Active antenna including a single-turn frame

Abstract

The invention relates to an active antenna including a single-turn loop, for example, an antenna for radio communications and / or electromagnetic field measurements. An active antenna according to the invention comprises: a first element (11), the first element being an electrical conductor; a second element (12), the second element being an electrical conductor; a third element (3), the third element being an electrical conductor, a first end of the third element being directly coupled to a first end of the first element and to a first end of the second element; a transformer having a primary, a secondary, and a ferrite core; and an amplifier, an input port of the amplifier being coupled to the secondary, and an output port of the active antenna being coupled to an output of the amplifier. Figure 5 is shown for the abstract.

Description

Title of the invention: Active antenna including a single-turn frame TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to an active antenna including a single-turn loop, for example an antenna for radio communications and / or electromagnetic field measurements. PREVIOUS STATE OF THE ART

[0002] In the following, “coupled” always refers to 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, and / or that the entities are indirectly coupled, an electrical interaction different from the direct coupling existing in this case between the entities, for example through one or more components.When this term is applied to two multi-terminal entities, such as access points, connectors, etc., “coupled” can indicate that the entities are directly coupled, with each terminal of one entity being directly coupled to one and only one terminal of the other entity, and / or that the entities are indirectly coupled, with an electrical interaction other than direct coupling existing between the terminals of the entities, for example, through one or more components. In the following, in accordance with circuit theory, an access point has exactly two terminals.

[0003] Loop antennas and shielded loop antennas are well known to specialists. They are used for radio reception applied to electromagnetic field measurements, radio direction finding, and radio communications. Characteristics and limitations of these antennas for these uses are explained in the article by F. Broydé and E. Clavelier entitled “Contribution to the Theory of Planar Wire Loop Antennas Used for Reception,” published in the journal “IEEE Transactions on Antennas and Propagation,” vol. 68, no. 3, in March 2020, and in the article by F. Broydé and E. Clavelier entitled “The Open-Circuit Voltage of a Planar Wire Loop Antenna Used for Reception,” published in the journal “Excem Research Papers in Electronics and Electromagnetics,” no. 6, in January 2023 (this article can be downloaded from https: / / doi.org / 10.5281 / zenodo.7498910).In particular, these articles explain to what extent it is possible to consider that these antennas measure a component of an incident magnetic field, that is, a magnetic component of an incident electromagnetic field. As indicated in these articles and in paragraph 5-4. From Chapter 5 of R.C. Johnson's book, "Antenna Engineering Handbook, 3rd Edition," published by McGraw-Hill in 1993, the shielding of a shielded frame typically functions like a frame. Shielded frames used for radio reception perform better than unshielded frames because they are not affected by common-mode current flowing in the cable connecting the antenna to a measuring device or radio receiver, induced by an incident electromagnetic field received as a signal, or by electromagnetic interference.

[0004] The antenna factor is defined as the magnitude of the ratio of the intensity of an incident field (expressed in V / m for an electric field or in A / m for a magnetic field) to the voltage developed by an antenna across a specified impedance. For electromagnetic field measurements, it is often preferable, when possible, to use an antenna with an antenna factor that is substantially independent of frequency over a wide frequency band. Specialists know that this result can be obtained, within a known frequency band, the known frequency band having an upper bound, the upper bound corresponding to a wavelength in a vacuum, by using, for radio reception, a prior art active antenna comprising a shielded loop, the shielded loop being a single-turn shielded loop, the shielded loop being small compared to said wavelength in a vacuum.

[0005] An example of such an active antenna from the prior art is shown in [Fig. 1], where the hidden (i.e., not directly visible) edges and the hidden contours are not shown. The active antenna shown in [Fig. 1] comprises: • a first element (51), the first element being a transmission line having an outer conductor and an inner conductor (513), the transmission line having a first end (511) and a second end (512), the outer conductor having a first end at the first end of the transmission line, the outer conductor having a second end at the second end of the transmission line, the inner conductor having a first end at the first end of the transmission line, the inner conductor having a second end at the second end of the transmission line, the first element being a part of the shielded frame (5), an access to the shielded frame being the second end of the transmission line, an impedance presented by this access being called the “shielded frame impedance”; • a second element (52), the second element being an electrical conductor, the second element having a first end (521) and a second end (522), the first end of the second element being coupled to the first end of the internal conductor, the second element being part of the shielded frame; • a part called “base” (6), the base providing an electrical contact between the second end of the external conductor and the second end of the second element, the base being part of the shielded frame; • an amplifier having an input and an output, the input of the amplifier having an impedance called the “amplifier input impedance”, a magnitude of the amplifier input impedance being, at any frequency in the known frequency band, much smaller than a magnitude of the shielded frame impedance, an amplifier output voltage being, at a given frequency in the known frequency band, equal to the product of an amplifier input current and a transimpedance, a magnitude of the transimpedance being substantially independent of the given frequency, the amplifier input being directly coupled to the shielded frame input; and • an active antenna access, the active antenna access being directly coupled to the amplifier output access.

