Microwave cavity sensor

1. A sensor for sensing a sample using a microwave signal at a wavelength of an excited electromagnetic wave, the sensor comprising:

a dielectric waveguide for guiding the microwave signal to allow formation of a standing wave at the wavelength of the excited electromagnetic wave within the dielectric waveguide; and

a dielectric reflector at an end of the dielectric waveguide to cause formation of a sensing field beyond an outer surface of the dielectric reflector or below an inner surface of the dielectric reflector,

wherein the dielectric reflector comprises a material which has a higher dielectric constant than that of the dielectric waveguide and has a thickness of at least λ g /20, where λ g is the wavelength of the excited electromagnetic wave in the dielectric reflector, to thereby maximize electromagnetic field intensity in the sensing field.

2. A sensor as claimed in claim 1 , wherein the standing wave is resonant at a fractional multiple of half of the excitation wavelength such as nλ g /4, where n is an integer.

3. A sensor as claimed in claim 2 , wherein the resonant modes are TM or TE modes.

4. A sensor as claimed in claim 1 , wherein the sensing field is an evanescent field.

5. A sensor as claimed in claim 1 , wherein the sensing field is a radiating field.

6. A sensor as claimed in claim 1 , further comprising a concentrator arranged around the dielectric waveguide for concentrating microwave energy in the dielectric waveguide.

7. A sensor as claimed in claim 6 , wherein the concentrator is a distributed Bragg reflector structure.

8. A sensor as claimed in claim 7 , wherein the Bragg reflector structure is a honey comb structure.

9. A sensor as claimed in claim 1 , wherein the excitation wavelength is in the microwave region between 0.3 GHz to 1 THz.

10. A sensor as claimed in claim 1 , further comprising means for detecting change in the sensing field.

11. A sensor as claimed in claim 10 , wherein the means for detecting change in the sensing field are operable to measure a frequency difference and/or a phase difference and/or an amplitude difference and/or Q factor difference.

12. A sensor as claimed in claim 10 , wherein the means for detecting changes in the sensing field are located at an end of the dielectric waveguide opposite the dielectric reflector.

13. A sensor as claimed in claim 1 , further comprising a microwave generator for generating the excitation signal.

14. An electron paramagnetic resonance spectroscope that includes a sensor as claimed in claim 1 .

15. A measurement system that includes a sensor as claimed in claim 1 and further comprises a nuclear magnetic resonance system for detecting NMR signals.

16. A measurement system as claimed in claim 15 , wherein the sensor and the NMR system are operable to take measurements simultaneously.

17. A sensor as claimed in claim 1 , wherein the sensor is a composition sensor, a continuous flow rate and composition sensor, a corrosion inspection sensor, a sensor for monitoring the thickness of pipes or surfaces, a sensor for monitoring the composition of a wet gas mixture flowing in a pipeline, or a sensor for a standalone laboratory and handheld configuration for detecting the composition of fluids, solid materials, gases enclosed in a sample chamber.

18. A sensor for sensing a sample using a microwave signal at an excitation wavelength, the sensor comprising:

a dielectric waveguide for guiding the microwave signal;

a dielectric reflector at an end of the dielectric waveguide to cause formation of a sensing field; and

a concentrator arranged around the dielectric waveguide for concentrating microwave energy in the dielectric waveguide, wherein the concentrator is a distributed Bragg reflector structure.

19. A sensor as claimed in claim 18 , wherein the Bragg reflector structure is a honey comb structure.