choke type bias supply line  5 ′ on the substrate  7  that is similar to that shown in  FIG. 3 . The substrate  7  connected to the PIN diode  3  is provided in the leading end portion of the dielectric waveguide  9  for modulation such that high frequency signals enter the PIN diode  3  from the dielectric waveguide  9  for modulation, and the nonreflective terminator  10  is provided in the extended direction of the dielectric waveguide  9  for modulation such that the high frequency signals having transmitted through the PIN diode  3  enter the nonrelfective terminator  10  but are terminated. The arrangement of the PIN diode  3  with respect to the dielectric waveguide  9  for modulation is the same as the arrangement of the PIN diode  3  with respect to the dielectric waveguide  1 ′ shown in  FIG. 2  described above. The arrangement of the PIN diode  3  with respect to the nonreflective terminator  10  is the same as the arrangement of the PIN diode  3  with respect to the dielectric waveguide  2 ′ shown in  FIG. 2  described above.  
      A selector switch of one embodiment of the invention shown in  FIG. 5  includes an input side high frequency transmission line  81  having an input terminal  81   a,  two output side high frequency transmission lines  82  and  83  having output terminals  82   a  and  83   a,  respectively. Hereinafter, the output terminal  82   a  of the one of the output side high frequency transmission lines  82  may be referred to as one output terminal  82   a,  and the output terminal  83   a  of the other of the output side high frequency transmission lines  83  may be referred to as another output terminal  83   a.  The PIN diodes  3  and  3 ″ are providedbetween the input terminal  81   a  and the one output terminal  82   a  and between the input terminal  81   a  and the other output terminal  83   a , in other words, between the input side high frequency transmission line  81  and the one of the output side high frequency transmission lines  82 , and between the input side high frequency transmission line  81  and the other of the output side high frequency transmission lines  83 , respectively. To the PIN diodes  3  and  3 ″ are connected bias supply circuits C 1  and C 2  that individually correspond to these PIN diodes  3  and  3 ″ and that apply a bias voltage. The bias supply circuit C 1  for applying a bias voltage is connected to the PIN diode  3 , and the bias supply circuit C 2  for applying a bias voltage is connected to the PIN diodes  3 ″. These supply circuits C 1  and C 2  include trimmable chip resistors  4  and  4 ″ as variable resistors for adjusting the bias current flowing through the PIN diodes  3  and  3 ″. By individually providing the bias supply circuits C 1  and C 2  with the trimmable chip resistors  4  and  4 ″, it is possible to individually adjust a current flowing through the PIN diodes  3  and  3 ″. In the selector switch shown in  FIG. 5 , the same components as in the configuration of the amplitude modulator described above bear the same reference numerals. The anode of the PIN diode  3 ″ is grounded, and the cathode is connected to the choke inductor  5 ″. The choke inductor  5 ″ is connected to the signal source  6 ″ via the trimmable chip resistor  4 ″. The bias supply circuit C 2  is constituted by the choke inductor  5 ″, the trimmable chip resistor  4 ″ and the signal source  6 ″.  
      The input side high frequency transmission line  81  has an input portion  181   a  having an input terminal  81   a , and two branched portions  181   b  and  181   c  that are branched from the end portion opposite to the input terminal  81   a  of the input portion  181   a . The PIN diode  3  is provided between the end portion opposite to the input portion  181   a  of the first branched portion  181   b  and the end portion opposite to the output portion  82   a  of the one of the output side high frequency transmission lines  82 . The PIN diode  3 ″ is provided between the end portion opposite to the input portion  181   a  of the second branched portion  181   c  and the end portion opposite to the output portion  83   a  of the other of output side high frequency transmission lines  83 . The arrangement relationship between the first branched portion  181   b , the PIN diode  3 , and the one of the output side high frequency transmission lines  82  and the arrangement relationship between the second branched portion  181   c , the PIN diode  3 ″, and the other of the output side high frequency transmission lines  83  are the same as the arrangement relationship between the one of the high frequency transmission lines  1 , the PIN diode  3 , and the other of the high frequency transmission lines  2  of the amplitude modulator shown in  FIG. 1  described above.  
      More specifically, the bias supply circuits C 1  and C 2  that are the same as the bias supply circuit C in the example of the amplitude modulator shown in  FIG. 1  are connected to the two PIN diodes  3  and  3 ″, respectively. The signal sources  6  and  6 ″ supply bias voltages such that a forward bias voltage is applied to either one of the PIN diodes  3  and  3 ″ and a reverse bias voltage is applied to the other, and that a high frequency signal input to the input terminal  81   a  is output from the one output terminal  82   a  or the other output terminal  83   a . That is to say, the signal sources  6  and  6 ″ supply bias voltages such that when a forward bias voltage is applied to either one of the PIN diodes  3  and  3 ″, a reverse bias voltage is applied to the other, and that when a forward bias voltage is applied to either of the other PIN diodes  3  and  3 ″, a reverse bias voltage is applied to the other. The signal sources  6  and  6 ″ supply bias voltages to the PIN diodes  3  and  3 ″ in the manner as described above, based on external control signals.  
      Even more specifically, in the amplitude modulators having the above-described configurations shown in FIGS.  1  to  4  and the selector switch shown in  FIG. 5 , as shown in  FIGS. 6A and 6B , for the trimmable chip resistor  4  (hereinafter, the same applies to the trimmable chip resistor  4 ′), a resistor layer  4   b  made of a resisting material such as a Ni—Cr (nickel—chrome) alloy is formed on a dielectric substrate  4   a  made of a dielectric such as alumina ceramics, namely on one surface  4 A in a thickness direction of the dielectric substrate  4   a , and electrodes  4   c   1  and  4   c   2  are formed so as to be connected to the opposite end portions of the resistor layer  4   b  and cover the opposite end portions of the dielectric substrate  4   a . The resistor layer  4   b  of the trimmable chip resistor  4  is irradiated with laser light from a YAG (yttrium—aluminum—garnet) laser or the like to oxidize an appropriate area of a portion of the resistor layer  4   b  to form a trimming portion  4   d  made of metal oxide having electrical insulating properties, and thus a resistance value between the electrodes  4   c   1  and  4   c   2  can be changed.  
      The both end portions of the resistor layer  4   b  are, that is, the both end portions in a predetermined direction along the one surface  4 A of the dielectric substrate  4   a  in the resistor layer  4   b , and are the both end portions in the longitudinal direction X 1 . The both end portions of the resistor layer  4   a  are, that is, the both end portions in a predetermined direction along the one surface  4 A of the dielectric substrate  4   a , and are the end portions in the longitudinal direction X 1 . The electrodes  4   c   1  and  4   c   2  are formed of a metal material having a lower specific resistance than that of the resistor layer  4   b , by plating solder, aluminum, copper or the like. The resistor layer  4   b  can be realized with a metal thin film having a rectangular parallelepiped shape. The resistor layer  4   b  is formed in a region excluding the peripheral portion on the one surface  4 A in the thickness direction of the dielectric substrate  4   a , and the both end portions in the longitudinal direction X 1  are in contact with the electrodes  4   c   1  and  4   c   2 .  