[0006] The specialist knows that a suitable amplifier may, for example, be a transimpedance amplifier or a transresistance amplifier similar to those discussed in paragraph 8.11 of the book by P. Horowitz and W. Hill entitled “The Art of Electronics, Third Edition”, published by Cambridge University Press in 2015, or to those used in United States patent number 5,352,984 entitled “Fault and splice finding System and method”, or in international application number PCT / DE2006 / 000415 (WO 2006 / 097071) entitled “Active reception antenna System”.

[0007] If we compare a first active antenna consisting of such a single-turn shielded frame and such an amplifier, to a second active antenna consisting of a single-turn frame having the same size as the shielded frame, and the same amplifier as that used in the first active antenna, we find that, as explained above, the immunity to a common-mode current flowing on the cable connecting the antenna to a measuring device or a radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic disturbances, is much better for the first active antenna than for the second active antenna.

[0008] Figure 2 is a graph showing the impedance magnitude of the shielded frame of The active antenna of [Fig. 1], as a function of frequency. [Fig. 3] is a graph showing a characteristic of the shielded frame of the active antenna of [Fig. 1], in This characteristic, expressed in siemens-meters, is a function of frequency and is equal to the magnitude of the ratio of the short-circuit current of the shielded frame to the intensity of an incident field expressed in V / m. This characteristic is substantially independent of frequency up to approximately 10 MHz. Since, at any frequency within the known frequency band, the amplifier's input impedance has a magnitude much smaller than the magnitude of the shielded frame impedance, and the transimpedance has a magnitude that is substantially independent of frequency, the specialist understands that the antenna factor is substantially independent of frequency up to approximately 10 MHz. An upper limit of the frequency band in which the antenna factor is substantially independent of frequency is therefore close to 10 MHz.

[0009] Unfortunately, this upper limit of the frequency band over which the antenna factor is substantially independent of frequency is much lower for the first active antenna than for the second active antenna. To compensate for this reduction, it is necessary to reduce the size of the shielded frame, which results, at frequencies below the upper limit, in an increase in the product of the antenna factor and the noise voltage density at the output access of the active antenna (this density being expressed in volts RMS per square root of hertz), whereas the designer wants this product to be as small as possible at these frequencies.

[0010] Thus, the prior art does not disclose an active antenna having good immunity to a common-mode current flowing on the cable connecting the antenna to a measuring device or a radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic disturbances, the active antenna having an antenna factor that is substantially independent of frequency in a known frequency band having an upper bound and a lower bound that is less than one-tenth of the upper bound, the active antenna being such that, in the known frequency band, a product of its antenna factor by the noise voltage density at the output access of the active antenna is smaller than that which would be obtained using a prior art active antenna having a shielded frame and having an antenna factor that is substantially independent of frequency in said known frequency band. Description of the invention

[0011] The invention relates to an active antenna including a single-turn frame, free from the limitations mentioned above of known techniques.

[0012] An active antenna according to the invention is an active antenna for radio reception in a known frequency band, the active antenna comprising: a first element, the first element being an electrical conductor, the first element having a first end and a second end, a central line of the first element extending from the first end of the first element to the second end of the first element, the first element being part of a single-turn frame; a second element, the second element being an electrical conductor, the second element having a first end and a second end, a centerline of the second element extending from the first end of the second element to the second end of the second element, the second element being part of the single-turn frame, a reference length being equal to an upper bound of the distance between any point on the centerline of the first element and any point on the centerline of the second element, a precision parameter being a positive or zero length less than or equal to one-tenth of the reference length; a third element, the third element being an electrical conductor, the third element having a first end and a second end, a centerline of the third element extending from the first end of the third element to the second end of the third element, the first end of the third element being directly coupled to the first end of the first element and to the first end of the second element, a length of the centerline of the third element being greater than or equal to one-third of the reference length, a line S and a plane P being such that the plane P contains the line S, and such that, for any point C belonging to the centerline of the third element, a plane orthogonal to the line S and containing the point C intersects the centerline of the first element at a point A, the centerline of the second element at a point B, and the line S at a unique point D,a distance between points C and D being less than the precision parameter, a distance between point A and plane P being less than the precision parameter, a distance between point B and plane P being less than the precision parameter, an absolute value of the difference between a first distance and a second distance being less than the precision parameter, the first distance being a distance between points A and D, and the second distance being a distance between points B and D; , a transformer having a primary, a secondary and a ferrite core, the primary having a first terminal and a second terminal, the secondary having a first terminal and a second terminal, the first terminal of the primary being coupled to the second end of the first element, the second terminal of the primary being coupled to the second end of the second element, a passive antenna comprising the first element, the second element, the third element and the transformer, an access of the passive antenna being made up of the first terminal of the secondary and the second terminal of the secondary; • an amplifier having an input port, an output port, and a reference node (ground), the input port of the amplifier having, at any frequency in the known frequency band, an impedance with a magnitude less than one magnitude of an impedance presented by the passive antenna port, the input port of the amplifier being coupled to the passive antenna port, the reference node being coupled to the second end of the third element; and • an active antenna output access, the active antenna output access being coupled to the amplifier output access.

