Electromagnetic radiation monitor

Abstract: An electromagnetic radiation monitor includes an electromagnetic radiation sensor (44), a conductive shield (48) spaced apart from the sensor and a backing of graduated lossy material (46). The electromagnetic radiation sensor (44) includes thin film resistive thermocouples (54). The backing (46) of graduated lossy material is interposed between the radiation sensor (44) and the conductive shield (48). The layer is progressively more conductive through its thickness from the side which faces the sensor to the side which faces the conductive shield (48). The invention may be provided in a form of a personal electromagnetic radiation monitor or an area (fixed position) electromagnetic radiation monitor.

Description: This invention relates to radiation monitors, and more specifically relates to an electromagnetic radiation monitor which may either be worn by persons who may be exposed to potentially harmful levels of electromagnetic energy or may be strategically positioned at a fixed location whereby persons will be alerted as to the presence of potentially harmful levels of electromagnetic radiation. Even more particularly, this invention relates to a personal or area (fixed position) electromagnetic radiation monitor which exhibits broadband frequency performance and polarization independence. Attempts have been made to male an electromagnetic radiation monitor which may be worn by a person working in areas where potentially harmful electromagnetic radiation may be present or which may be strategically positioned at a fixed location to detect excess leakage of RF radiation and alert persons to the leakage. Early studies, such as those reported by Beischer in his article Microwave Reflection, Diffraction and Transmission By Man , Department of Naval Aerospace Medical Research Lab, Pensacola, Florida, June, 1973, have shown that scattering from a body may produce errors greater than 2dB. This scattering becomes more significant where broadband frequency performance and independence of polarization are desired monitor characteristics. To this date, no practical device having a broadband frequency response and being independent of polarization, to the knowledge of the inventor, has been successfully marketed. It is also known from document GB-A-803792 an apparatus for radio frequency measuring comprising a transmitter and a receiver located in a chamber. The transmitter is placed at the focus of a curved reflecting wall, and walls with wave-absorbing layers are placed along the lateral walls of the chamber so as to ensure that only the direct wave from the transmitter and the reflected waves from the curved reflecting wall can reach the receiver. The apparatus also presents a backing made of a graduated lossy material. It is also known from document GB-A-998779 a portable measuring instrument for detecting high frequency electromagnetic radiation. The device comprises circular housing having two two pairs of slot aerials of different lenghts so as to detect the presence of electromagnetic radiations over a wide frequency range. A conducting surface may be placed behind the low frequency aerials to provide a capacitive shunt effect so as to reduce their response to high frequency components of radiation, and the device may be backed by material having an appreciable dielectric loss at the higher frequencies to reduce the response of the high frequency aerials at the lower frequencies, at which frequencies the lossy material will appear reactive. It is an object of the present invention to provide an electromagnetic radiation monitor which may be worn by persons who may be exposed to potentially harmful levels of electromagnetic energy. It is another object of the present invention to provide an electromagnetic radiation monitor which exhibits broadband frequency performance and substantial independence of polarization. It is a further object of the present invention to provide an unobtrusive, pocket-size personal electromagnetic radiation monitor which accurately detects RF radiation and sends out a warning the moment it senses that the wearer moves into a danger zone. It is yet another object of the present invention to provide an electromagnetic radiation monitor that includes an audible and visual alarm which will indicate a need to leave an area promptly. It is still another object of the present invention to provide a personal electromagnetic radiation monitor which includes a dual battery system and a low battery warning when battery replacement is required. It is yet a further object of the present invention to provide a personal electromagnetic radiation monitor which includes structure that allows sensitive electronic circuitry contained in the monitor to operate correctly while effectively "surrounded" by high level RF radiation, thereby producing minimal false alarms within or outside of the monitor's specified frequency range. It is still a further object of the present invention to provide a personal electromagnetic radiation monitor whose performance is substantially unaffected due to scattering when worn by a person. It is yet a further object of the present invention to provide an area (fixed position) electromagnetic radiation monitor by which an area near a source of high power RF or microwave energy can be monitored. It is still a further object of the present invention to provide an area (fixed position) electromagnetic radiation monitor which accurately detects RF or microwave energy within a broad frequency range independent of polarization and sends out a warning when irradiated by power exceeding a threshold amount. It is yet still a further object of the present invention to provide an area (fixed position) electromagnetic radiation monitor which includes an audio and visual alarm and includes structure that allows sensitive electronic circuitry contained in the monitor to operate correctly while irradiated. In accordance with the present invention as defined in Claim 1, a personal electromagnetic radiation monitor includes an electromagnetic radiation sensor, a backing of graduated lossy material and a conductive shield spaced apart from the sensor. The electromagnetic radiation sensor may include thin film resistive thermocouples. The radiation sensor is situated in proximity of the backing of graduated lossy material so that the backing is between the radiation sensor and one side of the conductive shield. The backing is progressively more conductive through its thickness from the front surface of the backing which faces the sensor to the back surface which faces the conductive shield. The radiation monitor also comprises an audible alarm transducer connected with the electromagnetic radiation sensor and an ear plug assembly in communication with the audible alarm transducer, the ear plug assembly being electrically non-conductive to prevent RF energy from being received by the assembly and radiated to the wearer's head. In accordance with a preferred embodiment, the radiation sensor, backing of lossy material and conductive shield are all mounted in a housing