      The trimmable chip resistor  4  may include a protective film having electrical insulating properties for covering the resistor layer  4   b  between the electrodes  4   c   1  and  4   c   2 . The protective film transmits about 99% of light of a YAG laser. Such a protective film eliminates the necessity of performing a process of protecting the resistor layer  4   b  separately after trimming, which facilitates the post treatment. Moreover, since the resistor layer  4   b  is protected by the protective film, the resistance value of the resistor layer  4   b  is prevented from changing so that the resistance value that is stable can be maintained in the trimmable chip resistor  4 .  
      This trimmable chip resistor  4  can be used as follows. As shown in  FIGS. 6A and 6B , a peripheral portion of the resistor layer  4   b  in which the electrodes  4   c   1  and  4   c   2  are not connected is irradiated with a YAG laser light in parallel with a width direction X 2  of the resistor layer  4   b  from the outer side toward the inner side so that linear oxidized portion is provided to form the trimming portion  4   d . The area of the linear oxidized portion changes the resistance value of the trimmable chip resistor  4 , and as this area increases, the cross sectional area of the cross section of the resistor layer  4   b  in which a current flows decreases, so that the resistance value can be increased.  
      When oxidizing the resistor layer  4   b , for example, in the region that has been irradiated with laser light, all the portions on one surface through the other surface in the thickness direction of the resistor layer  4   b  may be oxidized, or only one surface portion in the region that has been irradiated with laser light may be oxidized.  
      When the resistance value of the trimmable chip resistor  4  is adjusted, in general, it is possible to select a relatively small resistance value in a desired adjustment range as an initial value, and adjust the resistance value so as to be increased.  
      When increasing the area of the linear oxidized portion, the width of the trimming portion  4   d  is set to a predetermined width that can be determined by the spot size of the YAG laser light, and the YAG laser light scans in one direction, so that the area may be increased in the scanning direction. In this case, before the next scanning, the same portion is irradiated with the pulsed YAG laser light a plurality of times. With this, adjustment (trimming) of the resistance value can be performed with a high accuracy.  
      In this embodiment, the resistance value of the resistor layer  4   b  is changed by oxidizing a portion of the resistor layer  4   b . However, in another embodiment of the invention, the resistance value of the resistor layer  4   b  may be changed by cutting out a portion of the resistor layer  4   b  by a laser.  
      Besides the linear oxidized portion shown in  FIGS. 6A and 6B , a trimming portion  4   d  in which the liner oxidized portion is provided in the central portion of the resistor layer  4   b , as shown in a plan view of  FIG. 7A , may be formed. Alternatively, as shown in  FIG. 7B , after the linear oxidized portion is provided as a first oxidized portion  4   d   1 , a similar linear oxidized portion that is shorter than the first oxidized portion  4   d   1  may be provided as a second oxidized portion  4   d   2  in a position slightly apart from the first oxidized portion  4   d   1  (double oxidization). The direction in which the first oxidized portion  4   d   1  extends is parallel to the direction in which the second oxidized portion  4   d   2  extends. The first oxidized portion  4   d    1  and the second oxidized portion  4   d   2  are formed so as not to be connected, and it is preferable that the end of the first oxidized portion  4   d   1  on the second oxidized portion  4   d   2  side and the end of the second oxidized portion  4   d   2  on the first oxidized portion  4   d   1  side are formed apart by a predetermined distance in the direction perpendicular to the direction in which the first oxidized portion  4   d   1  and the second oxidized portion  4   d   2  extend and the thickness direction of the resistor layer  2   b,  that is, the longitudinal direction X 1  of the resistor layer  2   b.    
      As shown in  FIG. 7C , as opposed to such double oxidization, double oxidizations in which the second oxidized portion  4   d   2  is provided on the side opposite to the side on which the first oxidized portion  4   d   1  is provided may be formed. As shown in  FIG. 7D , the double oxidized portions  4   d   1  and  4   d   2  shown in  FIG. 7C  and the similar double oxidizations  4   d   3  and  4   d   4  may be provided in a comb shape (serpentine oxidization). When the trimming portions  4   d ,  4   d   1  to  4   d   4  are formed as shown in  FIGS. 7B  to  7 D, the second oxidized portions  4   d   2  and  4   d   4  serve to set the resistance value more delicately, so that higher accurate trimming can be performed. Furthermore, by forming the trimming portion  4   d  in this manner, the length of the line in the resistor layer  4   b  can be increased, so that resistance can be increased.  
      Furthermore, as shown in  FIG. 7E  as well, the first linear oxidized portion  4   d   5  formed parallel to the width direction X 2  and an L-shaped oxidized portion (L-oxidized portion) having a second linear oxidized portion  4   d   6  extending in the longitudinal direction X 1  of the resistor layer  4   b , that is formed by bending the first linear oxidized portion  4   d   5  substantially at a right angle at a certain point in the direction of laser light scanning may be provided. The length of the first linear oxidized portion  4   d   5  in the direction parallel to the width direction X 2  of the resistor layer  4   b  is selected so as to be ½ or less of the length of the resistor layer  4   b  in the width direction X 2 . The length of the second linear oxidized portion  4   d   6  in the extending direction, in other words, the length of the second linear oxidized portion  4   d   6  in the direction parallel to the longitudinal direction X 1  of the resistor layer  4   b  is selected so as to be longer than the length of the first linear oxidized portion  4   d   5  in the direction parallel to the width direction X 2  of the resistor layer  4   b .  
      In this case, the stress applied to the resistor layer  4   b  is reduced so that it becomes difficult that microcrack occur in the resistor layer  4   b  and thus drift caused by microcracks can be reduced.  
      Trimming can be performed with a sufficient adjustment width even with a single trimmable chip resistor  4 . However, a plurality of trimmable chip resistors  4  that are connected in series or in parallel may be used.  
      The trimmable chip resistor  4  is provided so as to be exposed to the outside when incorporating the amplitude modulator into a high frequency transmitting/receiving apparatus. The trimmable chip resistor  4  is provided so as to be exposed to the outside when incorporating the selector switch into a high frequency transmitting/receiving apparatus. Thus, the resistance value of the trimmable chip resistor  4  can be changed in the state in which the amplitude modulator is incorporated or the when the selector switch is incorporated.  
      The amplitude modulator of the invention shown in FIGS.  1  to  4  operate in the same manner as the conventional amplitude modulator in the following manner. A high frequency signal, which is a signal to be modulated, that is input to the dielectric waveguides  1 ′ and  1 ″ (high frequency transmission line  1 ) for input is amplitude-modulated in the high frequency modulating portion M by a modulating signal output from the signal source  6 , and the amplitude-modulated high frequency signal is output from the dielectric waveguides  2 ′ and  2 ″ (high frequency transmission line  2 ) for output. In this case, the transmission characteristics of the high frequency signal that is transmitted through the amplitude modulator (that is transmitted from the input terminal  1 ′ a  and  1 ″ a  of the dielectric waveguides  1 ′ and  1 ″ to the output terminal  2 ′ a  and  2 ″ a  of the dielectric waveguides  2 ′ and  2 ″) depend on the bias current flowing through the PIN diode  3 . In the amplitude modulator of the invention, the trimmable chip resistor  4 , which is a variable resistor, is provided between the signal source  6  and the PIN diode  3 , so that by adjusting (trimming) the resistance value of the trimmable chip resistor  4 , the bias current flowing through the PIN diode  3  can be adjusted and the transmission characteristics thereof can be adjusted (tuned) to the optimal state. For example, even after the amplitude modulator is incorporated into a module or the like, the characteristics of the modulator can be tuned in a simple manner with the trimmable chip resistor  4 .  
