FIELD OF THE INVENTION

The invention relates to a passive antenna comprising a screened loop aerial, for instance an antenna for producing a known radio frequency magnetic field in a laboratory.

The French patent application No. FR2206759 of 4 July 2022, entitled “Antenne passive incluant un cadre blinde” is incorporated by reference.

PRIOR ART

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 may be used to produce a known radio frequency magnetic field, for instance during electromagnetic immunity tests, or to calibrate a measuring antenna. As explained 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. To produce a known radio frequency magnetic field, a screened loop aerial provides better results than an unscreened loop aerial, because a properly utilized screened loop aerial is an antenna which does not produce a common-mode current flowing on the cable linking the antenna to a radio-frequency generator or a radio transmitter, induced by the electromagnetic field emitted by the antenna.

In what follows, we shall use “reference field level” to designate an absolute value of an intensity of a radio frequency magnetic field produced by an antenna, the radio frequency magnetic field being measured at a reference location with respect to the antenna, a generator being coupled to a port of the antenna, the generator producing a sinusoidal voltage at a given frequency, the generator having a known internal impedance at the given frequency, an opencircuit voltage of the generator being equal to two volts root mean square. The reference field level is expressed in A/m root mean square.

An example of a prior art passive antenna is shown in Fig. 1, where the hidden (that is to say, not directly visible) edges and the hidden outlines are not shown. The passive antenna shown in Fig. 1 comprises: a first element (1), the first element being a transmission line having an outer conductor (123) and an inner conductor (121), the transmission line having a first end (101) and a second end (102), 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 a screened loop aerial; a second element (2), the second element being an electric conductor, the second element having a first end (201) and a second end (202), the first end of the second element being directly 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” (3), 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; a port of the passive antenna, the port of the passive antenna having a first terminal and a second terminal, the first terminal of the port of the passive antenna being coupled to the second end of the inner conductor, the second terminal of the port of the passive antenna being coupled to the second end of the outer conductor.

Said screened loop aerial is a single-turn screened loop aerial, made of the first element, the second element and the base. The largest width of the passive antenna shown in Fig. 1 is about 133 mm. Figure 2 shows a plot of the absolute value of an impedance of this passive antenna, as a function of frequency. Figure 3 shows a plot of the reference field level of this passive antenna, as a function of frequency, for a reference location close to the passive antenna, the internal impedance of the generator being assumed to be equal to 50 ohms at any frequency. More precisely, the reference location used to obtain Fig. 3 is positioned on a reference axis of the passive antenna and at a distance of about 100 mm from a reference plane of the passive antenna, the reference axis being orthogonal to the reference plane.

For the calibration of some measuring antennas, it would be desirable to generate a known magnetic field by utilizing a passive antenna comprising a screened loop aerial and having a reference field level which is substantially independent of the frequency, over a wide frequency band, for a reference location close to the passive antenna. The specialist knows that this result maybe obtained by reducing the size of the screened loop aerial. However, this solution is often not acceptable because it entails that the magnetic field becomes less homogeneous in the vicinity of the passive antenna, and weaker far from the passive antenna.

SUMMARY OF THE INVENTION

The purpose of the invention is a simple and inexpensive passive antenna comprising a screened loop aerial, the passive antenna having a reference field level which, for a reference location close to the passive antenna, is substantially independent of the frequency at any frequency less than a least upper bound, the least upper bound being increased compared to the one which would be obtained using a prior-art passive antenna comprising a screened loop aerial of the same size.

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.

A passive antenna of the invention comprises: a first element, the first element comprising a “tube of the first element” and a transmission line, the tube of the first element being an electric conductor having a length, the transmission line having an outer conductor and an inner conductor, the transmission line having a first end, a second end and a length, 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, at least a portion of the length of the transmission line lying inside the tube of the first element, the outer conductor being, along a portion of the length of the tube of the first element, not in electric contact with the tube of the first element, the first end of the outer conductor being connected to the tube of the first element, the first element being a part of a screened loop aerial; a second element, the second element being an electric conductor, the second element having a first end and a second end, the second element being a part of the screened loop aerial; a single-port linear device having a first terminal and a second terminal, the first terminal of the single-port linear device being coupled to the first end of the second element, the second terminal of the single-port linear device being coupled to the first end of the inner conductor, the single-port linear device having a direct current resistance, the direct current resistance being greater than 10 ohms, the direct current resistance being less than 500 ohms; and a port of the passive antenna, the port of the passive antenna having a first terminal and a second terminal, the first terminal of the port of the passive antenna being coupled to the second end of the inner conductor, the second terminal of the port of the passive antenna being coupled to the second end of the outer conductor.

