APPARATUS FOR THE MEASUREMENT OF MICROWAVE RADIATION

APPAREIL DE MESURE DU RAYONNEMENT HYPERFREQUENCES

English 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.

French Abstract

On décrit un appareil qui sert à mesurer la puissance d'impulsions hyperfréquences intenses. L'appareil comprend une piste conductrice (3) formée sur un substrat isolant (1, 2) et couplée à un rayonnement émanant d'une source éloignée (10), par l'intermédiaire d'une antenne (7) et d'une ligne de transmission (6). Le chauffage subséquent de la piste (3) provoque une modification de sa résistance, laquelle est surveillée par une source de courant constant (8) et un voltmètre (9). Le degré de modification de la résistance donne une indication du contenu énergétique de l'impulsion hyperfréquences.

Claims

10
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 s constant current source and a voltmeter.
7. Apparatus according to any preceding claim and
including a resistive device, sensitive to ambient temperature

11
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.

Description

CA 02288660 1999-11-04
WO 98/52056 PCT/GB98/01326
1
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.

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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.
r ,.T

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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.

~CA'02288660 1999-11-04
WO 98152056 PCT/GB98/01326
4
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 a.s 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 a.s 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.
..._ ._ ..~ ~ ~ . ,

CA 02288660 1999-11-04
WO 98/52056 PCT/GB98/01326
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 C1 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 L1 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 L1 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 C1 and C2 isolate the transmission line fi
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.

ICA102288660 1999-11-04
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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-shat 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 dust 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
_..,v._... .. ,....,. . r . ~ , r ....

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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.

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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.
~ ~ . T.

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9
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.

Representative Drawing