GB2254440A

GB2254440A - Detecting microwave radiation

Info

Publication number: GB2254440A
Application number: GB9102074A
Authority: GB
Inventors: Stanley William Waterman
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-01-31
Filing date: 1991-01-31
Publication date: 1992-10-07
Also published as: GB9102074D0

Abstract

A device for detecting microwave radiation comprises an antenna (2) on a printed circuit board (1), a rectifying diode (3) bridging the terminals of the antenna (2), and an LED indicator (4) connected across the terminals. The LED may be arranged in series with a resistor (5 Fig 2 not shown) and in parallel with two further LEDs (6, 7). A plurality of detectors may form an array (Figs 3, 4 not shown) e.g. in a mesh, sheet or printed structure which prevents radiation from escaping. The antenna may be a folded dipole, a loop or spiral device (Figs 5a, 5b, 5c not shown), and reflectors may be used (Figs 6a, 6b, 6c, not shown). An output from the rectifying diode (2) may be amplified (15, Fig 7 not shown) before application to the LED (4). <IMAGE>

Description

METHOD AND APPARATUS FOR DETECTING AND INDICATING E1Fxnlw > tGNsTIc RADIATION According to one aspect of the present invention, there is provided a device for detecting and indicating electromagnetic radiation, comprising first converting means, which converts electromagnetic radiation substantially directly into electric current, and second converting means electrically connected to said first convertingdmeans and arranged to convert said electric current substantially directly into optically visible light.

According to another aspect of the present invention, there is provided a method of detecting and indicating electromagnetic radiation, comprising substantially directly converting said electromagnetic radiation into electric current and substantially directly converting said electric current into optically visible light.

According to a third aspect of the present invention, there is provided an array of devices for detecting and indicating electromagnetic radiation, each device comprising first converting means which converts electromagnetic radiation into electric current and second converting means connected to said first converting means and arranged to convert said electric current into optically visible light.

According to a fourth aspect of the present invention, there is provided a device for detecting and indicating electromagnetic radiation, comprising antenna means, first and second terminals of said antenna means, rectifying means bridqing said terminals, and indicating means connected substantially directly across said terminals and responsive to voltage produced across said rectifying means by detection of said electromagnetic radiation by said antenna means.

In order that the invention may be clearly understood and readily carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a device for detecting and indicating electromagnetic radiation, Figure la showing a side elevation of the device, Figure lb showing a plan view thereof, and Figure lc showing diagrammatically electrical circuitry of the device, Figure 2 shows a modified version of the device, Figure 2a showing a diagrammatic side elevation of the modified version, and Figure 2b showing diagrammatically electrical circuitry of the modified version, Figure 3 is a diaqrammatic perspective view of an array of devices each according to Figure 1 or 2, Figure 4 is a diagrammatic perspective view of an installation. incorporating an array of devices each according to Figure 1 or 2, Figures 5a to 5c are diagrammatic views of respective other modified versions of the device, particularly illustrating respective forms of antenna of the respective versions, Figures 6a to 6c are respective diagrammatic views of further modified versions of the device, particularly illustrating various forms of reflector of the modified versions, and Figure 7 shows diagrammatically a yet further modified version of the device.

The versions to be described with reference to Figures 1 to 7 are particularly designed for detecting and indicating microwave radiation.

Figure 1 shows a basic version which comprises a dipole antenna 2, which may or may not be on a printed circuit board 1, a suitable rectifying diode 3 bridging the terminals of the dipole 2, and a light emitting diode (LED) 4 connected directly across those terminals. The device senses the presence of an electromagnetic field in a range of frequencies determined by the dipole 2 and the diode 3 and indicates that the field is above a certain power density by the emission of optically visible light from the LED 4.

