TUNEABLE FLUID-FILLED DIELECTRIC RESONATOR ANTENNAS

Abstract: A dielectric resonator antenna (1) comprising a vessel (2) containing a volume of dielectric fluid (5) and at least one feed (4) for transferring energy to and from the dielectric fluid (5). The dielectric resonator antenna (1) may be tuned to different resonant frequencies by changing the volume of fluid (5) within the vessel (2).

Description: TUNEABLE FLUID-FILLED DIELECTRIC RESONATOR ANTENNAS The present invention relates to a fluid-filled dielectric resonator antenna (DRA) having a single or multiple feeds, and in particular to a fluid-filled DRA in which the frequency of resonance can be adjusted by some mechanism for changing the level of fluid within the DRA. Since the first systematic study of DRAs in 1983 [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical Dielectric Cavity Antenna", IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412], interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R.K. and BHARTIA, P.: "Dielectric Resonator Antennas - A Review and General Design Relations for Resonant Frequency and Bandwidth", International Journal of Microwave and Millimetre- Wave Computer-Aided Engineering, 1994, 4, (3), pp 230-247]. Most of the configurations reported have used a slab of dielectric material mounted on a ground plane excited by either an aperture feed in the ground plane or by a probe inserted into the dielectric material. The general concept of deploying a plurality of probes within a single liquid-filled dielectric resonator antenna, as pertaining to a cylindrical geometry, is described in the paper KINGSLEY, S.P. and O'KEEFE, S.G., "Beam Steering and Monopulse Processing of Probe-Fed Dielectric Resonator Antennas", IEE Proceedings - Radar, Sonar and Navigation, 146, 3, 121 - 125, 1999, the disclosure of which is incorporated into the present application by reference, as is the disclosure of US patent application serial no. 09/431,548 filed on 29 th October 1999 by the inventors in respect of the present invention. In general, a DRA consists of a volume of a dielectric material disposed on a grounded substrate, with energy being transferred to and from the dielectric material by, way of monopole probes inserted into the dielectric material or by way of monopole aperture feeds provided in the grounded substrate. Alternatively, dipole probes may be inserted into the dielectric material, in which case a grounded substrate is not required. By providing multiple feeds and exciting these sequentially or in various combinations, a continuously or incrementally steerable beam or beams may be formed. The resonant characteristics of a DRA depend upon the shape and size of the volume of dielectric material and also on the shape, size and position of the feeds thereto. According to a first aspect of the present invention, there is provided a dielectric resonator antenna comprising a vessel containing a volume of dielectric fluid and at least one feed for transferring energy to and from the dielectric fluid, characterised in that there is further provided means for changing the volume of fluid within the vessel so as to tune the antenna to at least one predetermined frequency. According to a second aspect of the present invention, there is provided a method for tuning a dielectric resonator antenna comprising a vessel containing a volume of dielectric fluid and at least one feed for transferring energy to and from the dielectric fluid, characterised in that the dielectric resonator is tuneable to different resonant frequencies by changing the volume of fluid within the vessel. For the avoidance of doubt, the expression "changing the volume of fluid" is intended to encompass changing the shape of the volume, of liquid, either by adding or removing liquid from the vessel or by changing the shape of the vessel, with or without attendant addition or removal of liquid. By changing the volume of fluid, it is possible to tune the DRA to particular resonant frequencies. Where the at least one feed comprises a monopole probe or an aperture feed, the volume of dielectric material is disposed upon or close to a grounded substrate. Alternatively, the at least one feed may comprise a dipole probe, in which case a grounded substrate is not required. By providing more than one feed and by activating the feeds either individually or in combination, it is possible to produce at