Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q7/00—Loop antennas with a substantially uniform current distribution around the loop and having a directional radiation pattern in a plane perpendicular to the plane of the loop
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/04—Adaptation for subterranean or subaqueous use
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
  • H01Q9/04—Resonant antennas
  • H01Q9/16—Resonant antennas with feed intermediate between the extremities of the antenna, e.g. centre-fed dipole
  • H01Q9/26—Resonant antennas with feed intermediate between the extremities of the antenna, e.g. centre-fed dipole with folded element or elements, the folded parts being spaced apart a small fraction of operating wavelength
  • H01Q9/265—Open ring dipoles; Circular dipoles

Definitions

• the present invention relates to a large-sized antenna for transmitting and / or receiving surface waves in a wide frequency band including in particular all or part of the low, medium and high frequencies between about 30 kHz and about 30 MHz, or waves. kilometers, MF and HF.
• the antenna can be incorporated for example into a high power transmission system, in particular for the broadcasting of radio or television program signals, a surface wave radar system or a reception and interception system.
• the patent application EP 1,594,186 A1 filed by the applicant discloses a large ground antenna for radiating a surface wave kilometer or hectometric.
• This antenna comprises a metal ground plane, a metal excitation loop, and a metal link element.
• the ground plane is buried horizontally near and under the surface of the ground.
• the excitation loop is longer than approximately 25 m for LF and MF wavelengths and open between two extremities and extends parallel to the ground plane and horizontally above the ground surface at a height greater than About 2 m from the ground plane.
• the metal connecting element is perpendicular to the loop and connects one end of the excitation loop to the ground plane.
• the excitation loop and the connecting element are each constituted by at least one thin cylindrical element.
• the opening of the excitation loop is small relative to the length of the loop to nearly eliminate any horizontal electric field component at the surface of the ground.
• the ground wave is due to the injection of high currents in the ground, a consequence of an ohmic resistance of the weak antenna, without any lateral radiation of a space wave compared to a pylon antenna.
• the patent application EP 1 594 186 A1 aims to significantly promote the propagation by surface waves and to minimize the radiation of a space wave by the radiating towers, in particular to avoid coupling of the antenna with structures close to the antenna above the ground, the ground antenna generates a significant space wave for angles close to the normal to the ground plane.
• This space wave has a much lower power than that of the surface wave and is evanescent a few tens of kilometers above the surface of the ground.
• the space wave can be reflected on layers of the ionosphere and cause fading phenomena in combination with a surface wave.
• the space wave can disturb useful signals received from the ionosphere by other antennas. Conversely, the operation in reception of the antenna can be disturbed by space wave recovery.
• the ground antenna has a large surface area and a relatively narrow bandwidth.
• the present invention aims to overcome the various problems mentioned above and particularly to provide a large surface wave antenna which has increased ionospheric protection over short and medium distances, and a structure conducive to a reduction in the congestion of the antenna according to at least one dimension of the space and to a widening of the bandwidth.
• a surface wave antenna comprising a metal excitation loop capable of being positioned at a height of at least about 1 m above the surface of a conductive medium and a feeding means capable of to be connected to the conducting medium, the loop having a length of approximately ⁇ / 2 and ⁇ denoting the operating wavelength of the antenna, is characterized in that the excitation loop comprises two approximately parallel portions and at least approximately ⁇ / 50 and able to extend approximately parallel to the surface of the conductive medium in a plane approximately perpendicular to said surface and to be traversed by currents of opposite directions, the most close to said surface having an opening between ends of the loop connected to the supply means.
• Said two portions of the excitation loop according to the invention are lower and upper portions relative to the surface of the conductive medium, such as earth or sea, and can constitute approximately half of the loop, the remaining portions of the loop each having a length of at most ⁇ / 50 approximately.
• the excitation loop is thus composed in large part of one or more pairs of lower and upper portions each extending in a plane approximately perpendicular to the surface of the conductive medium, the lower and upper portions of a pair being arranged in the loop so that they are the seat of currents of opposite directions.
