Active antenna with a single-turn loop

Abstract

The invention relates to an active antenna comprising 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; and a differential amplifier having a positive input terminal coupled to a second end of the first element, a negative input terminal coupled to a second end of the second element, and a reference node coupled to a second end of the third element, an output port of the active antenna being coupled to an output of the differential amplifier.Figure for abstract: Figure 4.

Description

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

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

[0002] 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, 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 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, and / or that the entities are indirectly coupled, a different electrical interaction than 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, a port 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 at 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 to say a magnetic component of a field. incident electromagnetic field. As discussed in these articles and 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. Shielded loops used for radio reception perform better than unshielded loops because they are not affected by a common-mode current flowing on a cable connecting the antenna to a measuring device or radio receiver, induced by an incident electromagnetic field received as a signal, or by electromagnetic disturbances.

[0004] An antenna factor is a module of a ratio of an intensity of an incident field (expressed in V / m for an electric field or in A / m for a magnetic field) to a voltage developed by an antenna at the terminals of a specified impedance. For measurements of electromagnetic fields, it is sometimes preferred, when possible, to use an antenna having an antenna factor substantially proportional to the inverse of the frequency, over a wide frequency band. The specialist knows that this result can be obtained, in a known frequency band, the known frequency band having an upper limit, the upper limit corresponding to a wavelength in a vacuum, by using, for radio reception, an active antenna of the prior art comprising a shielded frame, the shielded frame being a single-turn shielded frame, the shielded frame being small compared to said wavelength in a vacuum.

[0005] An example of such a prior art active antenna, which includes a shielded frame, is shown in [Fig.l], where the hidden edges (i.e. not directly visible) and the hidden contours are not shown. The active antenna shown in [Fig.l] 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 of the shielded frame being the second end of the transmission line; • 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 inner conductor, the second element being a part of the shielded frame; • a part called “base” (6), the base providing electrical contact between the second end of the external conductor and the second end of the second element, the base being a part of the shielded frame; • an amplifier having an input port, an output port and a reference node (ground), the input port of the amplifier being composed of an input terminal and the reference node, the input port of the amplifier having, at any frequency in the known frequency band, an admittance having a modulus which is less than one millisiemens, an output voltage of the amplifier being, at a given frequency in the known frequency band, equal to the product of an input voltage of the amplifier by a voltage gain, a modulus of the voltage gain being substantially independent of the given frequency, the input port of the amplifier being directly coupled to the port of the shielded frame; and • an active antenna output port, the active antenna output port being coupled to the amplifier output port.

[0006] If we compare a first active antenna consisting of such a single-turn shielded frame and such an amplifier, with 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.

[0007] [Fig. 2] is a graph showing a modulus of an effective length (in English, “effective length”), expressed in meters, of the shielded loop of the active antenna of [Fig. 1], as a function of the frequency. This modulus of this effective length is substantially proportional to the frequency up to about 9 MHz. The specialist understands that, since said admittance has a modulus which is less than one millisiemens, said input voltage of the amplifier is substantially equal to the product of the effective length of the shielded loop by an intensity of an incident electric field expressed in V / m. Said modulus of the voltage gain being, in the known frequency band, substantially independent of the frequency, the specialist understands that the antenna factor is substantially proportional to the inverse of the frequency up to about 9 MHz.An upper bound of the frequency band in which the antenna factor is substantially proportional to the inverse of the frequency is therefore . close to 9 MHz.

[0008] Unfortunately, this upper limit of the frequency band in which the antenna factor is substantially proportional to the inverse of the frequency is much smaller 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, in the known frequency band, in an increase in the product of the antenna factor times the density of the noise voltage at the output port of the active antenna (this density being expressed in rms volts per square root of hertz), whereas the designer wishes this product to be as small as possible at these frequencies.

[0009] 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 which is substantially proportional to the inverse of the frequency in a known frequency band having an upper limit and a lower limit which is less than one tenth of the upper limit, the active antenna being such that, in the known frequency band,a product of its antenna factor times the noise voltage density at the output port of the active antenna is smaller than that which would be obtained using a prior art active antenna comprising a shielded frame and having an antenna factor which is substantially proportional to the inverse of the frequency in said known frequency band. Statement of the invention

[0010] The subject of the invention is an active antenna comprising a single-turn frame, free from the limitations mentioned above of known techniques.

