Background of the Invention Time of flight ranging systems, are commonly used in level measurement applications, and are referred to as level measurement systems. Level measurement systems determine the distance to a reflector (i. e. reflective surface) by measuring how long after transmission of energy pulses or waves, an echo is received. Such systems typically utilize ultrasonic pulses, pulse radar signals, or microwave signals.

Microwave-based time of flight ranging systems are preferred in applications where the atmosphere in the container is subject to large temperatures changes, high humidity, dust and other type of conditions which can effect propagation, but to provide a sufficient receive response a high gain antenna is required. High gain translates into a large antenna size. Such applications also require a transmit antenna with a narrow beamwidth. However in view of the often limited space available in these types of time of flight ranging systems, severe constructional restraints are placed on the construction of the transmitting and receiving antenna. For example, many containers have small opening/flange sizes which limit the size of the level measurement system and the accompanying transmit/receive antenna. The challenge for microwave-based time of flight ranging systems is to provide a compact antenna design with both a high gain receive response and a small beamwidth for the transmit pattern.

Accordingly, there remains a need for an antenna design optimized and suitable for microwave-based time of flight ranging systems.

Brief Summary of the Invention The present invention provides an antenna structure suitable for use in microwave-base time of flight ranging systems and level measurement instruments.

The antenna structure comprises a horn antenna for transmitting and receiving electromagnetic signals. The horn antenna comprises a short horn antenna with a dielectric lens. The dielectric lens focusses the electromagnetic signals and allows the antenna to achieve a high gain response for receiving electromagnetic signals and also provides a small beamwidth transmit pattern.

The dielectric lens compensates for phase error and focusses the transmitted electromagnetic wave signals directed at the surface of the material to be measured.

In a first aspect, the present invention provides an antenna structure suitable for use in a level measurement system for measuring the level of a material held in a container, the antenna structure comprises: (a) a horn antenna, the horn antenna has a mouth and the mouth defines a small aperture size for fitting the horn antenna inside the container; and (b) a dielectric material, the dielectric material is mounted in the mouth of the horn antenna, and the dielectric material has a dielectric constant for producing a larger electrical aperture size for the horn antenna.

In a further aspect, the present invention provides a time of flight ranging system comprising: (a) a transducer for emitting electromagnetic energy and coupling reflected electromagnetic energy ; (b) a controller having a receiver component and a transmitter component; (c) the transducer having an input port operatively coupled to the transmitter component and being responsive to the transmitter component for emitting the electromagnetic energy, and the transducer includes an output port operatively coupled to the receiver component for outputting reflected electromagnetic energy coupled by the transducer; (d) the receiver component converts the reflected electromagnetic energy into corresponding electrical signals, and the controller includes a program component for determining the distance travelled by the electromagnetic energy; and (e) the transducer includes a horn antenna, the horn antenna has a mouth and the mouth defines an aperture size for fitting the horn antenna inside a container; and a dielectric material, the dielectric material is mounted in the mouth of the horn antenna, and the dielectric material has a dielectric constant for producing an effective larger electrical aperture size for the horn antenna.

In yet another aspect, the present invention provides a microwave level measurement system for determining the level of a material contained in a container, and the system comprises: (a) a transducer for emitting microwaves and coupling microwaves reflected from the surface of the material ; (b) a controller has a receiver component and a transmitter component ; (c) the transducer has an input port operatively coupled to the transmitter component and responsive to the transmitter component for emitting the microwaves, and the transducer includes an output port operatively coupled to the receiver component for outputting the reflected microwaves coupled by the transducer; (d) the receiver component converts the reflected microwaves into corresponding electrical signals, and the controller includes a program component for determining the distance travelled by the microwaves; and (e) the transducer includes a horn antenna, the horn antenna has a mouth and the mouth defines an aperture size for fitting the horn antenna inside the container; and a dielectric material, the dielectric material is mounted in the mouth of the horn antenna, and the dielectric material has a dielectric constant for producing a larger electrical aperture size for the horn antenna.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

Brief Description of the Drawings Reference will now be made to the accompanying drawings, which show, by way of example, a preferred embodiment of the present invention, and in which : Fig. 1 shows in diagrammatic form a microwave level measurement instrument with a microwave horn antenna according to the present invention; Fig. 2 shows a cross-sectional view of a microwave horn antenna according to the present invention.