[0013] In the preceding sentence, “line” always means an abstract mathematical concept (as opposed to a material object such as an electric wire or a transmission line). The centerline of the first element has a finite length since it extends from the first end of the first element to the second end of the first element. The centerline of the second element has a finite length since it extends from the first end of the second element to the second end of the second element. The centerline of the third element has a finite length since it extends from the first end of the third element to the second end of the third element. Similarly, point A, point B, point C, point D, line S, and plane P are abstract mathematical concepts. Line S is infinite. Plane P is infinite.

[0014] In the above sentence defining the active antenna according to the invention, the distance between point A and plane P is obviously the lower bound of the distance between point A and an arbitrary point belonging to plane P, and the distance between point B and plane P is obviously the lower bound of the distance between point B and an arbitrary point belonging to plane P.

[0015] For example, the active antenna may further comprise a single-access linear device, the single-access linear device being a part of said passive antenna, the single-access linear device having a first terminal and a second terminal, each terminal of the single-access linear device being coupled to a terminal of a transformer winding, said single-access linear device having, at a frequency above an upper limit of the known frequency band, a resistance and a reactance, the resistance being greater than an absolute value of the reactance. Said winding could, for example, be the primary. Said winding could, for example, be the secondary. For example, it is possible that the single-access linear device is a resistor.

[0016] For example, the active antenna may further include a passive linear filter having an input access and an output access, the input access of the passive linear filter being coupled to the access of the passive antenna, the input access of the amplifier being coupled to the output access of the passive linear filter, so that, in this case, the input access of the amplifier is indirectly coupled to the access of the passive antenna, through the passive linear filter. Brief description of the drawings

[0017] Other advantages and features will become clearer from the following description of particular embodiments of the invention, given by way of non-limiting examples, and shown in the accompanying drawings in which:

[0018] - the [Fig.1] is a drawing of an active antenna from the prior art;

[0019] - [Fig.2] is a graph showing the magnitude of the impedance, expressed in ohms, of the shielded frame of the active antenna of [Fig.1], depending on the frequency;

[0020] - [Fig. 3] is a graph showing a characteristic, expressed in siemens- meters, of the shielded frame of the active antenna of [Fig.1], as a function of frequency, this characteristic being equal to a modulus of a ratio of a short-circuit current of the shielded frame to an intensity of an incident field expressed in V / m.

[0021] - [Fig. 4] is a drawing of a part of the active antenna of the first mode of realization ;

[0022] - [Fig.5] is a front view of the active antenna of the first embodiment;

[0023] - [Fig.6] is a top view of the active antenna of the first embodiment;

[0024] - [Fig. 7] is a left-hand view of the active antenna of the first mode of realization ;

[0025] - [Fig.8] is a front view of the active antenna of the third embodiment;

[0026] - [Fig.9] is a top view of the active antenna of the third mode of realization ;

[0027] - Figure 10 is a graph showing the magnitude of the impedance, expressed in ohms, of the passive antenna of the active antenna of the third embodiment, depending on the frequency;

[0028] - [Fig. 1 1] is a graph showing a characteristic, expressed in siemens- meters, of the passive antenna of the active antenna of the third embodiment, as a function of the frequency, this characteristic being equal to a modulus of a ratio of a short-circuit current of this passive antenna to an intensity of an incident field expressed in V / m.

[0029] DETAILED DESCRIPTION OF CERTAIN IMPLEMENTATIONS

[0030] First embodiment.

