Active antenna comprising a single-turn loop aerial

Abstract

The invention relates to an active antenna comprising a single-turn loop aerial, for instance an antenna for radio communications and / or electromagnetic field measurements. An active antenna of the invention comprises: a first element (11), the first element being an electric conductor; a second element (12), the second element being an electric conductor; a third element (3), the third element being an electric 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.

Description

[0001] Active antenna comprising a single-turn loop aerial

[0002] FIELD OF THE INVENTION

[0003] The invention relates to an active antenna comprising a single-tum loop aerial, for instance an antenna for radio communications and / or electromagnetic field measurements.

[0004] The French patent application No. FR2401352 of 12 February 2024, entitled “Antenne active comportant un cadre monospire” is incorporated by reference.

[0005] PRIOR ART

[0006] In what follows, “coupled” always refers to an electrical coupling. When applied to two items such as terminals, conductors, nodes, etc, “coupled” may indicate that the items are directly coupled, that is to say connected (or, equivalently, in electrical contact) to one another, and / or that the items are indirectly coupled, in which case an electrical interaction different from direct coupling exists between the items, for instance through one or more components. When applied to two multi-terminal items, such as ports, connectors, etc, “coupled” may indicate that the items are directly coupled, in which case each terminal of one of the items is directly coupled to one and only one of the terminals of the other item, and / or that the items are indirectly coupled, in which case an electrical interaction different from direct coupling exists between the terminals of the items, for instance through one or more components. In what follows, in line with circuit theory, a port has exactly two terminals or nodes.

[0007] Loop aerials, more commonly referred to as “loop antennas”, and screened loop aerials, more commonly referred to as “shielded loop antennas”, are well known to specialists. They are used for radio reception applied to electromagnetic field measurements, to direction finding and to radio communications. Some characteristics and limitations of these antennas for these uses are explained in the article of F. Broyde 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 of F. Broyde 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 January2023 (available 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 an incident electromagnetic field. As explained in these articles and in paragraph 5-4 of chapter 5 of the book of R.C. Johnson entitled “Antenna Engineering Handbook, 3rd Edition”, published by McGraw-Hill in 1993, the screen (also referred to as “shield”) of a screened loop aerial typically operates as a loop aerial. Screened loop aerials used for radio reception provide better results than unscreened loop aerials, because they are not affected by a common-mode current flowing on a cable linking the antenna to a measuring instrument or a radio receiver, induced by an incident electromagnetic field received as a signal, or by electromagnetic disturbances.

[0008] One uses “antenna factor” to designate an absolute value 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 across a specified impedance . F or electromagnetic field measurements, one sometimes prefers, when it is possible, to use an antenna presenting an antenna factor that is substantially proportional to the inverse of the frequency, over a wide frequency band. The specialist knows that this result may be obtained, in a known frequency band, the known frequency band having a least upper bound, the least upper bound corresponding to a wavelength in vacuum, by utilizing, for radio reception, a prior art active antenna comprising a screened loop aerial, the screened loop aerial being a single-turn screened loop aerial, the screened loop aerial being small compared to said wavelength in vacuum.

[0009] An example of such a prior art active antenna, which includes a screened loop aerial, is shown in Fig. 1, where the hidden (that is to say, not directly visible) edges and the hidden outlines 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 screened loop aerial (5), a port of the screened loop aerial being the second end of the transmission line; a second element (52), the second element being an electric 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 screened loop aerial; a part referred to as “base” (6), the base providing an electric contact between the second end of the outer conductor and the second end of the second element, the base being a part of the screened loop aerial; 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 presenting, at any frequency in the known frequency band, an admittance having an absolute value that 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 and a voltage gain, an absolute value 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 screened loop aerial; and an output port of the active antenna, the output port of the active antenna being coupled to the output port of the amplifier.

[0010] If we compare a first active antenna consisting of such a single-turn screened loop aerial and such an amplifier, to a second active antenna consisting of a single-tum loop aerial having the same size as the screened loop aerial, and of the same amplifier as the one used in the first active antenna, we find that, as explained above, the immunity to a common-mode current flowing on the cable linking the antenna to a measuring instrument 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.

