DEVICE FOR MICROWAVE FIELD DETECTION

Abstract

An electromagnetic receiving device configured to discriminately react predominantly to an external electric field directed along an axis of said device, comprising a high-permittivity dielectric rod as receiving element, wherein said dielectric rod is oriented along said axis and has a receiving end to be directed towards an object under study, and wherein the device is configured to be essentially resonant with a circularly cylindrical TM00 mode.

Claims

1. An electromagnetic receiving device configured to discriminately react predominantly to an external electric field directed along an axis of the electromagnetic receiving device, comprising, as receiving element, a dielectric rod with permittivity in a range between 20 and 100, wherein the dielectric rod is oriented along the axis and has a receiving end to be directed towards an object under study, and wherein the dielectric rod has a diameter and a permittivity resulting in an essentially resonant condition of a circularly cylindrical TM.sub.00 mode.

2. The electromagnetic receiving device of claim 1, wherein the permittivity of the dielectric rod is higher than an average permittivity of an outer 5 mm of the object under study, in a frequency range 1 to 5 GHz.

3. The electromagnetic receiving device of claim 1, further comprising a transition to a coaxial line in a rear end of the dielectric rod opposite to the receiving end, wherein the coaxial line has an outer conductor that is separated by a gap to the dielectric rod and an inner conductor that is in direct contact with a metalized hole in a center of a circular end of the dielectric rod.

4. The electromagnetic receiving device of claim 3, in which a characteristic impedance of the coaxial line is higher than 50 .

5. The electromagnetic receiving device of claim 1, wherein the dielectric rod has a loss factor such that / exceeds about 50.

6. An apparatus comprising an electromagnetic transmitting device and one or more receiving devices according to at least claim 1, the apparatus being configured to collect diffracted signals from internal dielectric inhomogeneities in the object under study by using orthogonality relations between a primary magnetic field generated by the electromagnetic transmitting device, an electric field induced in the object under study by the primary magnetic field, and an electric field caused by diffraction by an internal dielectric inhomogeneity in the object under study.

7. The apparatus of claim 6, further comprising means for direct readout of the received signals as function of a receiving device position on the object under study, including means for computing, using the received signals and receiving device positions for providing a diffracted signal map over a large part of a surface of the object under study.

8. The apparatus of claim 6, wherein the permittivity of the dielectric rod is higher than an average permittivity of an outer 5 mm of the object under study, in a frequency range 1 to 5 GHz.

9. The apparatus of claim 6, further comprising a transition to a coaxial line in a rear end of the dielectric rod opposite to the receiving end, wherein the coaxial line has an outer conductor that is separated by a gap to the dielectric rod and an inner conductor that is in direct contact with a metalized hole in a center of a circular end of the dielectric rod.

10. The apparatus of claim 9, in which a characteristic impedance of the coaxial line is higher than 50 .

11. The apparatus of claim 6, wherein the dielectric rod has a loss factor such that / exceeds about 50.

12. The electromagnetic receiving device of claim 2, further comprising a transition to a coaxial line in a rear end of the dielectric rod opposite to the receiving end, wherein the coaxial line has an outer conductor that is separated by a gap to the dielectric rod and an inner conductor that is in direct contact with a metalized hole in a center of a circular end of the dielectric rod.

13. The electromagnetic receiving device of claim 2, wherein the dielectric rod has a loss factor such that / exceeds about 50.

14. The electromagnetic receiving device of claim 3, wherein the dielectric rod has a loss factor such that / exceeds about 50.

15. The electromagnetic receiving device of claim 4, wherein the dielectric rod has a loss factor such that / exceeds about 50.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the detailed description below, reference will be made to the accompanying drawings, on which

[0019] FIG. 1 schematically shows an electromagnetic receiving device according to the present invention;

[0020] FIG. 2 illustrates the overall momentary E field as arrows in logarithmic scale in a vertical plane of the device axis, when used in transmission mode; and

[0021] FIG. 3 is a graph showing the system reflection factor in conventional polar format.

DETAILED DESCRIPTION

[0022] FIG. 1 schematically shows a cross section of a rotationally symmetric device according to a preferred embodiment of the invention, for 3 GHz operation. A rod 1 with permittivity =70 is 9 mm long and has a diameter of 2.75 mm. The object under study (OUS) has a 2 mm thick skin layer 2, below which there is, in this example, breast fat 3. There is a protective holder 4 of a low-permittivity plastic, fitting the rod as well as the outside of the metallic outer coaxial conductor 5, having an inner diameter of about 2.5 mm or less. The inner 0.2 mm diameter coaxial conductor 6 is soldered to a 1.5 mm diameter metal filling 7 which is reliably in contact with the surrounding hole in the ceramic end of the rod opposite to the receiving end. One preferred way of obtaining good contact between the metal filling 7 and the surrounding hole is to use a lead-bismuth alloy, which has a slight expansion upon solidification due to the thermal properties of bismuth. There is an airgap 8 between the outer metal part 5 of the coaxial line and the rod 1, for obtaining the desired performance of the transition. The inner space above this is also air. Since there may be some unwanted external fields emanating from the OUS-contacting area, a metal holder 9 with a dielectric-filled quarterwave wavetrap may be affixed at the external coaxial conductor 5. Further, a compartment 11 for holding electronics such as a signal converter to DC, circuitry for wireless communication, etc. may also be provided.