[0011] In the following, in accordance with the “IEC multilingual dictionary of electricity” published by the “Central Bureau of the International Electrotechnical Commission” in 1983, the meaning of “differential input circuit” is: input circuit having two sets of input terminals intended to measure the difference between the values ​​of electrical quantities of the same nature applied to them. In the following, an amplifier having a differential input circuit is called a “differential amplifier”, and the two sets of input terminals are called “positive input access” and “negative input access”, respectively.

[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 member having a first end and a second end, a centerline of the first member extending from the first end of the first member to the second end of the first member, the first member being part of a single-spiral frame; a second member, the second member being an electrical conductor, the second member having a first end and a second end, a centerline of the second member extending from the first end of the second member to the second end of the second member, the second member being a portion 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 member and any point on the centerline of the second member, an accuracy 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 center 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 the first element and to the first end of the second element, a length of the center line of the third element being greater than or equal to one-third of the reference length, a straight line S and a plane P being such that the plane P contains the straight line S, and such that, for any point C belonging to the center line of the third element, a plane orthogonal to the straight line S and containing the point C intersects the center line of the first element at a point A, the center line of the second element at a point B, and the straight line 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 differential amplifier having a positive input port, a negative input port, an output port, and a reference node (ground), the positive input port consisting of a positive input terminal and the reference node, the negative input port consisting of a negative input terminal and the reference node, the positive input port, and the negative input port having, at any frequency in the known frequency band, an admittance matrix, the admittance matrix being square and of order 2, each element of the admittance matrix having a modulus which is less than one millisiemens, the positive input terminal being directly coupled to the second end of the first element, the negative input terminal being directly coupled to the second end of the second element, the reference node being coupled to the second end of the third element; and • an active antenna output port, the active antenna output port being coupled to the differential amplifier output port.

[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 having 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 having 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 having 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] Recall that a square matrix of order 2 is a matrix with 2 rows and 2 columns.

[0015] 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. 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 an active antenna of the prior art;

[0018] - [Fig.2] is a graph showing the modulus of the effective length of the frame shielded of the active antenna of [Fig.l], as a function of the frequency;

[0019] - [Fig.3] is a drawing of a part of the active antenna of the first mode of realization lization;

[0020] - [Fig.4] is a front view of the active antenna of the first embodiment;

[0021] - [Fig.5] is a top view of the active antenna of the first embodiment;

[0022] - [Fig.6] is a left view of the active antenna of the first mode of realization lization;

[0023] - [Fig.7] is a front view of the active antenna of the second embodiment;

[0024] - [Fig.8] is a top view of the active antenna of the second mode of realization lization;

[0025] - [Fig.9] is a graph showing the modulus of the effective length of the frame of the active antenna of the second embodiment, depending on the frequency.

[0026] DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

[0027] First embodiment.

[0028] As a first embodiment of a device according to the invention, given by way of non-limiting example, we have shown in Figures 3 to 6 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 center 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 center line 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 center line of the first element and any point on the center line 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 center 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 the first element and to the first end of the second element, a length of the center line of the third element being greater than or equal to one third of the reference length, a straight line S and a plane P being such that the plane P contains the straight line S, such that the center line of the third element is substantially rectilinear and is substantially a segment of the straight line S, such that the center line of the first element is substantially included in the plane P, such that the center line of the second element is substantially included in the plane P, and such that the center line of the second element is substantially the image of the center line of the first element by a rotation of 180 degrees around the straight line S; • a part called “base” (6); • a differential amplifier having a positive input port, a negative input port, an output port and a reference node, the positive input port being composed of a positive input terminal and the reference node, the negative input port being composed of a negative input terminal and the reference node, the positive input port and the negative input port having, at any frequency in the known frequency band, an admittance matrix, the admittance matrix being square and of order 2, each element of the admittance matrix having a modulus which is less than one millisiemens, the positive input terminal being directly coupled to the second end of the first element, the negative input terminal being directly coupled to the second end of the second element, the reference node being coupled to the second end of the third element; and • an active antenna output port, the active antenna output port being coupled to the differential amplifier output port.