For instance, it is possible that the single-port linear device comprises a resistor, and it is possible that the single-port linear device is a resistor. For instance, it is possible that the singleport linear device comprises a resistor and a capacitor. For instance, it is possible that the transmission line has a characteristic impedance, the direct current resistance being greater than one-half of a real part of said characteristic impedance, the direct current resistance being less than two times said real part of said characteristic impedance. For instance, said characteristic impedance may be a nominal characteristic impedance. For instance, said characteristic impedance may be a characteristic impedance measured at 10 MHz, or at 200 MHz.

BRIEF DESCRIPTION OF THE DRAWINGS

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 a passive antenna of the prior art;

Figure 2 is a plot of the absolute value of an impedance of the passive antenna shown in Fig. 1, as a function of frequency;

Figure 3 is a plot of a reference field level of the passive antenna shown in Fig. 1, as a function of frequency;

Figure 4 is a drawing of a passive antenna of the invention (first embodiment);

Figure 5 is a drawing of a part of the passive antenna shown in Figure 4 (first embodiment);

Figure 6 is a drawing of a passive antenna of the invention (second embodiment);

Figure 7 is a second drawing of the passive antenna shown in Figure 6 (second embodiment);

Figure 8 is a third drawing of the passive antenna shown in Figure 6 (second embodiment);

Figure 9 is a plot of the absolute value of an impedance of the passive antenna shown in Fig. 6, as a function of frequency; and

Figure 10 is a plot of a reference field level of the passive antenna shown in Fig. 6, as a function of frequency.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

First embodiment.

As a first embodiment of a device of the invention, given by way of non-limiting example, we have represented in figures 4 and 5 some drawings of a passive antenna of the invention, the passive antenna comprising: a first element (1), the first element comprising a tube (11) and a transmission line (12), the tube being an electric conductor referred to as “the tube of the first element”, the transmission line having an outer conductor (123) and an inner conductor (121), the transmission line having a first end (101) and a second end, 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 tube of the first element having a first end at the first end of the transmission line, at least a portion of the length of the transmission line lying inside the tube of the first element, the outer conductor being, along a portion of the length of the tube of the first element, not in electric contact with the tube of the first element, the first end of the outer conductor being connected to the first end of the tube of the first element; a second element (2), the second element being an electric conductor, the second element having a first end (201) and a second end; a base (3), the base providing an electric contact between a second end of the tube of the first element and the second end of the second element; a single-port linear device (4) having a first terminal and a second terminal, the first terminal of the single-port linear device being directly coupled to the first end of the second element, the second terminal of the single-port linear device being directly coupled to the first end of the inner conductor; and a port of the passive antenna, the port of the passive antenna having a first terminal and a second terminal, the first terminal of the port of the passive antenna being coupled to the second end of the inner conductor, the second terminal of the port of the passive antenna being coupled to the second end of the outer conductor.

Figure 4 shows the whole passive antenna. In Figure 4, the hidden edges and the hidden outlines are not shown. Said at least a portion of the length of the transmission line may for instance be the entirety of the length of the transmission line.

In this first embodiment, a cross-section of the transmission line comprises the inner conductor, located inside a hollow dielectric, the hollow dielectric being located inside the outer conductor, which has a hollow cross-section. Optionally, the cross-section of the transmission line may also comprise an insulating jacket having a hollow cross-section, the outer conductor being located inside the insulating jacket. The characteristic impedance of this transmission line is close to 75 ohms at 100 MHz.

In this first embodiment, a cross-section of the tube of the first element is rectangular and hollow. The transmission line is fixed in the tube of the first element by utilizing insulating parts. The outer conductor is connected to the tube of the first element at the first end of the tube of the first element, and only at the first end of the tube of the first element.