[0011] Figure 2 shows a plot of an absolute value of an effective length, expressed in meters, of the screened loop aerial of the active antenna shown in Fig. 1, as a function of the frequency. This absolute value of this effective length is substantially proportional to the frequency up to about 9 MHz. The person skilled in the art understands that, since said admittance has an absolute value that is less than one millisiemens, said input voltage of the amplifier is substantially equal to the product of the effective length of the screened loop aerial and an intensity of an incident electric field expressed in V / m. Said absolute value of the voltage gain being, in the known frequency band, substantially independent of the frequency, the person skilled in the art understands that the antenna factor is substantially proportional to the inverse of the frequency up to about 9 MHz. A least upper bound of the frequency band over which the antenna factor is substantially proportional to the inverse of the frequency is therefore close to 9 MHz.

[0012] Unfortunately, this least upper bound of the frequency band over 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 this reduction, it is necessary to reduce the size of the screened loop aerial, which entails, in the known frequency band, an increase of the product of the antenna factor and the noise voltage density at the output port of the active antenna (this density being expressed in r.m.s. volt per square root of hertz), whereas the designer wishes that this product is as small as possible at these frequencies.

[0013] Thus, the prior art does not disclose an active antenna having a good immunity to a common-mode current flowing on the cable linking the antenna to a measuring instrument 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 proportional to the inverse of the frequency in a known frequency band having a least upper bound and a greatest lower bound that is less than one tenth of the least upper bound, the active antenna being such that, in the known frequency band, a product of its antenna factor and the noise voltage density at the output port of the active antenna is smaller than the one which would be obtained using a prior-art active antenna comprising a screened loop aerial and having an antenna factor that is substantially proportional to the inverse of the frequency in said known frequency band. SUMMARY OF THE INVENTION

[0014] The purpose of the invention is an active antenna comprising a single-turn loop aerial, without the above-mentioned limitations of known techniques.

[0015] In what follows, in line with the “Dictionnaire CEI multilingue de 1’electricite - IEC multilingual dictionary of electricity” published by the “Bureau Central de la Commission Electrotechnique Internationale” in 1983, the meaning of “differential input” is: an 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 what follows, an amplifier having a differential input is called “differential amplifier”, and the two sets of input terminals are called “positive input port” and “negative input port”, respectively.

[0016] In what follows, “electric conductor” refers to a component intended to carry electric conduction current (referred to as “conductor” in said “Dictionnaire CEI multilingue de 1’electricite - IEC multilingual dictionary of electricity”), such that a d.c. current can easily flow between any two points of the electric conductor. Thus, it is not possible to consider that an electric conductor is made of several conducting parts that are insulated from one another.

[0017] An active antenna of 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 electric conductor, the first element having a first end 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, the first element being a part of a single-turn loop aerial; a second element, the second element being an electric conductor, the second element having a first end 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, the second element being a part of the single-tum loop aerial, a reference length being equal to a least upper bound of the distance between any point of the center line of the first element and any point of the center line of the second element, an accuracy parameter being a nonnegative length less than or equal to one tenth of the reference length; a third element, the third element being an electric 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 lying in the center line of the third element, a plane orthogonal to the straight line S and passing through 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 the points C and D being less than the accuracy parameter, a distance between the point A and the plane P being less than the accuracy parameter, a distance between the point B and the plane P being less than the accuracy parameter, an absolute value of the difference between a first distance and a second distance being less than the accuracy 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 differential amplifier having a positive input port, a negative input port, an output port and a reference node (ground), 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 presenting, at any frequency in the known frequency band, an admittance matrix having two rows and two columns, each entry of the admittance matrix having an absolute value that 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.

[0018] In the previous sentence, “line” always means an abstract mathematical concept (as opposed to a material object such as an electric wire or a transmission line), the center line 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 center line 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 center line 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. Likewise, the point A, the point B, the point C, the point D, the straight line S and the plane P are abstract mathematical concepts. The straight line S is infinite. The plane P is infinite.