The diode 3 is in effect acting as a half-wave rectifier and the impedance of the dipole 2 and the characteristics of the diode 3 are chosen to optimise the performance in this mode. By doing this, it becomes possible directly to drive the LED with incident microwave power densities much below recommended current safety levels and therefore to obtain a visible indication of the microwave energy present when that energy is much below a level currently thought to be hazardous. For example, the device shown in Figure 1 can be made sensitive to a power density at 2.5 GHz.

of less than 0.05 mW per square cm., which is a level 100 times lower than the exposure limit of 5 mW/square cm. currently recommended by the British National Radiological Protection Board at this frequency. Thus, the device of Figure 1 can constitute a very low cost, directly indicating, warning device which does not need a power supply, such as a battery.

In the version shown in Figure 2, both an indicating function and a warning function are built into the device by a relatively simple modification of the device of Figure 1. In this version, a resistor 5 is arranged in series with the LED 4 so that a larger voltage will be developed across the diode 3.

Two further LEDs 6 and 7 of the same rating as the LED 4 are connected in series directly across the two terminals of the dipole 2, so that when the incident microwave power and thus the voltage reach an appropriate higher level (about 3.2 to 3.6 volts depending upon the LED), they begin to emit light. Referring to Figure 2b, if V is the voltage across the diode 3, VL is the turn-on voltage for each of the 3 LEDs 4, 6 and 7, R is the resistance of the resistor 5, and I is the current through the LED 4 and the resistor 5, then for I x R < VL and V > VL, only the LED 4 is alight, but for I x R > VL all three LEDs are alright. Thus, the smaller R is, the higher the threshold power P required to turn-on the LEDs 6 and 7.With the arrangement described above with reference to Figure 2, the original sensitivity of the first LED 4 can be very nearly maintained and the turn-on level for the LED pair 6 and 7 can be set by choosing an appropriate value for the resistor 5, or the resistor 5 can be a variable resistor, whereby the turn-on level can be adjusted.

The LED 4 and the LED pair 6 and 7 may emit light of differing colours to enhance the distinguishability of the indications of the differing levels.

As illustrated in Figure 3, a plurality of the devices (each referenced 10 in Figure 3) may be arrayed on a surface 11, which need not be planar. Alternatively, the devices 10 may be arrayed in a volume for 3-dimensional display. Both array arrangements are used to give a visual indication of the spatial variation of the electromagnetic field. Such an array may be applied to antenna measurement, educational aids, visual displays and entertainment devices. For the last three applications and as shown in Figure 4, the radiation can be prevented from escaping by enclosure of the microwave source 12 in a suitable metal wire mesh or sheet metal structure, or equivalent printed structure, 13.

The e devices of each of these arrays can be arranged to emit light of differing colours and to have differing sensitivities to microwave radiation, so that the hazard warning application can, for example, be extended by having a number of devices scattered through an area (built into curtains, space dividers, on doors, hanging from the ceiling, etc.), some of high sensitivity to indicate merely the presence of microwave radiation and some of low sensitivity to emit light at a predetermined higher microwave power level as a warning. In those circumstances, devices according to Figure 2 would be arranged in arrays according to Figure 3 or 4.

The antenna 2 is not limited in form to the dipole shown in Figures 1 and 2. Instead, the antenna 2 may be in the form of a folded dipole as shown in Figure 5a, a loop antenna as shown in Figure 5b, or a spiral antenna as shown in Figure 5c, for example. These various shapes have their own particular merits in particular circumstances. For example, the spiral antenna shown in Figure Sc responds to a wider range of microwave frequencies and polarizations than do the others.

The sensitivity of the various versions of the device can be enhanced by the addition of reflecting screens, which can be of various forms. For example, the reflecting screen 14 can be of the part-cylindrical form shown in Figure 6a, the can form shown in Figure 6b, or the planar form shown in Figure 6c. The screens can be of solid metal, metal-plated plastics, a metal wire grid or a metal-plated grid.