• the opening of the excitation loop is very small compared to the perimeter of the loop to almost eliminate any component of electric field parallel to the surface of the conductive medium, and therefore horizontal.
• the antenna of the invention is very discreet and insensitive to any wind, blast, lightning, earthquake or explosion.
• the antenna also has a very low radar echo area (SER).
• the excitation loop may be flat and contained in a plane approximately perpendicular to the surface of the conductive medium.
• the excitation loop may be rectangular and comprise two long sides formed by the two lower and upper portions and long of at most ⁇ / 4 approximately.
• the size of the antenna can be reduced along longitudinal directions of the antenna by one or more folds of long portions of the excitation loop in planes perpendicular to the surface of the conducting medium.
• the excitation loop may be distributed approximately into two half-loops which are superimposed on two planes approximately parallel to the surface of the conducting medium and distant at most ⁇ / 50 and which each have two portions to almost parallel, capable of being traversed by currents of opposite meanings.
• Each of the half-loops may comprise more than two approximately parallel portions, two neighboring portions in each half-loop being able to be traversed by currents of opposite directions and two superposed portions of the half-loops being able to be traversed by currents in opposite directions.
• the excitation loop may be circumscribed to a parallelepiped having large faces approximately parallel to the surface of the conductive medium.
• the parallelepiped can be straight.
• each of the half loops may extend zigzag on one of the large faces.
• each of the half-loops may comprise two rectangular flat spirals having opposite directions and a common center and extending on one of the large faces.
• the excitation loop is circumscribed to a cylinder having bases approximately parallel to the surface of the conductive medium, and each of the half-loops comprises two circular flat spirals having opposite directions and one common center and extending on one of the bases.
• two portions of the excitation loop are approximately close parallel, superimposed and adjacent may be at least about ⁇ / 200 apart.
• the antenna may comprise at least one metallic intermediate element which is connected to lower and upper portions of the superimposed excitation loop in a plane able to be approximately perpendicular to the surface of the conductive medium and which is located near small sides of the excitation loop approximately perpendicular to the superimposed portions.
• the means for feeding the antenna may comprise a power supply device such as a transmission device if the antenna is transmitting, or a reception device if the antenna operates in reception, and one or two substantially vertical metallic connecting elements connecting the supply means to the propagation medium.
• the supply means comprises only a metal connection element, which may include a terminal impedance, for connecting the excitation loop to the conductive medium; the terminals of the power supply device are connected to the ends of the loop, and the metal connecting element has one end connected to the negative terminal of the supply device and another end adapted to be connected to the conductive medium.
• the supply means comprises two metal connection elements for connecting the excitation loop to the conductive medium; a metal connecting element has one end connected to one end of the loop and another end adapted to be connected to the conductive medium, the power supply device has a positive terminal connected to the other end of the loop , and another metal link element which may include an end impedance at one end connected to a negative terminal of the supply device and another end adapted to be connected to the conductive medium.
• the invention remedies to maintain the surface wave radiation properties of the antenna by burying a metal mass element near and under the surface of the conductive medium and having a surface at less equal to the projection of the surface of the excitation loop on the surface of the conductive medium.
• a metal connecting element which is unique according to the first embodiment, or which is one or the other of the metallic connection elements according to the second embodiment, then has its end adapted to be connected to the conductive medium, which is connected to the metal mass element.