In this first embodiment, a cross-section of the second element is a rectangular and hollow electric conductor, similar to the tube of the first element. Figure 5 shows a part of the passive antenna, in the vicinity of the first end of the transmission line and of the first end of the second element. In Figure 5, the hidden edges and the hidden outlines are shown. Figure 5 shows the inner conductor (121), the hollow dielectric (122), the outer conductor (123), and the optional insulating jacket (124). Figure 5 shows how the first terminal of the single-port linear device is directly coupled to the first end (201) of the second element, and how the second terminal of the single-port linear device is directly coupled to the first end of the inner conductor (121). Figure 5 shows connections (13) through which the first end of the outer conductor is coupled to the first end of the tube of the first element. Figure 5 shows that an impedance seen by the first end of the transmission line is affected by an impedance of the single-port linear device.

Figure 5 also shows an insulating part (15) used to support the transmission line (12) in the tube of the first element (11).

The specialist sees that the tube of the first element, the base and the second element form a single-turn polygonal winding, the winding being used as a loop aerial. The specialist also sees that the first element, the base and the second element form a prior-art screened loop aerial. The prior-art screened loop aerial is a single-turn screened loop aerial in which the inner conductor is shielded by two separate screens: the outer conductor and the tube of the first element. Thus, the prior-art screened loop aerial formed by the first element, the base and the second element is different from the one shown in Fig. 5-22(a) of paragraph 5-4 of chapter 5 of said book of R.C. Johnson, and from all the ones shown in Fig. 9 of the report of G.A. Morgan, Jr. entitled “Analysis and calibration of loop probes for use in measuring interference fields”, published by the Naval Research Laboratory as NRL Report R-3486, in June 1949. In contrast, the prior-art screened loop aerial formed by the first element, the base and the second element is similar to the “feed loop” which is a part of the “wide range tunable transmitting loop antenna” disclosed in the patent of the United States of America number 3,588,905. The prior-art screened loop aerial formed by the first element, the base and the second element is also similar to the “shielded loop antenna” described by Phillip Frost in an electronic document entitled “Is this shielded loop antenna balanced?”, dated 31 May 2013, and available in February 2020 from https ://electronics. stackexchange, com.

Consequently, the first element is a part of this prior-art screened loop aerial, the second element is a part of this prior-art screened loop aerial, and the base is a part of this prior-art screened loop aerial.

We note that neither said patent of the United States of America number 3,588,905 nor said electronic document entitled “Is this shielded loop antenna balanced?” mention a component having a first terminal and a second terminal, the first terminal of the component being coupled to the first end of the second element, the second terminal of the component being coupled to the first end of the inner conductor. Consequently, the specialist sees that the first element, the second element, the base and the single-port linear device form a new antenna, which may be viewed as a new screened loop aerial. Thus, the first element is a part of this new screened loop aerial, the second element is a part of this new screened loop aerial, and the base is a part of this new screened loop aerial.

The single-port linear device is a resistor having a value which is close to the characteristic impedance of the transmission line, for instance a 82-ohm resistor. Thus, the single-port linear device has a direct current resistance, the direct current resistance being greater than 10 ohms, the direct current resistance being less than 500 ohms.

A connector (not shown in Fig. 4) is fixed to the base. This connector is used as a port of the passive antenna. The base is hollow, and it contains means used to couple the first terminal of the port of the passive antenna to the second end of the inner conductor, and the second terminal of the port of the passive antenna to the second end of the outer conductor.

Experiments show that, for a reference location close to the passive antenna, said prior-art screened loop aerial has a reference field level which is substantially independent of the frequency up to about 25 MHz, whereas the passive antenna of this first embodiment has a reference field level which is substantially independent of the frequency up to about 70 MHz. Also, the difference between the cost of said prior-art screened loop aerial and the cost of the passive antenna of this first embodiment is very small.

