Passive antenna incorporating a shielded frame

Abstract

The invention relates to a passive antenna having a shielded frame, for example an antenna for producing a known radio-frequency magnetic field in a laboratory. A passive antenna according to the invention comprises: a first element (1), the first element being a transmission line having an outer conductor (11) and an inner conductor (12), the first element being a part of a shielded frame; a second element (2), the second element being a transmission line having an outer conductor (21) and an inner conductor (22), the second element being a part of the shielded frame, one end of the inner conductor of the transmission line of the second element being directly coupled to one end of the inner conductor of the transmission line of the first element; a base (3); a single-port linear device having a resistor and a capacitor; and a port of the passive antenna. Figure for abstract: Figure 6.

Description

Title of the invention: Passive antenna incorporating a shielded frame TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a passive antenna comprising a shielded frame, for example an antenna for producing a known radio-frequency magnetic field in a laboratory. STATE OF THE PRIOR ART

[0002] Loop antennas and shielded loop antennas are well known to those skilled in the art. They can be used to produce a known radio frequency magnetic field, for example during electromagnetic immunity tests, or to calibrate a measuring antenna. As explained in paragraph 5-4 of Chapter 5 of R.C. Johnson's book “Antenna Engineering Handbook, 3rd Edition,” published by McGraw-Hill in 1993, the shielding of a shielded loop typically functions as a loop. To produce a known radio frequency magnetic field, a shielded loop performs better than an unshielded loop, because a properly used shielded loop is an antenna that does not produce a common-mode current flowing on the cable connecting the antenna to a radio frequency generator or transmitter, induced by the electromagnetic field emitted by the antenna.

[0003] In the following, we will call “reference field level” a module of an intensity of a radio-frequency magnetic field produced by an antenna, the radio-frequency magnetic field being measured at a reference location relative to the antenna, a generator being coupled to an access of the antenna, the generator producing a sinusoidal voltage at a given frequency, the generator having a known internal impedance at the given frequency, an open-circuit voltage of the generator being equal to 2 volts rms. The reference field level is expressed in A / m rms.

[0004] In the following, “coupled” always refers to an electrical coupling. When this term is applied to two entities such as terminals, conductors, nodes, etc., “coupled” may indicate that the entities are directly coupled, i.e., connected (or, equivalently, in electrical contact) with each other, or that the entities are indirectly coupled, an electrical interaction different from direct coupling existing in this case between the entities, for example through one or more components. When this term is applied to two entities with several terminals, such as ports, connectors, etc., “coupled” may indicate that the entities are directly coupled, each terminal of one of the entities being in this case directly coupled to one and only one of the terminals of the other entity, or that the entities are indi directly coupled, an electrical interaction different from the direct coupling existing in this case between the terminals of the entities, for example through one or more components. In the following, in accordance with circuit theory, an access has exactly two terminals.

[0005] An example of a prior art passive antenna is shown in [Fig. 1], where hidden edges (i.e. not directly visible) and hidden contours are not shown. The passive antenna shown in [Fig.l] comprises: • a first element (1), the first element being a transmission line named Tl, Tl having an outer conductor (11) and an inner conductor (12), Tl having a first end (101) and a second end (102), the outer conductor of Tl having a first end at the first end of Tl, the outer conductor of Tl having a second end at the second end of Tl, the inner conductor of Tl having a first end at the first end of Tl, the inner conductor of Tl having a second end at the second end of Tl, the first element being a part of a shielded frame; • a second element (2), the second element being a transmission line named T2, T2 having an outer conductor (21) and an inner conductor (22), T2 having a first end (201) and a second end (202), the outer conductor of T2 having a first end at the first end of T2, the outer conductor of T2 having a second end at the second end of T2, the inner conductor of T2 having a first end at the first end of T2, the inner conductor of T2 having a second end at the second end of T2, the second element being a part of the shielded frame, the first end of the inner conductor of T2 being directly coupled to the first end of the inner conductor of T1; • a part called “base” (3), the base providing electrical contact between the second end of the external conductor of T2 and the second end of the external conductor of T1, the base being a part of the shielded frame; and • a passive antenna port, the passive antenna port having a first terminal and a second terminal, the first terminal of the passive antenna port being coupled to the second end of the inner conductor of Tl, the second terminal of the passive antenna port being coupled to the second end of the outer conductor of Tl.

[0006] The specialist sees that the external conductor of T1, the base and the external conductor of T2 form a winding having a single turn, the winding being used as a single-turn frame. Said single-turn frame is distinct from said shielded frame.

[0007] Said shielded frame is a single-turn shielded frame, consisting of the first element, the second element and the base. The largest width of the passive antenna shown in [Fig.l] is approximately 133 mm. The transmission line T1 has a nominal characteristic impedance, which is equal to 50 ohms. The transmission line T2 has a nominal characteristic impedance, which is equal to 50 ohms.