[0019] In the above sentence that defines the active antenna of the invention, the distance between the point A and the plane P is of course the greatest lower bound of the distance between the point A and an arbitrary point lying in the plane P, and the distance between the point B and the plane P is of course the greatest lower bound of the distance between the point B and an arbitrary point lying in the plane P.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Other advantages and characteristics will appear more clearly from the following description of particular embodiments of the invention, given by way of non-limiting examples, with reference to the accompanying drawings in which: Figure 1 is a drawing of an active antenna of the prior art;

[0022] Figure 2 is a plot of the absolute value of the effective length of the screened loop aerial of the active antenna shown in Fig. 1, as a function of the frequency;

[0023] Figure 3 is a drawing of a part of the active antenna of the first embodiment;

[0024] Figure 4 is a front view of the active antenna of the first embodiment;

[0025] Figure 5 is a top view of the active antenna of the first embodiment;

[0026] Figure 6 is a left-side view of the active antenna of the first embodiment;

[0027] Figure 7 is a front view of the active antenna of the second embodiment;

[0028] Figure 8 is a top view of the active antenna of the second embodiment;

[0029] Figure 9 is a plot of the absolute value of the effective length of the loop aerial of the active antenna of the second embodiment, as a function of the frequency.

[0030] DETAILED DESCRIPTION OF SOME EMBODIMENTS

[0031] First embodiment.

[0032] As a first embodiment of a device of the invention, given by way of non-limiting example, we have represented in Figures 3 to 6 some drawings of an active antenna of the invention for radio reception in a known frequency band, the known frequency band being the band 100 kHz to 80 MHz, the known frequency band having a least upper bound equal to 80 MHz, and a greatest lower bound that is less than one tenth of the least upper bound, the active antenna comprising: a first element (11), the first element being an electric 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 electric 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 a least upper bound of the distance between any point of the center line of the first element and any point of the center line of the second element; a third element (3), the third element being an electric 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 contained in the plane P, such that the center line of the second element is substantially contained 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 under a rotation of 180 degrees about the straight line S; a part referred to as “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 presenting, at any frequency in the known frequency band, an admittance matrix having two rows and two columns, each entry of the admittance matrix having an absolute value that 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.

[0033] Figure 3 only shows the first element (11), the second element (12), and the third element (3). Figures 4 to 6 show the whole active antenna. In figures 3 to 6, the hidden edges and the hidden outlines are not shown, except the hidden outline of the third element in Figure 5. Figure 4 is a front view of the active antenna, Figure 5 is a top view of the active antenna, and Figure 6 is a left-side view of the active antenna.

[0034] 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” entails that the first end of the second element is coupled to the first end of the first element.

[0035] We see in Figures 3 to 6 that, for any point C lying in the center line of the third element, a plane orthogonal to the straight line S and passing through 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. Consequently, the requirement according to which the straight line S and the plane P are “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 contained in the plane P, such that the center line of the second element is substantially contained 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 under a rotation of 180 degrees about the straight line S” entails that there exists an accuracy parameter which is a nonnegative length less than or equal to one thirtieth of the reference length, the accuracy parameter being such that a distance between the points C and D is less than the accuracy parameter, such that a distance between the point A and the plane P is less than the accuracy parameter, such that a distance between the point B and the plane P is less than the accuracy parameter, and such that an absolute value of the difference between a first distance and a second distance is less than the accuracy 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.

[0036] In this first embodiment, a cross-section of the first element is a rectangular and hollow electric conductor, and a cross-section of the second element is a rectangular and hollow electric conductor, substantially identical to the cross-section of the first element.

[0037] The base is hollow and conductive. The differential amplifier is installed inside the base. The second end of the third element is directly coupled to the base. A bushing (61) enables the first element to pass through a wall of the base, and provides an electrical insulation between the first element and the base. A bushing (62) enables the second element to pass through a wall of the base, and provides an electrical insulation between the second element and the base. Two connectors are attached to the base: a coaxial connector (71), which materializes the output port of the active antenna; and a power supply connector (72) allowing to power feed the differential amplifier.