least one incrementally or continuously steerable beam which may be steered through a predetermined angle. Further details of this beamsteering capability are disclosed in the present Inventors' copending US patent application serial number 09/431,548. In one embodiment of the present invention, therefore, there is provided a plurality of feeds for transferring energy into and from different regions of the dielectric fluid, the feeds being activatable either individually or in combination so as to produce at least one incrementally or continuously steerable beam which may be steered through a predetermined angle. Preferably, there is further provided electronic circuitry adapted to activate the feeds either individually or in combination so as to produce at least one incrementally or continuously steerable beam which may be steered through a predetermined angle. The means for changing the volume of fluid may be a pump adapted to add or remove fluid to or from the vessel. Alternatively or in addition, fluid may be added to the vessel from a reservoir and removed simply by being drained therefrom, with valves being provided to control the addition or removal of fluid. Alternatively or in addition, the vessel may be adapted to have a variable shape. For example, the vessel may have movable walls so as to change its volume and/or shape, thereby changing the volume of fluid. For example, a cylindrical vessel may be formed from a rectangular sheet of material by bending the sheet so as to bring two opposed edges thereof together. By allowing the opposed edges to overlap to a greater or lesser degree, the diameter of the cylindrical vessel may be made respectively smaller or larger. Advantageously, a sealing mechanism allowing movable overlap of the edges while maintaining a fluid-tight seal is provided. The sealing mechanism may be provided by forming a longitudinal slit near one of the edges and passing the other edge therethrough, the slit being lined with a resilient material so as to provide a fluid-tight seal. In an alternative example, the vessel may be polyhedral in cross-section, having side walls each having one free end and one end abutting a surface of an adjacent side wall i a fluid-tight manner. In this way, the side walls may be slid relative to each other so as to change the shape and/or volume of the vessel and thus to change the volume of fluid contained therein. Other mechanisms for changing the shape and/or volume of the vessel will be apparent to the skilled person. The shape of the vessel may be adjusted manually, but in preferred embodiments servo motors or the like are provided so as to move the side wall or walls under remote control, advantageously in response to control signals from a computer, microprocessor, microcontroller or other electronic control device. As is clear from the above, the vessel need not be circular in cross-section, but may take any appropriate shape. The dielectric fluid may be a liquid, a gas or a gel. Suitable liquid fluids include water, alcohols including butanol, polyethylene glycol, diethylene glycol, dimethyldigol, acetone, tetrahydrafuran and 1,4 dioxane. The last two examples in this list have been found to be particularly effective, since they have the lowest loss tangents. In embodiments where the DRA of the present invention is intended for use on a mobile platform, for example a vehicle such as an aircraft, ship or road vehicle, gels and gases are preferred since these will tend to maintain a desired volume and/or shape within the vessel regardless of any movement of the, platform. However, in some embodiments of the invention a support matrix may be provided within the vessel, the support matrix being adapted to hold and constrain a liquid. The support matrix may be made out of a solid foam material, for example of the sort used to carry fuel within the petrol tanks of racing cars. The solid foam material desirably has a low dielectric constant and loss tangent. Suitable foam materials include polyurethane "open pore" foams with precisely controlled pore sizes and without cell membranes. These materials have skeletal structures occupying about 3% of the tank volume and are designed to control fuel surges in many types of aircraft and vehicle. For use with the present invention, the solid foam material desirably has a low dielectric constant and loss tangent. Polyurethanes generally have relative