MEASUREMENT OF MICROWAVE RADIATION

MESURE DE RAYONNEMENT MICRO-ONDES

English 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.

French Abstract

Dispositif servant à mesurer la puissance d'impulsions micro-ondes intenses et comprenant un calorimètre (2), ainsi que des moyens de limitation sélective (3) de bande de fréquence. Le liquide réchauffé contenu dans le calorimètre (2), quand on le soumet à un rayonnement, se dilate dans un capillaire (5), ce qui indique le niveau d'énergie émise par une source micro-ondes (1). Les moyens de limitation de bande comportent un substrat diélectrique sur lequel est imprimé un groupement d'éléments conducteurs d'électricité. L'invention présente l'avantage de permettre des mesures répétées de l'énergie émise par une source pulsée (1) à l'intérieur d'une bande de fréquence particulière. Elle ne nécessite, de plus, aucuns moyens de déclenchement.

Claims

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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



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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.

Description

CA 02288860 1999-11-08
WO 98/52058 PCT/GB98/01333
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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.

CA 02288860 1999-11-08
WO 98/52058 PCT/GB98/01333
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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.

CA 02288860 1999-11-08
WO 98/52058 PCT/GB98/01333
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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.

CA 02288860 1999-11-08
WO 98/52058 PCT/GB98/01333
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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.

Representative Drawing