[0038] The person skilled in the art sees that the first element and the second element are parts of a single-turn polygonal winding, the winding being used as a single-turn loop aerial (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. Consequently, the first element is a part of this single-turn loop aerial, and the second element is a part of this single-tum loop aerial.

[0039] The person skilled in the art 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, entails that the active antenna can be proportioned 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 and the noise voltage density at the output port of the active antenna is smaller than the one which would be obtained using a prior-art active antenna comprising a screened loop aerial and having an antenna factor that is substantially proportional to the inverse of the frequency in said known frequency band.

[0040] If a common-mode current flows on a cable connected to said coaxial connector (for instance a cable linking the active antenna to a measuring instrument or a radio receiver) and / or on a cable connected to said power supply connector, it may continue on the third element and at any point of the single-tum loop aerial, without meeting a lumped impedance. The person skilled in the art understands that the fact that the center line of the second element is substantially the image of the center line of the first element under a rotation of 180 degrees about the straight line S entails that this common-mode current induces 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 person skilled in the art understands that a resulting common-mode voltage has an absolute value which is much less than the one which would appear if the third element was not present, so that said resulting common-mode voltage is effectively rejected by the differential amplifier. Consequently, said common-mode current is substantially without effect on a signal at the output port of the active antenna. This is why the active antenna has a good immunity to a common-mode current flowing on the cable linking the antenna to a measuring instrument or a radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic disturbances.

[0041] Consequently, the active antenna has a good immunity to a common-mode current flowing on the cable linking the antenna to a measuring instrument 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 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 and the noise voltage density at the output port of the active antenna is smaller than the one which would be obtained using a prior-art active antenna comprising a screened loop aerial and having an antenna factor that is substantially proportional to the inverse of the frequency in said known frequency band.

[0042] We observe that the prior art includes a two-tum polygonal winding having a grounded center tap, the two-tum polygonal winding being used as a multi-turn loop aerial. This device is for instance described in Fig. 105 of the book of R. Keen 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 the book of D. S. Bond entitled Radio Direction Finders, published by McGraw-Hill Book Company in 1944. This device is completely different from the combination of the first element, second element and third element of the invention. The person skilled in the art 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 single-turn winding, thereby dramatically reducing the frequency band over which an active antenna using this device could have an antenna factor substantially proportional to the inverse of the frequency.

[0043] Second embodiment (best mode).

[0044] As a second embodiment of a device of the invention, givenby way of non-limiting example and best mode of carrying out the invention, we have represented in Figures 7 and 8 some drawings of an active antenna of the invention for radio reception in a known frequency band, the known frequency band being the band 9 kHz to 40 MHz, the known frequency band therefore having a least upper bound equal to 40 MHz, and a greatest lower bound that is less than one tenth of the least upper bound, the active antenna comprising: a first element (11), the first element being an electric 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 electric 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 least upper bound of the distance between any point of the center line of the first element and any point of the center line of the second element, an accuracy parameter being a nonnegative length less than or equal to one tenth of the reference length; a third element (3), the third element being an electric 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 lying in the center line of the third element, a plane orthogonal to the straight line S and passing through 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 accuracy parameter, a distance between the point A and the plane P being less than the accuracy parameter, a distance between the point B and the plane P being less than the accuracy parameter, an absolute value of the difference between a first distance and a second distance being less than the accuracy 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 presenting, at any frequency in the known frequency band, an admittance matrix having two rows and two columns, each entry of the admittance matrix having an absolute value that 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 output port of the active antenna, the output port of the active antenna being directly coupled to the output port of the differential amplifier.

[0045] Figures 7 and 8 show the whole active antenna. In figures 7 and 8, the hidden edges and the hidden outlines are not shown, except the hidden outline of the third element in Figure 8. Figure 7 is a front view of the active antenna, and Figure 8 is a top view of the active antenna.