permittivities in the range 3-6 and can be relatively low-loss. Various proprietary materials such as SafeCrest® are thought to be suitable and a military specification aviation foam, B-83054, might also be suitable. In a further embodiment, the DRA may be constructed with a central core of a solid dielectric material having a relatively high dielectric constant, and providing a vessel in the form of a jacket surrounding the central core and having a volume into which a fluid dielectric material is supplied. The central splid core having a relatively high dielectric constant allows the DRA as a whole to be made relatively small. By changing the volume of the fluid within the jacket, the DRA may be tuned to a desired frequency. The feed or feeds may be inserted into the solid core or may be inserted into the fluid dielectric material in the jacket. The latter is preferred for reasons of improved directionality of any resultant beams. In all of the embodiments described above, the feeds for transferring energy into and from the dielectric fluid are advantageously adjustable together with the volume of fluid. For example, where the feeds are in the form of probes, the probes are advantageously adapted to have a variable effective length. This may be achieved through the use of mechanical adaptations such as telescopic mechanisms that allow the probe length to be varied preferably automatically by way of, for example, a servo motor so as to be well-matched to any particular volume of fluid. Another mechanical solution may be provided by providing a mechanism for replacing a probe of one length with a probe of another when the volume of fluid changes in a predetermined manner. This may be achieved by providing a plurality of probes of different lengths, for example in a region below or above the dielectric fluid, and raising or lowering the probes one at a time into the dielectric fluid as the volume of fluid changes. Alternatively, electrical mechanisms could be employed, where a plurality of probes of different lengths is immersed in the volume of fluid and wherein an electrical switching mechanism is provided so as to bring the different probes selectively on-line, the remaining probes then being open circuited. Similarly, where the feeds are aperture feeds, the effective length of the aperture may be varied mechanically or electrically so as to be well-matched to the volume of fluid. In all of these embodiments, selection of the appropriate probe length is preferably made automatically in accordance with a predetermined operating protocol by way of a control mechanism that also controls the volume of fluid. Applications for embodiments of the present invention include low frequency antennas where space is limited such as on ships or oilrigs. HF and VHF radar, communication and RDF systems may be constructed with full beamsteering and monopulse processing capabilities in about 11 percent of the space occupied by a simple conventional antenna without these capabilities. For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which: FIGURE 1 shows an outline plan view of a DRA according to an embodiment of the present invention; FIGURE 2 shows a side elevation of the DRA of Figure 1; FIGURE 3 is a graph showing the return loss of the DRA ; of Figures 1 and 2 at a resonant frequency of 55.5MHz; FIGURE 4 is a graph showing the variation of return loss with water depth for the DRA of Figures 1 and 2; FIGURE 5 is a graph showing the variation of return loss with resonant frequency for the DRA of Figures 1 and 2; and FIGURE 6 is a graph showing predicted and measured resonant frequencies against water depth for the DRA of Figures 1 and 2. Referring firstly to Figures 1 and 2, there is shown a DRA 1 comprising a cylindrical PNC outer wall 2, 5mm in thickness and 550mm in diameter, mounted on a grounded octagonal aluminium plate 3 of dimension 800mm between opposing sides. The DRA 1 is fitted with a single probe 4, 55mm from the outer wall 2, and filled with water 5. As shown in Figure 2, an outlet 6 is provided at a lower portion of the wall 2, the outlet 6 being connected by way of a pump 7 to a raised reservoir 8. The reservoir 8 contains a supply of water 5, and has an outlet 9 which passes to an inlet 10 at an upper portion of the wall 2 by way of a valve 11. By operating the pump 7 and the valve 11 in an appropriate manner, the level of water 5 within the wall 