[0023] FIG. 2 shows the overall momentary E field as arrows in logarithmic scale in a vertical plane of the device axis, when used in transmission mode. In a transmitter-receiver system this performance will of course be reciprocal, and the above-mentioned external fields without the wavetrap are seen. The dominant axial E field at the receiving rod end is also seen.

[0024] FIG. 3 shows the system reflection factor as modelled with a coaxial input line having a characteristic impedance of 110. The frequency span is from 2100 to 4000 MHz and the diametrical line across the circular part, used for determination of the loaded quality factor (Q value), has crosspoints at 2590 (closer to the left end in FIG. 3) and 3660 MHz, which correspond to a loaded Q value of 3.0 and an unloaded Q value thus being twice of that due to the matching at resonance, i.e. about 6. A low Q value is generally advantageous, but is needs to be sufficiently high in order to stabilize the TM.sub.00 mode field pattern. Hence, in embodiments of the present invention, the loaded Q value is preferably 3 or higher, corresponding to an unloaded Q value of 6 or higher. In general, an inherent Q value of the rod material should preferably be much higher in order not to disturb the system resonance. The material selected for the rod may, for example, have a real permittivity of and a corresponding loss factor such that / exceeds about 50.

[0025] In the preferred embodiment, the rod is in direct contact with a surface of the OUS during use. This gives the best coupling of the axial E field across the boundary. There will of course be a lower E axial field intensity in the rod than in the OUS when of the latter is lower than that of the rod, but in the case of e.g. a contacting thin skin layer with a lower- tissue below, the evanescence of the incoming surface-perpendicular E field will in total not be weakened much, due to the continuity of the corresponding D vector. However, in cases where the OUS has an uneven surface in the sub-millimeter scale, an intermediate liquid layer may be applied, as in the comparable case with ultrasound examinations, and then for avoiding air pockets. A microwave-adapted liquid for this purpose should then have an which is not much lower than about half that of the rod material, i.e. its is to be at least about 20 in the preferred embodiment.

[0026] The rod length is per se not a sensitive parameter, but one should consider the need for the TM.sub.00 mode with very low axial variation of the field to be established, and also the need for locating its top end transition part to the coaxial line sufficiently far away from the object contacting end, for avoiding the above-mentioned undesired emissions. There may also be a need for a free end region below the device holder 4, as shown in FIG. 1, for viewing of any marked contact spots on the OUS and also for cleaning purposes. A rod length of about 10 mm for 3 GHz operation may be sufficient under most conditions, with about half of this being covered by the holder 4.

[0027] As will be understood, there is a need for the coaxial outer conductor 5 to end some distance above the rod, in order for the creation of the TM.sub.00 field in the latter not to be significantly disturbed. The length of the free center conductor 6 has also an influence on the coupling field impedance. It has been found that this gap 8 is suitably between 2 and 4 mm, at 3 GHz. There will then of course, in the preferred embodiment, be an emission of a nearfield into free space from this junction region, but since the free length is much shorter than a quarter wavelength, this does not deteriorate the performance significantly, since most of the field will be non-radiating. However, the wavetrap 9, 10 above will reduce any unwanted interfering emissions.

[0028] Typical embodiments will include a holder 4 as seen in FIG. 1. Such holder should preferably have a minimal influence on the overall performance and should thus generally be made from a low loss, low- material. Typical suitable plastic materials have less than about 3. With such a choice, the material thickness can be several mm.

[0029] The coaxial inner conductor 6 has a diameter of only 0.2 mm and is in air, in the preferred embodiment for 3 GHz. This corresponds to a characteristic impedance of 150, which gives a coupling factor near 1 at resonance, as seen in FIG. 3. A thicker wire will provide undercoupling, with best coupling for slightly lower frequency. While the inner conductor is preferably protected from external influences, it does not need to be strained or tensed since a radial offset of the position does not influence its characteristic impedance much. In any case, its length does not need to be more than 20 mm and it is soldered at both ends.

[0030] The diameter and depth of the hole 7 with metal can be experimentally determined, for example, according to the design principle is discussed in U.S. Pat. No. 4,392,039. As shown in FIG. 3, critical coupling is achievable.

Signal Processing and Presentation

[0031] This section is provided for creating a complete picture of the system, with reference being made to the patent publication EP 3 373 808 B1 as background.

[0032] The coaxial line enters the compartment or box 11, which contains standard engineering sub-components such as for rectification, low-pass filtering, amplification, and then an output of a DC signal corresponding to the received averaged signal from the rod. This signal can be conveyed by a metallic cable to the system processing unit with DC feed to the amplifier in the cable, or a small battery can be used for energizing the internal subsystems. The box 11 may also include an ND converter and a Bluetooth or similar wireless transmitter.

[0033] In use, one or a few devices for field detection is/are moved over the OUS, for recording position-dependent signals. This means that positions of the devices should be combined with signal readings, for obtaining a kind of signal map over at least part of the object surface. It is also conceivable that several receiving devices are located in a predetermined geometric pattern over the surface of the OUS. If a transmitting device according to the previously referenced EP 3 373 808 B1 is used, also that device needs to have at least two different positions, due to the fact that there is no induced E field along its axis, thus not providing any diffraction signals from any dielectric irregularity in that region.