[0029] [Fig. 3] shows only the first element (11), the second element (12) and the third element (3). Figures 4 to 6 show the entire active antenna. In Figures 3 to 6, hidden edges and hidden contours are not shown, except for the hidden contour of the third element in [Fig. 5]. [Fig. 4] is a front view of the active antenna, [Fig. 5] is a top view of the active antenna, and [Fig. 6] is a left view of the active antenna.

[0030] The requirement “the first end of the third member being directly coupled to the first end of the first member and to the first end of the second member” implies that the first end of the second member is coupled to the first end of the first member.

[0031] We see in Figures 3 to 6 that, for any point C belonging to the center line of the third element, a plane orthogonal to the line S and containing the point C intersects the center line of the first element at a point A, the center line of the second element at a point B, and the line S at a single 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 center line of the third element is substantially rectilinear and is substantially a segment of the line S, such that the center line of the first element is substantially included in plane P, such that the center line of the second element is substantially included in plane P, and such that the center line of the second element is substantially the image of the center line of the first element by a rotation of 180 degrees about 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. ,

[0032] 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.

[0033] The base is hollow and conductive. The differential amplifier is installed in the base. The second end of the third element is directly coupled to the base. A feedthrough (61) serves to pass the first element through a wall of the base, and provides electrical insulation between the first element and the base. A feedthrough (62) serves to pass the second element through a wall of the base, and provides electrical insulation between the second element and the base. Two connectors are fixed on the base: a coaxial connector (71), which materializes the output access of the active antenna; and a power connector (72) for powering the differential amplifier.

[0034] The skilled person will see 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 the positive input terminal is directly coupled to the second end of the first element, and the negative input terminal is directly coupled to the second end of the second element. Therefore, the first element is a part of this single-turn frame, and the second element is a part of this single-turn frame.

[0035] The specialist understands that, the fact that the positive input terminal is directly coupled to the second end of the first element, and the negative input terminal is directly coupled to the second end of the second element, means that the active antenna can be dimensioned in such a way that its antenna factor is substantially proportional to the inverse of the frequency in the known frequency band, and that, in the known frequency band, a product of its antenna factor by the density of the noise voltage at the output port of the active antenna is smaller than that which would be obtained by using a prior art active antenna comprising a shielded frame and having an antenna factor which is substantially proportional to the inverse of the frequency in said known frequency band.

[0036] 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 to the third element and at any point of the single-turn frame, without encountering any localized impedance. The specialist understands that the fact that the center line of 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 causes this common mode current to induce a first complex voltage between the positive input terminal and the reference node (ground), and a second complex voltage between the negative input terminal 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 modulus that is 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 differential amplifier. Therefore, said common mode current has substantially no effect on a signal at the output port of the active antenna. Therefore, 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.

[0037] Therefore, 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 which is substantially proportional to the inverse of the frequency in the known frequency band, and the active antenna is such that, in the known frequency band, a product of its antenna factor times the noise voltage density at the output port of the active antenna is smaller than that which would be obtained by using a prior art active antenna comprising a shielded frame and having an antenna factor which is substantially proportional to the inverse of the frequency in said known frequency band.

[0038] We observe that the prior art includes a polygonal winding having two turns and a grounded center tap, the polygonal winding having two turns being used as a multi-turn frame. This arrangement is for example described in Figure 105 of R. Keen's book “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 “Radio Direction Finders”, published by McGraw-Hill Book Company in 1944. This device is completely different from the combination of the first element, the second element and the third element according to the invention. The skilled person will understand 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 a single turn, thereby dramatically reducing the frequency band in which an active antenna using this device could have an antenna factor substantially proportional to the inverse of the frequency.

[0039] Second embodiment.