      The selector switch of the invention shown in  FIG. 5  operates such that a high frequency signal that is input to the input terminal  81   a  is transmitted through either one of the PIN diodes  3  and  3 ″ so as to be output from the one output terminal  82   a  or the other output terminal  83   a . In this case, the transmission characteristics of the high frequency signal that is transmitted through the selector switch (that is transmitted from the input terminal  81   a  to the one output terminal  82   a  or from the input terminal  81   a  to the other output terminal  83   a ) depend on the bias current flowing through the PIN diode  3  or  3 ″. In the selector switch of the invention, the trimmable chip resistors  4  and  4 ″, which are variable resistors, are provided between the signal sources  6  and  6 ″ and the PIN diodes  3  and  3 ″, so that by adjusting (trimming) the resistance values of the trimmable chip resistors  4  and  4 ″, the bias current can be adjusted and the transmission characteristics thereof can be adjusted (tuned) to the optimal state as in the amplitude modulator shown in  FIG. 1 . For example, even after the selector switch is incorporated into a module or the like, the transmission characteristics of the selector switch can be tuned in a simple manner with the trimmable chip resistors  4  and  4 ″.  
      The same function of the trimmable chip resistor  4  can be obtained by using other variable resistors such as a trimmer resistor that is operated mechanically, for example, by rotation or with contact points, a potentiometer or a trimmer potentiometer, besides the trimmable chip resistor  4 . However, an irreversible resistor such as the trimmable chip resistor  4  is preferable, and the trimmable chip resistor  4  is particularly preferable because even if vibration is applied to the trimmable chip resistor  4 , the resistance value is not shifted or the reliability on temperature and humidity is high (the same applies to the trimmable chip resistor  4 ″).  
      Instead of the trimmable chip resistor  4 , the trimmer potentiometer  104  may be used.  
       FIG. 8  is a perspective view showing the configuration of the trimmer potentiometer  104 . The trimmer potentiometer  104  has lead wires  105  and a rotor  106 . A lead wire  105   a  or a lead wire  105   b  is connected to the signal source  6 , and a lead wire  105   c  is connected to a choke inductor  5 . The trimmer potentiometer  104  can change the resistance value between the lead wires  105   a,    105   b  and the lead wire  105   c  by engaging an engaging member in an engaging portion provided in the rotor  106  to rotate the rotor  106  around its axis. When the trimmer potentiometer  104  is used as a variable resistance, the trimmer potentiometer  104  functions so as to set the resistance value dynamically in accordance with the signal for control that is input from the outside, so that the trimmer potentiometer  104  can perform tuning so as to obtain a desired modulator characteristics against a change in the environmental conditions such as ambient temperatures or a temporal change in the characteristics of a high frequency modulating element. Therefore, the modulator characteristics can be further stabilized. As in the trimmable chip resistor  4 , trimmer potentiometer  104  is preferable in that the trimmer potentiometer  104  has the characteristics that the resistance value can be set variably and the once set resistance value is unlikely to change unexpectedly (the same applies to the trimmable chip resistor  4 ″).  
      Instead of the PIN diode  3 , other diodes such as Schottky barrier diodes or transistors such as field-effect transistors such as MESFETs and bipolar transistors may be used as the high frequency modulating element. However, it is preferable to use those that do not have a function of detecting a high frequency signal as a signal to be modulated, as the PIN diode  3  with respect to a high frequency signal in the millimeter band, because when modulating such a high frequency signal, even if the intensity of the high frequency signal that is input is changed, the bias current flowing through the high frequency modulating element is not changed thereby, and the transmission characteristics of the high frequency signal transmitted through the high frequency modulating element can be stabilized. As a result, even if the intensity of the high frequency signal that is input is changed, the modulator characteristics can be relatively stabilized.  
      Furthermore, instead of the PIN diodes  3  and  3 ″, other diodes such as Schottky barrier diodes or transistors such as field-effect transistors such as MESFETs (Metal Semiconductor Field Effect Transistors) and bipolar transistors may be used as the switching element. However, it is preferable to use those which do not have a function of detecting a high frequency signal, as the PIN diodes  3  and  3 ″ with respect to a high frequency signal in the millimeter band, because when switching such a high frequency signal, even if the intensity of the high frequency signal that is input is changed, the bias current flowing through the switching element is not changed thereby, and the transmission characteristics of the high frequency signal transmitted through the switching element can be stabilized. As a result, even if the intensity of the high frequency signal that is input is changed, the transmission characteristics of the selector switch can be relatively stabilized.  
      Furthermore, when a high frequency modulating element that does not have a function of detecting a high frequency signal as a signal to be modulated is used, it is preferable to form the island-shaped conductors  5 ′ e apart from the choke type bias supply line  5 ′ on the substrate  7  on both sides in the width direction thereof (or one side) for the following reason. A capacitance is formed between the island-shaped conductor  5 ′ e  and the line conductor  5   d  and the vicinity of the PIN diode  3 , which is the high frequency modulating element of the choke type bias supply line, and this capacitance serves to confine the electric field of the high frequency signal so that the electric field of the high frequency signal is not leaked to the dielectric waveguide  2 ′ or the nonreflective terminator  10  side. Therefore, in the amplitude modulator shown in  FIG. 1  and  FIGS. 2A and 2B , the high frequency signal is hardly input to the dielectric waveguide  2 ′ side at the time of OFF, namelywhen the signal source  6  applies the voltage to the PIN diode  3  so as to be a reverse bias, so that the ON/OFF ratio of the amplitude modulator can be high. Furthermore, in the amplitude modulator shown in  FIG. 4 , the high frequency signal is hardly input to the nonreflective terminator  10  side at the time of ON, namely when the signal source  6  applies the voltage to the PIN diode  3  so as to be a forward bias, so that the output of the high frequency signal output from the dielectric waveguide  2 ″ at the time of ON can be large.  
      The amplitude modulator may be constituted as follows: the amplitude modulator having the above-described configuration and another similar amplitude modulator whose the input terminals  1   a,    1 ′ a  and  1 ″ are connected to the output terminals  2   a,    2 ′ a  and  2 ″ a  are included, and the resistance value of the trimmable chip resistor  4  as a variable resistor of the bias supply circuit provided with each of the amplitude modulators is different from each other. In this case, the frequency characteristics of the ON/OFF ratio is different between the amplitude modulators, so that the frequency bandwidth that can provide at least a predetermined ON/OFF ratio of the frequency characteristics can be enlarged by combining two different characteristics of different frequencies that provide a high ON/OFF ratio to obtain frequency characteristics having an ON/OFF ratio obtained by summing their ON/OFF ratios. Therefore, the frequency bandwidth that can provide a predetermined ON/OFF can be increased.  
      In the amplitude modulator of the invention, besides the nonradiative dielectric waveguide, a strip line, a microstrip line, a coplanar line, a coplanar line provided with a ground, a slot line, a wave guide, a dielectric wave guide, and the like may be used as the high frequency transmission line. However, it is preferable to use the nonradiative dielectric waveguide, a wave guide, a dielectric wave guide, and the like as the high frequency transmission line for the following reason. A circuit for transmitting a high frequency signal as a signal to be modulated and the bias circuit C for transmitting a modulating signal function substantially independently, so that the variable resistor provided in the bias circuit C functions with respect to the high frequency modulating element and hardly affects directly on the high frequency signal as a signal to be modulated. Therefore, tuning can be performed with good controllability in a simple configuration.  