2 may be varied as desired. An experiment was performed with the DRA 1 of Figures 1 and 2. The temperature of the water 5 started at 8°C, but may have risen slightly during the experiment, and the relative permittivity was estimated to be 87. The, water depth was altered, generally in 10mm steps, and the resonant frequency of the HEMπ δ mode was measured using a HP 8714B network analyser (not shown) operated in reflection (Sπ) mode. Radiation patterns for this antenna have already been disclosed in co- pending US patent application serial no. 09/431,548. Initially a fixed probe 4 of length 175mm was used and the water 5 depth was 276mm. The water 5 depth was then reduced in 10mm steps to 156mm, with a final reading at 151mm. This change in depth caused a corresponding change in resonant frequency from 50MHz to 68.6MHz. The return loss was better than 15dB across this frequency range, and was at its best towards the centre of the range, as shown in Figure 3. At a depth of 151mm, the return loss was getting quite poor at only -15.5dB, and so the probe 4 was shortened before proceeding. The remaining readings were taken with a fixed probe 4 of length 134mm and the water 5 depth was reduced from 151mm in mainly 10mm steps to 95mm. This change in depth caused a corresponding change in resonant frequency from 68.3MHz. to 96.8MHz. In a practical or commercial system a change in probe 4 length could be achieved by electronic means or by switching between two different probes 4 (and leaving the unused probe 4 open circuited). Likewise there are means of tuning slot feeds to achieve the same end result. Figure 4 shows the variation in return loss plotted against water 5 depth. The discontinuity 12 at the centre of the plot is where the probe 4 length was changed, with the right-hand section of the plot being for the 175mm probe 4 and the left-hand section for the 134 mm probe 4. Figure 5 shows the same return loss values plotted against resonant frequency. The discontinuity 13 at the centre of the plot is where the probe 4 length was changed, with the right-hand section of the plot being for the 175mm probe 4 and the left-hand section for the 134 mm probe 4. The double-trough nature of the two parts of the plot of Figure 5 is caused by an interesting result: when the aspect ratio (depth radius) of the DRA 1 is high, say around 0.8, the best return loss is obtained when the top of the probe 4 lies well below the water 5 level. This is the deep trough 14 on the left of Figure 5 at 55MHz. The second trough 15 at 64MHz is caused by the probe 4 breaking the surface as the water 5 level falls but, as the aspect ratio is high (say 0.6) at this stage, this trough 15 does not represent such a good match as the trough 14 at 55MHz. However, as the water 5 level continues to go down, the DRA 1 develops a low aspect ratio and the next trough 16 at 76MHz, now for the shortened probe 4 well below the surface of the water 5, is not a particularly good match at -24dB return loss. When the probe 4 does break the surface, a better match of -30dB is obtained at 87MHz. These results demonstrate that the probe 4 is advantageously shorter than the depth of dielectric material for high aspect ratios and comparable with the depth of dielectric material for low aspect ratios, at least for a material with a high dielectric constant. An important result is shown in Figure 6. This is the variation of resonant frequency plotted against water 5 depth. The resonant frequencies expected from theory are also shown in Figure 6 and the agreement between experiment and theory can be seen to be very good. The formula for calculating the resonant frequency of a cylindrical DRA 1 can be found in MONGIA, R.K. and BHARTIA, P.: "Dielectric Resonator Antennas - A Review and General Design Relations for Resonant Frequency and Bandwidth", International Journal of Microwave and Millimetre-Wave Computer- Aided Engineering, 1994, 4, (3), pp 230-247. Throughout the experiment the bandwidth of the DRA 1, as measured at the lOdB level of the return loss, remained about 5%. Such narrow bandwidths remain a disadvantage of DRAs 1. However, the results presented here show that the DRA 1 can be easily tuned over a bandwidth of 32% using a single probe 4 of fixed length and 64% using two probes 4 or a single probe 4 of switchable length. Varying the probe 4 length over a greater range would clearly extend the results.