[0031] By way of a first embodiment of a device according to the invention, given by way of non-limiting example, we have shown in Figures 4 to 7 drawings of an active antenna according to the invention for radio reception in a known frequency band, the known frequency band being the 100 kHz to 80 MHz band, the known frequency band having an upper limit equal to 80 MHz, and a lower limit which is less than one-tenth of the upper limit, the active antenna comprising: • a first element (11), the first element being an electrical conductor, the first element having a first end (111) and a second end (112), a central line of the first element extending from the first end of the first element to the second end of the first element; • a second element (12), the second element being an electrical conductor, the second element having a first end (121) and a second end (122), a centerline of the second element extending from the first end of the second element to the second end of the second element, a reference length being an upper bound of the distance between any point on the centerline of the first element and any point on the centerline of the second element; • a third element (3), the third element being an electrical conductor, the third element having a first end (31) and a second end (32), a centerline of the third element extending from the first end of the third element to the second end of the third element, the first end of the third element being directly coupled to the first end of the first element and to the first end of the second element, a length of the centerline of the third element being greater than or equal to one-third of the reference length, a line S and a plane P being such that the plane P contains the line S, such that the centerline of the third element is substantially straight and is substantially a segment of the line S, such that the centerline of the first element is substantially included in the plane P, such that the centerline of the second element is substantially included in the plane P,and such that the central line of the second element is substantially the image of the central line of the first element under a rotation of 180 degrees around the line S; • a part called “base” (6); • a transformer having a primary, a secondary and a ferrite core, the primary having a first terminal and a second terminal, the secondary having a first terminal and a second terminal, the first terminal of the primary being coupled to the second end of the first element, the second terminal of the primary being coupled to the second end of the second element, a passive antenna comprising the first element, the second element, the third element and the transformer, an access of the passive antenna being made up of the first terminal of the secondary and the second terminal of the secondary; • an amplifier having an input port, an output port, and a reference node, the input port of the amplifier having, at any frequency in the known frequency band, an impedance with a magnitude less than one magnitude of an impedance presented by the passive antenna port, the input port of the amplifier being directly or indirectly coupled to the passive antenna port, the reference node being coupled to the second end of the third element; and • an active antenna output access, the active antenna output access being coupled to the amplifier output access.

[0032] Figure 4 shows only the first element (11), the second element (12), and the third element (3). Figures 5 to 7 show the entire active antenna. In Figures 4 to 7, hidden edges and contours are not shown, except for the hidden contour of the third element in Figure 6. Figure 5 is a front view of the active antenna, Figure 6 is a top view of the active antenna, and Figure 7 is a left view of the active antenna.

[0033] The requirement “the first end of the third element being directly coupled to the first end of the first element and to the first end of the second element” implies that the first end of the second element is coupled to the first end of the first element.

[0034] We see in Figures 4 to 7 that, for any point C belonging to the centerline of the third element, a plane orthogonal to the line S and containing the point C intersects the centerline of the first element at a point A, the centerline of the second element at a point B, and the line S at a unique point D. Therefore, the requirement that the line S and the plane P are “such that the plane P contains the line S, such that the centerline of the third element is substantially straight and is substantially a segment of the line S, such that the centerline of the first element is substantially included in the plane P, such that the centerline of the second element is substantially included in the plane P, and such that the centerline of the the second element is substantially the image of the center line of the first element by a rotation of 180 degrees around the line S” implies that there is a precision parameter which is a positive or zero length less than or equal to one thirtieth of the reference length, the precision parameter being such that a distance between points C and D is less than the precision parameter, such that a distance between point A and plane P is less than the precision parameter, such that a distance between point B and plane P is less than the precision parameter, and such that an absolute value of the difference between a first distance and a second distance is less than the precision parameter, the first distance being a distance between points A and D, and the second distance being a distance between points B and D.

[0035] In this first embodiment, a cross-section of the first element is a rectangular and hollow electrical conductor, and a cross-section of the second element is a rectangular and hollow electrical conductor, substantially identical to the cross-section of the first element.

[0036] The base is hollow and conductive. The amplifier is installed in the base. The second end of the third element is directly coupled to the base. A feedthrough (61) allows the first element to pass through a wall of the base and provides electrical insulation between the first element and the base. A feedthrough (62) allows the second element to pass through a wall of the base and provides electrical insulation between the second element and the base. Two connectors are fixed to the base: a coaxial connector (71), which provides the output access for the active antenna; and a power connector (72) for supplying power to the amplifier.

[0037] The specialist sees that the first element and the second element are parts of a polygonal winding having a single turn, the winding being used as a single-turn frame (1) because:

[0038] - the first terminal of the primary is coupled to the second end of the first element ;

[0039] - the second terminal of the primary is coupled to the second end of the second element ;

[0040] - the passive antenna access consists of the first terminal of the secondary and of the second terminal of the secondary; and

[0041] - the amplifier input access is coupled to the passive antenna access.

[0042] Therefore, the first element is a part of this single-turn frame, and the second element is a part of this single-turn frame. Furthermore, said passive antenna comprises this single-turn frame, the third element, and the transformer.

[0043] The specialist understands that the fact that the amplifier's input access is coupled to the passive antenna's access means that the active antenna can be sized in such a way that its antenna factor is substantially independent of the frequency in the known frequency band, and that, in the known frequency band, a product of its antenna factor by the noise voltage density at the output access of the active antenna is smaller than that which would be obtained using a prior art active antenna having a shielded loop and having an antenna factor that is substantially independent of frequency in said known frequency band.