• FIG. 1 is a schematic vertical front view of an antenna with a rectangular loop and a power supply circuit according to a first embodiment of the invention, having a single connecting element connected to a conductive medium of high electrical conductivity;
• FIG. 2 is a schematic vertical front view of an antenna with a rectangular loop according to the first embodiment and a supply circuit according to a second embodiment of the invention, having two connecting elements connected to a conductivity conducting medium. high electric;
• FIGS. 3 and 4 are schematic vertical face views of an antenna respectively according to variants of the embodiments shown in FIGS. 1 and 2, for a conductive medium of low electrical conductivity;
• FIG. 5 is a schematic vertical front view of an antenna according to another variant of the antenna shown in Figure 1, for expanding the bandwidth of the antenna;
• FIG. 6 is a schematic perspective view of an antenna with a loop according to a second embodiment of the invention which is intended to reduce the longitudinal size of the antenna compared to the first embodiment of the loop, by folding the following way a central zenith axis with a loop of FIG. 1;
• FIGS. 7 and 8 are respectively a front-end view and a right-side view along perpendicular vertical planes XOZ and YOZ of the antenna shown in FIG. 6;
• FIG. 9 is a schematic perspective view of an antenna with a folded loop according to a third embodiment of the invention, intended to further reduce the longitudinal size of the antenna;
• FIGS. 10, 11 and 12 are respectively a top view, a front view and a right side view of the antenna shown in FIG. 9;
• FIG. 13 is a schematic perspective view of an antenna with a loop contained in a parallelepiped and folded following Archimedean spirals according to a fourth embodiment of the invention.
• FIGS. 14 and 15 are respectively a view from above and a front view of the antenna shown in FIG. 13;
• FIG. 16 is a schematic perspective view of an antenna with a loop contained in a cylinder and folded according to Archimedean spirals according to a fifth embodiment of the invention.
• FIG. 17 and 18 are respectively a top view and a side view of the antenna shown in Figure 16.
• a surface wave antenna according to the invention is able to operate at a useful wavelength ⁇ of emission or reception.
• the useful wavelength ⁇ corresponds to the central frequency of the antenna bandwidth, which corresponds at least partially to Telec and / or MF and / or HF wavelengths.
• the antenna according to the first embodiment essentially comprises a metal excitation loop B1 that is approximately vertical, and a power supply circuit comprising a power supply device A and a metal conductive connection element L1 n, which is approximately vertical. connecting the excitation loop to a conductive medium M of surface SM.
• the term "approximately vertical" means that the excitation loop or the connecting element may extend in a plane perpendicular to the surface SM or in an oblique plane at an angle of a few degrees with a plane perpendicular to the surface.
• the conducting medium M acts as a surface wave propagation vector transmitted or received by the antenna.
• the medium M can have a high electrical conductivity like the sea, a swamp salt or salt lake, or lower electrical conductivity, such as soil or sand.
• a reference sign to the drawings comprising the letter p, respectively n, designates an element or a portion of element or excitation loop connected to the positive or negative terminal of the power supply device. A or located on the side thereof along the excitation loop.
• the metal excitation loop B1 extends approximately vertically above the surface SM at a height between h and H.
• the loop B1 is rectangular and composed of two long sides. 11 p-11 n and S1 roughly horizontal and two sides roughly vertical V1 p and V1 n significantly smaller.
• the large lower side 11 p-11 n is located at the height h relative to the surface SM.
• the large upper side S1 is located at the height H relative to the surface SM.
• the difference in height H - h is the length of the short sides V1 p and V1 n which is at least equal to ⁇ / 200 so as to reduce the coupling between the long sides 11 p-11 n and S1 of the loop to the origin of the creation of a mode of transmission for two-wire line decreasing the output of the antenna.
• the difference in height H - h is at most equal to ⁇ / 50 so that the large sides 11 p-11 n and S1 of the loop B1 are close and the currents in them are in opposite directions.
• the shape of the loop is not limited to a rectangle and is determined according to the purity of the essentially vertical polarization of a surface wave and the omnidirectionality at the SM surface desired for the antenna.
• Height H is at least 2 m for LF and MF and at least 1 m for HF.
• the average distance (H + h) / 2 between the loop B1 and the surface SM must not be too great in order to couple as much radioelectric energy as possible to the surface SM so that the antenna radiates a surface wave over above the SM surface.
• the Heights h and H are not necessarily constant over the length of the loop, just as the difference H - h which is not necessarily constant; therefore the long sides 11 p-11 n and S1 are "nearly parallel" to each other and each of them is “nearly parallel” to the surface SM.
• the discontinuity between the air and the conductive medium M at the periphery of the excitation loop favors a vertical polarization of the electric field with respect to which the horizontal electric field component is negligible in the surface wave propagation by the antenna , especially since the excitation loop is regular and almost closed.
