MEASUREMENT OF MICROWAVE RADIATION

Abstract: Apparatus for measuring the power of intense microwave pulses comprises a calorimeter (2) and frequency selective band limiting means (3). Heating of liquid contained within the calorimeter (2) when irradiated, expands in a capillary (5) thus indicating the amount of energy emitted by a microwave source (1). The band limiting means comprises a dielectric substrate on which an array of electrically-conductive elements are printed. The invention has the advantage of allowing repeated measurements of energy emitted by a pulsed source (1) within a particular frequency band. Furthermore, it requires no triggering means.

Description: MEASUREMENT OF MICROWAVE RADIATION This invention relates to apparatus for the measurement of the power and spectra of microwave radiation. The invention 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 and 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 that it requires no triggering mechanism. Further, the invention provides a means for measurement of the microwave energy emitted by a pulsed device within a particular band of frequencies. By employing a bank of such instruments, total energy output and spectrum of a microwave pulse can be measured. The present invention comprises apparatus for the measurement of microwave radiation including calorimetric means, thermally responsive to incident microwave radiation emitted from a remote source, and a frequency selective filter located between said calorimetric means and said source for passing a pre-defined band of microwave frequencies. The invention thus allows the measurement of total microwave power only in that frequency band which passes through the filter. It has the advantage that it can be reused indefinitely, providing a continuous scale of absorbed energy. The calorimetric means may be of the type in which the incident microwave energy is absorbed by a liquid (eg ethanol). The liquid then expands along a capillary tube and the expansion of the liquid is used to infer the change in its temperature and (indirectly) the amount of microwave energy absorbed. Preferably the calorimeter is insulated from other sources of heat and vibration or mechanical shock. One suitable type of calorimeter is described in Conference Proceedings "BEAMS 96" pp461-464 by A.L. Lisichkin et al. 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. An embodiment of the invention will now be described, by way of example only with reference to the drawings. Therein, a source 1 of brief, intense microwave pulses irradiates an ethanol filled calorimeter 2 via a band-pass filter 3. The calorimeter 2 comprises a reservoir 4 and capillary 5. The band-pass filter 3 is a layered frequency selective surface assembly having a plurality of dielectric substrates each with an array of electrically conductive elements disposed thereon. The geometry of the arrays and the spacing between substrates is chosen to match the frequency pass-band of interest. Those frequencies which are allowed to pass through the filter 3 irradiate the ethanol contained within the calorimeter 2 causing heating and expansion of the liquid. The expansion of liquid is measured by monitoring its level in the capillary 5. This can be done by bonding two metallic strips 6 to the outer surface of the capillary and measuring the capacitance change.

Claims: CLAIMS 1. Apparatus for the measurement of microwave radiation including calorimetric means, thermally responsive to incident microwave radiation emitted from a remote source, and a frequency selective filter located between said calorimetric means and said source for passing a pre-defined band of microwave frequencies. 2. Apparatus according to claim 1 in which the calorimetric means comprises a reservoir of liquid whose expansion in response to irradiation of microwave energy is measured by monitoring its level in a capillary tube. 3. Apparatus according to claim 2 in which the level of liquid in the capillary tube is determined by measuring the capacitance of the capillary tube. 4. Apparatus according to either of claims 2 or 3 in which the liquid is ethanol. 5. Apparatus according to any preceding claim in which the frequency selective filter comprises at least one dielectric substrate on which is printed an array of electrically conducting elements. 6. Apparatus according to claim 5 in which the frequency selective filter comprises at least two such dielectric substrates and means for moving one surface relative to another. 7. Apparatus for the measurement of microwave radiation substantially as hereinbefore described with reference to the drawing.

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: G01R22
IPC codes: G01R21, G01R29, G01R31