      In the selector switch of the invention, besides the nonradiative dielectric waveguide, a strip line, a microstrip line, a coplanar line, a coplanar line provided with a ground, a slot line, a wave guide, a dielectric wave guide, and the like may be used as the high frequency transmission line. However, it is preferable to use the nonradiative dielectric waveguide, a wave guide, a dielectric wave guide, and the like as the high frequency transmission line for the following reason. A circuit for transmitting a high frequency signal as a signal to be modulated and the bias circuits C 1  and C 2  for transmitting a selector switch control signal function substantially independently, so that the variable resistors provided in the bias circuits C 1  and C 2  function with respect to the PIN diodes  3  and  3 ′ and hardly affect directly on the high frequency signal. Therefore, tuning can be performed with good controllability in a simple configuration.  
      Next, one example of a high frequency transmitting/receiving apparatus  110  according to a first embodiment of the invention, as shown in a block circuit diagram of  FIG. 9 , includes a high frequency oscillator  11  for generating high frequency signals; a branching device  12  that is connected to the high frequency oscillator  11  for branching the high frequency signal and outputting the signal to one output terminal  12   b  and the other output terminal  12   c ; an amplitude modulator  13  that is connected to the one output terminal  12   b  and is either one of the examples of the embodiment of the invention having the above-described configurations for modulating the high frequency signal that has been branched to the one output terminal  12   b  and outputting a high frequency signal for transmission; a circulator  14  that includes a first terminal  14   a , a second terminal  14   b  and a third terminal  14   c  around a magnetic body, and outputs the high frequency signal that is input from one terminal from the next terminal adjacent in this order, the output of the amplitude modulator  13  being input to the first terminal  14   a ; an antenna  15  for transmission/reception connected to the second terminal  14   b  of the circulator  14 ; and mixer  16  connected between the other output terminal  12   c  of the branching device  12  and the third terminal  14   c  of the circulator  14  for mixing the high frequency signal branched to the other output terminal  12   c  and the high frequency signal received by the antenna  15  for transmission/reception and outputting an intermediate frequency signal.  
      In other words, the branching device  12  has two output portions  112   b ,  112   c , and an input portion  112   a  is connected to the high frequency oscillator  11 , so that the high frequency signal supplied from the high frequency oscillator  11  is branched and output from one output portion  112   b  and another output portion  112   c . When the amplitude modulator shown in  FIG. 1  is used, the input terminal  13   a  is connected to the one output portion  112   b , and the first terminal  14   a  of the circulator  14 ,which is a signal divider, is connected to the output terminal  13   b.  When the amplitude modulator shown in  FIG. 2  is used, the input terminal  1 ″ a  is connected to the one output portion  112   b , and the first terminal  14   a  of the circulator  14 , which is a signal divider, is connected to the output terminal  2 ″ a.    
      The amplitude modulator  13  modulates the high frequency signal branched by this one output portion  112   b  and outputs a high frequency signal for transmission. When the circulator  14 , which is a signal divider, receives the high frequency signal for transmission from the modulator  13  at its first terminal  14   a , the high frequency signal for transmission input from the first terminal  14   a  is output from the second terminal  14   b , and the high frequency signal for transmission input from the second terminal  14   b  is output from the third terminal  14   c . In the mixer  16 , its first input terminal  16   a  is connected to the other output portion  112   c  of the branching device  12 , and its second input terminal  16   b  is connected to the third terminal  14   c , and thus the branched high frequency signal that is output from the other output portion  112   c  is mixed with a high frequency signal received at the antenna  15  for transmission/reception so that an intermediate signal is output.  
      The high frequency transmitting/receiving apparatus  110  according to the first embodiment of the invention shown in  FIG. 9  uses a nonradiative dielectric waveguide as the high frequency transmission line for connecting the components as described above. The basic configuration of the nonradiative dielectric waveguide is the same as that shown by the partially cut perspective view of  FIG. 18 .  
      More specifically, as shown in a plan view of  FIG. 10 , the high frequency transmitting/receiving apparatus  110  according to the first embodiment of the invention shown in  FIG. 9  includes, between plate conductors  21  (the other plate conductor is not shown) that are arranged in parallel with a gap of ½ or less of the wavelength of a high frequency signal, a high frequency oscillator  11 , to which one end of a first dielectric waveguide  22  is connected, for frequency-modulating a high frequency signal output from the high frequency diode and letting the high frequency signal propagate the first dielectric waveguide  22  and outputting the high frequency signal; an amplitude modulator  13  that is connected to the other end of the first dielectric waveguide  22  and is either one of the examples of the embodiment of the invention having the above-described configurations for reflecting the high frequency signal to the input terminal  13   a  side or transmitting the high frequency signal to the output terminal  13   b  side, depending on the pulse signal; a second dielectric waveguide  23  whose one end is connected to the output terminal  13   b  of the amplitude modulator  13 ; a circulator  14  that includes a first terminal  24   a , a second terminal  24   b  and a third terminal  24   c,  which are each an input/output terminal of the high frequency signal, in the peripheral portion of a ferrite plate  24  provided in parallel with the plate conductors  21 , and outputs the high frequency signal that is input from one terminal from the next terminal adjacent in this order, the first terminal  24   a  being connected to the other end of the second dielectric waveguide  23 ; a third dielectric waveguide  25  and a fourth dielectric waveguide  26  that are provided radially in the peripheral portion of the ferrite plate  24  of the circulator  14  and whose ends are connected to the second terminal  24   b  and the third terminal  24   c,  respectively; an antenna  15  for transmission/reception connected to the other terminal of the third dielectric waveguide  25 ; a fifth dielectric waveguide  27  whose middle point is positioned close or joined to a middle point of the first dielectric waveguide  22 , that is, whose intermediate portion in the extending direction of the line is positioned close or joined to an intermediate portion in the extending direction of the first dielectric waveguide  22 , for branching a part of the high frequency signal propagated on the first dielectric waveguide  22  and propagating the high frequency signal; a nonreflective terminator  28  connected to one end of the fifth dielectric waveguide  27  on the high frequency oscillator  11  side; and a mixer  16  connected between the other end of the fourth dielectric waveguide  26  and the other end of the fifth dielectric waveguide  27  for mixing the high frequency signal input from the fifth dielectric waveguide  27  and the high frequency signal received by the antenna  15  for transmission/reception and input from the circulator  14  and outputting an intermediate frequency signal. In this configuration, the portion in which the first dielectric waveguide  22  and the fifth dielectric waveguide  27  are positioned close or are joined constitutes the branching device  12 .  
      In  FIG. 10 , the first terminal  24   a , the second terminal  24   b  and the third terminal  24   c  correspond to the first terminal  14   a , the second terminal  14   b  and the third terminal  14   c  in  FIG. 9 .  
      In this configuration, in the mixer  16 , as shown in a perspective view of  FIG. 11 , a high frequency detecting portion is such that a diode  43  as a high frequency modulating element is connected to a connection terminal  42  formed in a portion where the choke type bias supply line  41  formed on one surface in the thickness direction of the substrate  40  is interrupted. This high frequency signal detecting portion is provided in the other end of the fourth dielectric waveguide  26  and the other end of the fifth dielectric waveguide  27  such that the high frequency signals output from the fourth dielectric waveguide  26  and the fifth dielectric waveguide  27  enter the diode  43 . The choke type bias supply line  41  has the same shape as that of the choke type bias supply line  5 ′ shown in  FIG. 3  described above. In  FIG. 11 , for clarification, the choke type bias supply line  41  is hatched. In this configuration, for the diode  43  as a high frequency detecting element, a Schottky barrier diode can be used.  
