APPARATUS FOR THE MEASUREMENT OF MICROWAVE RADIATION

Abstract: Apparatus for measuring the power of intense microwave pulses comprises a conducting track (3) formed on an insulating substrate (1, 2) and coupled to radiation from a remote source (10) via an antenna (7) and transmission line (6). The subsequent heating of the track (3) brings about a change in its resistance, which is monitored by a constant current source (8) and voltmeter (9). The degree of resistance change gives an indication of the energy content of the microwave pulse.

Description: APPARATUS FOR THE MEASUREMENT OF MICROWAVE RADIATION This invention relates to measurement of microwave energy and has particular application to the measurement of brief, intense microwave pulses. In certain areas of engineering design and research, there is a need to characterise the outputs of devices which generate brief (often sub-microsecond) but very powerful (often gigawatt) bursts of broadband microwave energy. In such cases, two of the key features to be determined are often total pulse energy and spectrum. It is widely acknowledged in the microwave measurement community that such measurements are very difficult to perform reliably and accurately, especially in those cases where the source device is destroyed in the microwave-emission process and can therefore emit only one pulse. One of the difficulties in this measurement task is that all conventional microwave pulse measurement techniques require a triggering signal; basically to determine the moment at which data capture begins. This can be a serious problem with expensive one-shot devices since the triggering process usually itself requires some empirical adjustment The present invention has the advantage of requiring no triggering mechanism. According to the- present invention, apparatus for the measurement of microwave radiation emanating from a remote source comprises; a resistive element, and means for coupling microwave radiation from the source to the resistive element. The resistive element may conveniently take the form of an electrically conducting track or semi-conducting track deposited on a suitable insulating substrate. The coupling means may comprise an antenna and microstrip transmission line. Optionally, electrical circuit means may be provided for measuring the electrical resistance of said element. As a further option, any fluctuations in ambient temperature in the vicinity of the resistive element may be recorded by providing a further resistive device whose resistance can be monitored as a function of temperature. In some applications it may be desirable to band-limit the radiation received by the antenna. In such cases a frequency selective filter can be located between the antenna and the remote source. This feature allows the measurement of microwave energy within a particular band of frequencies. When a pulse of_ microwave energy is emitted by the source, the energy coupled to the resistive element via the coupling means causes heating of the element. This heating effect may be sufficient to vaporise the element completely and permanently, to melt it temporarily or to bring about a temperature-dependent change in resistance without altering its physical state. Vaporisation or melting and re-freezing of the element can be determined by visual inspection (with the aid of a microscope if necessary). Alternatively, vaporisation, melting and refreezing or a temperature-dependent change in resistance can be monitored by measuring the electrical resistance of the element. Suitable electrical circuitry for doing this could comprise a constant current source and a voltmeter. Whether the element vaporises, melts or merely heats up depends on its resistivity and the strength of the microwave source, these values being a matter of design choice. The vaporisation and melting thresholds and the resistance variation with temperature for a particular element can be determined by calculation or by a calibration procedure. Some embodiments of the invention will now be described, by way of example only, with reference to the drawings of which; Figure 1 is a schematic plan view of a resistive element formed on a substrate in accordance with the invention, Figure 2 is a schematic circuit diagram of apparatus for the measurement of microwave radiation in accordance with the invention, and Figure 3 is a schematic diagram of a second, alternative embodiment. It is convenient to employ microfabrication techniques in the construction of the resistive element and therefore Figure 1 shows a silicon wafer 1 which acts as a mechanical support. On top of the silicon wafer 1 is spun a layer of polyimide 2. The polyimide layer 2 acts as an electrical and thermal insulator. On to the polyimide layer 2 is deposited a titanium track which forms the resistive element 3. Aluminium pads 4 are deposited at either end of the titanium track 3 and connecting wires 5 are bonded to the pads 4. Optionally a further layer of insulator, such as polyimide, may be applied over the metal track and pads. Polyimide has the advantage of a greater dielectric strength and therefore a higher break-down voltage than air. Referring now to Figure 2. Each connecting wire 5 is connected via one of two capacitors Cl and C2 to a transmission line 6 whose end remote from the resistive element 3 terminates in an antenna 7. The antenna 7 can be any conventional device receptive to the microwave frequencies of interest. Each connecting wire 5 is also connected, via one of two inductors LI and L2 to a constant current source 8. A DC voltmeter 9 monitors the voltage across the resistive element 3. The purpose of the inductors LI and L2 is to isolate the constant current source 8 and voltmeter 9 from the microwave signal flowing through the resistive element 3. The capacitors Cl and C2 isolate the transmission line 6 from the DC current provided by the constant current source 8. Certain types of antennna may not dictate a need for these capacitors, however. Preferably the dimensions of the resistive element 3 are chosen so that its electrical impedance matches that of the transmission line 6 e.g. 50ohms. In a first mode of operation, a short and intense microwave pulse is emitted from a remote microwave source 10. Microwave energy detected by the antenna 7 is coupled into the resistive element 3 via the transmission line 6 and causes sufficient heating for the element to vaporise. Once the element has vaporised, the voltmeter will indicate that an open circuit exists. (Alternatively this can be inferred from visual inspection of the element ) . This will give an indication that the electrical energy deposited in the resistive element 3 has exceeded a certain threshold. It will be evident that in this mode of operation the apparatus is used as a one-shot device. In the second mode of operation, the heating energy coupled into the element 3 from the remote source 10 for the duration of a pulse is just enough to melt the element. It can be shown by calculation that the energy required to melt a resistive element is typically a tenth of that required to vaporise it. It has also been observed that when the resistive element melts then refreezes, a change in its geometrical configuration occurs. This change may be detectable by visual inspection. However it has also been observed that this change in geometrical configuration gives rise to a permanent change in the resistance. Thus by measuring the resistance of the element 3 before and after application of a microwave pulse, one can ascertain whether or not the melting threshold has been exceeded. In a third mode of operation, the resistive element is designed so that it will not melt at the anticipated heating levels. In this case, the resistance value of the element 3, monitored by the voltmeter 9 changes as a function of temperature. Thus, measured resistance value gives an indication of the energy received by the element 3 from the remote source 10. In this third mode of operation, the apparatus can be reused indefinitely. For each mode of operation, it is preferred that the heating process is confined to the resistive element i.e. that the substrate does not act as an effective heat sink. Therefore, it is recommended that the substrate 1 is provided with an insulator which has a long thermal time scale compared with the length of the microwave pulses of interest. Polyimide fulfils this role for some applications. Figure 3 shows a further embodiment of the invention in which a second resistive element 11 is included on the same substrate 1 as the first element 3. This second element 11 is not coupled to the radiation source 10 but is connected to a constant current source 12 and DC voltmeter 13. Any fluctuations in ambient temperature are monitored by measuring resistance changes in the element 11 by the voltmeter 13. These measurements can then be used to compensate for resistance changes in the radiation sensitive element 3 due to ambient temperature fluctuations. In some applications it may be desirable to band-limit the microwave frequencies incident on the antenna 7 from the remote source 10. If so, a frequency selective filter shown ghosted in Figure 2 and 3 and designated 14 may be placed between said antenna 7 and source 10. Preferably the frequency selective filter comprises a dielectric surface on which arrays of conducting elements are printed. Two or more such dielectric surfaces may be stacked to form a composite assembly. The spacing and size of the conducting elements and the spacing of the surfaces dictate the frequency band limiting properties of the filter. US 4307404 describes a similar device for antenna applications. The filter may optionally incorporate means for moving one surface relative to another thereby modulating its frequency characteristics. This modulation of the frequency characteristic is described in detail in European patent EP-B-468623 . Such a filter may be incorporated with either of the embodiments of Figures 2 or 3 and employed in any of the three modes of operation described above.