of the monitor. On the inside of one of the housing sides is mounted an audible alarm transducer, which transducer is situated in alignment with an opening formed through the thickness of the housing side. A first hook and loop fastener patch is mounted on the outside surface of the housing side and surrounding the opening. Preferably, the monitor ear plug assembly has an elongated pneumatic tube having opposite ends, a pneumatic ear plug mounted on one end of the tube and a hollow housing defining an interior cavity mounted on the other end. The cavity housing has an opening formed through its thickness, and includes a second hook and loop fastener patch surrounding the opening. The second fastener patch is adapted to mate with the first fastener patch of the monitor housing to allow the cavity housing of the ear plug assembly to be mounted adjacent to the transducer. In this manner, the non-conductive ear plug assembly may be quickly and easily secured to the monitor housing so that the wearer of the monitor may use the ear plug assembly in high noise environments. In accordance with another preferred embodiment, the monitor includes an alarm circuit and a dual battery system, and a low battery sensor. When a low battery voltage is detected on one battery, the audible alarm transducer will chirp at a predetermined interval. At an extremely low battery level, the audible alarm will emit a continuous tone. The second battery is used substantially independently of the first battery to illuminate a warning light in dangerous radiation environments. If the warning light circuit fails, the audible alarm circuit will continue to function. In a further preferred embodiment, the sensitive electronic circuitry of the monitor is protected from high level RF radiation by including a second section of graduated lossy material situated in the housing and at least partially surrounding the batteries of the monitor. In this way, RF energy entering the monitor will be absorbed by this second section before it is "picked up" by the battery leads between the circuitry and the battery. In accordance with a further embodiment, an area electromagnetic radiation monitor is provided. The area electromagnetic radiation monitor includes a radiation sensor proximately situated to a backing of graduated lossy material, a sensor and an extruded, non-conductive housing structure which is affixed to a conductive shield and encases the radiation sensor and graduated lossy material. In accordance with another preferred solution, the backing of graduated lossy material comprises a laminate formed from at least first, second and third layers of lossy material having different resistivities, wherein the first layer is more resistive than the second layer and wherein the second layer is more resistive than the third layer. The third layer being positioned closest to the conductive shield and the first layer being tapered so as to minimize field distortion. These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings, in which: Fig. 1 is a front perspective view of a personal electromagnetic radiation monitor formed in accordance with the present invention; Fig. 2 is a rear perspective view of the radiation monitor of the present invention; Fig. 3 is an exploded view, in perspective, of a portion of the monitor shown in Fig. 1; Fig. 4 is a front view of a preferred form of a radiation sensor for use in the monitor of the present invention; Fig. 5 is a cross-sectional view of an ear plug assembly for use with the monitor of the present invention; Fig. 6 is a graph illustrating the performance of the radiation monitor of the present invention over frequency in free space and when worn by a user; Fig. 7 is a preferred form of an electronic circuit used in conjunction with the monitor of the present invention; Fig. 8 is an elevated perspective view of an area electromagnetic radiation monitor formed in accordance with a second form of the present invention; Fig. 9 is a cross-sectional view of the area electromagnetic radiation monitor shown in Figure 8. A first embodiment of the electromagnetic radiation monitor of the present invention is shown in Figs. 1 and 2 of the drawings, it will be seen that a personal electromagnetic radiation monitor constructed in accordance with one form of the present invention includes a two piece housing 2. Mounted on the inside surface of the front wall 4 of the two piece housing is an audible alarm transducer 6 (see Fig. 3), such as a piezo ceramic transducer, which, as will be explained, provides a warning of high level RF radiation or that the battery used in the monitor is at a low voltage. During an initial turn-on test, the audible alarm 6 preferably provides a one second burst. Above a preset threshold of the detected RF energy, the alarm provides a periodic nominal one second burns with the repetition rate increasing with the level of exposure. When the monitor battery is at a low voltage, the alarm 6 provides an audible chirp every forty seconds to a continuous warble (as the battery voltage drops). If the battery voltage is so low that the electromagnetic radiation sensor used in the monitor fails, the audible alarm provides a continuous tone. The personal electromagnetic radiation monitor of the present invention further includes a visual display in the form of a light emitting diode (LED) 8. The LED 8 is mounted on the upper wall 10 of the mated housing. During an initial turn-on test, the circuitry included in the monitor will illuminate the LED 8. When a predetermined amount of RF energy is detected by the monitor, the electronic circuitry will cause the LED 8 to light and the audible alarm 6 to beep, indicating a need for the wearer of the monitor to leave the area promptly. The monitor further includes an on/off switch 12 mounted on a side wall 14 of the mated housing, as well as a resilient clip 16 mounted on the back wall 18 of the housing to allow the monitor to be carried by the wearer on his belt or shirt pocket. The radiation monitor of the present invention also includes an ear plug assembly 20 so that the monitor may be used in high noise environments. As shown in Figs 1 and 5 of the drawings, a Velcro (TM) hook and loop fastener patch 22 is mounted on the outside surface of the housing wall 4 on which the transducer 6 is mounted and situated to surround an opening 23 formed in the wall and aligned with the transducer. The ear plug assembly 20 includes an elongated hollow tube 24. A pneumatic ear piece or ear plug 26 is mounted on one end of the tube 24, and a hollow housing 28 defining an interior cavity 30 is mounted on the other end. The cavity housing 28 has an opening 32 formed through its thickness, and includes a second hook and loop fastener patch 34 surrounding the opening 32 which is adapted to mate with the first hook and loop fastener patch 22 of the monitor housing. This will allow the cavity housing 28 to be mounted adjacent to the transducer 6 so that the tone emitted by the transducer will be carried by air pressure through the tube to the pneumatic ear plug 26. One advantage of the ear