      The high frequency transmitting/receiving apparatus  110  according to the first embodiment of the invention shown in  FIGS. 9 and 10  that is configured as above operates in the same manner as a conventional high frequency transmitting/receiving apparatus. However, since the amplitude modulator of the invention is provided as the amplitude modulator  13 , the amplitude modulator  13  serves to tune the modulator characteristics in accordance with the characteristics of the PIN diode  3  or its mounting state, resulting in a high performance high frequency transmitting/receiving apparatus in which good transmission output can be obtained stably. Furthermore, the trimmable chip resistor can hold a predetermined resistance value stably under an environment in which vibration or temperature changes are extreme, and therefore, even in such an environment, good modulator characteristics can be maintained. Therefore stable performance can be obtained constantly.  
      In the above configuration, it is possible that a potentiometer or a trimmer potentiometer is used as the variable resistor connected as a component of the bias circuit C of the amplitude modulator  13 , and that a part of a detected output that has been detected by the high frequency detecting portion of the mixer  16  on the other output terminal  12   c  side (the fifth dielectric waveguide  27  side) is input as a control signal to the control terminal for controlling the resistance value of the potentiometer or the trimmer potentiometer. In this case, the resistance value of the potentiometer or the trimmer potentiometer may be set during the time when the high frequency detecting portion of the mixer  16  is detecting almost only the high frequency signals output from the other output terminal  12   c  of the branching device  12 . With this configuration, the intensity of the high frequency signal output from the high frequency oscillator  11  is monitored, so that a high frequency signal for transmission that has been adjusted to a desired output intensity can be output from the amplitude modulator  13  in accordance with the variation in the intensity.  
      Furthermore, in the above configuration, preferably, a switch  17  for switching in accordance with a switching control signal from the outside maybeprovided at an output terminal of the mixer  16 . When the switch  17  for switching in accordance with a switching control signal from the outside is provided at an output terminal of the mixer  16 , that is, the output portion  16   c  from which a generated intermediate frequency signal is output, even if a part of a high frequency signal for transmission is leaked to the third terminal  14   c  of the circulator  14  because of insufficient isolation between the first terminal  14   a  and the third terminal  14   c  of the circulator  14 , the switch  17  can be operated to block an intermediate frequency signal with respect to the leaked high frequency signal so that such an intermediate frequency signal is not output. This makes it easy to identify a high frequency signal to be received on the receiving side.  
      Next, a high frequency transmitting/receiving apparatus  120  according to a second embodiment of the invention, as shown in a block circuit diagram of  FIG. 12 , includes a high frequency oscillator  11  for generating high frequency signals; a branching device  12  that is connected to the high frequency oscillator  11  for branching the high frequency signal and outputting the signal to one output terminal  12   b  and the other output terminal  12   c ; an amplitude modulator  13  that is connected to the one output terminal  12   b  and is either one of the examples of the embodiment of the invention having the above-described configurations for modulating the high frequency signal that has been branched to the one output terminal  12   b  and outputting a high frequency signal for transmission; an isolator  18  whose input terminal  18   a  is connected to the output terminal  13   b  of the amplitude modulator  13  for transmitting a high frequency signal for transmission to the output terminal  18   b  from the input terminal  18   a;  an antenna  19  for transmission connected to the isolator  18 ; an antenna  20  for reception connected to the other output terminal  12   c  of the branching device  12 ; and mixer  16  whose two input terminals  16   a ,  16   b  are connected to the other output terminal  12   c  of the branching device  12  and the antenna  20  for reception, respectively, for mixing the high frequency signal branched to the other output terminal  12   c  and the high frequency signal received by the antenna  20  for reception and outputting an intermediate frequency signal. In the high frequency transmitting/receiving apparatus  120 , the same components as in the previous embodiment bears the same reference numeral and description thereof may be omitted.  
      The high frequency transmitting/receiving apparatus  120  according to the second embodiment of the invention shown in  FIG. 12  uses a nonradiative dielectric waveguide as the high frequency transmission line for connecting the components as described above. The basic configuration of this nonradiative dielectric waveguide is the same as that shown by the partially cut perspective view of  FIG. 15 .  
      More specifically, as shown in a plan view of  FIG. 13 , the high frequency transmitting/receiving apparatus  120  according to the second embodiment of the invention shown in  FIG. 12  includes, between plate conductors  31  (the other plate conductor is not shown) that are arranged in parallel with a gap of ½ or less of the wavelength of a high frequency signal, a high frequency oscillator  11 , to which one end of a first dielectric waveguide  32  is connected, for frequency-modulating a high frequency signal output from the high frequency diode and letting the high frequency signal propagate the first dielectric waveguide  32  and outputting the high frequency signal; an amplitude modulator  13  that is connected to the other end of the first dielectric waveguide  32  and is either one of the examples of the embodiment of the invention having the above-described configurations for reflecting the high frequency signal to the input terminal  13   a  side or transmitting the high frequency signal to the output terminal  13   b  side, depending on the pulse signal; a second dielectric waveguide  33  whose one end is connected to the output terminal  13   b  of the amplitude modulator  13 ; a circulator  1  that includes a first terminal  34   a , a second terminal  34   b  and a third terminal  34   c,  which are each an input/output terminal of the high frequency signal, in the peripheral portion of a ferrite plate  34  provided in parallel with the plate conductors  31 , and outputs the high frequency signal that is input fromone terminal from the next terminal adjacent in this order, the first terminal  34   a  being connected to the other end of the second dielectric waveguide  33 ; a third dielectric waveguide  35  and a fourth dielectric waveguide  36  that are provided radially in the peripheral portion of the ferrite plate  34  of the circulator  1  and whose ends are connected to the second terminal  34   b  and the third terminal  34   c,  respectively; an antenna  19  for transmission connected to the other end of the third dielectric waveguide  35 ; a fifth dielectric waveguide  37  whose middle point is positioned close or joined to a middle point of the first dielectric waveguide  32 , for branching a part of the high frequency signal propagated on the first dielectric waveguide  32  and propagating the high frequency signal; a nonreflective terminator  38   a  connected to the other end of the fourth dielectric waveguide  36 ; a nonreflective terminator  38   b  connected to one end of the fifth dielectric waveguide  37  on the high frequency oscillator  11  side; a sixth dielectric waveguide  39  whose one end is connected to the antenna  20  for reception; and a mixer  16  connected between the other end of the fifth dielectric waveguide  37  and the other end of the sixth dielectric waveguide  39  for mixing the high frequency signal input from the fifth dielectric waveguide  37  and the high frequency signal received by the antenna  20  for reception and input from the sixth dielectric waveguide  39  and outputting an intermediate frequency signal. In this configuration, the portion in which the first dielectric waveguide  32  and the fifth dielectric waveguide  37  are positioned Close or are joined constitutes the branching device  12 . The isolator  18  is constituted by including circulator  14 , the fourth dielectric waveguide  36 , and the nonreflective terminator  38   a.    
