EP0980529A1

EP0980529A1 - Apparatus for the measurement of microwave radiation

Info

Publication number: EP0980529A1
Application number: EP98920653A
Authority: EP
Inventors: Christopher Matra BAe Dynamics BUDD (UK) Limited; Stewart R. Chesterfield
Original assignee: Matra Bae Dynamics UK Ltd
Current assignee: Matra Bae Dynamics UK Ltd
Priority date: 1997-05-09
Filing date: 1998-05-08
Publication date: 2000-02-23
Also published as: NO995464D0; GB9709306D0; WO1998052056A1; GB2325054A; NO995464L; CA2288660A1; JP2000513105A

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

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.