[0044] If a common-mode current flows on a cable connected to said coaxial connector (for example, a cable connecting the active antenna to a measuring device or a radio receiver) and / or on a cable connected to said power connector, it can extend into the third element and at any point of the single-turn loop without encountering any localized impedance. It is understood that the fact that the centerline of the second element is substantially the image of the centerline of the first element under a 180-degree rotation around the line S means that this common-mode current induces a first complex voltage between the first terminal of the primary and the reference node (ground), and a second complex voltage between the second terminal of the primary and the reference node, the first complex voltage being substantially equal to the second complex voltage.The specialist understands that a resulting common-mode voltage has a magnitude much smaller than that which would appear if the third element were not present, so that said resulting common-mode voltage is effectively rejected by the transformer, despite the inevitable parasitic capacitance between the primary and secondary windings. Consequently, said common-mode current has virtually no effect on a signal at the output access of the active antenna. This is why the active antenna has good immunity to a common-mode current flowing in the cable connecting the antenna to a measuring instrument or a radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic interference.

[0045] Consequently, the active antenna has good immunity to a common-mode current flowing on the cable connecting the antenna to a measuring device or a radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic disturbances, the active antenna has an antenna factor that is substantially independent of frequency in the known frequency band, and the active antenna is such that, in the known frequency band, a product of its antenna factor by the noise voltage density at the output access of the active antenna is smaller than that which would be obtained using a prior art active antenna having a shielded frame and having an antenna factor that is substantially independent of frequency in said known frequency band.

[0046] We observe that the prior art includes a polygonal winding having two turns and a grounded central tap, the polygonal winding having two turns being used as a multi-turn frame. This device is, for example, described in Figure 105 of R. Keen's book entitled “Wireless Direction Finding, Third and Enlarged Edition”, published by Iliffe & Sons Limited in 1938 and reprinted in 1943. This device is also described in Fig. 4.17 of D.S. Bond's book entitled “Radio Direction Finders”, published by McGraw-Hill Book Company in 1944. This device is completely different from the combination of the first, second, and third elements according to the invention. The expert understands that this device is also unsuitable for the purpose of the invention, because a winding having more than one turn has a much lower resonant frequency than a winding having only one turn, thereby dramatically reducing the frequency band in which an active antenna using this device could have an antenna factor substantially independent of frequency.

[0047] Second embodiment.

[0048] The second embodiment of a device according to the invention, given by way of non-limiting example, also corresponds to the active antenna shown in Figures 4 to 7, and all the explanations provided for the first embodiment are applicable to this second embodiment.

[0049] In this second embodiment, the active antenna further comprises a single-access linear device, the single-access linear device being a part of said passive antenna, the 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 first terminal of the primary, the second terminal of the single-access linear device being coupled to the second terminal of the primary. Thus, each terminal of the single-access linear device is coupled to a terminal of the primary. The single-access linear device has, at a frequency above the upper limit of the known frequency band, a resistance and a reactance, the resistance being greater than an absolute value of the reactance. The single-access linear device reduces the variations of said impedance presented by the access of the passive antenna as a function of frequency.For example, the single-access linear device could be a resistor, for example a 100-ohm resistor.

[0050] Third embodiment.

[0051] By way of a third embodiment of a device according to the invention, given by way of non-limiting example, we have shown in [Fig. 8] and [Fig. 9] drawings of an active antenna according to the invention for radio reception in a known frequency band, the known frequency band being the 20 kHz to 50 MHz band, the known frequency band therefore having an upper limit equal to 50 MHz, and a lower limit which is less than one-tenth of the upper limit, the active antenna comprising: a first element (11), the first element being an electrical conductor, the first element having a first end (111) and a second end, a central line of the first element extending from the first end of the first element to the second end of the first element; a second element (12), the second element being an electrical conductor, the second element having a first end (121) and a second end, a centerline of the second element extending from the first end of the second element to the second end of the second element, a reference length being the upper bound of the distance between any point on the centerline of the first element and any point on the centerline of the second element, a precision parameter being a positive or zero length less than or equal to one tenth of the reference length; a third element (3), the third element being an electrical conductor, the third element having a first end (31) and a second end (32), a centerline of the third element extending from the first end of the third element to the second end of the third element, the first end of the third element being directly coupled to the first end of the first element and to the first end of the second element, a length of the centerline of the third element being greater than or equal to one-third of the reference length, a line S and a plane P being such that the plane P contains the line S, and such that, for any point C belonging to the centerline of the third element, a plane orthogonal to the line S and containing the point C intersects the centerline of the first element at a unique point A, the centerline of the second element at a unique point B, and the line S at a unique point D,a distance between points C and D being less than the precision parameter, a distance between point A and plane P being less than the precision parameter, a distance between point B and plane P being less than the precision parameter, an absolute value of the difference between a first distance and a second distance being less than the precision parameter, the first distance being a distance between points A and D, and the second distance being a distance between points B and D; , a base (6), the second end of the third element being directly coupled to the base; a transformer having a primary, a secondary and a ferrite core, the primary having a first terminal and a second terminal, the secondary having a first terminal and a second terminal, the first terminal of the primary being coupled to the second end of the first element, the second terminal of the primary being coupled to the second end of the second element, a passive antenna comprising the first element, the second element, the third element and the transformer, an access of the passive antenna being made up of the first terminal of the secondary and the second terminal of the secondary; • a single-access linear device, the single-access linear device being a part of said passive antenna, the 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 first terminal of the secondary, the second terminal of the single-access linear device being coupled to the second terminal of the secondary; • a passive linear filter having an input access and an output access, the input access of the passive linear filter being directly coupled to the access of the passive antenna; • an amplifier having an input port, an output port, and a reference node, the input port of the amplifier presenting, at any frequency in the known frequency band, an impedance having a magnitude that is less than the magnitude of an impedance presented by the passive antenna port, the input port of the amplifier being directly coupled to the output port of the passive linear filter (such that the input port of the amplifier is indirectly coupled to the passive antenna port, through the passive linear filter); and • an active antenna output access, the active antenna output access being coupled to the amplifier output access.

