NL1010457C2 - Large loop antennas. - Google Patents

Description

- 1 - 5 Large loop antennas.

Loop antennas are used to generate an alternating magnetic field for inductive communication, identification and detection systems. This magnetic switching field is necessary to interrogate and often power a transponder. This transponder sends out a response signal, which signal can be received back with the aid of the same loop antenna or with the aid of a second loop antenna.

The respective alternating fields have a high frequency. Examples of known operating frequencies are 120 kHz, 3.25 MHz, 6.78 MHz, 8.2 MHz, 13.56 MHz and 27.12 MHz.

Examples of inductive systems utilizing these loop antennas are described in patents NL 9202158 and EUR 0608961.

The following figures have been added to illustrate the description of the invention:

Figure 1 shows a loop antenna according to the prior art.

Figure 2 schematically illustrates dispersed self-induclies and capacities in a prior art loop antenna.

Figure 3 shows the current and voltage distribution on an extreme example of an open loop antenna: an electric half-wave dipole.

Figure 4 shows the top view of a passenger passageway with a loop antenna on either side with associated magnetic field lines.

Figure 5 shows the schematic view of a loop antenna according to the invention.

Figure 6 shows the top view of a personnel passageway with a loop antenna, which is walked through, with associated field lines.

30 The state of the art.

For relatively low operating frequencies, such as 120 kHz, the loop antenna usually consists of many turns and the dimensions may be more than 2 by 2 meters.

For the medium-high frequencies, such as 8.2 MHz, the loop antenna consists of one winding with smaller dimensions, for example approx. 0.5 by 1.4 meters.

For the highest frequencies, such as 13.56 MHz, the dimensions are even more limited and the loop antenna also consists of one turn with dimensions of, for example, approximately 0.5 x 1.0 m.

At high frequencies, the dimensions and number of turns are limited by the total length of the loop.

40 A loop antenna is shown in figure 1. The loop 1 contains one winding and is connected at the top to 0 4. 3 71 - 2 - a capacitor 2, which resonates the loop antenna at the operating frequency.

The connection points of the loop with the capacitor can also be the points with which the antenna is connected to a transmission and / or reception circuit, but this can also be done in many other ways. The manner of connecting to the transmitting and / or receiving circuit is not relevant to the invention and is therefore further disregarded.

For the relatively low frequency, the loop antenna, together with the tuning capacitor, forms a resonant circuit consisting of a concentrated self-induction formed by self-induction of the conductive loop, and of the capacitance of the tuning capacitor.

At higher operating frequencies, the loop transmission line will start to behave due to the loop consisting of a dispersed self-inductance and a dispersed capacitance. Figure 2 gives a schematic view of this.

The consequence of the dispersed self-inductance and capacitance is that the current through the loop is not the same everywhere. In the symmetrical example, as shown in Figure 2, on the node at the bottom center, opposite the tuning capacitor 2, the AC voltage is zero and the current is maximum. The tension increases on both sides of the loop. As a result, a capacitive current will flow through the lateral capacitance capacities 3 and through the mutual capacitances 4. This current reduces the current through the loop portions above these capacities. Ultimately, the current is at least in place of the 20 tuning capacity 2.

An extreme situation occurs when the total length of the loop equals half a wavelength corresponding to the operating frequency. The current is now again maximum in the middle of the loop and the total current disappears in the space capacities. The capacitance of the tuning capacitor has become zero. In figure 3 this loop is drawn in a stretch (6) and the similarity with a half wave dipole antenna is clear. This figure also shows the current distribution 7 and the voltage distribution 8.

There are the following drawbacks when using an inductive loop antenna with a length that is not negligibly small compared to half a wavelength: 1. The current is not constant over the circumference. This means that the resulting magnetic field is not regular, but is distorted because the top part of the loop contributes less to the field.

35 2. The tuning capacity will have a low value; the space capacities 3 and 4 contribute relatively much to the total resonance capacity. As a result, the tuning becomes very sensitive to environmental influences. It is also no longer possible to build in the loop in materials with a relative permeability> 1, unless the dimensions are even more limited.

The great influence of the space capacities also makes the loop sensitive to damping due to 40 dielectric losses in the mounting material. Moisture in these materials in particular can have a very negative effect on the effect 1 o i o · ε7 "- 3 -.

3. The relatively small resonance capacity, together with the fact that the loop antenna consists of one winding, so that a high current flows in order to achieve the required magnetic field strength, makes this over the tuning capacitor, and thus on those parts of the loop antenna near the tuning capacitor, large high frequency voltages arise.

4. Because the dimensions of the loop are very limited, an antenna loop can only be placed on the side of a passage.

10

Figure 4 shows such a passage in which arrow 9 indicates the direction of passage. A loop antenna 1 is placed on either side of the passage. The magnetic field lines are indicated by 10, as are the common axis 11 of the loop antennas. It is known that on the axis of the loop antenna, the field strength decreases very sharply with the distance to the plane of the loop antenna as soon as this distance exceeds half the diameter of the loop antenna.