[0008] In a first variant of the prior art passive antenna shown in [Fig. 1], the second end of the inner conductor of T2 is directly coupled to the second end of the outer conductor of T2, and is therefore directly coupled to the base. The skilled person will see that in this first variant, said single-turn shielded frame is similar to that shown in Fig. 9E of the report by G.A. Morgan, Jr. entitled “Analysis and calibration of loop probes for use in measuring interference fields,” published by the Naval Research Laboratory under number NRL Report R-3486, in June 1949. The skilled person will understand that [Fig. 2] shows an equivalent electrical schematic of the first variant of the prior art passive antenna shown in [Fig. 1]. In [Fig.2], we see: an ideal transmission line (41) corresponding to T1; an ideal transmission line (42) corresponding to T2; the access of the passive antenna (43); a voltage source (44) which represents a voltage induced in said single-turn loop when the passive antenna is used for reception (i.e., the voltage induced between the first end of the external conductor of T2 and the first end of the external conductor of T1, when the passive antenna is used for reception); an impedance (45) of said single-turn loop (i.e., an impedance which exists between the first end of the external conductor of T2 and the first end of the external conductor of T1); and the connection (80) existing between the second end of the internal conductor of T2 and the second end of the external conductor of T2. [Fig.4] is a graph showing the reference field strength of the first variant of the prior art passive antenna shown in [Fig.l], as a function of frequency, for a reference location close to the passive antenna, the internal impedance of the generator being assumed to be 50 ohms at any frequency. More precisely, the reference location used to obtain [Fig.4] is located on a reference axis of the passive antenna and at a distance of about 100 mm from a reference plane of the passive antenna, the reference axis being orthogonal to the reference plane.

[0009] A second variant of the passive antenna of the prior art shown in [Fig.l] further comprises a 50 ohm resistor, this resistor having a first terminal and a second terminal, the first terminal of the resistor being coupled to the second end of the inner conductor of T2, the second terminal of the resistor being coupled to the second end of the outer conductor of T2. Thus, in this second variant of the passive antenna of the prior art shown in [Fig.l], the second end of the inner conductor of T2 is indirectly coupled to the second end of the outer conductor of T2, through the resistor. Thus, in this second variant of the prior art passive antenna shown in [Fig.l], the second end of the inner conductor of T2 is indirectly coupled to the base, through the resistor. The skilled person sees that, in this second variant, the said single-turn shielded loop is similar to that shown in [Fig.2].17(a) of J. Goedbloed's book “Electromagnetic Compatibility”, published by Prentice-Hall in 1990. The skilled person understands that [Fig.3] shows an equivalent electrical diagram of the second variant of the prior art passive antenna shown in [Fig.l]. In [Fig.3], we see: an ideal transmission line (41) corresponding to T1; an ideal transmission line (42) corresponding to T2; passive antenna access (43);a voltage source (44) which represents a voltage induced in said single-turn loop when the passive antenna is used for reception (i.e., the voltage induced between the first end of the outer conductor of T2 and the first end of the outer conductor of T1, when the passive antenna is used for reception); an impedance (45) of said single-turn loop (i.e., an impedance which exists between the first end of the outer conductor of T2 and the first end of the outer conductor of T1);and said resistor (81), said resistor having a first terminal and a second terminal, the first terminal of the resistor being coupled to the second end of the inner conductor of T2, the second terminal of the resistor being coupled to the second end of the outer conductor of T2. [Fig. 5] is a graph showing the reference field strength of the second variant of the prior art passive antenna shown in [Fig. 1], as a function of frequency, for a reference location near the passive antenna, the internal impedance of the generator being assumed to be 50 ohms at any frequency. The reference location used to obtain [Fig. 5] is the same as the reference location used to obtain [Fig. 4], so that we can meaningfully compare [Fig. 5] to [Fig. 4]. ;

[0010] For the calibration of certain measuring antennas, it would be desirable to generate a known magnetic field using a passive antenna comprising a shielded loop and having a reference field level which is substantially independent of frequency, in a wide frequency band, for a reference location close to the passive antenna. The specialist knows that this result can be achieved by reducing the size of the shielded loop. However, this solution is often not acceptable because it implies that the magnetic field becomes less homogeneous near the passive antenna, and weaker far from the passive antenna. Statement of the invention

[0011] The invention relates to a simple and economical passive antenna comprising a shielded frame, the passive antenna having a reference field level which, for a reference location close to the passive antenna, is substantially independent of the frequency at any frequency lower than an upper limit, the upper limit being increased compared to that which would be obtained by using a passive antenna of the prior art comprising a shielded frame of the same dimensions.

[0012] A passive antenna according to the invention comprises: • a first element, the first element comprising a transmission line named Tl, Tl having an outer conductor and an inner conductor, Tl having a first end and a second end, the outer conductor of Tl having a first end at the first end of Tl, the outer conductor of Tl having a second end at the second end of Tl, the inner conductor of Tl having a first end at the first end of Tl, the inner conductor of Tl having a second end at the second end of Tl, the first element being a part of a shielded frame; • a second element, the second element comprising a transmission line named T2, T2 having an outer conductor and an inner conductor, T2 having a first end and a second end, the outer conductor of T2 having a first end at the first end of T2, the outer conductor of T2 having a second end at the second end of T2, the inner conductor of T2 having a first end at the first end of T2, the inner conductor of T2 having a second end at the second end of T2, the second element being a part of the shielded frame, the first end of the inner conductor of T2 being directly coupled to the first end of the inner conductor of T1; • a single-access linear device having a first terminal and a second terminal, the first terminal of the single-port linear device being coupled to the second end of the inner conductor of T2, the second terminal of the single-port linear device being coupled to the second end of the outer conductor of T2, the single-port linear device comprising a resistor and a capacitor; and • a passive antenna port, the passive antenna port having a first terminal and a second terminal, the first terminal of the passive antenna port being coupled to the second end of the inner conductor of Tl, the second terminal of the passive antenna port being coupled to the second end of the outer conductor of Tl.