All of the versions so far described have no provision of external power, except of course that provided by the electromagnetic field being sensed. This obviously limits the minimum microwave power intensity that can be indicated, although, as already mentioned, these versions can generally indicate power levels well below established safety levels.

However, the provision of external power can enhance the performance in various ways. A version incorporating an external power supply is illustrated in Figure 7, in which a high currentqain transistor 15 connected in series with the LED 4 and a battery 16 can improve the sensitivity by a factor of between 10 and 50. With this version, n significant current is drawn from the battery when the device is not in use, so that the battery 16 can be built-in and last for several years with intermittent use.

Alternatively, an amplifier or comparator can be used to give a greater variety of performance and sensitivity.

The indicating means does not have to be an LED 4r it could instead be a liquid crystal display (LCD), for example.

It is believed to be possible ,using the basic approach above described, to construct devices which will successfully detect and indicate microwave frequencies from well below 1 GHz.

to over 100 GHz.

A particular advantage of the devices described above with reference to the drawings is that they give a visual indication of electromagentic field intensity relatively simply and thus relatively cheaply. In particular, the combination of the three elements of the dipole 2, the diode 3 and the LED 4 is very simple and cheap. The simplicity lies in the direct conversion of microwave radiation into electric current and the direct conversion of that electric current into optically visible light.

The above-described versions with the simple dipole antenna are particular suitable for applications where the device has only to respond to one freguency and one polarization, for example where the device is to measure leakage from microwave ovens.

Claims

1. A device for detecting and indicating electromagnetic radiation, comprising first converting means, which converts electromagnetic radiation substantially directly into electric current, and second converting means electrically connected to said first converting means and arranged to convert said electric current substantially directly into optically visible light.

2. A device as claimed in claim 1, wherein said first converting means comprises antenna means, first and second terminals of said antenna means and rectifying means bridging said terminals.

3. A device as claimed in claim 2, wherein said second converting means is connected substantially directly across said terminals and responsive to voltage produced across said rectifying means by detection of said electromagnetic radiation by said antenna means.

4. A device as claimed in claim 2 or 3, wherein said rectifying means comprises a diode.

5. A device as claimed in claim 2, 3 or 4, wherein said antenna means is provided on a printed circuit board.

6. A device as claimed in any one of claims 2 to 5, wherein said antenna means comprises a dipole antenna.

7. A device as claimed in claim 6, wherein said antenna is a folded dipole antenna.

8. A device as claimed in any one of the claims 2 to 5, wherein said antenna means comprises a loop antenna.

9. A device as claimed in any one of claims 2 to 5, wherein said antenna means comprises a spiral antenna.

10. A device as claimed in any one of claims 2 to 9, and further comprising a reflecting screen serving to reflect said electromagnetic radiation towards said antenna means.

11. A device as claimed in claim 10 as appended to claim 6 or 7, wherein said screen is of part-cylindrical form.

12. A device as claimed in claim 10, wherein said screen is of can form.

13. A device as claimed in claim 10, wherein said screen is of planar form.

14. A device as claimed in any one of claims 10 to 13, wherein said screen is of metal-plated plastics.

15. A device as claimed in any preceding claim, wherein said second converting means comprises an LED or LCD.

16. A device as claimed in any one of claims 1 to 14, wherein said second converting means comprises indicating means arranged to convert said electric current into optically visible light, resistance electrically in series with said indicating means, and warning means electrically in parallel with said resistance and said indicating means and arranged to convert said electric current into optically visible light upon the voltage across said warning means exceeding a predetermined value.

17. A device as claimed in claim 16, wherein said indicating means comprises an LED.

18. A device as claimed in claim 16 or 17, wherein said warning means comprises a plurality of LEDs.

19. A method of detecting and indicating electromagnetic radiation, comprising substantially directly converting said electromagnetic radiation into electric current and substantially directly converting said electric current into optically visible light.

20. A method as claimed in claim 19, wherein the substantially direct conversion of said current into said light comprises converting said current into optically visible indicating light for lower levels of current and converting said current into optically visible warning light, distinguishable from said indicating light, for higher levels of current.