      In  FIG. 13 , the first terminal  34   a , the second terminal  34   b  and the third terminal  34   c  correspond to the input terminal  18   a,  the input terminal  18   b  and the input terminal  18   c  in  FIG.12 .  
      In this configuration, in the mixer  16 , as shown in a perspective view of  FIG. 11 , a high frequency detecting portion is such that a diode  43  as a high frequency modulating element is connected to a connection terminal  42  formed in a portion where the choke type bias supply line  41  formed on one surface in the thickness direction of the substrate  40  is interrupted. This high frequency signal detecting portion is provided in the other end of the fifth dielectric waveguide  27  and the other end of the sixth dielectric waveguide  39  such that the high frequency signals output from the fifth dielectric waveguide  27  and the sixth dielectric waveguide  39  enter the diode  43 . In this configuration, for the diode  43  as a high frequency detecting element, a Schottky barrier diode can be used.  
      The high frequency transmitting/receiving apparatus  120  according to the second embodiment of the invention shown in  FIGS. 12 and 13  that is configured as above operates in the same manner as a conventional high frequency transmitting/receiving apparatus. However, since the amplitude modulator of the invention is provided as the amplitude modulator  13 , the amplitude modulator  13  serves to tune the modulator characteristics in accordance with the characteristics of the PIN diode  3  or its mounting state, resulting in a high performance high frequency transmitting/receiving apparatus in which good transmission output can be obtained stably. Furthermore, the trimmable chip resistor can hold a predetermined resistance value stably under an environment in which vibration or temperature changes are extreme, and therefore, even in such an environment, good modulator characteristics can be maintained. Therefore stable performance can be obtained constantly.  
      In the above configuration, it is possible that a potentiometer or a trimmer potentiometer is used as the variable resistor connected as a component of the bias circuit C of the amplitude modulator  13 , and that a part of a detected output that has been detected by the high frequency detecting portion of the mixer  16  on the other output terminal  12   c  side (the fifth dielectric waveguide  37  side) is input as a control signal to the control terminal for controlling the resistance value of the potentiometer or the trimmer potentiometer. In this case, the resistance value of the potentiometer or the trimmer potentiometer may be set during the time when the high frequency detecting portion of the mixer  16  is detecting almost only the high frequency signals output from the other output terminal  12   c  of the branching device  12 . With this configuration, the intensity of the high frequency signal output from the high frequency oscillator  11  is monitored, so that a high frequency signal for transmission that has been adjusted to a desired output intensity can be output from the amplitude modulator  13  in accordance with the variation in the intensity.  
      Furthermore, in the above configuration, preferably, a switch  17  for switching in accordance with a switching control signal from the outside may be provided at an output terminal of the mixer  16 . When the switch  17  for switching in accordance with a switching control signal from the outside is provided at an output terminal of the mixer  16 , that is, the output portion  16   c  from which a generated intermediate frequency signal is output, even if a part of a high frequency signal for transmission is leaked to the antenna  20  for reception because of insufficient isolation between the antenna  19  for transmission and the antenna  20  for reception, the switch  17  can be operated to block an intermediate frequency signal with respect to the leaked high frequency signal so that such an intermediate frequency signal is not output. This makes it easy to identify a high frequency signal to be received on the receiving side.  
      Next, a high frequency transmitting/receiving apparatus  130  according to a third embodiment of the invention shown in  FIG. 14  includes a high frequency oscillator  11  for generating high frequency signals; the selector switch  71  according to one embodiment of the invention having the above configuration, whose input terminal  71   a  is connected to the high frequency oscillator  11 , for switching a high frequency signal so that,either a high frequency signal RFt for transmission is output to the one output terminal  71   b  or a local signal L 0  is output to the other output terminal  71   c ; a second selector switch  72  having an input terminal  72   b , an output terminal  72   c  and an input/output terminal  72   a , whose input terminal  72   b  is connected to the one output terminal  71   b , for switching the input/output terminal  72   a  to the input terminal  72   b  or the output terminal  72   c  for connection; an antenna  15  for transmission connected to the input/output terminal  72   a  of the second selector switch  72 ; and a mixer  16  connected between the other output terminal  71   c  of the selector switch  71  and the terminal  72   c  of the second selector switch  72  for mixing the local signal L 0  output to the other output terminal  71   c  and the high frequency signal received by the antenna  15  for transmission/reception and outputting an intermediate frequency signal. In the high frequency transmitting/receiving apparatus  130  according to the third embodiment of the invention, the same components as in the previous embodiments bear the same reference numerals and description thereof may be omitted.  
      The second selector switch  72 , which is a signal divider, has a first terminal  172   b  in which the input terminal  72   b  is formed, a second terminal  172   a  in which the input/output terminal  72   a  is formed and a third terminal  172   c  in which the output terminal  72   c  is formed. The second selector switch  72  switches the connection state between the first terminal  172   b , the second terminal  172   a  and the third terminal  172   c , so that a high frequency signal for transmission is supplied from the selector switch  71  to the first terminal  172   a , and the high frequency signal input from the first terminal  172   b  is output to the second terminal  172   a , and the high frequency signal input from the second terminal  172   a  is output to the third terminal  172   c . The mixer  16  is connected to the other output terminal  71   c  of the selector switch  71  and the third terminal  172   c  of the second selector switch  72 .  
      When outputting a high frequency signal for transmission from the antenna  15  for transmission/reception, a control signal from the outside is supplied to the selector switch  71  and the second selector switch  72  so that in the selector switch  71 , the high frequency signal supplied to the input terminal  71   a  is output from the one output terminal  71   b , and that in the second selector switch  72 , the high frequency signal supplied to the first terminal  172   b  is supplied to the second terminal  172   a . When receiving a high frequency signal by the antenna  15  for transmission/reception, a control signal from the outside is supplied to the selector switch  71  and the second selector switch  72  so that in the selector switch  71 , the high frequency signal supplied to the input terminal  71   a  is output from the other output terminal  71   c , and that in the second selector switch  72 , the high frequency signal supplied to the second terminal  172   a  is supplied to the third terminal  172   c.    
      The input terminal  71   a , the one output terminal  71   b , and the other output terminal  71   c  in  FIG. 14  correspond to the input terminal  81   a , the one output terminal  82   a , and the other output terminal  83   a  in  FIG. 5 .  
      Next, a high frequency transmitting/receiving apparatus  140  according to a fourth embodiment of the invention shown in  FIG. 15  includes a high frequency oscillator  11  for generating high frequency signals; the selector switch  71  of an embodiment of the invention having the above configuration whose input terminal  71   a  is connected to the high frequency oscillator  11 , for switching a high frequency signal so that either a high frequency signal RFt for transmission is output to the one output terminal  71   b  or a local signal L 0  is output to the other output terminal  71   c ; an antenna  19  for transmission connected to the one output terminal  71   b ; an antenna  20  for reception connected to the other output terminal  71   c  side of the selector switch  71 ; and a mixer  16  connected between the other output terminal  71   c  of the selector switch  71  and the antenna  20  for reception for mixing the local signal L 0  output to the other output terminal  71   c  and the high frequency signal received by the antenna  20  for reception and outputting an intermediate frequency signal. In the high frequency transmitting/receiving apparatus  140 , the same components as in the previous embodiments bear the same reference numerals and description thereof may be omitted.  