Fig. 3 shows a cross-sectional view of the microwave horn antenna of Fig.

2 with a modified horn according to another aspect of the invention; Fig. 4 shows a cross-sectional view of a microwave horn antenna according to a second embodiment of the present invention; and Fig. 5 shows a cross-sectional view of a microwave horn antenna according to a third embodiment of the present invention.

Detailed Description of the Preferred Embodiment Reference is first made to Fig. 1, which shows in diagrammatic form a microwave level measurement device 10 with a microwave horn antenna according to the present invention. It will be appreciated that the microwave level measurement system 10 is one type of time of flight ranging system.

As shown in Fig. 1, the level measurement device 10 is mounted on top of a container 1 holding a material 2, e. g. liquid, and the level measurement device 10 is used to determine the level of the material 2 in the container 1. The container 1 has a flange 4 around the opening and the level measurement device 10 is coupled, e. g. bolted, to the flange 4. The level measurement device 10 comprises a housing 12 and a waveguide 14. The housing 12 contains electrical/electronic components as described below. The waveguide 14 extends into the interior of the container 2 and includes an antenna structure 16 in accordance with the present invention.

The microwave level measurement device 10 comprises a microwave transducer (i. e. the waveguide 14), a microcontroller unit 11, and an analog-to- digital converter 13. The microwave level measurement device 10 may also include a current (4-20mA) loop interface module 15. The transducer 14 is coupled to the microcontroller unit 11 through a transmitter 17. The microcontroller unit 11 uses the transmitter 17 to excite the transducer 14 to emit electromagnetic energy in the form of microwaves. The reflected electromagnetic energy, i. e. reflected microwaves, are coupled by the transducer 14 and converted into an electrical signal in a receiver 19.

The microwave level measurement device 10 is installed in the container 1, for example a tank, containing a material, such as the liquid 2, with a level determined by the top surface of the liquid 2. The top surface of the liquid 2 provides a reflective surface or reflector, indicated by reference 3, which reflects the microwaves generated from the emitter on the transducer 14, i. e. the waveguide 12 and the antenna structure 16. The reflected electromagnetic wave is coupled by the transducer 14 and converted by the receiver 19 into an electrical signal. The received electromagnetic wave is sampled and digitized by an A/D converter 13 for further processing by the microcontroller unit 11. The microcontroller unit 11 executes an algorithm which identifies and verifies the receive signal and calculates the range of the reflective surface 3, i. e. the time it takes for the reflected waveform or signal to travel from the reflective surface 3 to the receiver 19 on the transducer 14. From this calculation, the distance to the surface of the liquid 2 and thereby the level of the liquid is determined. The microcontroller 11 also controls the transmission of data and control signals through the current loop interface 15. The microcontroller 11 is suitably programmed to perform these operations as will be within the understanding of those skilled in the art. These techniques are described in prior patents of which U. S. Pat. No. 4,831,565 and U. S. Pat. No. 5,267,219 are exemplary.

As shown in Fig. 1, the waveguide 14 extends from the flange 4 into the interior of the container 1. The antenna structure 16 is attached to the end of the flange 4 and functions as the transducer 14 to provide a combined transmitting and receiving device. The antenna structure 16 transmits electromagnetic signals onto the surface 3 of the liquid 2 in the container 1 whose level is to be measured. The electromagnetic signal is reflected by the surface 3 of the liquid 2, and an echo signal is received by the antenna structure 16. The echo signal is processed as described above to calculate the level of the liquid 2 in the container 1.