[0013] For example, it is possible that the single-port linear device has a DC resistance, the DC resistance being greater than 10 ohms, the DC resistance being less than 500 ohms.

[0014] For example, it is possible for Tl to have a characteristic impedance, the DC resistance being greater than half a real part of the characteristic impedance of Tl, the DC resistance being less than twice said real part of the characteristic impedance of Tl. For example, the characteristic impedance of Tl may be a nominal characteristic impedance. For example, the characteristic impedance of Tl may be a characteristic impedance measured at 10 MHz, or at 200 MHz.

[0015] For example, it is possible for T1 to have a characteristic impedance and for T2 to have a characteristic impedance, a real part of the characteristic impedance of T2 being greater than half of a real part of the characteristic impedance of T1, said real part of the characteristic impedance of T2 being less than twice of said real part of the characteristic impedance of T1. For example, the characteristic impedance of T2 may be a nominal characteristic impedance. For example, the characteristic impedance of T2 may be a characteristic impedance measured at 10 MHz, or at 200 MHz. Brief description of the drawings

[0016] Other advantages and characteristics will emerge more clearly from the following description of particular embodiments of the invention, given as non-limiting examples, and represented in the appended drawings in which:

[0017] - [Fig.l] is a drawing of a passive antenna of the prior art;

[0018] - [Fig.2] is an equivalent electrical diagram of a first variant of the antenna passive of the prior art shown in [Fig.l];

[0019] - [Fig.3] is an equivalent electrical diagram of a second variant of the antenna passive of the prior art shown in [Fig.l];

[0020] - [Fig.4] is a graph showing a reference field level of the first variant of the passive antenna shown in [Fig.l], as a function of frequency;

[0021] - [Fig.5] is a graph showing a reference field level of the second variant of the passive antenna shown in [Fig.l], as a function of frequency;

[0022] - [Fig.6] is a drawing of a passive antenna according to the invention (first mode of realization) ;

[0023] - [Fig.7] is an equivalent electrical diagram of a passive antenna according to the invention (first embodiment);

[0024] - [Fig.8] is a drawing of a passive antenna according to the invention (second mode of realization) ;

[0025] - [Fig.9] is an equivalent electrical diagram of a passive antenna according to the invention (second embodiment and third embodiment);

[0026] - [Fig.10] is a drawing of a passive antenna according to the invention (third mode of realization) ;

[0027] - [Fig.11] is a second drawing of the passive antenna shown in [Fig.10] (third embodiment);

[0028] - [Fig. 12] is a third drawing of the passive antenna shown in [Fig. 10] (third embodiment);

[0029] - [Fig. 13] is a graph showing the modulus of an antenna impedance passive shown in [Fig. 10], as a function of frequency;

[0030] - [Fig. 14] is a graph showing a reference field level of the antenna passive shown in [Fig. 10], as a function of frequency; and

[0031] - [Fig.15] is a drawing of a passive antenna according to the invention (fourth embodiment of realization).

[0032] DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

[0033] First embodiment.

[0034] As a first embodiment of a device according to the invention, given as a non-limiting example, we have shown in [Fig.6] a drawing of a passive antenna according to the invention, the passive antenna comprising: • a first element (1), the first element being a transmission line named Tl, Tl having an outer conductor (11) and an inner conductor (12), Tl having a first end (101) and a second end (102), the outer conductor of Tl having a first end at the first end of Tl, the outer conductor of Tl having a second end at the second end of Tl, the inner conductor of Tl having a first end at the first end of Tl, the inner conductor of Tl having a second end at the second end of Tl; • a second element (2), the second element being a transmission line named T2, T2 having an outer conductor (21) and an inner conductor (22), T2 having a first end (201) and a second end (202), the outer conductor of T2 having a first end at the first end of T2, the outer conductor of T2 having a second end at the second end of T2, the inner conductor of T2 having a first end at the first end of T2, the inner conductor of T2 having a second end at the second end of T2, the first end of the inner conductor of T2 being directly coupled to the first end of the internal conductor of Tl; • a base (3), the base providing electrical contact between the second end of the external conductor of T2 and the second end of the external conductor of T1; • a single-port linear device having a first terminal and a second terminal, the first terminal of the single-port linear device being coupled to the second end of the inner conductor of T2, the second terminal of the single-port linear device being coupled to the second end of the outer conductor of T2; and • a passive antenna port, the passive antenna port having a first terminal and a second terminal, the first terminal of the passive antenna port being coupled to the second end of the inner conductor of Tl, the second terminal of the passive antenna port being coupled to the second end of the outer conductor of Tl.

[0035] [Fig.6] shows the entire passive antenna. In [Fig.6], hidden edges and hidden contours are not shown.