      When outputting a high frequency signal for transmission from the antenna  19  for transmission, a control signal from the outside is supplied to the splector switch  71  so that in the selector switch  71 , the high frequency signal supplied to the input terminal  71   a  is output from the one output terminal  71   b . When receiving a high frequency signal by the antenna  20  for reception, a control signal from the outside is supplied to the selector switch  71  so that in the selector switch  71 , the high frequency signal supplied to the input terminal  71   a  is output from the other output terminal  71   c.    
      The high frequency transmitting/receiving apparatuses  130  and  140  according to the third and fourth embodiments of the invention shown in  FIGS. 14 and 15  also can use a nonradiative dielectric waveguide as the high frequency transmission line for connecting the components as described above. The basic configuration of the nonradiative dielectric waveguide is the same as that shown by the partially cut perspective view of  FIG. 18 .  
      According to the high frequency transmitting/receiving apparatuses  130  and  140  according to the third and fourth embodiments of the invention shown in  FIGS. 14 and 15 , since the selector switch of the invention is provided as the selector switch  71 , the selector switch  71  serves to tune the transmission characteristics of the selector switch in accordance with the characteristics of the PIN diodes  3  and  3 ″ or their mounting state, resulting in a high performance high frequency transmitting/receiving apparatus in which good transmission output can be obtained stably. Furthermore, trimmable chip resistor can hold a predetermined resistance value stably under an environment vibration or temperature changes are extreme, and therefore, even in such an environment, good modulator characteristics can be maintained. Therefore stable performance can be obtained constantly.  
      In the above configuration, it is possible that a potentiometer or a trimmer potentiometer is used as the variable resistor connected as a component of the bias circuits C 1  and C 2  of the selector switch  71 , and that a part of a detected output that has been detected by the high frequency detecting portion of the mixer  16  on the other output terminal  71   c  side is input as a control signal to the control terminal for controlling the resistance value of the potentiometer or the trimmer potentiometer. In this case, the resistance value of the potentiometer or the trimmer potentiometer may be set during the time when the high frequency detecting portion of the mixer  16  is detecting almost only the high frequency signals output from the other output terminal  71   c  of the selector switch  71 . With this configuration, the intensity of the high frequency signal output from the high frequency oscillator  11  is monitored, so that a high frequency signal RFt for transmission that is adjusted to a desired output intensity can be output from the selector switch  71  in accordance with the variation in the intensity.  
      Also in the high frequency transmitting/receiving apparatuses  130  and  140  according to the third and fourth embodiments of the invention, as shown in  FIGS. 14 and 15 , a switch  17  constituted by a semiconductor such as CMOS (complementary metal oxide semiconductor) that blocks intermediate frequency signals in response to a control signal from the outside may be connected to the output terminal of the mixer  16 . In this case, the switch  17  operates so as to block an unwanted intermediate frequency signals corresponding toa high frequency signal that is transmitted between the first terminal  172   b  of the second selector switch  72  and the third terminal  172   c  or between the antenna  19  for transmission and the antenna  20  for reception, leaked and input to the mixer  16 , so that unwanted noise is not mixed to an intermediate frequency signal to be received, and therefore the receiving performance can be improved.  
      Next, in the high frequency transmitting/receiving apparatus of the invention, the first to the sixth dielectric waveguides  22 ,  23 ,  25  to  27 ,  32 ,  33 ,  35  to  37  and  39  are preferably made of, for example, resins such as tetrafluoroethylene and polystyrene, or ceramics such as cordierite (2MgO-2Al 2 O 3 -5SiO 2 ) ceramics having a low dielectric, alumina (Al 2 O 3 ) ceramics and glass ceramics, and these materials provide low loss in high frequency signals in the millimeter wave band.  
      The cross-sectional shape of the first to the sixth dielectric waveguides  22 ,  23 ,  25  to  27 ,  32 ,  33 ,  35  to  37  and  39  in one virtual plane that is perpendicular to the extending direction is basically substantially rectangular, but may be rounded at the corners of a rectangle, and various cross-sectional shapes used for transfer of high frequency signals can be used.  
      As the material for the ferrite plates  24  and  34 , among ferrite, zinc nickel iron oxide (Zn a Ni b Fe c O x ) is preferable to high frequency signals.  
      Furthermore, the shape of the ferrite plates  24  and  34  is, in general, circular, but besides that, the shape viewed from the top may be regular polygonal, in other words, the shape viewed from one side in the thickness direction may be regular polygonal. In this case, taking the number of dielectric waveguides to be connected as n (n is an integer of 3 or more), it is preferable that the shape viewed from the top is regular polygonal having m sides (m is an integer of 3 or more and larger than n).  
      As the material for the plate conductors  21  and  31  and the other plate conductors that are not shown in the drawings, conductor plates of Cu, Al, Fe, Ag, Au, Pt, SUS (stainless steel) and brass (Cu—Zn alloy) are preferable because of high electrical conductivity, good processibility and the like. Alternatively, layers of these conductors formed on a surface of an insulating plate made of ceramics, resin and the like may be used.  
      The nonreflective terminators  28 ,  38   a  and  38   b  can be constituted by allowing a film-like resistor or wave absorber to adhere onto an internal surface of the dielectric waveguide  53  that is parallel to the plate conductors  51  and  52  with respect to such a dielectric waveguide  53  as shown in  FIG. 18 , for example. In this case, as the material of the resistor, nickel chrome alloys, carbons and the like are preferable. As the material of the wave absorber, permalloy, sendust and the like are preferable. With these materials, millimeter wave signals can be attenuated efficiently. Any other materials that can attenuate millimeter wave signals can be used.  
      For the substrates  7  and  40 , a substrate obtained by forming a choke type bias supply lines  5 ′ and  41  made of a strip conductor or the like formed of aluminum (Al), gold (Au) , copper (Cu) or the like on one principal surface of a plate-like substrate made of tetrafluoroethylene, polystyrene, glass ceramics, glass epoxy resin, epoxy resin, thermoplastic resins or the like, such as so-called liquid crystal polymer or the like can be used.  
      The high frequency transmitting/receiving apparatus of the invention is characterized in that at least one of the amplitude modulator and the selector switch of the invention is provided, and as the high frequency transmission line connecting between the circuit elements, besides the nonradiative dielectric waveguide, a wave guide, a dielectric wave guide, a strip line, a microstrip line, a coplanar line, a slot line, a coaxial line, high frequency transmission lines obtained by transforming these lines may be selected in accordance with the frequency band to be used or the applications. The frequency band to be used is effective, not only for the millimeter wave band, but also for the microwave band or smaller frequency band.  
      Instead of the circular  14 , a duplexer, a switch, a hybrid circuit and the like may be used. For the high frequency oscillator, the modulator and the mixer, bipolar transistors, field effect transistors (FETs) and integrated circuits (CMOS, MMIC, etc.) which integrate these above, and the like can be used, instead of diodes.  
      Next, a radar apparatus of the invention, and a radar apparatus-mounting vehicle and a radar apparatus-mounting small boat on which a radar apparatus is mounted will be described.  
      One example of an embodiment of the radar apparatus of the invention has a configuration including either one of the high frequency transmitting/receiving apparatuses  110 ,  120 ,  130 , and  140  according to the first to the fourth embodiments of the invention and a distance information detecting device for detecting in formation on the distance up to an object to be detected by processing an intermediate frequency signal output from the high frequency transmitting/receiving apparatuses  110 ,  120 ,  130 , and  140 .  