Reference is next made to Fig. 2 which shows in more detail the antenna structure 16 according to the present invention. The antenna structure 16 as shown comprises a horn antenna 100 with an aperture or horn mouth, indicated by reference 102. According to this aspect, the horn mouth 102 is loaded with a dielectric material denoted by reference 110. According to another aspect of the invention, the dielectric material fills the horn antenna as described below with reference Fig. 5. As will be described in more detail below, the horn antenna 16 loaded with the dielectric lens 110 according to this aspect of the invention provides a high gain receive response and a small beamwidth transmit pattern, and with a small aperture. Advantageously, the small aperture configuration allows the antenna 16 to slip through a flange 4 of small size and therefore the device 10 can be installed on a large range of existing container sizes. The narrow beamwidth also benefits the measurement calculations for the device 10.

The receive gain of the horn antenna 16 is determined according to equation (1) as follows : alec G = lOlog 7. 5 z (1) In equation (1), G represents the gain in dB of the antenna 16, the parameter Aelec represents the electrical aperture of the horn antenna 16, and the parameter A represents the wavelength of the center frequency. The electrical aperture is determined by Aztec = Aphy Er with the parameter Er representing the dielectric constant of the used material for the dielectric lens 110 and the parameter Aphy representing the physical aperture of the horn antenna 16.

Preferably, to compensate for the phase deviation in the horn antenna 16 the inserted dielectric material 110 is formed on one side as a lens as indicated by reference 112 in Fig. 2.

The beamwidth of the antenna 16 is related to the gain G of the antenna 16. The beamwidth can be determined using equation (2) as follows : Equation (2) assumes that the beamwidth is identical in both the E and H- planes.

From both equations it can be seen that without the dielectric lens 110, a large horn mouth (Aphy = rr D2/4) is required to achieve a small. beamwidth, respectively a high gain antenna, which is required in microwave level measurement instrumentation. High gain antennas are used to avoid scattered signals from unwanted targets in the containers, which can be metal ladders or blades for instance.

Additionally, the dielectric lens 110 seals the mouth of the horn antenna 16. Advantageously, this prevents the build-up of dust inside the horn of the antenna 16 and thereby maintains the sensitivity of the antenna 16 under the operating conditions typically encountered in various container applications.

According to another aspect, a quarter-wavelength transmission line transformer with a material which has a dielectric constant of £rm =5 is included to reduce reflection from the transition between the dielectric lens 110 and the air boundary. As shown in Fig. 3, one implementation of the quarter- wavelength transmission line transformer, i. e. anti-reflection layer, comprises applying a grooved surface indicated by reference 120 to the lens 110. The effect of the grooves is to reduce the dielectric constant of the dielectric material 110.

Another implementation for the quarter-wavelength transmission line transformer is shown in Fig. 4 and comprises applying a material layer 130 to the dielectric lens 110. The material layer 130 comprises a material different from the material of the lens 110 but having the required dielectric constant. The thickness for both the grooved layer 120 and the material layer 130 can be calculated according to equation (3) as follows : In equation (3), the parameter Ao represents the wavelength in air.

Reference is next made to Fig. 5, which shows an antenna structure 140 according to another embodiment of the invention. The antenna structure 140 comprises a horn antenna 142 and a dielectric material 150. According to this aspect, the dielectric material 150 fills a substantial portion or entire horn of the antenna 142 to provide a high electrical aperture size. If angular opening of the horn antenna 142 is small, for example up to 30 degrees, then additional phase compensation is not needed. As described above, an anti-reflection layer 120 or 130 can be added to the dielectric material 150 to reduce reflection from the transition between the dielectric lens 150 to air as described above.

Suitable material compositions for the dielectric 150 include PTFE, ceramic, glass, and plastic materials. The anti-reflection layer 120,130 may comprise any of the above materials having a dielectric constant which is lower than the dielectric constant for the lens.

The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Certain adaptations and modifications of the invention will be obvious to those skilled in the art.

Therefore, the presently discussed embodiments are considered to be illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.