Consequently, the invention is a simple and inexpensive passive antenna comprising a screened loop aerial, the passive antenna having a reference field level which, for a reference location close to the passive antenna, is substantially independent of the frequency at any frequency less than a least upper bound, the least upper bound being increased compared to the one which would be obtained using a prior-art passive antenna comprising a screened loop aerial of the same size.

Second embodiment (best mode).

As a second embodiment of a device of the invention, given by way of non-limiting example and best mode of carrying out the invention, we have represented in figures 6, 7 and 8 some drawings of a passive antenna of the invention, the passive antenna comprising: a first element (1), the first element comprising a tube (11) and a transmission line (12), the tube being an electric conductor referred to as “the tube of the first element”, the tube of the first element having a length, the transmission line having an outer conductor and an inner conductor, the transmission line having a first end and a second end, 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 tube of the first element having a first end at the first end of the transmission line, the transmission line being totally or partially inserted in the tube of the first element, the outer conductor being, over a portion of the length of the tube of the first element, not in electric contact with the tube of the first element, the first end of the outer conductor being connected to the tube of the first element, the first element being a part of a screened loop aerial; a second element (2), the second element being an electric conductor, the second element having a first end and a second end, the second element being a part of the screened loop aerial; a part referred to as “base” (3), the base providing an electric contact between a second end of the tube of the first element and the second end of the second element, the base being a part of the screened loop aerial; a single-port linear device (4) having a first terminal and a second terminal, the first terminal of the single-port linear device being coupled to the first end of the second element, the second terminal of the single -port linear device being coupled to the first end of the inner conductor; and a port of the passive antenna, the port of the passive antenna having a first terminal and a second terminal, the first terminal of the port of the passive antenna being coupled to the second end of the inner conductor, the second terminal of the port of the passive antenna being coupled to the second end of the outer conductor.

In figures 6, 7 and 8, the hidden edges and the hidden outlines are not shown, except the hidden outline of the transmission line (12) in Figure 7. Figure 6 is a front view of the passive antenna. Figure 8 is a back view of the passive antenna, showing a coaxial connector (7) which materializes the port of the passive antenna.

The base comprises a fastening ring (5). The fastening ring is used to fasten the passive antenna to an antenna mast (9). The base (3) is a box. This box is conducting. It may for instance be a metallic box. The first terminal of the port of the passive antenna is the center conductor of the coaxial connector. The second terminal of the port of the passive antenna is the peripheral conductor of the coaxial connector, which is directly coupled to the base. Figure 7 is a front view of the passive antenna without the lid of the base, and without the screws used to fasten the lid of the base to the base. Figure 7 shows how the transmission line goes from the first end of the tube of the first element to the coaxial connector, where the first terminal of the port of the passive antenna is coupled to the second end of the inner conductor, and where the second terminal of the port of the passive antenna is coupled to the second end of the outer conductor.

In this second embodiment, the transmission line is a coaxial cable whose characteristic impedance is close to 50 ohms at 100 MHz. The transmission line may for instance be a rigid or semi-rigid coaxial cable, suitably bent. The transmission line may for instance be a flexible coaxial cable. The transmission line may for instance comprise a center conductor, a dielectric surrounding the center conductor, a metallic braid surrounding the dielectric, and an insulating j acket surrounding the metallic braid, said inner conductor being the center conductor, said outer conductor being the metallic braid. In this second embodiment, the tube of the first element is a rigid metallic tube, a crosssection of the rigid metallic tube being circular and hollow. The outer conductor is connected to the tube of the first element, at the first end of the tube of the first element, but the outer conductor is not anywhere else in electric contact with the tube of the first element.

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

The single-port linear device (4) is made of a resistor (41) of 49.9 ohms connected in parallel with a capacitor (42) of 47 pF. Thus, the single-port linear device has a direct current resistance, the direct current resistance being close to 49.9 ohms. Thus, we can say that: the direct current resistance is close to the characteristic impedance of the transmission line; the direct current resistance is greater than one-half of the real part of the characteristic impedance of the transmission line; and the direct current resistance is less than two times the real part of the characteristic impedance of the transmission line.