plug assembly 20 of the present invention is that it is completely electrically non-conductive. Thus, the ear plug assembly will not pick up RF energy which might have otherwise affected the electronic circuitry of the monitor, as a conventional electrical transducer would, and further provides a safety feature in that the user of the monitor does not wear an electrically conductive device on his head, to prevent electrical shock and to prevent RF energy from being picked up by the ear plug assembly and being radiated to the wearer's head. The mating Velcro (TM) hook and loop fastener patches 34, 22 on the ear plug assembly and the monitor housing allow the ear plug assembly to be quickly and easily connected non-electrically to the radiation monitor. Referring now to Fig. 3 of the drawings, it will be seen that a preferred form of the personal electromagnetic radiation monitor of the present invention includes a first housing half 36 on which is mounted the transducer 6, as mentioned previously, and a second housing half 38 which includes a battery compartment 40, and on which is mounted the on/off switch 12. Sandwiched between the housing halves is an electromagnetic radiation sensor 44, a backing of graduated lossy material 46, a conductive shield 48 which acts as a back or ground plane, and a printed circuit board 50 containing the electronic circuitry for the monitor. The electromagnetic radiation sensor 44 preferably includes thin film resistive thermocouples. In one form of the present invention, the electromagnetic radiation sensor includes a pair of mutually orthogonally disposed antenna elements 52 residing coplanarly to each other, but more preferably includes four interconnected antenna elements 52, adjacent antenna elements being at right angles to each other, all four antenna elements 52 residing in the same plane, as shown in Fig. 4. Each antenna element 52 extends along a respective longitudinal axis, and includes an array of the thin film resistive thermocouples 54 arranged along the axis, and a plurality of spaced apart conductive elements 56 of discrete length arranged transversely to the axis of the array. Each thermocouple 54 of each antenna element is disposed between adjacent conductive elements 56. The antenna arrangement used in the radiation monitor of the present invention is very similar to that disclosed in U.S. Patent No. 4.518.912. The thin film elements function as both antenna and detector. Preferably, the hot and cold junctions of the thermocouples are separated by only a 0,5 millimetre, thereby eliminating offsets due to changes in ambient temperature, either spatial or temporal. The use of the thin film resistive thermocouples 54 in the particular arrangement described above provides broadband, true RMS detection of electromagnetic energy. The electromagnetic radiation sensor 44 is mounted against a thin Mylar (TM), or more preferably, Kapton (TM) spacer sheet 58. The spacer 58 is preferably about 0,0127 centimetres in thickness, and is provided to prevent shorting of the sensor on the lossy material backing 46. The lossy material backing 46 is graduated in resistivity. More specifically, the backing includes a front surface 60 facing the sensor 44 and an opposite back surface 62 facing the conductive shield 48. The backing is progressively more conductive through its thickness from the front surface 60 to the back surface 62. In one form of the invention, the backing may be a laminate or sandwich of two or more layers of lossy material having different resistivities, as shown in Fig. 3. Alternatively, the backing may be formed from a lossy material which is continuously more conductive through its thickness from the front surface to the back surface. When a backing of a laminate of two layers of lossy material is used, the first layer 64 closest to the sensor is preferably a 0,635 centimetres thick high resistivity material, and the second layer 66 closest to the conductive shield is preferably a 0,635 centimetres thick low resistivity material. The first layer 64 preferably has a resistivity of about 15kΩ (kilo ohms) per cubic centimetre, and has a surface resistivity of approximately 20.000 ohms per square centimetre. The second layer 66 has a lower resistivity of approximately 600 ohms per cubic centimetre, and has a surface resistivity of approximately 2.000 ohms per square centimetre. The higher resistivity surface of the lossy material backing is positioned to face the radiation sensor 44, while the lower resistance surface of the backing is positioned to face the conductive shield plate 48 such that the resistive material becomes progressively more conductive as the electric field penetrates deeper into the backing. Thus, the lossy material backing is terminated with the substantially totally reflective plate 48. The conductive shield plate 48 also provides some of the shielding for the electronic circuitry of the monitor. The distributed resistance of the sensor 44, in combination with the sensor's being mounted on the lossy material backing 46 which gradually decreases in resistivity towards the conductive shield plate, provides broadband operation. The resistance of the sensor being large relative to the antenna reactance maintains a constant sensitivity with frequency over the operating band. Fig. 6 compares the variation in the threshold which will sound the monitor alarm over the operating frequency, which is preferably between 2 Gigahertz and 18 Gigahertz when the monitor is worn by a user and when it is standing in free space. It is evident from the graph shown in Fig. 6 that the broadband personal monitor of the present invention is made essentially independent of its operative environment. The scattering from the wearer's body has little affect on the performance of the monitor. As essentially flat frequency response with less than ± 1,5 dB variation and an independence of field perturbation due to scattering from the subject's body results from the utilization in the monitor of the thin film resistive thermocouple elements 54 mounted adjacent the graduated lossy material backing 46 which itself is backed with the conductive shield plate 48. The difference in response from when the monitor is irradiated by electromagnetic energy on a person from when it is supported in free space on a low dielectric material is less than 0,5 dB. The radiation sensor 44 is coupled to the electronic circuitry of the monitor by using a flexible transmission line 68 consisting of at least two leads of carbon impregnated TFE material. The two leads of this transmission line 68 carry the DC output from the sensor to the electronic circuitry. These leads have a resistance of approximately 3937 ohms per centimetre. The two DC leads are connected to one of the four antenna elements 52 and in particular to respective conductive elements 56 at the most outward end of the antenna element, as shown in Fig. 4 of the drawings. Fig. 7 illustrates one form of an electronic circuit for use with the radiation monitor. Integrated circuit U1 acts as a quasi-regulated voltage source, and provides regulated -3 