[0036] In this first embodiment, Tl and T2 are rectangular transmission lines similar to those studied in the article by T.-S. Chen entitled “Determination of the Capacitance, Inductance and Characteristic Impedance of Rectangular Lines”, published in September 1960 in the journal IEEE Transactions on Microwave Theory and Techniques. A cross-section of Tl has the inner conductor of Tl, which has a rectangular or circular cross-section, placed inside the outer conductor of Tl, which has a rectangular and hollow cross-section. The inner conductor of Tl is fixed in the outer conductor Tl using insulating pieces. Tl has a characteristic impedance, the characteristic impedance of Tl being close to 94 ohms at 100 MHz. A cross-section of T2 has the inner conductor of T2, which has a rectangular or circular cross-section, placed inside the outer conductor of T2, which has a rectangular and hollow cross-section.The inner conductor of T2 is fixed in the outer conductor T2 using insulating pieces. T2 has a characteristic impedance, the characteristic impedance of T2 being close to 94 ohms at 100 MHz. Thus, a real part of the characteristic impedance of T2 is greater than half of a real part of the characteristic impedance of T1, and said real part of the characteristic impedance of T2 is less than twice of said real part of the characteristic impedance of T1.

[0037] The specialist sees that the outer conductor of T1, the base and the outer conductor of T2 form a polygonal winding having a single turn, the winding being used as a single-turn frame. The specialist also sees that the first element, the base and the second element are the parts of a shielded frame of the state of the art prior art, which is a single-spiral armored frame. Therefore, the first element is a part of this prior art armored frame, the second element is a part of this prior art armored frame, and the base is a part of this prior art armored frame.

[0038] The single-port linear device (not shown in [Fig.6]) comprises a first resistor, a second resistor and a capacitor, the first resistor having a value which is close to the characteristic impedances of Tl and T2, for example a value of 100 ohms, this first resistor being connected in parallel with a dipole consisting of the capacitor connected in series with the second resistor. [Fig.7] shows an equivalent electrical diagram of the passive antenna of this first embodiment. In [Fig.7], we see: an ideal transmission line (41) corresponding to Tl; an ideal transmission line (42) corresponding to T2; the port of the passive antenna (43);a voltage source (44) which represents a voltage induced in said single-turn loop when the passive antenna is used for reception (i.e., the voltage induced between the first end of the outer conductor of T2 and the first end of the outer conductor of T1, when the passive antenna is used for reception); an impedance (45) of said single-turn loop (i.e., an impedance which exists between the first end of the outer conductor of T2 and the first end of the outer conductor of T1); said first resistor (81), this first resistor having a first terminal and a second terminal, the first terminal of the first resistor being coupled to the second end of the inner conductor of T2, the second terminal of the first resistor being coupled to the second end of the outer conductor of T2;said capacitor (82), said capacitor having a first terminal and a second terminal, the second terminal of the capacitor being coupled to the second end of the outer conductor of T2; and said second resistor (83), said second resistor having a first terminal and a second terminal, the first terminal of the second resistor being coupled to the second end of the inner conductor of T2, the second terminal of the second resistor being coupled to the first terminal of the capacitor. Thus, the single-port linear device (8) comprises a resistor and a capacitor, and the single-port linear device (8) has a DC resistance, the DC resistance being greater than 10 ohms, the DC resistance being less than 500 ohms. ;

[0039] The specialist sees that the first element, the second element, the base and the single-access linear device form a new antenna, which can be seen as a new shielded frame. Thus, the first element is a part of this new shielded frame, the second element is a part of this new shielded frame, and the base is a part of this new shielded frame.

[0040] A connector (not shown in [Fig.6]) is fixed to the base. This connector is used as the passive antenna port. The base is hollow, and it contains means used for coupling the first terminal of the passive antenna port to the second end of the inner conductor of Tl, and for coupling the second terminal of the passive antenna port to the second end of the outer conductor of Tl.

[0041] If the capacitor (82) and the second resistor (83) were omitted, an antenna similar to said second variant of the passive antenna of the prior art would be obtained. If the first resistor (81) were replaced by a short circuit, an antenna similar to said first variant of the passive antenna of the prior art would be obtained.Experiments show that, for a reference location close to the passive antenna, said antenna similar to said first variant of the passive antenna of the prior art has a reference field level which is substantially independent of the frequency up to about 15 MHz, and said antenna similar to said second variant of the passive antenna of the prior art has a reference field level which is substantially independent of the frequency up to about 35 MHz, whereas, if the values ​​of the capacitor and the second resistor are well chosen, the passive antenna of this first embodiment has a reference field level which is substantially independent of the frequency up to about 60 MHz. Moreover, the difference between the cost of these antennas similar to passive antennas of the prior art and the cost of the passive antenna of this first embodiment is very small.

[0042] Accordingly, the invention is a simple and economical passive antenna comprising a shielded frame, the passive antenna having a reference field level which, for a reference location close to the passive antenna, is substantially independent of frequency at any frequency below an upper bound, the upper bound being increased over that which would be obtained using a prior art passive antenna comprising a shielded frame of the same dimensions.

[0043] Second embodiment.