• the electric field lines are distributed almost uniformly to all the azimuths around the Z1 -Z1 axis of the loop which means that the antenna is omnidirectional.
• the loop has a perimeter equal to the useful half-wavelength ⁇ / 2, to within ⁇ ⁇ / 8, or a length L / 2 ⁇ KIA of the long sides 11 p-11 n and S1 of the order from 25 m to 250 m for a central hectometric frequency of the useful band.
• the shape of the excitation loop B1 is elongate and polygonal or elliptical so that two long portions such as the long sides 11 p-11 n and S1 are approximately parallel in a plane roughly perpendicular to the surface SM of the conductive medium M.
• the profile of the loop is designed in such a way that the portions of the loop, such as the sides 11 p-11 n and S1 of a rectangular loop, located approximately parallel to the surface SM and of dimensions at least greater than approximately ⁇ / 50 are the seat of currents of opposite directions .
• the large lower side 11 p-11 n consists of two portions approximately colinear 11 p and U n between the facing ends E1 p and E1 n of the loop B1 which define a small opening E1 p-E1 n whose width is very small vis-à-vis the wavelength ⁇ .
• the opening E1 p-E1 n can be practiced anywhere along the long side 11 p-11 n. According to Figure 1, the opening E1 p-E1 n is in the middle of the large lower side 11 p-11 n. Given the narrowness of the opening with respect to the length of the loop, the loop is considered "closed".
• the excitation loop B1 can be supported in a plane perpendicular to the surface SM by insulating posts (not shown) regularly distributed along the loop.
• each pole supports both the long sides 11 p-11 n and S1.
• the insulating posts may be fixed in the conductive medium M if the depth of the medium lends itself to it, or may be fixed on a support floating on the surface SM if the medium is water.
• the excitation loop B1 is made of tube or multi-strand or single-strand wire.
• the conductive connecting element L1 is approximately vertical and connects one E1 n of the ends of the loop B1 at the opening E1 p-E1 n to the conductive medium M.
• the element L1 n closes the loop B1 on the conducting medium M located below the SM surface.
• the element L1 n can be constituted by a peg or a metal tube of diameter preferably between 5 and 50 mm and having a lower end plunging a few tens of centimeters in the conductive medium M under the surface SM.
• the material constitution of the excitation loop and the connecting element can also be realized according to other variants described in the patent application EP 1 594 186 A1, such as a sheet or a cage of parallel metal wires.
• the link element L1 n may include a terminal impedance Zt which is optional and which may be replaced by a simple short circuit.
• Terminal impedance can be reactive or resistive. It can be adjustable as needed to adjust the frequency of operation of the antenna corresponding to ⁇ , adjust the antenna bandwidth or adjust the input impedance of the antenna.
• the influence of the capacitive and / or inductive and / or resistive character of the terminal impedance Zt on the operating characteristics of the antenna, such as the operating frequency, the bandwidth and the impedance matching, is similar to that described in patent application EP 1 594 186 A1.
• the power supply device A feeds the loop B1 and can be a transmitting or receiving device depending on whether the antenna operates in transmission or reception.
• the supply device A has positive and negative terminals respectively connected to the ends E1 p and E1 n of the loop B1 at the opening E1 p-E1 n, where appropriate by one or two d metallic intermediate elements L2p and L2n which may be electrical wires or have a constitution similar to that of the connecting element L1 n.
• at least one of the intermediate elements L2p and L2n has a zero length and the corresponding terminal of the supply device A is directly connected to one end of the excitation loop B1.
• another conductive connecting element L3n connects the negative terminal of the supply device A to the conducting medium M situated under the surface SM, as the second end of the connecting element L1 n opposite to the end E1 n of the excitation loop B1 and plunging into the conducting medium M under the surface SM.
• the lengths of the connecting elements L2p and L3n are determined so that the real part of the impedance of the antenna brought back to the terminals of the supply device A is equal to the characteristic impedance of the supply device.