[0052] Figures 8 and 9 show the entire active antenna. In Figures 8 and 9, hidden edges and contours are not shown, except for the hidden contour of the third element in [Fig. 9]. [Fig. 8] is a front view of the active antenna, and [Fig. 9] is a top view of the active antenna.

[0053] The requirement “the first end of the third element being directly coupled to the first end of the first element and to the first end of the second element” implies that the first end of the second element is coupled to the first end of the first element.

[0054] The specialist sees that the first element and the second element are parts of a circular winding having a single turn, the winding being used as a single-turn frame (1) because:

[0055] - the first terminal of the primary is coupled to the second end of the first element ;

[0056] - the second terminal of the primary is coupled to the second end of the second element ;

[0057] - the passive antenna access consists of the first terminal of the secondary and of the second terminal of the secondary; and

[0058] - the amplifier input access is coupled to the passive antenna access.

[0059] Therefore, the first element is a part of this single-turn frame, and the second element is a part of this single-turn frame. Furthermore, said passive antenna comprises this single-turn frame, the third element, the transformer, and the single-access linear device.

[0060] The base includes a mounting ring (65). The mounting ring is used to attach the active antenna to a hollow antenna mast (66). The base is a conductive box, the box containing the amplifier. This box can, for example, be a metal box. Thus, the base can provide shielding for the amplifier, that is, electromagnetic shielding of the amplifier. A feedthrough (61) provides electrical insulation between the first element and the base. A feedthrough (62) provides electrical insulation between the second element and the base. The amplifier is powered by a battery or a rechargeable battery.

[0061] The reference node is directly coupled to the base. Since the second end of the third element is directly coupled to the base, we see that the reference node is coupled to the second end of the third element.

[0062] A coaxial connector, which provides the output access for the active antenna, is attached to the lower part of the base and concealed by the retaining ring. This coaxial connector is intended to be connected to a coaxial cable installed inside the hollow antenna mast. It is understood that this configuration improves the immunity of the active antenna to common-mode current flowing in the cable connecting the antenna to a measuring instrument or radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic interference.

[0063] The accuracy parameter is less than or equal to one-thirtieth of the reference length, and the centerline of the third element is substantially straight and is substantially a segment of the line S. Furthermore, the centerline of the first element is substantially contained in the plane P, the centerline of the second element is substantially contained in the plane P, and the centerline of the second element is substantially the image of the centerline of the first element under a rotation of 180 degrees around the line S. If a common-mode current flows on a cable connected to said coaxial connector (for example, a cable connecting the active antenna to a measuring device or a radio receiver), this common-mode current may also The current flows freely through the third element and at every point of the single-turn loop, without encountering any localized impedance, thus inducing only a small common-mode voltage across the primary winding. This small common-mode voltage is effectively rejected by the transformer. Consequently, the active antenna exhibits good immunity to common-mode current flowing in the cable connecting the antenna to a measuring instrument or radio receiver, whether induced by an incident electromagnetic field received as a signal or by electromagnetic interference.

[0064] The upper limit of the known frequency band corresponds to a wavelength in a vacuum equal to the speed of light in a vacuum divided by the upper limit of the known frequency band. The sum of the centerline length of the first element and the centerline length of the second element is less than one-quarter of said wavelength in a vacuum. Therefore, said single-turn frame is electrically small.

[0065] The first element has a circular cross-section. The first element may, for example, comprise a rigid metal bar, the rigid metal bar being curved. The first element may, for example, comprise a rigid metal tube, the rigid metal tube being curved. The second element has a circular cross-section. The second element may, for example, comprise a rigid metal bar, of the same diameter as the cross-section of the first element, the rigid metal bar being curved. The second element may, for example, comprise a rigid metal tube, of the same diameter as the cross-section of the first element, the rigid metal tube being curved.

