An antenna system

Abstract: This invention relates to an antenna system for use in, for example, registration and identification applications. The antenna system includes a plurality of antenna loops which are adapted to act as single charge up loop antenna during a transmit cycle and individual antennae during a read cycle. One application of the invention is in automatic vehicle identification systems.

Description: This invention relates to an antenna system for use with a registration and identification system as defined in the precharacterizing part of claim 1. One example of a typical registration and identification system is an automatic vehicle identification (AVI) system. The AVI system is used to monitor vehicles for various applications such as for example motorway toll charging, speed monitoring, access to restricted areas of only certain vehicles, crime prevention etc. The AVI system typically includes a transponder on the vehicle, for example the transponder described in our co-pending application number GB 9220409.8 (TI-16821 UK); and an antenna system for monitoring the transponder and to register the relevant information relating to the vehicle on which the transponder is mounted. Two typical systems are described in our co-pending applications GB 9220411.4 (TI-16817 UK) and GB 9220412.2 (TI-17341). In AVI systems for monitoring motorway traffic there are potentially many vehicles approaching at any one time. If for example the system is being used for motorway toll charging it is important that each vehicle is accurately identified and the relevant information stored. For this type of application, it is necessary to have multiple antennae covering the area. Generally, each antenna comprises a tuned Loop or LC Circuits. The antennae and feeder cables typically need to be constructed of litze wire and are designed such that the inductivity of the antenna is about 27µH ± 1µH. The close proximity of two antennae can cause dead zones in the area to be covered. Forming a multiple antennae of tuned antennae would produce an over critical coupled series of tuned LC networks which could result in detuning of the individual antennae and heavy damping. Obviously this would mean that the system is not capable of registering and identifying all transponders in the field of view of the antennae. The problems caused can, to some extent, be overcome by critical on-location antenna pre-tuning to ensure that the resonant dead zones are minimised. This can be time consuming, costly and inconvenient. An auto-tuning system for a tuned antenna system has been used to solve the problem of tuning-on-location and detuning due to metal objects by adding circuitry. This solution is expensive due to the complex circuitry required. From EP-A-0 414 628 an electronic system is known including a tranmitting antenna system and a receiving antenna system having two or more loops each. Each loop of the transmitting antenna system is individually connected to a combiner network in the receiver. Half the loops of the transmitting antenna system are driven so as to be in phase opposition to the other loops of the transmitting antenna system. The two antennna systems are disposed in spaced relationship with respect to each other so that the security system can sense the presence of a resonant tag circuit passing through the space between the antenna systems. From GB-A-2 235 337 a non-resonant loop antenna is known which comprises a plurality of smaller loops connected in parallel and in phase thereby reducing the effective overall loop inductance. One object of the present invention is to provide an antennae system which overcomes at least some of the disadvantages of known systems. According to the present invention, there is provided an antenna system comprising the features of the characterizing part of claim 1. Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagram of an example of an antennae system according to the present invention; Figure 2 is a diagram of an antenna configuration for the system of Figure 1 for example; Figure 3 is a circuit diagram of an amplification stage for each antenna of the Figure 2 configuration; Figure 4 is a block diagram of a 4-loop transmission part of the antenna system; and Figure 5 is a block diagram of the receiver end for one part of the 4-loop transmission of Figure 4. Referring to Figure 1, a recognition and identification system is shown generally at 10. The system shown is an automatic vehicle identification system, but other systems are equally applicable. In the example shown , the system is used to identify vehicles on a six lane highway 12. Each lane of the highway has an antenna 14 associated therewith, which antenna is used to transmit and receive signals capable of identifying vehicles which may or may not be carrying transponders. The antennae 14 are linked to a reader box 16 by respective feeder cables 18. Typically the antenna are square loops of about 3.3m by 3.5m, one over each lane of the highway. Figure 2 shows three of the antennae in more detail. The antenna 14, 14' and 14'' are adjacent non-resonant loops that are fed by non-resonant HF amplifiers (not shown in Figure 2). One of the amplifiers is however, shown in Figure 3. The field lines 20 add in areas where the field generated by each loop is parallel, i.e. at 22 and cancel out in the areas where the field lines 24 of respective antennae run in opposite directions, i.e. at 26. Refering to figure 3, each amplifier 29 is a class-A-B power amplifier formed from push-pull source followers, providing a simple, low-distortion power amplifier, Class A or Class A-B power amplifiers are generally of low efficiency but make very good drivers. For this application therefore, Class A or Class A-B amplifiers are ideal. A sine wave 30 is input on the HF of a transformer 32. In the present case the sine wave has a frequency of about 134.2 kHz although this may be varied as required. The transformer 32 is a step up transformer which generates a high voltage on section 34 of the amplifier circuit. This high voltage is converted in to the low impedance output of emitter followers 36 and 38. This low impedance output is then used by transformer 40 to drive the antenna 42. The voltage provided to the antenna is typically around 300V peak to peak. The antenna impedance is provided between 10 and 40 µH by connecting the required point 44. The antenna 42 is an in-ground loop antenna and includes resistive element of about 0.5 to 10 Ω. The amplifier also includes a counter balance circuit 46 which counter balances the impedance in the loop. This ensures that there are not heavy losses in the amplifier and also improves the Q-factor of the amplifier. Figure 3 illustrates the mode of operation of the circuitry during antenna transmit cycles. Each antenna will transmit an investigation signal which is received by an appropriate transponder. The transponder will to some extent change the signal and return it to the antenna. The change in the signal is used to identify the unique nature of each transponder. Each change will be readable by the antenna to enable information regarding the transponder to be read and stored as appropriate. For multiple antenna configurations as in Figure 1, the individual amplifiers (one for each loop antenna) all run from the same sine wave source, which creates phase synchronous signals on all individual antennae. This allows antennae to be close together, as is Shown in Figures 1 and 2, without the problems that would normally occur using tuned antennae. Figure 2 shows the field distribution of the adjacent antennae sections. The phase and current are of such a nature (same current, 180 degree phase shifted) that the fields cancel each other in the areas 26 of the antenna system. In this way, the whole antennae row built up from individual loop antennae, acts as a giant charge-up loop, with the same performance of one loop, having the outer dimensions of the whole stack formed by the individual antennae. This creates a continuous field with no dead spots covering the lanes of the highway or any other area on which the system is used. The antennae are adapted to both transmit as described above and receive as will be explained in more detail below. This ability to transmit and receive forms part of the interrogation cycle of the system. The receive part of the interrogation cycle includes the steps transport, telegram, transmit. Referring to figure 4, the Readout set up is shown. Four antennae 140, 140', 140." and 140''' are shown, as are associated driving amplifier of each 142, 142', 142" and 142'''. Any signal received by the antennae will be fed back to the drive transformers 144, 144', 144" and 144''' and be detected by the receiver transformer loops 148, 146', 146" and 148'''. The detected information is then processed and stored so that the information transmitted by the transponder can be used for its required purpose. The loops 146, 146', 146" and 146"' are connected to the receiver front end circuits as are shown in figure 5. A low-bit and high-bit frequencies are determined by the receiver filters 50, 52. The former is low-bit turned to about 122kHz and the latter is high-bit turned to about 134.2kHz. The pass frequency of the system is determined by these filter and not the antenna. Since the antennae are not tuned, it is easy to switch the antennae using, for example, MOSFETS during transition from transmit to receive and vice-versa, therefore offering system flexibility in terms of RF multiplexing if needed. This is because the additional resistance to the network introduced by the MOSFET's on-resistance has virtually no effect for the untuned antenna system. Another very important advantage of the above over-tuned equipment is the fact that a damping circuit (to damp away the power pulse at the beginning of receive cycle for tuned interrogation systems) is not needed. The un-tuned nature of the antennae of the present invention makes the field drop from maximum to zero in the region of microseconds. Antenna tuning is also unnecessary for the receiver. The untuned loop is hooked-up to the receiver, and as previously indicated, the low-bit and high-bit frequencies are determined by the receiver filters, not the antenna. The circuitry shown is only one example of possible implementation of the system the skilled man will identify alternative arrangements which fall within the scope of the invention. This system avoids the whole concept of tuned antennae, so no complex circuitry is necessary. Other advantages offered by this system include the following which have been described detail above. Long feeder cables being usable and not diminishing the performance of the system; RF electronic switching (multiplexing) possible without performance loss; Adjacent antennae allowed; No dead zones in the charge-up field due to configuration and untuned nature of the antennae; No litze-wires required; Antenna impedance not critical for either transmit and receive; The phase of each antenna is always the same as would be expected since the stability does not depend on antenna tuning; and No noise sensitivity during telegram receive due to multiple loops. This is usable in, for example, Automatic Vehicle Identification applications, but may be used in all recognition and identification applications that require readout coverage over a large area.