• the antenna is operated above an imperfect conductive medium M with a low electrical conductivity such as earth or sand, located under the surface SM, as shown in FIGS. 3 and 4.
• a metallic mass element EM is buried near and under the surface SM.
• the metallic mass element EM is connected to the second end of the connecting element L1 n according to FIG. 3 corresponding to the first embodiment of the supply circuit, or to the ends of the connecting elements L3n and L1n in the medium.
• M according to Figure 4 corresponding to the second embodiment of the supply circuit.
• the depth to which the EM mass element is buried below the SM surface is relatively small, some tens of centimeters, to promote a surface wave above the SM surface and disadvantage any wave under the surface SM.
• the EM mass element may be a metal wire or rod, or a solid plate or screen according to embodiments described in patent application EP 1 594 186 A1. It provides excellent electrical continuity to contribute to the omnidirectional nature of the antenna and thus maintain the surface wave radiation properties of the antenna.
• the mass element P can be galvanized metal or coated in a plastic sheath, and be insensitive to chemical attack in the middle M.
• the mass element EM may have various contours of the circular or polygonal type so that it covers a surface at least equal to or even much greater than the projection of the surface of the excitation loop on the surface SM. This feature avoids electric field edge effects between the excitation loop and the ground element and improves the confinement of the electric field lines under the excitation loop.
• the plane element EM For an excitation loop extending in a vertical plane XOZ as shown in FIGS. 3 and 4, the plane element EM has a length at least equal to the length L / 2 of the long sides 11 p-11 n and S1 of the loop B1, greater than about half the length of the loop, and a width of at least a few tens of centimeters.
• At least one intermediate metallic element Vip, Vin is connected, for example by welding, to the long sides 11 p-11 n and S1 of the excitation loop B1, as shown in FIG. Figure 5.
• the metal intermediate element is approximately perpendicular to the long sides and may have a constitution similar to that of the loop B1.
• one or more intermediate elements Vip are placed in one side of the loop B1 with respect to the opening E1 p-E1 n of the loop, and / or one or more intermediate elements Vin are placed in the other side. of the loop in relation to the opening.
• the intermediate metal elements Vip and Vin are located near the longitudinal ends of the excitation loop B1, for example a few meters from the short sides V1 p and V1 n.
• the intermediate elements are intended to widen the bandwidth of the antenna around the resonance frequency of the antenna, without significant changes in the antenna radiation characteristics.
• each of these excitation loops may comprise one or more intermediate elements such as the Vip and Vin elements shown in FIG. 5, between the lower and upper portions of the excitation loop, or more generally between "half" loops. lower and upper excitation loop, to expand the bandwidth of the antennas.
• the excitation loop B2 of an antenna is based on a folding of a first half of the excitation loop B1 comprising the portion U n of the large lower side. , the short side V1 n and one half of the large upper side S1 towards the second half of the loop B1 around the central zenith axis Z1 -Z1 of the loop B1, as indicated by the arrow F2 in Figure 5.
• the loop B2 thus comprises approximately two "half" loops on the front faces ( Figure 7) and rear or the lower and upper faces of a long narrow parallelepiped almost straight. This parallelepiped enveloping the loop B2 has a length L / 4 approximately and a height H - h.
• the parallelepiped extends not only longitudinally along a vertical plane XOZ ( Figure 7), but also laterally along a vertical plane YOZ ( Figure 8) perpendicular to the plane XOZ.
• Two longitudinal upper portions S2p and S2n of the loop B2 corresponding to the two halves of the upper portion S1 of the loop B1 are connected by a short horizontal portion S21 p.
• the end of the lower portion I2n of the loop B2 corresponding to the upper portion U n of the loop B1 folded backwards is connected by a short horizontal portion 121 n which is parallel to the portion S21 p and located with the ci on a lateral vertical side of the parallelepiped.