volts on circuit terminal E2 (-V) and unregulated, approximately 9 volts on circuit terminal E1 (+V). Battery B1, which is preferably a 12 volt miniature battery, is coupled across terminals E1 and E2. Diode CR1 acts as a zener diode in a starved condition and provides approximately 3 volts as a reference voltage for integrated circuit U1. Potentiometer R28 provides an adjustment of the regulated -3 volts. Integrated circuit U4 acts as a comparator. It triggers on a positive going pulse from integrated circuit U3, as will be explained, and latches up through hysteresis (i.e., feedback resistor R7) to cause LED CR3 (which is the visual alarm LED 8) to remain illuminated. The output of circuit U4 is coupled to the base of driver transistor Q1, whose collector is coupled to LED CR3. LED CR3 is powered by an auxiliary 3 volt battery. Separate 12 volt and 3 volt batteries are used in the monitor to provide a fail-safe measure. Since the LED draws the most current, that is, approximately 500 milliamps, if the LED fails due to a low battery, the rest of the circuit which is powered by the 12 volt battery continues to operate to provide a warning to the user that high RF energy is present. Since the exposure light (i.e., the LED) is powered from a separate battery, maximum life is provided for the battery which powers the audible alarm. The battery for the audible alarm is envisioned to last approximately 30 days in a "sleep" mode and 6 hours in a continuous alarm state. The exposure light will last approximately 100 hours in a continuous lighted state. Integrated circuit U2 is an operational amplifier configured as a conditioning amplifier with a gain of approximately 1.000. Potentiometer R9 is provided as a gain adjustment. The amplifier amplifies the signal from the radiation sensor which is coupled to circuit terminals E7 and E8, and amplifies that signal before providing it to integrated circuit U3. Resistor R10, which is coupled to one leg of potentiometer R9, is a sensitor (i.e., a thermally sensitive resistor, or thermistor) and is provided to compensate for temperature variations so that conditioning amplifier U2 will provide more or less gain, as needed, as the temperature varies. Integrated circuit U3 is a conventional circuit used in smoke detectors. Smoke detector circuit U3 provides a regulated voltage on its pin 1 which, in the case of the monitor circuit, is a 3 volt reference voltage to ground. Resistor R19 and R20 comprise a resistor network which provides a 1 volt alarm threshold on pin 2 of circuit U3. A standard piezo electric transducer 6 is coupled through terminals E4, E5 and E6 to pin numbers 8, 9 and 10 of circuit U3. A suitable transducer which may be used is Part No. PKM 11-6A0 manufactured by Murata-Erie Co. The resistor circuit shown in dashed line A, in conjunction with potentiometer R26, provides an adjustment to correct the offset bias of conditioning amplifier U2. The circuit of the radiation monitor of the present invention operates in the following manner. The radiation sensor 44 generates 1 millivolt per 1 milliwatt per square centimetre of RF energy which irradiates it. This signal is carried by the transmission line 68 described previously to the inputs of conditioning amplifier U2. U2 amplifies the 1 millivolt signal from the radiation sensor (that is, when the sensor is irradiated with 1 milliwatt per square centimetre of energy) by 1.000 to provide an output signal which is approximately 1 volt. The 1 volt signal is provided to pin 3 of the smoke detector circuit U3. If the amplified signal from conditioning amplifier U2 is greater than the 1 volt threshold on pin 2 of circuit U3, the output of circuit U3 of pin 12 will provide a positive going pulse through diode CR2 to the non-inverting input (pin 3) of comparator U4. In response to this pulse, the output of comparator U4, at pin 6, will go to a positive voltage and bias transistor Q1 on. Transistor Q1 will conduct current through LED CR3 to illuminate the LED of the radiation monitor. Hysteresis will keep comparator U4 latched until the circuit is reset. Also, smoke detector circuit U3 sounds the piezo electric alarm 6 (Fig. 3) when the threshold is exceeded. Circuit U3 further monitors the battery voltage. When the battery voltage drops to approximately 7,5 volts, circuit U3 will cause the alarm to emit a chirp every 40 seconds. If the battery drops further in voltage, the chirps emitted by the alarm will become more frequent until a battery voltage is reached which causes the alarm to emit a continuous warble tone. The monitor of the present invention further provides a self-test upon turn on. Capacitor C2, which is connected between the input (pin 3) of the conditioning amplifier U2 and the regulated 3 volt output of circuit U3 (at pin 1), is initially uncharged, thus providing a test voltage to be applied to the inputs (pins 3 and 2) of conditioning amplifier U2 through the resistor divider network comprised of resistors R16 and R17 and the input resistor R15. This test voltage is approximately 1 millivolt, which is comparable to the voltage which would be provided by the radiation sensor when the sensor is illuminated with RF energy of 1 milliwatt per square centimetre. The test voltage is amplified by circuit U2, and smoke detector circuit U3 sounds the alarm and causes comparator U4 to go to a positive state on its output, thereby turning on transistor Q1 and illuminating LED CR3. Comparator U4 is not latched under these test conditions. This is because capacitor C8, which is coupled between the regulated -3 volts and the positive supply voltage input (pin 7) of comparator U4 and one side of resistor R8 whose other side is connected to the unregulated 9 volt supply, is initially uncharged. Capacitor C8 prevents the positive supply voltage from being supplied to pin 7 of comparator U4. Circuit U4 will note latch up through hysteresis feedback resistor R7 under these conditions until capacitor C8 has become charged. At that time however, capacitor C2 has become fully charged and effectively removes the test voltage from the inputs of conditioning amplifier U2. The output of amplifier U2 thereby falls below the 1 volt threshold, and the output signal from circuit U3 returns to a low level. This, in turn, causes the output signal of comparator U4 to return to a low logic level, thereby cutting off transistor Q1 and turning off warning LED CR3. To ensure that the various capacitors and other components in the electronic circuitry of the monitor are fully discharged when the monitor is shut off, which thereby prevents false alarms as well as prevents comparator U4 from latching, a single pole, double throw switch is used as the on/off switch 12. The positive side of battery B1 is coupled to one pole (S1-1) (fig. 3) of the switch. The wiper terminal (S1-2) (Fig. 3) of the switch is coupled to the E1 terminal of the electronic circuit board. The E2 terminal is connected directly to the negative terminal of the battery and to the other pole (S1-3) (Fig. 3) of the switch. Accordingly, when the switch 12 is in the on position, wiper S1-2 