Claims: A transmit/receive antenna system for use with a registration and identification system (10), having a signal source and comprising: a plurality of adjacent non-resonant antenna loops (14) connected to said registration and identification system, each one of said loops (14) laying in the same selected plane, where one of said loops (14) is adjacent another one of said loops (14), such adjacent loops being located such that a first portion of one loop is located alongside a second portion of an adjacent loop; characterized in that said plurality of adjacent antenna loops (14) forms a row that extends across at least two adjacent and parallel pathways, each one of said loops (14) associated with a single pathway and each one of said loops (14) including a normal portion extending normal to the direction in which said pathway extends and first and second parallel portions extending parallel with or in substantially the same direction on said pathway; said antenna loops (14) are adapted to both transmit and receive; said system further comprises a switching means for selectively switching said registration and identification system (10) from a transmit to receive mode and from a receive to transmit mode; and said signal source is connected to each of said first and second portions of each of said plurality of antenna loops (14) such that the field lines (20) generated by each antenna loop are additive for said normal portions of adjacent antennas and such that the field lines (24) generated by a first portion parallel to a second portion cancel each other so that said plurality of antenna loops (14) act as a single large charge-up loop antenna during a transmit cycle and act as an individual antenna during a receive cycle. The antenna system of claim 1, wherein each antenna loop (140, 140', ...) is driven by an associated amplifier (142, 142', ...). The antenna system of claim 2, wherein the amplifier is a class A amplifier. The antenna system of claim 2, wherein the amplifier is a class A-B amplifier (29). The antenna system of any preceding claim, wherein the switching means comprises a MOSFET device. The antenna system of any preceding claim, wherein said registration and identification system (10) is an automatic vehicle identification system and said selected pathway is a roadway. The antenna system of any preceding claim, wherein said signal source is a sine-wave source such that phase synchronous signals are created at each of the individual antennae.

Inventors: D HONT LOEK, D'HONT, LOEK
Applicants: TEXAS INSTRUMENTS HOLLAND, TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS HOLLAND B.V., TEXAS INSTRUMENTS INCORPORATED
Application year: 1993
Main classification: H01Q7
IPC codes: G01S13, H01Q1, H01Q21, H01Q7