Figure 9 shows a plot of the absolute value of an impedance of the passive antenna of this second embodiment, as a function of frequency. Figure 10 shows a plot of a reference field level of the passive antenna of this second embodiment, as a function of frequency, for a reference location close to the passive antenna, the internal impedance of the generator coupled to the port of the passive antenna being assumed to be equal to 50 ohms at any frequency.

The prior art passive antenna shown in Fig. 1 and the passive antenna shown in Fig. 6 are based on the same loop aerial, that is to say: the outer conductor of Fig. 1 is the same as the tube of the first element of Fig. 6; the second elements are the same in Fig. 1 and Fig. 6; and the bases are the same in Fig. 1 and Fig. 6. An important difference between these passive antennas is that, in the passive antenna shown in Fig. 1, the first end of the second element is directly coupled to the first end of the inner conductor, whereas, in the passive antenna shown in Fig. 6, the first end of the second element is indirectly coupled to the first end of the inner conductor, through the single-port linear device comprising a resistor and a capacitor, the resistor being connected in parallel with the capacitor.

The reference location used to obtain Fig. 10 is the same as the reference location used to obtain Fig. 3, so that we can meaningfully compare Fig. 10 to Fig. 3. This comparison shows that the characteristics of the screened loop aerial and of the single-port linear device interact in such a way that, for a reference location close to the passive antenna, the passive antenna shown in Fig. 6 has a reference field level which is substantially independent of the frequency up to about 50 MHz, whereas the passive antenna shown in Fig. 1 has a reference field level which is substantially independent of the frequency only up to about 10 MHz.

Consequently, the invention is a simple and inexpensive passive antenna comprising a screened loop aerial, the passive antenna having a reference field level which, for a reference location close to the passive antenna, is substantially independent of the frequency at any frequency less than a least upper bound, the least upper bound being greater than the one which would be obtained using a prior-art passive antenna comprising a screened loop aerial of the same size.

A comparison of Fig. 2 with Fig. 9 shows that the absolute value of the impedance of the passive antenna shown in Fig. 6 is substantially independent of the frequency up to about 20 MHz, whereas the absolute value of the impedance of the passive antenna shown in Fig. 1 is not substantially independent of the frequency in any part of the frequency range 100 kHz to 1000 MHz. This shows that the characteristics of the screened loop aerial and of the single-port linear device interact to produce an impedance stabilization effect up to about 20 MHz. Moreover, a comparison of Fig. 9 with Fig. 10 shows that the impedance stabilization effect, which makes the absolute value of the impedance of the passive antenna shown in Fig. 6 substantially independent of the frequency up to about 20 MHz, is distinct from the phenomenon by which the characteristics of the screened loop aerial and of the single -port linear device interact in such a way that, for a reference location close to the passive antenna, the passive antenna shown in Fig. 6 has a reference field level which is substantially independent of the frequency up to a least upper bound, the least upper bound being close to 50 MHz.

This least upper bound corresponds to a wavelength in vacuum, equal to the velocity of light in vacuum divided by the least upper bound. Since the largest width of the passive antenna shown in Fig. 6 is about 133 mm, we can say that a sum of the length of the tube of the first element, a length of the second element, measured from its first end to its second end, and a length, measured on the base, from the second end of the tube of the first element to the second end of the second element, is less than a quarter of said wavelength in vacuum. The screened loop aerial, comprising the first element, the second element and the base, is consequently electrically small at any frequency less than said least upper bound.

INDICATIONS ON INDUSTRIAL APPLICATIONS

The port of the passive antenna of the invention may be coupled to an output of a radiofrequency generator, to obtain a known radio frequency magnetic field. In this application, the main advantage of the passive antenna of the invention is that the reference field level is, for a reference location close to the passive antenna, substantially independent of the frequency at any frequency less than a least upper bound, the least upper bound being greater than the one which is obtained using another passive antenna comprising a screened loop aerial of the same size.

The port of the passive antenna of the invention may also be coupled to an input of a radio communication receiver, a measuring receiver, or a spectrum analyzer, to measure a radio frequency magnetic field. In this application, the main advantage of the passive antenna of the invention is that it can be used in such a way that the voltage delivered by the passive antenna of the invention is substantially proportional to a time derivative of the radio frequency magnetic field, over a broad frequency range.