contacts pole S1-1 to provide voltage from battery B1 across terminals E1 and E2. When the switch is in the off position, wiper S1-2 contacts the opposite pole S1-3 and provides a short circuit across terminals E1 and E2. Because transistor Q1 remains cut off, when LED CR3 is not illuminated, negligible current is drawn from battery B2 when the monitor is off. A parts list for the circuit shown in Fig. 7 is provided below. Aslo, the pin members shown in Fig. 7 for integrated circuits U1-U4 relate to the parts specified in the list although, of course, it is envisioned that components comparable to those listed below, connected differently from that shown in Fig. 7, may be suitable for use. PARTS LIST FOR CIRCUIT SHOWN IN FIG. 7 Part Description Reference Designation TRANSISTOR 2N4124 Q1 WIRE, BUSS W1-2 CAPACITOR 22 µF C7-8 CAPACITOR 0, 1 µF C2,C3,C6 CAPACITOR 0,01 µF C4,C5 CAPACITOR 0,001 µF C1 LIGHT EMITTING DIODE CR3 DIODE CR2 ZENER DIODE CR1 IC CHIP - CA3169A U3 IC CHIP - CA3078 U1,U4 IC CHIP - OP22EZ U2 CAPACITOR 1 µF C9 POTENTIOMETER 1 MΩ R27,R28 POTENTIOMETER 10 kΩ R26 RESISTOR 2,4 kΩ R32 RESISTOR 360 kΩ R9 RESISTOR 62 Ω R23 rESISTOR 3 MΩ R15,22 RESISTOR 200 kΩ R8,18 RESISTOR 20 MΩ R17,31 RESISTOR 6,2 MΩ R16 RESISTOR 510 kΩ R13-14, R20 RESISTOR 10 kΩ R9,R11,R12 SENSITOR 680 Ω R10 RESISTOR 2 MΩ R7 RESISTOR 3,3 MΩ R6 RESISTOR 5,1 MΩ R21 RESISTOR 820 kΩ R2,R5 RESISTORS 1 MEG Ω R1,3,4,19,29 RESISTOR 100 kΩ R24 RESISTOR 51 kΩ R30 Battery B1 is a 12 volt miniature battery and is received by a receptacle 70 mounted in a battery compartment 40 of the housing. Battery B2, in one form of the invention, is comprised of a pair of 1,5 volt button batteries connected in series and mounted in a separate receptacle 72 containing a spring 74 for biasing the batteries against contacts in the housing. To further ensure that no RF energy affects the performance of the monitor, the battery compartment 40 which houses batteries B1 and B2 is provided with additional lossy material 76 so that the receptacles 70, 72 which receive batteries B1 and B2, respectively, are at least partially surrounded by the material. The material 76 is preferably progressively more conductive through its thickness from the back wall 18 of the housing to the conductive shield plate 48, as with the lossy material backing 46 described previously. The radiation monitor of the present invention accurately detects RF radiation without being affected by body scattering caused by the wearer's body. Its compact size allows it to be worn on a belt using the clip 16 (Fig. 2) provided or in the wearer's pocket. Its broadband frequency performance and independence of polarization make the monitor perfectly adaptable for use in a variety of RF environments. The audible alarm 6 provides a warning of RF exposure, and the LED 8 provides a visual indication as well. The LED latches so as to provide a record that the wearer was exposed to RF energy, in the event the wearer did not hear the audible alarm before he left the danger zone. The personal electromagnetic radiation monitor of the present invention is further quite suitable for use in high ambient noise environments. The ear plug assembly 20 includes ear plugs which may be used in conjunction with ear phones, and is non-electrically conductive to prevent injury to the wearer and misreadings or damage to the electronic circuitry of the monitor. The Velcro (TM) hook and loop fastener patches used on the ear plug assembly and the housing of the monitor allows the user to quickly and easily connect the ear plug assembly to the transducer on the housing with no electrical connection required. An alternative embodiment of the electromagnetic radiation monitor of the present invention, specifically an area (fixed position) electromagnetic radiation monitor 100, is shown in Figure 8. The monitor includes an electromagnetic radiation sensor 121 (shown in fig. 9), a conductive shield 101, a housing 102, a conductive electronic casing 103 mounted upon the conductive shield, and a backing of graduated lossy material in the form of a first layer 118, a second layer 119 and a third layer 120 (as shown in Fig. 9). The backing of graduated lossy material is interposed between the radiation sensor and the conductive shield. The backing is progressively more conductive through its thickness from the first layer 118 to the third layer 120. The area (fixed position) electromagnetic radiation monitor 100 is designed to monitor a region near a source of high power radio frequency (RF) or microwave energy. In the event that a break or component failure occurs, the monitor will generate an alarm if it detects RF power exceeding a predetermined threshold. The operating frequency range of the present invention is 0,5 Gigahertz to 40 Gigahertz. The area electromagnetic radiation monitor is designed with a backing of graduated lossy material 118,119,120 which becomes progressively more conductive through its thickness with increasing distance from the radiation sensor 121. The backing consists of three layers. The first layer 118, closest to the radiation sensor, has a volume resistivity of approximately 15kΩ per cubic centimetre and a surface resistance of approximately 20kΩ per square centimetre. The second layer 119, which is contiguous with the first layer 118, and a third layer 120, which is contiguous with the second layer 119, have volume resistivities and surface resistances of approximately 600Ω per cubic centimetre, 150Ω per cubic centimetre, 2000Ω per square centimetre and 500Ω per square centimetre respectively. The third layer 120, which is the least resistive layer, is affixed to the conductive shield 101. The housing 102 is a substantially rectangular one-piece extrusion having a top level portion 111, a first side 107, a second side 104, a third side 105 opposite the first side and a fourth side 106 opposite the second side. The four sides are disposed adjacent to and orthogonal to the edges of conductive shield 101. Contiguous with the second side 104 is a tapered surface 108 which extends up at an angle to meet top level portion 111. In a like manner, a tapered surface 109 and a tapered surface 110 are respectively contiguous with third and fourth sides 105 and 106, extending therefrom at an angle to meet top level portion 111. The optimal taper angle of surface 108, 109 and 110 is 45° so as to provide minimal field distortion. Top level portion 111 has a cylindrical extrusion or boss 112 thereon which encloses the electromagnetic radiation sensor 121. The first side 107 of housing 102 does not have a tapered portion similar to tapered portions 108, 109, 110. First side 107 abuts conductive electronic casing 103. Electrical conductive casing 103 contains electronics similar to those of the personal electromagnetic radiation monitor as depicted in figures 1-7 and as previously described. Conductive electronic casing 103, however, may also contain a battery 122 (shown in Figure 9) within a battery compartment 113. The battery 122 is surrounded by a second backing 114 which can be similar to the graduated lossy material contained within housing 