• the loop B2 From the end E2p of the excitation loop B2 connected to the positive terminal of the supply device A, the loop B2 comprises a long longitudinal lower portion I2p, a short vertical portion V2p of height H - h, a long upper portion longitudinally S2p located above the portion I2p and delimiting with the portions I2p and V2p the front face of the parallelepiped, a short lateral portion S21 p, a long upper longitudinal portion S2n and delimiting with the portions S2p and S21 p the upper face of the parallelepiped, a short vertical portion V2n of height H - h located with the short portion V2p in a plane perpendicular to the longitudinal portions, a long longitudinal lower portion I2n located below the portion S2n and delimiting with the portions S2n and V2n the face rear of the parallelepiped, and a short lateral portion 121 n located below the portion S21 p delimiting with the portions I 2p and I2n the lower face of the
• the length of the approximately horizontal lateral portions 121 n and S 21 p defines the width W of the loop B2 in a vertical plane Y 0 Z which is much smaller than ⁇ so that the two parallel portions situated in each of the longitudinal faces of the parallelepiped are traveled by currents of opposite meanings. Under these conditions, the secondary components of the electric field generated in horizontal planes are very strongly restricted in directions close to the central zenith axis Z2-Z2 of the B2 loop.
• the length of the lateral portions 121 n and S 21 p is however at least equal to approximately ⁇ / 200 in order to avoid too high couplings between the longitudinal portions I2p and I2n and S2p and S2n which cause a significant reduction in the antenna efficiency. .
• the involute of the folded excitation loop B2 is longer than the involute of the excitation loop B1.
• the length of the involute of the folded loop B2 shown in FIG. 6 is a function of the length of the portions 121 n and S21 p.
• the bandwidth is also reduced due to the increase in the quality factor of the antenna. However this bandwidth reduction can be compensated by the addition of intermediate metal elements Vip between the lower portions I2p and upper S2p and / or intermediate metal elements Vin between the lower portions I2n and upper S2n, as those shown in FIG.
• the principle of the folding of the excitation loop on itself can be extended to multiple successive folds by means of a proportional increase of the involute of the antenna and a reduction of the bandwidth. for the same resonance frequency.
• the excitation loop B3 of an antenna according to the third embodiment is based on folds of the thirds situated to the left and to the right of the loop B1 in FIG. rear of the central third of the loop B1.
• the left third of the excitation loop B3 is located in a vertical front plane located in front of the central third of the loop B1 after folding around a zenith axis of the loop B1 located at the left end of the central third, as indicated by the arrow F3p in Figure 5.
• the right third of the excitation loop B3 is located in a rear vertical plane located behind the central third of the loop B1 after folding around a zenith axis of the loop B1 located at the the right-hand end of the central third, as indicated by the arrow F3n in Figure 5.
• the excitation loop B3 according to the third embodiment thus comprises approximately three thirds I3p-S3p ( Figure 1 1), I3cp-I3cn-S3c and I3n- S3n loop on each front vertical faces, central and rear of a narrow parallelepiped almost straight.
• This parallelepiped enveloping the loop B3 has a length L / 6 approximately and a height H - h.
• the loop B3 consists of approximately two "half" loops I3p-I3cp-I3cn-I3n and S3p-S3c-S3n (FIG. 11) on each of the large horizontal lower and upper faces of the long parallelepiped.
• a left end of the lower front portion I3p of the loop B2 corresponding to the left third of the lower portion 11 p of the loop B1 folded forward and a left end of the upper front portion S3p of the loop B2 corresponding to the left third of the upper portion S1 p of the loop B1 folded forwardly are respectively connected by two short horizontal side portions 131 p and S31 p which are parallel and located in a left vertical side of the parallelepiped.
• a right end of the lower rear portion I3n of the loop B2 corresponding to the right third of the lower portion U n of the loop B1 folded backwards and a right end of the upper rear portion S3n of the loop B2 corresponding to the right third of the upper portion S1 p of the loop B1 folded backwards are respectively connected by two short horizontal side portions 131 n and S31 n which are parallel and located in a vertical right side of the parallelepiped.