[0046] 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” entails that the first end of the second element is coupled to the first end of the first element.

[0047] The person skilled in the art sees that the first element and the second element are parts of a single-turn circular winding, the winding being used as a single -turn loop aerial (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. Consequently, the first element is a part of this single-turn loop aerial, and the second element is a part of this single-tum loop aerial.

[0048] The base comprises a fastening ring (65). The fastening ring is used to fasten the active antenna to an hollow antenna mast (66). The base is a conductive box, the box containing the differential amplifier. This box may for instance be a metallic box. Thus, the base may provide a shielding of the differential amplifier, that is to say an electromagnetic screening of the differential amplifier. A bushing (61) provides an electrical insulation between the first element and the base. A bushing (62) provides an electrical insulation between the second element and the base. The differential amplifier is power fed by a battery or a rechargeable battery.

[0049] 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.

[0050] A coaxial connector, which materializes the output port of the active antenna, is attached to the bottom of the base and hidden by the fastening ring. This coaxial connector is intended to be connected to a coaxial cable installed inside the hollow antenna mast. The person skilled in the art understands that this configuration improves the immunity of the active antenna to a common-mode current flowing on the cable linking the antenna to a measuring instrument or a radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic disturbances.

[0051] The accuracy 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. Moreover, the center line of the first element is substantially contained in the plane P, the center line of the second element is substantially contained in the plane P, and the center line of the second element is substantially the image of the center line of the first element under a rotation of 180 degrees about the straight line S. If a common-mode current flows on a cable connected to said coaxial connector (for instance a cable linking the active antenna to a measuring instrument or a radio receiver), this common-mode current may also flow on the third element and at any point of the single-turn loop aerial, without meeting a lumped impedance, so that it only induces a small common-mode voltage at the input ports of the differential amplifier. This small common-mode voltage is effectively rej ected by the differential amplifier. Consequently, the active antenna has a good immunity to a common-mode current flowing on the cable linking the antenna to a measuring instrument or a radio receiver, induced by an incident electromagnetic field received as a signal or by electromagnetic disturbances.

[0052] The least upper bound of the known frequency band corresponds to a wavelength in vacuum, equal to the velocity of light in vacuum divided by the least upper bound 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 loop aerial is consequently electrically small.

[0053] The first element has a circular cross-section. The first element may for instance comprise a rigid metallic bar, the rigid metallic bar being bent. The first element may for instance comprise a rigid metallic tube, the rigid metallic tube being bent. The second element has a circular cross-section. The second element may for instance comprise a rigid metallic bar, of the same diameter as the cross-section of the first element, the rigid metallic bar being bent. The second element may for instance comprise a rigid metallic tube, of the same diameter as the cross-section of the first element, the rigid metallic tube being bent.

[0054] Figure 9 shows a plot of an absolute value of an effective length, expressed in meters, of the single-turn loop aerial, as a function of the frequency. This absolute value of this effective length is substantially proportional to the frequency up to about 40 MHz. The person skilled in the art understands that, since each entry of said admittance matrix has an absolute value 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 screened loop aerial and an intensity of an incident electric field expressed in V / m. An absolute value 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 person skilled in the art understands that an antenna factor of the active antenna is substantially proportional to the inverse of the frequency up to about 40 MHz.

[0055] The screened loop aerial of the prior-art active antenna shown in Fig. 1 has the same dimensions as the single-turn loop aerial of the active antenna shown in Fig. 7, so that the person skilled in the art understands why the curve of Fig. 2 and the curve of Fig. 9 show absolute values of effective lengths that are substantially equal from 10 kHz to 9 MHz. The person skilled in the art understands that, to obtain an antenna factor that is substantially proportional to the inverse of the frequency up to about 40 MHz by utilizing a screened loop aerial as in the active antenna shown in Fig. 1, it would be necessary to reduce the size of the screened loop aerial, until an absolute value of its effective length become substantially proportional to the frequency up to about 40 MHz, which would entail that this absolute value of this effective length would be, from 10 kHz to 40 MHz, much smaller than the one shown in Fig. 9. Consequently, a product of the antenna factor and the noise voltage density at the output port of the active antenna comprising a screened loop aerial having a reduced size would be much greater than a product of the antenna factor and the noise voltage density at the output port of the active antenna of this second embodiment.