102. Also shown is an alarm test button 115 mounted upon the conductive electronic casing 103. When an alarm electromagnetic radiation threshold is set and that value is exceeded, a piezoelectric horn 117 will sound and a light emitting diode (LED) 116 will illuminate. In a safe environment, the LED 116 can be arranged to flash intermittently, such as once every forty seconds to indicate to the user that the device is currently operating and monitoring the environment. Also, the horn 117 may be arranged to sound intermittently, such as once every forty seconds, if the battery voltage drops below a preset minimum such as 7,5 volts. A failsafe circuit, as previously described, may be included within the electronics so as to produce an alarm state if the sensor opens or fails. The electronics housed within conductive electronic casing 103 may be surrounded by second backing material 114 which also surrounds battery 122. Referring now to Figure 9, a cross-sectional view of the above-described embodiment of the present invention is shown. Figure 9 shows an area (fixed position) electromagnetic radiation monitor 100 including a conductive shield 101 upon which is mounted the housing 102 and the conductive electronic casing 103. The boss 112 is shown encasing sensor element 121 which is electrically coupled to conductive electronic casing 103 through conduit 124. It can be seen from the figure that a backing of graduated lossy material 118, 119, 120 extends from conductive shield 101 to the base or vertex of radiation sensor 121. In an alternative embodiment, a second battery 123 may be secured to conductive shield 101 and surrounded by the backing material. Second battery 123, which is shown in Figure 9, is an optional back-up battery. Sensor element 121 consists of 3 mutually orthogonal thin film resistive thermocouple elements similar to those described in U.S. Patents 3.641.439, 3.794.914 and 4.518.912 and similar to the configuration described with regard to the personal electromagnetic radiation monitor. The higher frequency range of the area (fixed position) monitor is provided by the array configuration as described in U.S. Patent No. 4.629.978. Specifically, the vertex of the thermocouple elements is positioned at the outer surface of the highest resistivity backing, which is then tapered at its outer edges. The taper provides a wider incidence angle for reception of incoming energy thereby preventing or minimizing field distortion. The response of radiation sensor 121 is relatively independent of polarization or the direction of propagation of the electromagnetic radiation except when the energy is propagated from behind the sensor unit and through the conductive shield 101. The monitor 100 acts as an isotropic antenna, receiving all electromagnetic energy within the space on the backing side of the plane of the conductive shield surface. Altough illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention.

Claims: An electromagnetic radiation monitor, which comprises: an electromagnetic radiation sensor (44; 121); a conductive shield (48; 101), spaced apart from the sensor; and a backing of graduated lossy material, (46; 118, 119, 120), the backing being interposed between the radiation sensor (44; 121) and the conductive shield (48; 101) and having a front surface (60; 118) facing the sensor (44) and an opposite back surface (62; 120) facing the conductive shield (48; 101); characterized in that the backing is progressively more conductive through the thickness thereof from the front surface (60; 118) to the back surface (62; 120), the radiation monitor further comprising: an audible alarm transducer (6) connected with the electromagnetic radiation sensor (44; 121), and an ear plug assembly (20) in communication with the audible alarm transducer (6; 117), the ear plug assembly (20) being electrically non-conductive to prevent RF energy from being received by the assembly (20) and radiated to the wearer's head. An electromagnetic radiation monitor as defined by claim 1, wherein the electromagnetic radiation sensor (44) includes a pair of mutually orthogonally disposed antenna elements (52) residing coplanarly to each other, each antenna element extending along a respective longitudinal axis and including an array of thin film resistive thermocouples (54) arranged along the axis, and a plurality of spaced apart conductive elements (55) of discrete length arranged transversely to the axis of the array, each thermocouple (54) of each antenna element (52) being disposed between adjacent conductive elements (56). An electromagnetic radiation monitor as defined by claim 1, which further includes a sheet (58) of insulating material interposed between the sensor (44) and the backing of graduated lossy material (46). An electromagnetic radiation monitor as defined by claim 1, wherein the backing of graduated lossy material (46) comprises a laminate formed from at least first (64) and second (66) layers of lossy material having different resistivities, the first layer (64) having a higher resistivity than the second layer (66). An electromagnetic radiation monitor as defined by claim 1, wherein the backing (46) is formed from a lossy material which is continuously more conductive through the thickness thereof from the front surface (60) to the back surface (62). An electromagnetic radiation monitor as defined by claim 1, which further includes a housing (2), wherein the sensor (44), backing of graduated lossy material (46) and conductive shield (48) are mounted in the housing. An electromagnetic radiation monitor as defined by claim 6, wherein the audible alarm transducer (6) is mounted on a housing side; wherein the housing includes a hook and loop fastener patch (22) mounted on the outer surface of the housing side and positioned in proximity to the transducer (6); and wherein the electrically non-conductive ear plug assembly (20) includes an elongated pneumatic tube (24) having opposite first and second ends, a pneumatic ear plug (26) mounted on the first end of the tube and a hollow housing (28) defining an interior cavity (30) mounted on the second end of the tube, the hollow housing (28) having an opening (32) formed through the thickness thereof and including a second hook and loop fastener patch (34) surrounding the opening (32) and which is adapted to mate with the first hook and loop fastener patch (22) of the monitor housing to allow the hollow housing (28) to be mounted adjacent to the transducer (6), the ear plug (26) being in communication with the transducer (6) through the pneumatic tube (24), cavity and opening when the hollow housing (28) is mounted adjacent to the transducer (6). An electromagnetic radiation monitor as defined by claim 6, wherein the conductor shield (48) has opposite first and second sides, the lossy material backing (46) being mounted adjacent to the first side of the shield (48); and wherein the monitor further comprises an alarm circuit, the alarm circuit being mounted in the housing (2) on the second side of the shield (48). An electromagnetic radiation monitor as defined by claim 