• the loop B3 From the end E3p of the excitation loop B3 connected to the positive terminal of the supply device A, the loop B3 comprises the "half" -entrtion central longitudinal lower I3cp, the short lower side portion 131 p, the long portion lower longitudinal front I3p, a short vertical portion V3p of height H - h, the long upper longitudinal front portion S3p, the short upper lateral portion S31 p, the long upper central longitudinal portion S3c, the short upper lateral portion S31 n, the long longitudinal upper rear portion S3n, a short vertical portion V3n of height Hh, the long longitudinal rear lower portion I3n, the short lower side portion 131n and the "half" - lower central horizontal longitudinal portion I2n terminated by the other end E3n of the excitation loop B3.
• the length of the horizontal lateral portions 131p, 131n, S31p and S31n defines the half-width W of the loop B3 in a vertical plane YOZ which is between ⁇ / 200 and ⁇ / 50 and therefore much smaller than ⁇ of in such a way that the two parallel longitudinal portions situated in each of the three longitudinal front, intermediate and rear faces and two adjacent parallel longitudinal portions of three lying in each of the two central and upper longitudinal faces of the parallelepiped are traversed as closely as possible by currents of opposite directions.
• the length of the superimposed lateral portions 131p and S31p may be different from the length of the superimposed lateral portions 131n and S31n, and the face vertical containing the parallel longitudinal portions I3cp, I3cn and S3c may be at different distances from the front and rear faces.
• the excitation loop B4 of an antenna according to the fourth embodiment shown in FIGS. 13 to 15 comprises approximately a "lower" half-loop. formed by two rectangular flat spirals I4p and I4n having opposite directions and a common center and an "upper half-loop" formed by two rectangular flat spirals S4p and S4n having opposite directions and a common center.
• the half-loops I4p-I4n and S4p-S4n are respectively circumscribed to the large lower and upper faces of a parallelepiped approximately right height H - h, length 5xp1 and width 4xp2 according to the example shown in Figure 14
• the longitudinal pitch p1 and the lateral pitch p2 of the turns of the spirals may be a priori different and are clearly less than ⁇ , for example between ⁇ / 120 and ⁇ / 80.
• the large upper and lower faces of the parallelepiped are approximately parallel to the surface SM of the conductive medium M.
• the upper spirals S4p and S4n are approximately superposed vertically on the lower spirals I4p and I4n, respectively.
• Short vertical portions V4p and V4n of the excitation loop B4 have a height H - h and respectively connect peripheral ends of the spirals I4p and S4p and the peripheral ends of the spirals I4n and S4n.
• the ends E4p and E4n of the opening of the loop B4 situated in the center of the half-loop I4p-I4n, the lower spirals I4p and I4n and the upper spirals S4p and S4n are respectively symmetrical with respect to a central zenith axis Z4-Z4 of the loop B4 passing through the centers of the spirals and the lower and upper faces of the parallelepiped.
• the pitch may be variable for example to form lower and upper logarithmic spirals of the loop.
• a variable pitch for each turn of the half-loops can be chosen insofar as the restrictions on the distance between the turns are respected so as to maintain a significant radiation efficiency of the same order of magnitude as in loops B2 and B3 obtained by folding.
• the loop B5 according to the fifth embodiment shown in FIGS. 15 to 18 comprises approximately a lower half-loop formed by two circular circular Archimedean spirals I5p and I5n having opposite directions and a common center and a lower half-loop formed by two spirals.
• circular circular Archimedean S5p and S5n having opposite directions and a common center.
• the half loops I5p and I5n and S5p and S5n are respectively circumscribed to the lower and upper bases of a cylinder having a height H - h, a radius p and a zenith axis Z5 - Z5 passing through the centers of the spirals and the opening E5p-E5n of the loop B5 located in the center of the lower half-loop I5p-I5n.
• the bases of the cylinder are approximately parallel to the surface SM of the conducting medium M and are for example circular or elliptical, or the cylinder is replaced by a prism polygonal bases.
• Short vertical portions V5p and V5n of the excitation loop B5 have a height H - h and respectively connect peripheral ends of the spirals I5p and S5p and peripheral ends of the spirals I5n and S5n.

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