21. A method as claimed in claim 19 or 20, wherein said electromagnetic radiation is microwave radiation.

22. An array of devices for detecting and indicating electromagnetic radiation, each device comprising first converting means which converts electromagnetic radiation into electric current and second converting means connected to said first converting means and arranged to convert said electric current into optically visible light.

23. An array as claimed in claim 22, wherein each first converting means comprises antenna means, first and second terminals of said antenna means and rectifying means bridging said terminals.

24. An array as claimed in claim 23, wherein, in each device, said second converting means is connected substantially directly across said terminals and responsive to voltage produced across said rectifying means by detection of said electromagnetic radiation by said antenna means.

25. An array as claimed in claim 23 or 24, wherein said rectifying means comprises a diode.

26. An array as claimed in claim 23, 24 or 25, wherein said antenna means is provided on a printed circuit board.

27. An array as claimed in any one of claims 23 to 26, wherein each antenna means comprises a dipole antenna.

28. An array as claimed in claim 27, wherein each antenna is a folded dipole antenna.

29. An array as claimed in any one of claims 23 to 26, wherein each antenna means comprises a loop antenna.

30. An array as claimed in any one of claims 23 to 26, wherein each antenna means comprises a spiral antenna.

31. An array as claimed in any one of claims 23 to 30, and further comprising reflecting screen means serving to reflect said electromagnetic radiation towards the antenna means.

32. An array as claimed in claim 31 as appended to claim 27 or 28, wherein said screen means is of part-cylindrical form.

33. An array as claimed in claim 31, wherein said screen means is of can form.

34. An array as claimed in claim 31, wherein said screen means is of planar form.

35. An array as claimed in any one of claims 31 to 34, wherein said screen means is of metal-plated plastics.

36. An array as claimed in any one of claims 22 to 35, wherein said second converting means comprise an LED or LCD.

37. An array as claimed in any one of claims 22 to 35, wherein each second converting means comprises indicating means arranged to convert said electric current into optically visible light, resistance electrically in series with said indicating means, and warning means electrically in parallel with said resistance and said indicating means and arranged to convert said electric current into optically visible light upon the voltage across said warning means exceeding a predetermined value.

38. An array as claimed in claim 37, wherein said indicating means comprises an LED.

39. An array as claimed in claim 37 or 38, wherein said warning means comprises a plurality of tEDs.

40. An array as claimed in any one of claims 22 to 39, and further comprising external power supply means electrically connected to said devices, each device further comprising sensitivity-improving means serving to utilize the externally supplied power to improve the sensitivity of said devices.

41. An array as claimed in claim 40, wherein each sensitivityimproving means comprises a high current-gain transistor.

42. An array as claimed in claim 40, where each sensitivityimproving means comprises an amplifier.

43. An array as claimed in claim 40, wherein each sensitivityimproving means comprises a comparator.

44. An array as claimed in any one of claims 22 to 43, wherein said devices are arrayed in a volume for three-dimensional display.

45. An array as claimed in any one of claims 22 to 44, wherein said devices emit light of differing colours and have differing sensitivities to said electromagnetic radiation.

46. A device for detecting and indicating electromagnetic radiation, comprising antenna means, first and second terminals of said antenna means, rectifying means bridging said terminals, and indicating means connected substantially directly across said terminals and responsive to voltage produced across said rectifying means by detection of said electromagnetic radiation by said antenna means.

47. A device for detecting and indicating electromagnetic radiation, substantially in accordance with any example hereinbefore described with reference to the accompanying drawings.

48. A method of detecting and indicating electromagnetic radiation, substantially in accordance with any example hereinbefore described with reference to the accompanying drawings.

49. An array of devices for detecting and indicating electromagnetic radiation, substantially in accordance with any example hereinbefore described with reference to the accompanying drawings.