Communications: directive radio wave systems and devices (e.g. – Radar transponder system – Radar transponder only
Reexamination Certificate
1999-07-30
2001-06-12
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar transponder system
Radar transponder only
C342S042000, C342S043000
Reexamination Certificate
active
06246356
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microwave transponder for automatic identification systems that have a back scatter function.
2. Related Art
A back scatter system can be defined as a system in which the data carrier, the so-called transponder, includes antennas and modulation circuits. These components modulate information sidebands with the data to be read from a microwave signal falling on the transponder from a reading unit, thus generating a reflected signal containing information for reception and decoding by the reading unit, without supplying fresh energy. Certain embodiments also enable data to be written into the transponder by modulating the amplitude of the signal from the reading unit with detection data, and memory programming in the transponder. This technique is well known and will not therefore be described in detail here.
One problem with microwave-based transponders of this kind, i.e. transponders that operate at 0.9 GHz and even higher frequencies, is that they are normally unable to communicate omnidirectionally, due to the directivity of the antenna system embodied in the transponder. This makes it necessary to orientate the transponder so that a given side of the transponder will face towards the reading unit, which is difficult to achieve when, e.g., the receiving unit is concealed in a package, a handbag, pocket or like receptacle.
Although the earth plane can be reduced to approximately the same size as the antenna plane with the aid of a patch antenna, so as to obtain equal sensitivity in both forward and rearward directions, transponder readings may become sensitive to reflections from a rearwardly lying reflection plane, should this plane be located at a distance that causes leakage of the information carrying signal from the transponder antenna through the signal reflected via the reflection plane. Packaged objects contained in handbags and the like are often reflective and cause uncertain readings. The same applies to the human body, when the transponder is carried in a pocket.
The necessity of taking into consideration the orientation of the data carrier and its proximity to surrounding objects in order to obtain positive readings is a troublesome limitation in each of these cases. Transponders equipped with dipolar antennas are subjected to similar problems, since these transponders are also influenced by the possible presence of a reflection plane in the proximity of the antenna.
Another drawback with antennas of this type is their low antenna gain, i.e. only a minor part of the incident signal is reflected back to the reading unit, due to lack of directivity, which shortens the range of the system.
SUMMARY OF THE INVENTION
The present invention fully solves these problems and provides a transponder that can be read from a short distance and from both sides of the antenna.
In accordance with one preferred embodiment, there is provided a transponder with which communication will not be disturbed by the presence of a rearwardly located reflection plane.
The present invention thus relates to a transponder which is adapted for use in automatic identifying systems and like systems, wherein the transponder includes a patch antenna which functions to reflect transponder data in the form of information sidebands to a microwave signal falling on the patch antenna and sent from a reading unit, and wherein the transponder is characterized in that it includes two mutually parallel but oppositely directed path antennas.
In one preferred embodiment, the transponder obtains an almost fully isotropic function. In other words, this means that it can be in communication with in all directions in the absence of the troublesome neutral settings found, for instance, in the rod directions of dipoles. Corresponding blind directions are, of course, also found with patch antennas that have a small earth plane, since isotropic antennas are theoretically impossible.
In another particular embodiment of the invention, data can be written into the transponder, e.g., by coding amplitude modulation of the microwave signal, e.g. with a so-called Manchester code, with the data to be written. Such a bit stream in the form of an amplitude modulated microwave signal is captured by detection circuits in the transponder, rectified, amplified and caused to influence the memory register of the transponder.
The present invention employs two patch antennas that operate from respective sides of a common earth plane, or alternatively towards mutually separate earth planes that are separated solely by means of a thin foil. In one aspect of the invention, each of these two antennas is provided with an individual modulation circuit that is supplied from a common application-specific integrated circuit (ASIC), in which the transponder data is stored. This antenna is turned towards the reading unit and reflects signals back to said unit with a directivity, or beaming effect, that provides a very good range.
This enables the transponder to be read from both directions. It also enables the transponder to be produced cheaply, e.g. by laminating the antenna in a known manner. Another advantage is that the ASIC, which is often the most expensive component of the transponder, is common to both antennas.
Accordingly, two separate antennas are employed which, in one preferred embodiment, are not mutually coupled by microwave techniques, but are, instead, mutually connected to respective modulation circuits by low-frequency signal conductors. Because no microwave connection is required between the two antennas, the transponder can be manufactured much more simply.
In another particular embodiment, the transponder is communicated with circular-polarized microwaves, which further enhances the orientation tolerance. One of the antennas may be turned through 90° when the antenna are linearly polarized and in communication with circular-polarized signals. This provides additional freedom on orientation, since only the E-field and not the H-field has neutral settings in a transponder with a small earth plane. The E-field and H-field from patch antennas with a small earth plane are disclosed in the literature, from which the effect referred to is clearly apparent.
The directional characteristic of the transponder is essentially the same, irrespective of whether it is read or written, and consequently remarks concerning reading of the transponder will also apply to writing in the transponder.
With the intention of reducing the size of the transponder, its battery can be placed within the circle circumscribed by the patch antennas, instead of on one side thereof.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
REFERENCES:
patent: 4922259 (1990-05-01), Hall et al.
patent: 5258762 (1993-11-01), Cabon et al.
patent: 5552790 (1996-09-01), Gunnarsson
patent: 5619207 (1997-04-01), D'Hont
patent: 5771021 (1998-06-01), Veghte et al.
patent: 5825329 (1998-10-01), Veghte et al.
patent: 60-334565 (1994-12-01), None
patent: 9209906A1 (1992-06-01), None
Gregory Bernarr E.
Tagmaster AB
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