[0056] Consequently, the active antenna of this second embodiment has a good immunity to a common-mode current flowing on the cable linking the antenna to a measuring instrument 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 proportional to the inverse of the frequency in the known frequency band having a least upper bound and a greatest lower bound that is less than one tenth of the least upper bound, and this active antenna is such that, in the known frequency band, a product of its antenna factor and the noise voltage density at the output port of the active antenna is smaller than the one which would be obtained using a prior-art active antenna comprising a screened loop aerial and having an antenna factor that is substantially proportional to the inverse of the frequency in said known frequency band.

[0057] INDICATIONS ON INDUSTRIAL APPLICATIONS

[0058] The output port of the active antenna of the invention may for instance be connected to an end of a feeder (i.e., feed line), the feeder having another end which is coupled to a radio communication receiver, a measuring receiver, or a spectrum analyzer. The active antenna of the invention is particularly suitable for radio communications and direction finding. The active antenna of the invention is particularly suitable for electromagnetic field measurements, for instance in the technical area of electromagnetic compatibility (EMC).

[0059] The active antenna of the invention may also comprise other devices, for instance a device which indicates when the level of the signals applied to the differential amplifier produces, or could produce, a non-linear behavior leading for instance to saturation or overload at the output.

Claims

CLAIMS1. An active antenna for radio reception in a known frequency band, the active antenna comprising: a first element (11), the first element being an electric 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 electric 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 a least upper bound of the distance between any point of the center line of the first element and any point of the center line of the second element, an accuracy parameter being a nonnegative length less than or equal to one tenth of the reference length; a third element (3), the third element being an electric 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 lying in the center line of the third element, a plane orthogonal to the straight line S and passing through 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 the points C and D being less than the accuracy parameter, a distance between the point A and the plane P being less than the accuracy parameter, a distance between the point B and the plane P being less than the accuracy parameter, an absolute value of the difference between a first distance and a second distance being less than the accuracy 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 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 presenting, at any frequency in the known frequency band, an admittance matrix having two rows and two columns, each entry of the admittance matrix having an absolute value that is less than one millisiemens, the positive input terminal beingdirectly 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. The active antenna of claim 1, further comprising a part referred to as “base” (6), the base being a conductive box, the box containing the differential amplifier.

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

4. The active antenna of any one of the claims 2 or 3, wherein the base provides an electromagnetic screening of the differential amplifier.

5. The active antenna of any one of the claims 1 to 4, wherein the first element is a part of a single-turn loop aerial (1), and the second element is a part of the single-turn loop aerial.

6. The active antenna of any one of the claims 1 to 5, wherein the known frequency band has a least upper bound, the least 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.

7. An active antenna for radio reception in a known frequency band, the active antenna comprising: a first element (11), the first element being an electric 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 electric 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 a least upper bound of the distance between any point of the center line of the first element and any point of the center line of the second element; a third element (3), the third element being an electric 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 aplane 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 contained in the plane P, such that the center line of the second element is substantially contained 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 under a rotation of 180 degrees about the straight 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 presenting, at any frequency in the known frequency band, an admittance matrix having two rows and two columns, each entry of the admittance matrix having an absolute value that 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.

8. The active antenna of claim 7, further comprising a part referred to as “base” (6), the base being a conductive box, the box containing the differential amplifier.

9. The active antenna of claim 8, wherein the second end of the third element is directly coupled to the base.

10. The active antenna of any one of the claims 8 or 9, wherein the base provides an electromagnetic screening of the differential amplifier.

11. The active antenna of any one of the claims 7 to 10, wherein the first element is a part of a single-turn loop aerial (1), and the second element is a part of the single-turn loop aerial.

12. The active antenna of any one of the claims 7 to 11, wherein the known frequency band has a least upper bound, the least 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.

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