[0066] The primary and secondary windings have the same number of turns. At low frequencies, the self-inductance of the primary and the self-inductance of the secondary are approximately 1.33 microhenries. It is very important that the ferrite core provides close coupling between the primary and secondary windings.

[0067] Each terminal of the single-access linear device is coupled to a terminal of the secondary winding. The single-access linear device is a 75-ohm resistor. Figure 10 is a graph showing the magnitude of the impedance presented by the passive antenna access as a function of frequency. It will be understood by those skilled in the art that Figure 10 shows that the single-access linear device reduces the variations in said impedance presented by the passive antenna access as a function of frequency.

[0068] The passive linear filter is a low-pass filter designed to provide a cutoff frequency of 100 MHz if its input has an impedance close to 70 ohms and its output has an impedance much lower than 70 ohms. It will be understood that [Fig. 10] indicates that this filter can effectively attenuate signals at frequencies above 100 MHz. Thus, the active antenna is not easily disturbed by a radio signal at a frequency higher than 100 MHz. Furthermore, the passive linear filter is such that it can be practically ignored when studying the behavior of the active antenna at frequencies below 50 MHz. Thus, the passive linear filter does not introduce undesirable variations in the antenna factor as a function of frequency at frequencies below or equal to 50 MHz.

[0069] Figure 11 is a graph showing a characteristic of the passive antenna as a function of frequency. This characteristic, expressed in siemens-meters, is equal to the magnitude of a ratio of the short-circuit current of the passive antenna to the intensity of an incident field expressed in V / m. This characteristic is substantially independent of frequency from approximately 20 kHz up to approximately 50 MHz.The specialist understands that, since the amplifier's input presents, at any frequency within the known frequency band, an impedance with a magnitude much smaller than the magnitude of an impedance presented by the passive antenna's input, the magnitude of an input current to the amplifier is substantially equal to the product of this characteristic and the magnitude of an incident electric field strength expressed in V / m. As the magnitude of the amplifier's transimpedance is, within the known frequency band, substantially independent of frequency, the specialist understands that the antenna factor of the active antenna is substantially independent of frequency from approximately 20 kHz to approximately 50 MHz.

[0070] The shielded frame of the prior art active antenna shown in [Fig. 1] has the same dimensions as the single-turn frame of the active antenna shown in [Fig. 8], so that the specialist understands why the curve in [Fig. 3] and the curve in [Fig. 11] show values ​​of said characteristic that are approximately equal from 20 kHz up to 9 MHz. The specialist understands that, in order to obtain an antenna factor substantially independent of frequency up to about 50 MHz using a shielded frame as in the active antenna shown in [Fig. 1], it would be necessary to reduce the size of the shielded frame until a modulus of said characteristic becomes substantially independent of frequency up to about 50 MHz, which would result in this modulus of this characteristic being, from 10 kHz up to 50 MHz, much smaller than that shown in [Fig. 11].Therefore, a product of the antenna factor by the noise voltage density at the output access of an active antenna with a small shielded frame would be much larger than a product of the antenna factor by the noise voltage density at the output access of an active antenna of this third embodiment.

[0071] Consequently, the active antenna of this third embodiment has good immunity to a common-mode current flowing on the cable connecting the antenna to a measuring device or a radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic disturbances; this active antenna has an antenna factor that is substantially independent of frequency in the known frequency band having an upper bound and a lower bound which is less than one tenth of the upper bound, and this active antenna is such that, in the known frequency band, a product of its antenna factor by the noise voltage density at the output access of the active antenna is smaller than that which would be obtained using a prior art active antenna having a shielded frame and having an antenna factor which is substantially independent of frequency in said known frequency band.

[0072] INDICATIONS ON INDUSTRIAL APPLICATIONS

[0073] The output of the active antenna according to the invention can, for example, be connected to one end of an antenna link, the antenna link having another end that is coupled to a radio communication receiver, a measurement receiver, or a spectrum analyzer. The active antenna according to the invention is particularly suitable for radio communications and radio direction finding. The active antenna according to the invention is particularly suitable for electromagnetic field measurements, for example in the technical field of electromagnetic compatibility (EMC).

[0074] The active antenna according to the invention may also include other devices, for example a device indicating when the level of the signals applied to the amplifier produces, or risks producing, non-linear operation leading for example to saturation or overload at the output.