[0040] As a second embodiment of a device according to the invention, given by way of non-limiting example, we have shown in [Fig.7] and [Fig.8] drawings of an active antenna according to the invention for radio reception in a known frequency band, the known frequency band being the 9 kHz to 40 MHz band, the known frequency band therefore having an upper limit equal to 40 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 center 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 center line 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 center line of the first element and any point on the center line 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 straight line S and a plane P being such that the plane P contains the straight line S, and such that, for any point C belonging to the center line of the third element, a plane orthogonal to the straight line S and containing the point C intersects the center line of the first element at a single point A, the center line of the second element at a single point B, and the straight line S at a single point D, a distance between the points C and D being less than the precision parameter, a distance between the point A and the plane P being less than the precision parameter, a distance between the point B and the 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 the points A and D, and the second distance being a distance between the points B and D; • a base (6), the second end of the third element being directly coupled to the base; • a differential amplifier having a positive input port, a negative input port, an output port and a reference node, the positive input port being composed of a positive input terminal and the reference node, the negative input port being composed of a negative input terminal and the reference node, the positive input port and the negative input port having, at any frequency in the known frequency band, an admittance matrix, the admittance matrix being square and of order 2, each element of the admittance matrix having a modulus which is less than one millisiemens, the positive input terminal being directly coupled to the second end of the first element, the negative input terminal being directly coupled to the second end of the second element; and • an active antenna output port, the active antenna output port being directly coupled to the differential amplifier output port.

[0041] Figures 7 and 8 show the entire active antenna. In Figures 7 and 8, hidden edges and hidden contours are not shown, except for the hidden contour of the third element in [Fig. 8]. [Fig. 7] is a front view of the active antenna, and [Fig. 8] is a top view of the active antenna.

[0042] The requirement “the first end of the third member being directly coupled to the first end of the first member and to the first end of the second member” implies that the first end of the second member is coupled to the first end of the first member.

[0043] The skilled person will see 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 the positive input terminal is directly coupled to the second end of the first element, and the negative input terminal is directly coupled to the second end of the second element. Therefore, the first element is a part of this single-turn frame, and the second element is a part of this single-turn frame.

[0044] The base comprises a fixing ring (65). The fixing ring is used to fix the active antenna on a hollow antenna mast (66). The base is a conductive box, the box containing the differential amplifier. This box can for example be a metal box. Thus, the base can provide shielding of the differential amplifier, that is to say electromagnetic screening of the differential amplifier. A bushing (61) provides electrical insulation between the first element and the base. A bushing (62) provides electrical insulation between the second element and the base. The differential amplifier is powered by a battery or a rechargeable accumulator.

[0045] 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.

[0046] A coaxial connector, which materializes the output access of the active antenna, is fixed to the lower part of the base and hidden by the fixing ring. This coaxial connector is intended to be connected to a coaxial cable installed inside the hollow antenna mast. The specialist understands that this configuration improves the immunity of the active antenna 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.

[0047] The precision parameter is less than or equal to one thirtieth of the reference length, and the center line of the third element is substantially rectilinear and is substantially a segment of the straight line S. In addition, the center line of the first element is substantially included in the plane P, the center line of the second element is substantially included in the plane P, and the center line of the second element is substantially the image of the center line of the first element by a rotation of 180 degrees around the straight 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 can also flow on the third element and at any point of the single-turn frame, without encountering any localized impedance, so that it induces only a low common mode voltage at the input ports of the differential amplifier.This low common mode voltage is effectively rejected by the differential amplifier. Therefore, the active antenna has good immunity to a common mode current flowing on 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 disturbances.

[0048] The upper limit of the known frequency band corresponds to a wavelength in vacuum, equal to the velocity of light in vacuum divided by the upper limit of the known frequency band. A sum of a length of the center line of the first element and a length of the center line of the second element is less than a quarter of said wavelength in vacuum. Said single-turn frame is therefore electrically small.

[0049] 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.

[0050] [Fig.9] is a graph showing a modulus of an effective length, expressed in meters, of the single-turn frame, as a function of frequency. This modulus of this effective length is substantially proportional to the frequency up to about 40 MHz. The specialist understands that, since each element of said admittance matrix has a modulus which is less than one millisiemens at any frequency in the known frequency band, a differential voltage at the input ports of the differential amplifier is substantially equal to the product of the effective length of the single-turn frame by an intensity of an incident electric field expressed in V / m.A module of the voltage gain of the differential amplifier (for a differential voltage at the input ports of the differential amplifier) ​​being, in the known frequency band, substantially independent of the frequency, the specialist understands that an antenna factor of the active antenna is substantially proportional to the inverse of the frequency up to about 40 MHz.