[0044] A second embodiment of a device according to the invention, given by way of non-limiting example, is a passive antenna shown in the drawings of [Fig.l] and [Fig.8], the passive antenna comprising: • a first element (1), the first element comprising a transmission line named Tl, Tl having an outer conductor (11) and an inner conductor (12), Tl having a first end (101) and a second end (102), the outer conductor of Tl having a first end at the first end of Tl, the outer conductor of Tl having a second end at the second end of Tl, the inner conductor of Tl having a first end at the first end of Tl, the conductor internal of Tl having a second end at the second end of Tl, the first element being part of a shielded frame; • a second element (2), the second element comprising a transmission line named T2, T2 having an outer conductor (21) and an inner conductor (22), T2 having a first end (201) and a second end (202), the outer conductor of T2 having a first end at the first end of T2, the outer conductor of T2 having a second end at the second end of T2, the inner conductor of T2 having a first end at the first end of T2, the inner conductor of T2 having a second end at the second end of T2, the second element being a part of the shielded frame, the first end of the inner conductor of T2 being directly coupled to the first end of the inner conductor of T1; • a base (3), the base providing electrical contact between the second end of the external conductor of T2 and the second end of the external conductor of T1, the base being a part of the shielded frame; • a single-access linear device (8) having a first terminal and a second terminal, the first terminal of the single-access linear device being coupled to the second end of the inner conductor of T2, the second terminal of the single-access linear device being coupled to the second end of the outer conductor of T2, the single-access linear device comprising a resistor (81) and a capacitor (82); and • a passive antenna port, the passive antenna port having a first terminal and a second terminal, the first terminal of the passive antenna port being coupled to the second end of the inner conductor of Tl, the second terminal of the passive antenna port being coupled to the second end of the outer conductor of Tl.

[0045] In [Fig.l] and [Fig.8], the hidden edges and hidden contours are not shown, except the hidden contours of the inner conductor of Tl and the inner conductor of T2 in [Fig.8].

[0046] [Fig.l] is a front view of the passive antenna. The base (3) is a box. This box is conductive. It can for example be a metal box. [Fig.8] is a front view of the passive antenna without the base cover, and without the screws used to fix the base cover to the base. [Fig.8] shows that the single-port linear device is located in the base. [Fig.8] shows a coaxial connector (7) which materializes the passive antenna port. The first terminal of the passive antenna port is the center conductor of the coaxial connector, which is coupled to the second end of the inner conductor of TL. The second terminal of the port of the passive antenna is the peripheral conductor of the coaxial connector, which is directly coupled to the base, and thus coupled to the second end of the outer conductor of Tl, through the base.

[0047] T1 has a nominal characteristic impedance, which is equal to 50 ohms. T2 has a nominal characteristic impedance, which is equal to 50 ohms.

[0048] The specialist sees that the outer conductor of T1, the base and the outer conductor of T2 form a winding having a single turn, the winding being used as a single-turn frame. The specialist also sees that the first element, the base and the second element are the parts of a shielded frame of the prior art, which is a single-turn shielded frame. Said single-turn frame should not be confused with said shielded frame.

[0049] The resistor (81) has a value which is close to the nominal characteristic impedances of T1 and T2, for example a value of 47 ohms. The resistor (81) is connected in parallel with the capacitor (82). [Fig. 9] shows an equivalent electrical diagram of the passive antenna of this second embodiment. In [Fig.9], we see: an ideal transmission line (41) corresponding to T1; an ideal transmission line (42) corresponding to T2; the access of the passive antenna (43); a voltage source (44) which represents a voltage induced in said single-turn loop when the passive antenna is used for reception; an impedance (45) of said single-turn loop; said resistor (81), this resistor having a first terminal and a second terminal, the first terminal of the resistor being coupled to the second end of the inner conductor of T2, the second terminal of the resistor being coupled to the second end of the outer conductor of T2; and said capacitor (82), this capacitor having a first terminal and a second terminal, the first terminal of the capacitor being coupled to the second end of the inner conductor of T2, the second terminal of the capacitor being coupled to the second end of the outer conductor of T2.So the single port linear device has a DC resistance, the DC resistance being greater than 10 ohms, the DC resistance being less than 500 ohms.

[0050] Third embodiment.

[0051] As a third embodiment of a device according to the invention, given by way of non-limiting example, we have shown in Figures 10, 11 and 12 drawings of a passive antenna according to the invention, the passive antenna comprising: • a first element (1), the first element comprising a transmission line named Tl, Tl having an external conductor (11) and an internal conductor (12), Tl having a first end (101) and a second end (102), the external conductor of Tl having a first end at the first end of Tl, the external conductor of Tl having a second end at the second end of Tl, the inner conductor of Tl having a first end at the first end of Tl, the inner conductor of Tl having a second end at the second end of Tl, the first element being a part of a shielded frame; • a second element (2), the second element comprising a transmission line named T2, T2 having an outer conductor (21) and an inner conductor (22), T2 having a first end (201) and a second end (202), the outer conductor of T2 having a first end at the first end of T2, the outer conductor of T2 having a second end at the second end of T2, the inner conductor of T2 having a first end at the first end of T2, the inner conductor of T2 having a second end at the second end of T2, the second element being a part of the shielded frame, the first end of the inner conductor of T2 being directly coupled to the first end of the inner conductor of T1; • a base (3), the base providing electrical contact between the second end of the external conductor of T2 and the second end of the external conductor of T1, the base being a part of the shielded frame; • a single-access linear device (8) having a first terminal and a second terminal, the first terminal of the single-access linear device being coupled to the second end of the inner conductor of T2, the second terminal of the single-access linear device being coupled to the second end of the outer conductor of T2; and • a passive antenna port, the passive antenna port having a first terminal and a second terminal, the first terminal of the passive antenna port being coupled to the second end of the inner conductor of Tl, the second terminal of the passive antenna port being coupled to the second end of the outer conductor of Tl.

[0052] In Figures 10, 11 and 12, the hidden edges and hidden contours are not shown, except the hidden contours of the inner conductor of T1 and the inner conductor of T2 in [Fig.l 1].