Claims

1. Demands An active antenna for radio reception in a known frequency band, the active antenna comprising: • a first element (11), the first element being an electrical conductor, the first element having a first end (111) and a second end (112), a central line of the first element extending from the first end of the first element to the second end of the first element; • a second element (12), the second element being an electrical conductor, the second element having a first end (121) and a second end (122), a centerline of the second element extending from the first end of the second element to the second end of the second element, a reference length being equal to an upper bound of the distance between any point on the centerline of the first element and any point on the centerline of the second element, a precision parameter being a positive or zero length less than or equal to one tenth of the reference length; • a third element (3), the third element being an electrical conductor, the third element having a first end (31) and a second end (32), a centerline of the third element extending from the first end of the third element to the second end of the third element, the first end of the third element being directly coupled to the first end of the first element and to the first end of the second element, a length of the centerline of the third element being greater than or equal to one-third of the reference length, a line S and a plane P being such that the plane P contains the line S, and such that, for any point C belonging to the centerline of the third element, a plane orthogonal to the line S and containing the point C intersects the centerline of the first element at a point A, the centerline of the second element at a point B, and the line

2. S at a single point D, a distance between points C and D being less than the precision parameter, a distance between point A and plane P being less than the precision parameter, a distance between point B and plane P being less than the precision parameter, an absolute value of the difference between a first distance and a second distance being less than the precision parameter, the first distance being a distance between points A and D, and the second distance being a distance between points B and D; • a transformer having a primary, a secondary and a ferrite core, the primary having a first terminal and a second terminal, the secondary having a first terminal and a second terminal, the first terminal of the primary being coupled to the second end of the first element, the second terminal of the primary being coupled to the second end of the second element, a passive antenna comprising the first element, the second element, the third element and the transformer, an access of the passive antenna being made up of the first terminal of the secondary and the second terminal of the secondary; • an amplifier having an input port, an output port, and a reference node, the input port of the amplifier having, at any frequency in the known frequency band, an impedance with a magnitude less than one magnitude of an impedance presented by the passive antenna port, the input port of the amplifier being coupled to the passive antenna port, the reference node being coupled to the second end of the third element; and • an active antenna output access, the active antenna output access being coupled to the amplifier output access. Active antenna according to claim 1, further comprising a part called “base” (6), the base being a conductive box, the box containing the amplifier, the base providing electromagnetic shielding of the amplifier.

3. Active antenna according to claim 2, wherein the second end of the third element is directly coupled to the base.

4. Active antenna according to any one of claims 1 to 3, wherein the first element is a part of a single-turn frame (1), and the second element is a part of the single-turn frame.

5. Active antenna according to any one of claims 1 to 4, wherein the known frequency band has an upper bound, the upper bound of the known frequency band corresponding to a wavelength in vacuum, a sum of a length of the centerline of the first element and a length of the centerline of the second element being less than one-quarter of said wavelength in vacuum.

6. Active antenna for radio reception in a known frequency band, the active antenna comprising: • a first element (11), the first element being an electrical conductor, the first element having a first end (111) and a second end (112), a centerline of the first element extending from the first end of the first element to the second end of the first element; • a second element (12), the second element being an electrical conductor, the second element having a first end (121) and a second end (122), a centerline of the second element extending from the first end of the second element to the second end of the second element, a reference length being equal to an upper bound of the distance between any point on the centerline of the first element and any point on the centerline of the second element;• a third element (3), the third element being an electrical conductor, the third element having a first end (31) and a second end (32), a central line of the third element extending from the first end of the third element to the second end of the third element, the first end of the third element being directly coupled to the first; end of first element and at the first end of second element, a length of the centerline of third element being greater than or equal to one third of the reference length, a line S and a plane P being such that plane P contains line S, such that the centerline of the third element is substantially straight and is substantially a segment of line S, such that the centerline of the first element is substantially included in plane P, such that the centerline of the second element is substantially included in plane P, and such that the centerline of the second element is substantially the image of the centerline of the first element by a rotation of 180 degrees around line S; a transformer having a primary, a secondary and a ferrite core, the primary having a first terminal and a second terminal, the secondary having a first terminal and a second terminal, the first terminal of the primary being coupled to the second end of the first element, the second terminal of the primary being coupled to the second end of the second element, a passive antenna comprising the first element, the second element, the third element and the transformer, an access of the passive antenna being made up of the first terminal of the secondary and the second terminal of the secondary; an amplifier having an input access, an output access and a reference node, the input access of the amplifier presenting, at any frequency in the known frequency band, an impedance having a magnitude that is less than a magnitude of an impedance presented by the access of the passive antenna, the input access of the amplifier being coupled to the access of the passive antenna, the reference node being coupled to the second end of the third element; And an output access of the active antenna, the output access of the active antenna being coupled to the output access of the amplifier.

7. Active antenna according to claim 6, further comprising a part called “base” (6), the base being a conductive box, the box containing the amplifier, the base providing electromagnetic shielding of the amplifier.

8. Active antenna according to claim 7, wherein the second end of the third element is directly coupled to the base.

9. Active antenna according to any one of claims 6 to 8, wherein the first element is a part of a single-turn frame (1), and the second element is a part of the single-turn frame.

10. Active antenna according to any one of claims 6 to 9, wherein the known frequency band has an upper bound, the upper bound of the known frequency band corresponding to a wavelength in vacuum, a sum of a centerline length of the first element and a centerline length of the second element being less than one-quarter of said wavelength in vacuum.