[0051] 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. 7], so that the skilled person understands why the curve of [Fig. 2] and the curve of [Fig. 9] show effective length modules that are substantially equal from 10 kHz up to 9 MHz. The skilled person understands that, in order to obtain an antenna factor substantially proportional to the inverse of the frequency up to about 40 MHz using a shielded frame as in the active antenna shown in [Fig. in [Fig.l], the size of the shielded frame would have to be reduced, until a module of its effective length becomes substantially proportional to the frequency up to about 40 MHz, which would result in this module of this effective length being, from 10 kHz up to 40 MHz, much smaller than that shown in [Fig.9]. Therefore, a product of the antenna factor times the noise voltage density at the output port of the active antenna having a reduced-size shielded frame would be much larger than a product of the antenna factor times the noise voltage density at the output port of the active antenna of this second embodiment.

[0052] Therefore, the active antenna of this second 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 which is substantially proportional to the inverse of the frequency in the known frequency band having an upper limit and a lower limit which is less than one tenth of the upper limit, and this active antenna is such that, in the known frequency band,a product of its antenna factor times the noise voltage density at the output port of the active antenna is smaller than that which would be obtained using a prior art active antenna comprising a shielded frame and having an antenna factor which is substantially proportional to the inverse of the frequency in said known frequency band.

[0053] INDICATIONS ON INDUSTRIAL APPLICATIONS

[0054] The output of the active antenna according to the invention may for example be connected to one end of an antenna link, the antenna link having another end which 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).

[0055] The active antenna according to the invention may also comprise other devices, for example a device indicating when the level of the signals applied to the differential amplifier produces, or risks producing, non-linear operation, leading for example to saturation or overload at the output.

Claims

1. Claims 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 center 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 center line 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 center line of the first element and any point on the center line 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 center 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 the first element and to the first end of the second element, a length of the center line of the third element being greater than or equal to one third of the reference length, a straight line S and a plane P being such that the plane P contains the straight line S, and such that, for any point C belonging to the center line of the third element, a plane orthogonal to the straight line S and containing the point C intersects the center line of the first element at a point A, the center line of the second element at a point B, and the straight line S at a single point D,a distance between points C and D being less than the parameter, of precision, 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 differential amplifier having a positive input port, a negative input port, an output port and a reference node, the positive input port being composed of a positive input terminal and the reference node, the negative input port being composed of a negative input terminal and the reference node, the positive input port and the negative input port having, at any frequency in the known frequency band, an admittance matrix, the admittance matrix being square and of order 2, each element of the admittance matrix having a modulus which is less than one millisiemens, the positive input terminal being directly coupled to the second end of the first element, the negative input terminal being directly coupled to the second end of the second element, the reference node being coupled to the second end of the third element;and • an output port of the active antenna, the output port of the active antenna being coupled to the output port of the differential amplifier.;

2. An active antenna according to claim 1, further comprising a portion called a “base” (6), the base being a conductive box, the box containing the differential amplifier, the base providing electromagnetic screening of the differential amplifier.

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

4. An 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. An active antenna according to any one of claims 1 to 4, in

6. which 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 center line of the first element and a length of the center line of the second element being less than a quarter of said wavelength in vacuum. 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 center 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 center line 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 center line of the first element and any point on the center line 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 center 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 the first element and to the first end of the second element, a length of the center line of the third element being greater than or equal to one third of the reference length, a straight line S and a plane P being such that the plane P contains the straight line S, such that the center line of the third element is substantially rectilinear and is substantially a segment of the line S, such that the center line of the first element is substantially included in the plane P, such that the center line of the second element is substantially included in the plane P, and such that the center line of 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;• a differential amplifier having a positive input port, a negative input port, an output port and a reference node, the positive input port being composed of a positive input terminal and the reference node, the negative input port being composed of a negative input terminal and the reference node, the positive input port and the negative input port having, at any frequency in the known frequency band, an admittance matrix, the admittance matrix being square and of order 2, each element of the admittance matrix having a modulus which is less than one millisiemens, the positive input terminal being directly coupled to the second end of the first element, the negative input terminal being directly coupled to the second end of the second element, the reference node being coupled to the second end of the third element;and • an output port of the active antenna, the output port of the active antenna being coupled to the output port of the differential amplifier.;

7. An active antenna according to claim 6, further comprising a portion called a “base” (6), the base being a conductive box, the box containing the differential amplifier, the base providing electromagnetic screening of the differential amplifier.

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

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

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