For a label to be read, the magnetic field strength in the center of that passage must be high enough to allow the label to differentiate. In combination with the limited size of the loop, this means that the magnetic field strength closer to the loop must be very much greater; strength ratios of a factor of 10 or more are very common.

Since high voltages occur on the antenna loop at the locations near the tuning capacitor, high electric field strengths will also be present on site.

These excessive magnetic and / or electric field strengths at the present high operating frequencies can threaten those carrying an implanted pacemaker (pacemaker) or other medical implant. Also, excessive magnetic and electric field strengths can interact with the tissue of the human body to such an extent that it is adversely affected.

It is the object of the invention to overcome this limitation in dimensions of inductive loop antennas for the higher frequencies. The object of the invention is also to create the possibility of also being able to package inductive loop antennas for higher frequencies in various non-conductive materials and constructions without these seriously affecting the operation.

Finally, it is the object of the invention to enable such an antenna construction that it is possible to largely cover a person's passage for reading a label, without excessive magnetic and / or electric field strengths occurring there, which could adversely affect field strengths. are for the carriers of medical implants, or what field strengths interact with the tissue of the human body to an adverse effect.

The invention is based on the fact that the reactance of a self-reduction, X1, is opposite to the reactance of a capacity, Xc. This means that if a self-reduction and a capacitance are connected in series, the reactances of both elements partly compensate each other. At one frequency 40 this compensation is complete, so C / *; } - 4 - XL = -Xc.

This frequency can be derived from the equation of the absolute values of both reactances: 2ji * f * L = 1 / (2ji * f * C) 5 In the inductive loop antenna this principle can be used by interrupting the loop at a number of places and by connecting through a series capacitor. In addition, the capacitor should have that value at which its reactance compensates for the reactance of the zell reduction of the intermediate loop parts. By choosing the length of the conductor segments briefly, the capacitance of the series capacitors becomes large and, therefore, the influence of the room capacities is small.

10

The table below gives an indication of the relationship between the length of the conductor segments and the capacitance of the series capacitors at an operating frequency of 13.56 MHz:

Length 1.0 0.75 0.5 0.25 0.1 m.

Capacity: 138 184 276 551 1378 pF.

An additional advantage when using a high capacitance value and thus short conductor segments is that at equal current the developed high-frequency voltage across the series capacitors, and thus on the entire loop antenna Lo.v. the environment is low.

In principle, such an infinitely long row of conductor segments with intermediate, inductive reactance compensating capacitors can be built up without the reactance of the entire loop becoming high, or the loop transmission line starting to exhibit properties.

Figure 5 schematically shows a loop antenna according to the invention. Here, 13 indicates the conductor segments, and 12 the series capacitors.

In this way, antenna forms are possible in which an antenna loop is placed around a passage, so-called loop antennas. Figure 6 shows such a loop-through antenna arrangement. Arrow 9 indicates the run-through direction which intersects the loop antenna 1. It will be immediately apparent that the physical dimensions of the loop antenna must be such that persons can walk through the loop, and that when using high operating frequencies a solution as indicated in this invention is necessary.

When such an antenna is used, the labels to be detected or read out pass the plane of the antenna loop, in which the / magnetic field strength of the antenna loop is maximum. This maximum field strength can now be much lower than in the case of antenna loops on the side of a passage. This minimizes risks of influencing medical implants, as well as adverse influences on the human body.

35

Figure 5 shows such an inductive loop 1. The series capacitors are indicated by 12 and the self-inductance of the intermediate conductive loop segments by 13.

The impedance of such a loop antenna is capacitive for all frequencies below the resonance frequency and is inductive above it. Only for frequencies where the individual loop parts have dimensions 40 that are greater than λ / 2π it is no longer possible to select such a capacity that the loop extends - 5 - no longer behaves like a transmission line.

In a further embodiment of the invention, the loop parts are integrated with the intermediate capacitors, so that a cable is produced that is reactance-free for one frequency. In principle, loop antennas of unlimited dimensions can be constructed with this cable.

Antenna loops, built up according to the combination of loop parts and series capacitors described above, can be part of more extensive antenna designs. This may include, for example, distorted null forms such as the octagonal antenna and the Maxwell pair 10 (two loops placed one behind the other, fed in phase opposition). Other complex antenna shapes can also be utilized, for example to form specific distributions of the magnetic field.

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Claims (4)

1. A loop antenna for use as a magnetic antenna in inductive communication, identification and detection systems, characterized in that the loop is interrupted in several places by an intermediate capacitor. 2. A loop antenna according to feature 1, characterized in that the capacitance of the intermediate capacitors has such a value that the reactance of that capacitance largely or completely compensates for the reactance of the self-induction of loop part located between two capacitors at the operating frequency. 3. A cable intended to form a loop antenna, characterized in that the cable consists of conductive segments separated by series-connected capacitors of such a value that the reactance of that capacitance by the reactance of the self-inductance of the two capacitors located cable segment at the operating frequency is largely or fully compensated. A cable intended for forming a loop antenna according to claim 3, characterized in that the capacitors are integrated in this cable, whereby large lengths of cable are created, parts of which can be cut to size for any application. / Ί 0 1 ü 4 5 7 j