[0053] [Fig. 10] is a front view of the passive antenna. The base has a fixing ring (5). The fixing ring is used to fix the passive antenna to an antenna mast (9). The base (3) is a conductive box, for example a metal box. [Fig.l 1] is a front view of the passive antenna without the base cover, and without the screws used to fix the base cover to the base. [Fig. 12] is a rear view of the passive antenna, showing a coaxial connector (7) which materializes the access of the passive antenna. [Fig.l 1] shows that the linear device to single port is located in the second element. The first terminal of the passive antenna port is the center conductor of the coaxial connector, which is coupled to the second end of the inner conductor of Tl. The second terminal of the passive antenna port is the peripheral conductor of the coaxial connector, which is directly coupled to the base, and thus coupled to the second end of the outer conductor of Tl through the base.

[0054] T1 and T2 are rigid transmission lines. Each of these transmission lines is a coaxial transmission line having a characteristic impedance which is close to 50 ohms at 100 MHz. At least one of the transmission lines T1 and T2 may for example comprise a rigid or semi-rigid coaxial cable, suitably curved. At least one of the transmission lines T1 and T2 may for example comprise a flexible coaxial cable inserted into a rigid tube made of a dielectric material comprising a polymer, the rigid tube having a circular cross-section and being curved.At least one of the transmission lines T1 and T2 may for example comprise a flexible coaxial cable having a central conductor, a dielectric surrounding the central conductor, and a metal braid surrounding the dielectric, the coaxial cable having no insulating sheath surrounding the metal braid, the coaxial cable being inserted into a rigid metal tube, the rigid metal tube having a length, the rigid metal tube having a circular cross-section and being curved, the metal braid being in electrical contact with the rigid metal tube over the entire length of the rigid metal tube, the inner conductor of said at least one of the transmission lines T1 and T2 being the central conductor, the outer conductor of said at least one of the transmission lines T1 and T2 consisting of the metal braid in electrical contact with the rigid metal tube.

[0055] The specialist sees that the external conductor of T1, the base and the external conductor of T2 form a winding having a single turn, the winding being used as a single-turn frame. Said single-turn frame should not be confused with said shielded frame.

[0056] The single-port linear device consists of a 49.9 ohm resistor connected in parallel with a 39 pF capacitor. The resistor and the capacitor are miniature components. [Fig.9] shows an equivalent electrical diagram of the passive antenna of this third embodiment. In [Fig.9], we see: an ideal transmission line (41) corresponding to T1; an ideal transmission line (42) corresponding to T2; the port of the passive antenna (43); a voltage source (44) which represents a voltage induced in said single-turn loop when the passive antenna is used for reception; an impedance (45) of said single-turn loop; said resistor (81); and said capacitor (82). Thus, the single-port linear device has a DC resistance, the DC resistance DC being close to 49.9 ohms. Thus, we can say that: the DC resistance is close to the characteristic impedance of Tl; the DC resistance is greater than half the real part of the characteristic impedance of Tl; the DC resistance is less than twice the real part of the characteristic impedance of Tl; the DC resistance is close to the characteristic impedance of T2; the DC resistance is greater than half the real part of the characteristic impedance of T2; and the DC resistance is less than twice the real part of the characteristic impedance of T2.

[0057] [Fig. 13] is a graph showing the modulus of an impedance of the passive antenna of this third embodiment, as a function of frequency. [Fig. 14] is a graph showing a reference field level of the passive antenna of this third embodiment, as a function of frequency, for a reference location close to the passive antenna, the internal impedance of the generator coupled to the port of the passive antenna being assumed to be 50 ohms at any frequency.

[0058] The prior art passive antenna shown in [Fig.l] and the passive antenna shown in [Fig. 10] are based on the same shielded frame, i.e., the same first element, the same second element, and the same base. The reference location used to obtain [Fig. 14] is the same as the reference location used to obtain [Fig.4] and [Fig.5], so that we can meaningfully compare [Fig. 14] with [Fig.4] and [Fig.5]. This comparison shows that the characteristics of the shielded frame and the single-port linear device interact in such a way that, for a reference location close to the passive antenna, the passive antenna shown in [Fig. 10] has a reference field level which is substantially independent of frequency up to about 50 MHz, while the first variant of the prior art passive antenna shown in [Fig.l] has a reference field level which is substantially independent of frequency only up to about 10 MHz, and that the second variant of the prior art passive antenna shown in [Fig.l] has a reference field level which is substantially independent of frequency only up to about 30 MHz.

[0059] Accordingly, the invention is a simple and economical passive antenna comprising a shielded frame, the passive antenna having a reference field level which, for a reference location close to the passive antenna, is substantially independent of frequency at any frequency below an upper bound, the upper bound being higher than that which would be obtained using a prior art passive antenna comprising a shielded frame of the same dimensions.

[0060] [Fig. 13] shows that the impedance modulus of the passive antenna shown in [Fig. 10] is substantially independent of frequency up to about 20 MHz. The skilled artisan therefore understands that the characteristics of the shielded frame and the single-port linear device interact to produce an impedance stabilizing effect up to about 20 MHz. Furthermore, a comparison of [Fig. 13] with [Fig. 14] shows that the impedance stabilizing effect, which makes the impedance modulus of the passive antenna shown in [Fig. 10] substantially independent of frequency up to about 20 MHz, is distinct from the phenomenon whereby the characteristics of the shielded frame and the single-port linear device interact in such a way that, for a reference location close to the passive antenna, the passive antenna shown in [Fig.10] has a reference field level which is substantially independent of frequency up to an upper limit, the upper limit being close to 50 MHz.

