Communications: radio wave antennas – Antennas – Measuring signal energy
Reexamination Certificate
2000-02-02
2001-07-24
Phan, T. (Department: 2821)
Communications: radio wave antennas
Antennas
Measuring signal energy
C343S7000MS, C343S770000, C342S124000, C324S644000
Reexamination Certificate
active
06266022
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device for determining the filling level of a filling material in a container. In particular, the invention relates to a device for determining the filling level in a container that provides effective overvoltage protection. The invention also relates to a method for fastening an input coupling unit to an antenna that can be used in the device according to the invention.
BACKGROUND OF THE INVENTION
It is preferred to use planar antennas to radiate a preferred wave mode. A planar antenna which can be used in connection with the device according to the invention is described in the book entitled “Einführung in die Theorie und Technik planarer Mikrowellenantennen in Mikrostreifenleitungstechnik” (“Introduction to the Theory and Technology of Planar Microwave Antennas in Microstrip Line Technology”), Gregor Gronau, Verlagsbuchhandlung Nellissen-Wolff or in the journal article “Impedance of a Radiating Slot in the Ground Plane of a Microstrip Line”, IEEE Trans. Antennas Propagat., vol. AP-30, 922-926, May 1982.
A planar antenna comprises, for example, a dielectric substrate on one side of which the antenna structure is provided, and on the other side of which a conductive coating is provided. Openings are arranged in the conductive coating in such a way that the antenna radiates only electromagnetic waves of the desired mode.
Special protective measures are to be taken when the filling level meter is used in an area subject to explosion hazards. Here, it is imperative to ensure that overvoltages cannot be discharged through the explosive medium onto a neighboring surface at ground potential. Thus, in particular, no discharge spark may be allowed to jump over from the antenna onto the flange with the aid of which the filling level meter is fastened on the container. Overvoltages on the antenna are caused, for example, by a lightning strike.
A known protective measure provides that before it strikes the filling level meter an overvoltage is intercepted by an additional device, a so-called lightning protection system. This solution is, of course, relatively expensive. It has also become known to implement overvoltage protection in an electronic way.
SUMMARY OF THE INVENTION
It is the object of the invention to propose a cost effective overvoltage protection of high quality for a planar antenna, and to propose a method for fastening an input coupling unit on such an antenna.
The object is achieved with respect to the device by virtue of the fact that the antenna comprises at least two dielectric layers. The first dielectric layer has at least one cutout for holding the input coupling unit. The second dielectric layer bears an antenna structure on the side facing the first dielectric layer and has a conductive coating with openings on the side averted from the first dielectric layer. Contacts are provided in the second dielectric layer which connect the input coupling unit to the conductive coating. Furthermore, the region of space bounded by the contacts, the conductive coating and the antenna housing form a Faraday cage. The openings which are provided in the conductive coating are preferably of slot-shaped construction. The cutout in the first dielectric layer can be comprised of one or more through-connections or one or more openings.
In accordance with an advantageous development of the device according to the invention, the contacts are arranged in blind bores. The use of blind bores into which the contacts have been or will be introduced produces a very reliable, permanent connection and thus a high mechanical stability between the input coupling unit and the planar antenna.
In accordance with a first configuration of the device according to the invention, the blind bores have a conductive inner coating. This configuration permits the use of a radio-frequency connector, for example an SMA connector as the input coupling unit. In accordance with one configuration of the device according to the invention, the radio-frequency connector has a plurality of frame contact pins and at least one contact pin, arranged essentially centrally, for the inner conductor. It is usual to provide four contact pins which—seen in cross section—are arranged at the corners of a square, the contact pin for the inner conductor being located at the center of the square. The contact pins of the radio-frequency connector are inserted into the blind bores with the conductive inner coating, producing an electric connection to the conductive coating; the contact pin for the inner conductor is connected in a conductive fashion to the antenna structure. It is preferred to use the reflow method for the purpose of permanent electric connection between the input coupling unit and antenna structure or conductive coating. A preferred development of the device according to the invention provides, in addition, that the contact pin for the inner conductor of the input coupling unit, in particular of the radio-frequency connector (SMA connector), is shorter by at least the thickness of the first dielectric layer than the frame contact pins.
Furthermore, an advantageous development of the device according to the invention provides that the conductive coating is at ground potential through contact with the antenna housing and/or the connecting flange.
Thus, the conductive coating at ground potential with the preferably slot-shaped openings is provided in front of the inner conductor via which the measuring signals are fed from the signal-generating unit onto the antenna. Upon the occurrence of an overvoltage on the antenna, the discharge is conducted from the antenna structure onto the conductive coating, which is at ground potential. The discharge path is thus located completely within the antenna, and the discharge spark does not come into contact with the gas mixture of the area subject to explosion hazards.
The contacts are provided for the purpose of rendering redundant the diversion of the overvoltage from the conductive coating. Since the contacts do not penetrate the dielectric protective layer situated in front of the conductive coating, this also eliminates the otherwise customary soldered joints which are situated outside the protective layer and are relatively unstable chemically and must therefore be covered in principle by a further protective layer.
Since, moreover, the inner conductor does not penetrate the conductive coating connected to ground, no discharge spark can break through into the area subject to explosion hazards: the contacts, the antenna housing and the conductive coating form a Faraday cage.
In order to achieve optimum impedance matching between the input coupling unit and the antenna, the opening in the first dielectric layer, in which the input coupling unit is arranged, is filled up at least partially with a dielectric material. This dielectric material is selected to minimize the jump in impedance which usually occurs upon transition from one medium to another.
An advantageous development of the device according to the invention provides a dielectric protective layer which is connected to the second dielectric layer in such a way that the protective coating is arranged between the two dielectric layers. As the name already says, the layer protects the conductive coating situated therebelow against contamination and corrosion. The thickness of the conductive layer is preferably selected to provide protection against diffusion. The dielectric layers, in particular the protective layer, are preferably produced from Teflon. Thus, it is possible for the Teflon layer to be connected directly to the second dielectric layer, for example by means of optical laser welding. An additional adhesive layer is superfluous. The dielectric layers, in particular the first and second dielectric layer can, moreover, comprise a Teflon-ceramic composite or a Teflon-ceramic-glassfiber composite.
In accordance with an advantageous development of the device according to the invention, a funnel-shaped adapter is provided which connects the antenna to a second waveg
Birgel Dietmar
Hauptvogel Karl-Peter
Malzahn Thomas
Muller Roland
Bose McKinney & Evans LLP
Endress + Hauser GmbH + Co.
Phan T.
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