8, wherein the housing defines a battery compartment (40) situated on the second side of the shield (48); and wherein the monitor further includes a battery receptacle (70) situated in the battery compartment (40), and a second layer of graduated lossy material (76) situated in the compartment (40) and at least partially surrounding the battery receptacle, the lossy material (76) being progressively more conductive through its thickness from a backwall (18) of the housing (2) to the shield (48). An electromagnetic radiation monitor as defined by claim 9, wherein the monitor further comprises a latching circuit and a light (8) mounted on the housing (2), the latching circuit being coupled to the alarm circuit and to the light (8), the latching circuit causing the light (8) to illuminate when a predetermined level of radiation is detected by the sensor (44), and further causing the light (8) to remain illuminated when the radiation detected by the sensor (44) falls below the predetermined level. An electromagnetic radiation monitor as defined by claim 1, wherein the electromagnetic radiation sensor (44) includes a pair of mutually orthogonally disposed antenna elements (52) residing coplanarly to each other, each antenna element extending along a respective longitudinal axis and including an array of thin film resistive thermocouples (54) arranged along the axis, and a plurality of spaced apart conductive elements (56) of discrete length arranged transversely to the axis of the array, each thermocouple (54) of each antenna element being disposed between adjacent conductive elements (56); the conductive shield has opposite first and second sides; the back surface (62) of the backing of graduated lossy material (46) faces the first side of the conductive shield (48); and wherein said electromagnetic radiation monitor further comprises: a housing (2), the sensor (44) and backing of graduated lossy material (46) and conductive shield (48) being contained within the housing; the audible alarm transducer (6) being mounted on a housing side, the housing (2) including a hook and loop fastener patch (22) mounted on the outer surface of the housing side and positioned in proximity to the transducer (6); the electrically non-conductive ear plug assembly (20), including an elongated pneumatic tube (24) having opposite first and second ends, a pneumatic ear plug (26) mounted on the first end of the tube and a hollow housing (28) defining an interior cavity (30) mounted on the second end of the tube, the hollow housing (28) having an opening formed through the thickness thereof and including a second hook and loop fastening patch (34) surrounding the opening which is adapted to mate with the first hook and loop fastener patch (22) of the monitor housing to allow the hollow housing (28) to be mounted adjacent to the transducer (6), the ear plug (20) being in communication with the transducer (6) through the pneumatic tube (24), cavity and opening when the hollow housing (28) is mounted adjacent to the transducer (6); a warning light (8) mounted on the housing (2); and an alarm circuit and a latching circuit, the alarm circuit and the latching circuit being mounted in the housing (2) adjacent to the second side of the shield (48), the alarm circuit being connected to the transducer (6) and being adapted to cause the transducer (6) to emit an audible sound when a predetermined level of radiation is detected by the sensor (44), the latching circuit causing the light (8) to illuminate when the predetermined level of radiation is detected by the sensor (44), and further causing the light (8) to remain illuminated when the radiation detected by the sensor (44) decreases in magnitude to below the predetermined level; the housing defining a battery compartment (40) situated on the second side of the shield (48), the monitor further including a battery receptacle (70) and a second backing of graduated lossy material (76) situated in the compartment and at least partially surrounding the battery receptacle, the lossy material (76) being progressively more conductive through its thickness from a backwall (18) of the housing (2) to the shield (48). An electromagnetic radiation monitor as defined by claim 1, wherein the electromagnetic radiation sensor (121) includes three mutually orthogonally disposed antenna elements (52), each antenna element extending from a vertex formed at the front surface of the backing and including an array of thin film resistive thermocouples (54) arranged along the axis of each antenna element (52), and a plurality of spaced apart conductive elements (56) of discrete length arranged transversely to the axis of the array, each thermocouple (54) of each antenna element (52) being disposed between adjacent conductive elements (56). An electromagnetic radiation monitor as defined by claim 1, wherein the backing of graduated lossy material (46; 118, 119, 120) comprises a laminate formed from at least first (118), second (119) and third (120) layers of lossy material having different resistivities, the first layer (118) having a higher resistivity than the second layer (119), the second layer (119) having a higher resistivity than the third layer (120). An electromagnetic radiation monitor as defined by claim 13, wherein the first (118), second (119) and third (120) layers have volume resistivities of substantially 15kΩ per cubic centimetre, 600Ω per cubic centimetre and 150Ω per cubic centimetre, respectively, and surface resistances of substantially 20kΩ per square centimetre, 2kΩ per square centimetre and 500Ω per square centimetre, respectively. An electromagnetic radiation monitor as defined by claim 1, further comprising an electronic control section having an alarm circuit, and a housing (102) mounted on the conductive shield (101) in which the sensor (121) and backing (46; 118, 119, 120) of graduated lossy material are disposed, the electronic control section being contained within a conductive electronic casing (103) and mounted on the conductive shield (101) adjacent to the housing (102); wherein the monitor further includes a visual alarm transducer (116) and an alarm test button (115), all of which are mounted on the conductive electronic casing (103). An electromagnetic radiation monitor as defined by claim 15, wherein the housing (102) is a rectangular shaped extrusion comprising a housing top and four sides, a first side (107) abutting the conductive electronic casing (103), the second (104), third (105) and fourth (106) sides being disposed adjacent to and orthogonal to the edges of the conductive shield (101); and wherein the housing top comprises three tapered portions (108, 109, 110) extending up at an angle from each of the second (104), third (105) and fourth (106) sides, each tapered portion being contiguous with a level portion (111) of the housing top upon which is disposed a cylindrical boss (112), the level portion (111) of the housing top also being contiguous with the first side (107). An electromagnetic radiation monitor as defined by claim 15, wherein the conductive electronic casing (103) includes a