[0061] This upper bound corresponds to a wavelength in vacuum, equal to the velocity of light in vacuum divided by the upper bound. Since the greatest width of the passive antenna shown in [Fig. 10] is about 133 mm, we can say that a sum of a length of Tl, measured between the first end of Tl and the second end of Tl, of a length of T2, measured between the first end of T2 and the second end of T2, and of a length, measured on the base between the second end of the external conductor of Tl and the second end of the external conductor of T2, is less than a quarter of said wavelength in vacuum. The shielded frame, comprising the first element, the second element and the base, is therefore electrically small at any frequency below said upper bound.

[0062] Fourth embodiment.

[0063] As a fourth embodiment of a device according to the invention, given as a non-limiting example, we consider a passive antenna according to the invention, which is made from a section of coaxial cable (i.e., a segment of coaxial cable), a resistor, a capacitor and a little solder, the passive antenna being shown in [Fig. 15], the passive antenna comprising: • a first element (1), the first element being a transmission line named Tl, Tl having an outer conductor (11) and an inner conductor (12), Tl having a first end (101) and a second end (102), the outer conductor of Tl having a first end at the first end of Tl, the outer conductor of Tl having a second end at the second end of Tl, the inner conductor of Tl having a first end at the first end of Tl, the inner conductor of Tl having a second end at the second end of Tl, the first element being a first part of the coaxial cable section; a second element (2), the second element being a transmission line named T2, T2 having an outer conductor (21) and an inner conductor (22), T2 having a first end (201) and a second end (202), the outer conductor of T2 having a first end at the first end of T2, the outer conductor of T2 having a second end at the second end of T2, the inner conductor of T2 having a first end at the first end of T2, the inner conductor of T2 having a second end at the second end of T2, the first end of the inner conductor of T2 being directly coupled to the first end of the inner conductor of T1, the second end of the outer conductor of T2 being directly coupled to the second end of the outer conductor of T1, the second element being a second part of the coaxial cable section;a single-port linear device (8), the single-port linear device being composed of the resistor and the capacitor connected in parallel, the single-port linear device having a first terminal and a second terminal, the first terminal of the single-port linear device being directly coupled to the second end of the inner conductor of T2, the second terminal of the single-port linear device being directly coupled to the second end of the outer conductor of T1 (so that the second terminal of the single-port linear device is coupled to the second end of the outer conductor of T2); and; a passive antenna port (6), the passive antenna port having a first terminal and a second terminal, a third portion (65) of the coaxial cable section being used to couple the first terminal of the passive antenna port to the second end of the inner conductor of Tl, and to couple the second terminal of the passive antenna port to the second end of the outer conductor of Tl.

[0064] In this fourth embodiment, the first element is a part of an armored frame, and the second element is a part of the armored frame.

[0065] This fourth embodiment is a very simple and very economical passive antenna comprising a shielded frame, the passive antenna having a reference field level which, for a reference location close to the passive antenna, is substantially independent of the frequency at any frequency below an upper limit, the upper limit being increased relative to that which would be obtained using a passive antenna of the prior art comprising a shielded frame of the same dimensions.

[0066] Fifth embodiment.

[0067] In a fifth embodiment of a device according to the invention, given in by way of non-limiting example, a passive antenna according to the invention comprises: • a first element, the first element comprising a “first element tube” and a transmission line named Tl, the first element tube being an electrical conductor having a length, Tl having an outer conductor and an inner conductor, Tl having a first end, a second end and a length, the outer conductor of Tl having a first end at the first end of Tl, the outer conductor of Tl having a second end at the second end of Tl, the inner conductor of Tl having a first end at the first end of Tl, the inner conductor of Tl having a second end at the second end of Tl, at least a portion of the length of Tl being in the first element tube, the outer conductor of Tl being, over a portion of the length of the first element tube, not in electrical contact with the first element tube,the first end of the outer conductor of Tl being connected to the tube of the first element, the first element being part of a shielded frame; , • a second element, the second element comprising a “second element tube” and a transmission line named T2, the second element tube being an electrical conductor having a length, T2 having an outer conductor and an inner conductor, T2 having a first end, a second end and a length, the outer conductor of T2 having a first end at the first end of T2, the outer conductor of T2 having a second end at the second end of T2, the inner conductor of T2 having a first end at the first end of T2, the inner conductor of T2 having a second end at the second end of T2, at least a portion of the length of T2 being in the second element tube, the outer conductor of T2 being, over a portion of the length of the second element tube, not in electrical contact with the second element tube,the first end of the outer conductor of T2 being connected to the tube of the second element, the second element being a part of the shielded frame, the first end of the inner conductor of T2 being directly coupled to the first end of the inner conductor of T1; , • a single-access linear device having a first terminal and a second terminal, the first terminal of the single-access linear device being coupled to the second end of the inner conductor of T2, the second terminal of the single-access linear device being coupled to the second end of the outer conductor of T2, the single-access linear device comprising a resistor and a capacitor; and • a passive antenna port, the passive antenna port having a first terminal and a second terminal, the first terminal of the passive antenna port being coupled to the second end of the inner conductor of Tl, the second terminal of the passive antenna port being coupled to the second end of the outer conductor of Tl.