Claims: CLAIMS 1. Apparatus for the measurement of microwave radiation emanating from a remote source comprising: a resistive element, and means for coupling microwave radiation from the source to the resistive element. 2. Apparatus according to claim 1 in which the resistive element is an electrically conducting track formed on an insulating substrate. 3. Apparatus according to claim 2 in which the insulating substrate includes a layer of polyimide. 4. Apparatus according to any preceding claim in which the means for coupling microwave radiation from the source to the resistive element comprises an antenna and microstrip transmission line. 5. Apparatus according to any preceding claim and including means for measuring the electrical resistance of the resistive element. 6. Apparatus according to claim 5 in which the means for measuring the electrical resistance of the resistive element comprise a constant current source and a voltmeter. 7. Apparatus according to any preceding claim and including a resistive device, sensitive to ambient temperature fluctuations, and means for monitoring the resistance of said device. 8. Apparatus according to any preceding claim and further including a frequency selective filter for band-limiting the radiation received by the resistive element from the remote source. 9. Apparatus according to claim 9 in which the frequency selective filter comprises at least one dielectric substrate on which is printed an array of electrically conducting elements. 10. Apparatus for the measurement of microwave radiation substantially as hereinbefore described with reference to the drawings.

Inventors: BUDD CHRISTOPHER, CHESTERFIELD STEWART ROBERT, BUDD, CHRISTOPHER, CHESTERFIELD, STEWART, ROBERT
Applicants: BUDD CHRISTOPHER, CHESTERFIELD STEWART ROBERT, MATRA BAE DYNAMICS UK LTD, BUDD, CHRISTOPHER, CHESTERFIELD, STEWART, ROBERT, MATRA BAE DYNAMICS (UK) LIMITED
Application year: 1998
Main classification: G01R21
IPC codes: G01R21, G01R29