battery compartment (113) for securing a battery (122) therein, and wherein the battery compartment (113) further includes a battery receptacle situated therein, and a second backing (114) of graduated lossy material situated in the compartment (113) to at least partially surround the battery receptacle. An electromagnetic radiation monitor as defined by claim 17, wherein the monitor further comprises a latching circuit mounted on the housing, the latching circuit being coupled to the alarm circuit and to the visual alarm transducer (116), the latching circuit causing the visual alarm transducer (116) to activate when a predetermined level of electromagnetic radiation is detected by the sensor (121), and further causing the visual alarm transducer (116) to remain activated when the radiation level detected by the sensor (121) falls below the predetermined level. An electromagnetic radiation monitor as defined by claim 1, wherein: the electromagnetic radiation sensor (121) includes three mutually orthogonally disposed antenna elements (52) extending from a vertex, each antenna element extending along a respective longitudinal axis and including an array of thin film resistive thermocouples (54) arranged along the axis, and a plurality of spaced apart conductive elements (56) of discrete length arranged transversely to the axis of the array, each thermocouple (54) of each antenna element (52) being disposed between adjacent conductive elements (56); the conductive shield (101) has opposite first and second sides; the back surface (62) of the backing of graduated lossy material (118, 119, 120) faces the first side of the conductive shield (101) and wherein said electromagnetic radiation monitor further comprises: a conductive electronic casing (103) attached to the conductive shield (101); a housing (102), the sensor (121) and backing of graduated lossy material (46; 118, 119, 120) being contained within the housing and mounted to the conductive shield (101), at least one side of the housing being adjacent to the conductive electronic casing (103); the audible alarm transducer (117) being mounted on the conductive electronic casing; a visual alarm (116) mounted on the conductive electronic casing (103); and an alarm circuit and a latching circuit, the alarm circuit and the latching circuit being mounted in the conductive electronic casing (103), the alarm circuit being connected to the audible alarm transducer (117) and being adapted to cause the audible alarm transducer (117) to emit an audible sound when a predetermined level of radiation is detected by the sensor, the latching circuit causing the visual alarm transducer (116) to illuminate when the predetermined level of radiation is detected by the sensor (121), and further causing the visual alarm transducer (116) to remain illuminated when the radiation level detected by the sensor (121) decreases in magnitude to below the predetermined level; and the conductive electronic casing (103) having a battery compartment (113) situated therein, and further including a battery receptacle and the casing (103) having a second backing (114) of graduated lossy material situated in the compartment (113) which at least partially surrounds the battery receptacle, the lossy material being progressively more conductive through its thickness in a similar manner as in the backing (118, 119, 120). An electromagnetic radiation monitor as defined by claim 15, wherein the alarm circuit, via the audible alarm transducer (117), provides a low battery warning indicating that a battery associated with the monitor is operating at a low level. An electromagnetic radiation monitor as defined by claim 17, further comprising a second battery (B2) for providing power to the visual alarm transducer (116) mounted on the conductive electronic casing (103). An electromagnetic radiation monitor as defined by claim 1, wherein the electromagnetic radiation sensor (121) includes a pair of mutually orthogonally disposed antenna elements (52), each antenna element extending from a vertex formed at the front surface of the backing and including an array of thin film resistive thermocouples (54) arranged along the axis of each antenna element (52), and a plurality of spaced apart conductive elements (56) of discrete length arranged transversely to the axis of the array, each thermocouple (54) of each antenna element (52) being disposed between adjacent conductive elements (56). An electromagnetic radiation monitor as defined by claim 1, wherein: the electromagnetic radiation sensor (121) includes a pair of mutually orthogonally disposed antenna elements (52) extending from a vertex, each antenna element extending along a respective longitudinal axis and including an array of thin film resistive thermocouples (54) arranged along the axis, and a plurality of spaced apart conductive elements (56) of discrete length arranged transversely to the axis of the array, each thermocouple (54) of each antenna element (52) being disposed between adjacent conductive elements (56); the conductive shield (101) has opposite first and second sides; the back surface (62) of the backing of graduated lossy material (118, 119, 120) faces the first side of the conductive shield (101); and wherein said electromagnetic radiation monitor further comprises: a conductive electronic casing (103) attached to the conductive shield (101); a housing (102), the sensor (121) and backing of graduated lossy material (46; 118, 119, 120) being contained within the housing and mounted to the conductive shield (101), at least one side of the housing being adjacent to the conductive electronic casing (103); the audible alarm transducer (117) being mounted on the conductive electronic casing; a visual alarm (116) mounted on the conductive electronic casing (103); and an alarm circuit and a latching circuit, the alarm circuit and the latching circuit being mounted in the conductive electronic casing (103), the alarm circuit being connected to the audible alarm transducer (117) and being adapted to cause the audible alarm transducer (117) to emit an audible sound when a predetermined level of radiation is detected by the sensor, the latching circuit causing the visual alarm transducer (116) to illuminate when the predetermined level of radiation is detected by the sensor (121), and further causing the visual alarm transducer (116) to remain illuminated when the radiation level detected by the sensor (121) decreases in magnitude to below the predetermined level; and the conductive electronic casing (103) having a battery compartment (113) situated therein, and further including a battery receptacle and the casing (103) having a second backing (114) of graduated lossy material situated in the compartment (113) which at least partially surrounds the battery receptacle, the lossy material being progressively more conductive through its thickness in a similar manner as in backing (118, 119, 120).

Inventors: ASLAN EDWARD, ASLAN,EDWARD
Applicants: L 3 COMMUNICATIONS INC, L-3 COMMUNICATIONS INC.
Application year: 1992
Main classification: G01R29
IPC codes: G01R29, G08B21