[0068] The specialist understands that, in this fifth embodiment of a device according to the invention, the first element is identical to the first element of the active antenna disclosed in claim 1 of international application number PCT / IB2021 / 053502 (WO 2021 / 234479) entitled “Active antenna including a screened loop aerial”. The specialist understands that certain aspects of the embodiments of said international application number PCT / IB2021 / 053502 could therefore be applied to the first element and the second element of this fifth embodiment of a device according to the invention.

[0069] INDICATIONS ON INDUSTRIAL APPLICATIONS

[0070] The access of the passive antenna according to the invention can be coupled to an output of a radio-frequency generator, to obtain a known radio-frequency magnetic field. In this application, the main advantage of the passive antenna according to the invention is that the reference field level is, for a reference location close to the passive antenna, substantially independent of the frequency at any frequency lower than an upper limit, the upper limit being higher than that which is obtained by using another passive antenna comprising a shielded loop of the same dimensions.

[0071] The passive antenna according to the invention is therefore particularly suitable for the calibration of magnetic field probes and the calibration of antennas, for example in the technical field of electromagnetic compatibility (EMC).

[0072] The access of the passive antenna according to the invention can also be coupled to an input of a radio communication receiver, a measurement receiver, or a spectrum analyzer, to measure a radio-frequency magnetic field. In this application, the main advantage of the passive antenna according to the invention is that it can be used in such a way that the voltage delivered by the passive antenna according to the invention is substantially proportional to a time derivative of the radio-frequency magnetic field, over a wide frequency band.

[0073] The passive antenna according to the invention is therefore particularly suitable for measurements of the time derivative of a radio-frequency magnetic field, for example in the technical field of electromagnetic compatibility (EMC).

Claims

1. Claims Passive antenna comprising: • a first element (1), the first element comprising a transmission line named Tl, Tl having an outer conductor (11) and an inner conductor (12), Tl having a first end (101) and a second end (102), the outer conductor of Tl having a first end at the first end of Tl, the outer conductor of Tl having a second end at the second end of Tl, the inner conductor of Tl having a first end at the first end of Tl, the inner conductor of Tl having a second end at the second end of Tl, the first element being a part of a shielded frame; • a second element (2), the second element comprising a transmission line named T2, T2 having an outer conductor (21) and an inner conductor (22), T2 having a first end (201) and a second end (202), the outer conductor of T2 having a first end at the first end of T2, the outer conductor of T2 having a second end at the second end of T2, the inner conductor of T2 having a first end at the first end of T2, the inner conductor of T2 having a second end at the second end of T2, the second element being a part of the shielded frame, the first end of the inner conductor of T2 being directly coupled to the first end of the inner conductor of T1; • a single-access linear device (8) having a first terminal and a second terminal, the first terminal of the single-access linear device being coupled to the second end of the inner conductor of T2, the second terminal of the single-access linear device being coupled to the second end of the outer conductor of T2, the single-access linear device comprising a resistor (81) and a capacitor (82); and • a passive antenna port, the passive antenna port having a first terminal and a second terminal, the first terminal of the passive antenna port being coupled to the second end of the inner conductor of Tl, the second terminal of the passive antenna access being coupled to the second end of the external conductor of Tl.

2. A passive antenna according to claim 1, wherein the single-port linear device has a DC resistance, the DC resistance being greater than 10 ohms, the DC resistance being less than 500 ohms.

3. A passive antenna according to any one of claims 1 or 2, further comprising a base (3), the base providing electrical contact between the second end of the outer conductor of T2 and the second end of the outer conductor of T1, the base being a part of the shielded frame.

4. A passive antenna according to claim 3, wherein the base is a box.

5. A passive antenna according to any one of claims 1 or 2, wherein the second end of the outer conductor of T2 is directly coupled to the second end of the outer conductor of T1.

6. A passive antenna according to claim 2, wherein Tl has a characteristic impedance, the DC resistance being greater than half a real part of the characteristic impedance of Tl, the DC resistance being less than twice said real part of the characteristic impedance of Tl.

7. A passive antenna according to claim 1, wherein Tl has a characteristic impedance and T2 has a characteristic impedance, a real part of the characteristic impedance of T2 being greater than half of a real part of the characteristic impedance of Tl, said real part of the characteristic impedance of T2 being less than twice said real part of the characteristic impedance of Tl.

8. A passive antenna according to any one of claims 1 to 7, wherein the resistor is connected in parallel with the capacitor.

9. A passive antenna according to any one of claims 1 to 8, wherein at least one of the transmission lines T1 and T2 comprises a flexible coaxial cable inserted into a rigid tube made of a dielectric material comprising a polymer.

10. A passive antenna according to any one of claims 1 to 8, wherein at least one of the transmission lines T1 and T2 comprises a flexible coaxial cable having a central conductor, a dielectric surrounding the central conductor, and a metal braid surrounding the dielectric, the coaxial cable being inserted into a rigid metal tube, the rigid metal tube having a length, the metal braid being in electrical contact with the rigid metal tube over the entire length of the rigid metal tube, the inner conductor of said at least one of the transmission lines T1 and T2 being the central conductor, the outer conductor of said at least one of the transmission lines T1 and T2 consisting of the metal braid in electrical contact with the rigid metal tube.