Claims: CLAIMS: 1. A dielectric resonator antenna comprising a vessel containing a volume of dielectric fluid and at least one feed for transferring energy to and from the dielectric fluid, characterised in that there is further provided means for changing the volume of fluid within the vessel so as to tune the antenna to at least one predetermined frequency. 2. An antenna as claimed in claim 1, wherein the means for changing the volume of fluid includes a pump adapted to add or remove fluid to or from the vessel. 3. An antenna as claimed in claim 1 or 2, wherein the means for changing the volume of fluid includes a reservoir. 4. An antenna as claimed in claim 1, 2 or 3, wherein the means for changing the volume of fluid includes at least one valve adapted to control a flow of fluid to or from the vessel. 5. An antenna as claimed in any preceding claim, wherein the vessel is adapted to have a variable shape and/or volume. 6. An antenna as claimed in claim 5, wherein the vessel has at least one movable wall which may be moved so as to change the shape and/or volume of the vessel. 7. An antenna as claimed in claim 6, wherein the shape and/or volume of the vessel is varied by way of at least one servo motor adapted to move the at least one wall. 8. An antenna as claimed in any preceding claim, wherein the at least one feed has a dimension that may be varied so as to maintain or improve tuning characteristics of the antenna as the volume of fluid is changed. 9. An antenna as claimed in any preceding claim, wherein a plurality of differently-dimensioned feeds is provided, and wherein an electrical switching mechanism is provided so as to select between the plurality of differently- dimensioned feeds as the volume of fluid is changed. 10. An antenna as claimed in any preceding claim, wherein a plurality of differently-dimensioned feeds is provided, and wherein a mechanical switching mechanism is provided so as to select between the plurality of differently- dimensioned feeds as the volume of fluid is changed. 11. An antenna as claimed in any preceding claim, wherein there is further provided a core made out of a solid dielectric material having a relatively high dielectric constant, and wherein the vessel surrounds the core and contains a fluid dielectric material having a relatively low dielectric constant. 12. An antenna as claimed in any preceding claim, wherein the fluid is a gas. 13. An antenna as claimed in any one of claims 1 to 11, wherein the fluid is a gel. 14. An antenna as claimed in any one of claims 1 to 11, wherein the fluid is a liquid. 15. An antenna as claimed in claim 14, wherein a support matrix is provided in the vessel, the support matrix being adapted to hold and constrain the liquid. 16. An antenna as claimed in claim 15, wherein the support matrix is a solid foam material. 17. A method for tuning a dielectric resonator antenna comprising a vessel containing a volume of dielectric fluid and at least one feed for transferring energy to and from the dielectric fluid, characterised in that the dielectric resonator is tuneable to different resonant frequencies by changing the volume of fluid within the vessel. 18. A method according to claim 17, wherem the volume of fluid is changed by pumping fluid to or from the vessel. 19. A method according to claim 17 or 18, wherein the volume of fluid is changed by removing fluid from the vessel to a reservoir and/or by supplying fluid to the vessel from a reservoir. 20. A method according to any one of claims 17 to 19, wherein the volume of fluid is changed by changing the shape and/or volume of the vessel. 21. A method according to claim 20, wherein the vessel has at least one movable wall provided with a servo motor which is operable to change the shape and/or volume of the vessel. 22. A method according to any one of claims 17 to 21, wherein the at least one feed has a dimension that is varied as the volume of fluid is changed. 23. A method according to claim 22, wherein the dimension of the at least one feed is varied by way of a telescopic mechanism. 24. A method according to claim 22 or 23, wherein a plurality of differently- dimensioned feeds is provided, and wherein at least one of the plurality of feeds is selected for immersion into the fluid by way of a mechanical mechanism. 25. A method according to claim 22 or 23, wherein a. plurality of differently- dimensioned feeds is provided, and wherein at least one of the plurality of feeds is selected for operation by way of an electrical switching mechanism. 26. A dielectric resonator antenna as claimed in any one of claims 1 to 16, wherem there is provided a plurality of feeds for transferring energy into and from different regions of the dielectric fluid, the feeds being activatable either individually or in combination so as to produce at least one incrementally or continuously steerable beam which may be steered through a predetermined angle. 27. A dielectric resonator antenna as claimed in claim 26, further comprising electronic circuitry adapted to activate the feeds either individually or in combination so as to produce at least one incrementally or continuously steerable beam which may be steered through a predetermined angle.

Inventors: KINGSLEY SIMON PHILIP, O'KEEFE STEVEN GREGORY, KINGSLEY, SIMON, PHILIP, O'KEEFE, STEVEN, GREGORY
Applicants: ANTENOVA LTD, KINGSLEY SIMON PHILIP, O'KEEFE STEVEN GREGORY, ANTENOVA LIMITED, KINGSLEY, SIMON, PHILIP, O'KEEFE, STEVEN, GREGORY
Application year: 2001
Main classification: H01Q9
IPC codes: H01Q9