      The radar apparatus of the invention has the configuration as above, and therefore the high frequency transmitting/receiving apparatus of the invention transmits high frequency signals for transmission stably at good transmission output, so that a radar apparatus that can detect an object to be detected fast and reliably and detect an object to be detected that is near or far away fast and reliably can be obtained. Furthermore, with the high frequency transmitting/receiving apparatus of the invention that provides stable performance, even in an environment in which vibration, temperature changes or the like are extreme, a radar apparatus that operates reliably under such extreme conditions can be obtained.  
      The radar apparatus-mounting vehicle of the invention has a configuration that includes the radar apparatus of the invention and in which the radar apparatus is used to detect an object to be detected.  
      The radar apparatus-mounting vehicle of the invention has such a configuration, so that similarly to a conventional radar apparatus-mounting vehicle, the behavior of the vehicle can be controlled based on the distance information detected by the radar apparatus, or the driver can be warned of, for example, an obstacle on a street or other vehicles, with sound, light or vibration. However, in the radar apparatus-mounting vehicle of the invention, an obstacle on a street or other vehicles can be detected fast and reliably so that appropriate control of the vehicle and appropriate warning to the driver can be performed without causing the vehicle to perform a sudden action.  
      The radar apparatus-mounting vehicle of the invention can be used, to be specific, in not only vehicles for transporting passengers or cargo such as railway trains, electric trains, and automobiles, but also bicycles, motor bicycles, and rides in an amusement park and carts in golf links.  
      The radar apparatus-mounting small ship of the invention has a configuration that includes the radar apparatus of the invention and in which the radar apparatus is used to detect an object to be detected.  
      The radar apparatus-mounting small ship of the invention has such a configuration, so that similarly to a conventional radar apparatus-mounting vehicle, in the small ship, the behavior of the small ship can be controlled based on the distance information detected by the radar apparatus, or the operator can be warned of, for example, obstacles such as a reef, other vessels or other small boats, with sound, light or vibration. However, in the radar apparatus-mounting small ship of the invention, obstacles such as a reef, other vessels or other small boats can be detected fast and reliably so that appropriate control of the small ship and appropriate warning to the operator can be performed without causing the small ship to perform a sudden action.  
      The radar apparatus-mounting small boat of the invention is, to be specific, a boat that can be operated with or without a license of a small vessel, and can be used in a hand-worked fishing boat, dinghy, water motorbike, small bus-boat with an outboard motor, inflatable boat(rubber boat) with an outboard motor, fishing vessel, pleasure and fishing boat, work boat, barge, towing boat, sports boat, fishing boat, yacht, offshore yacht, cruiser, and pleasure boat with a gross tonnage of 20 tons or more.  
      Next, the transmission characteristics of a high frequency signal in the amplitude modulator shown in  FIG. 1  and the selector switch shown in shown in  FIG. 5  will be described, Table 1 shows the results of experiments in which the attenuation amount of the high frequency signal was measured when the resistance value of the trimmable chip resistor  4  was changed in the amplitude modulator shown in  FIG. 1  and the selector switch shown in  FIG. 5 .  
                                       TABLE 1                               Diode       Trimmable   Trimmable               Diode   direct   Bias   resistance   resistance       Id   applied   current   Voltage   voltage   value   Attenuation       (mA)   Vd (V)   R (Ω)   (V)   Vr (V)   Rr (Ω)   amount (dB)                                                            0.0   0.00   —   5   5.00   —   16.25       0.2   1.14   5700   5   3.86   19300   4.83       0.4   1.17   2925   5   3.83   9575   2.95       0.7   1.20   1714   5   3.80   5429   1.88       1.0   1.22   1220   5   3.78   3780   1.37       1.5   1.24   827   5   3.76   2507   0.93       2.0   1.25   625   5   3.75   1875   0.69       3.0   1.27   423   5   3.73   1243   0.44       4.0   1.28   320   5   3.72   930   0.30       6.0   1.30   217   5   3.70   617   0.16       8.0   1.31   164   5   3.69   461   0.08       10.0   1.32   132   5   3.68   366   0.05       12.0   1.33   111   5   3.67   306   0.00                  
 
      In Table 1, Id (unit: mA) refers to the bias current flowing through the PIN diode  3 ; diode applied Vd (unit: V) refers to the bias voltage applied to the PIN diode  3 ; diode direct current R (unit: Ω) refers to the direct current resistance of the PIN diode  3 ; the bias voltage (unit: V) refers to the voltage supplied from the signal source  6 ; the trimmable resistance voltage Vr (unit: V) refers to the voltage applied to the trimmable chip resistor  4 ; the trimmable resistance value Rr (unit: Ω) refers to the resistance value of the trimmable chip resistor  4 ; and the attenuation amount (unit: dB) refers to the attenuation amount of the high frequency signal output from the output terminal with respect to the high frequency signal input from the input terminal.  
       FIG. 16  is a graph showing the relationship between the bias voltage applied to the PIN diode  3  shown in Table 1 and the bias current. In  FIG. 16 , the horizontal axis shows the bias voltage (unit: V), and the vertical axis shows the bias current (unit: A). When a voltage is applied to the PIN diode  3  so as to be a forward direction bias, current does not flow until a predetermined voltage, in this embodiment, until about 1.14 V, but when exceeding this predetermined voltage, current flows rapidly.  
       FIG. 17  is a graph showing the relationship between the resistance value of the trimmable chip resistor  4  shown in Table 1 and the attenuation amount of the high frequency signal. In  FIG. 17 , the horizontal axis shows the resistance value (unit: Ω) of the trimmable chip resistor  4 , and the vertical axis shows the attenuation amount of the high frequency signal. The resistance value of the trimmable chip resistor  4  is referred to as “trimmable resistance value”. In the amplitude modulator and the selector switch, when the trimmable resistance value increases, the attenuation amount of the high frequency signal increases. That is to say, the amplitude of the high frequency signal can be reduced by increasing the trimmable resistance value in the amplitude modulator. The trimmable chip resistor  4  is an irreversible resistor, so that when adjusting the amplitude of the high frequency signal, the current flowing through the PIN diode  3  is changed to one direction, that is, reduced in this embodiment.  
      Thus, according to the invention, an amplitude modulator can be provided in which the bias supply circuit of a high frequency modulating element, which is a component of the amplitude modulator, is provided with a variable resistor, and with this variable resistor, the modulator characteristics can be tuned in a simple manner. Furthermore, a high performance high frequency transmitting/receiving apparatus that can stabilize the high frequency signal for transmission with a predetermined output intensity in a simple configuration can be provided by including at least either one of the amplitude modulator and the selector switch, and a radar apparatus provided with such a high performance high frequency transmitting/receiving apparatus and a radar apparatus-mounting vehicle and radar apparatus-mounting small boat that are provided with such a radar apparatus can be provided.  
      The present invention is not limited to the examples of the embodiments described above, and can be modified to various other forms in the scope of the gist of the invention. For example, as the variable resistor, an element in which a contact point of a fixed resistor network in which a plurality of fixed resistors are connected is switched with a relay can be used. In this case, it is possible that the resistance value of the fixed resistor network can be set dynamically, and for example, the fixed resistor network is synchronized with the operation of the amplitude modulator  13  so that the operation of the amplitude modulator  13  becomes appropriate against changes of ambient conditions, so that the bias current of the amplitude modulator  13  is dynamically changed.  
      The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.