FR2925233A1 - VERY BROADBAND ACTIVE ANTENNA FOR PASSIVE RADAR. - Google Patents

Abstract

The present invention relates to the field of passive VHF-UHF radars. It relates more particularly to the production of high bandwidth, omnidirectional, VHF-UHF antennas that can operate in a network without altering their radiation patterns. The invention consists of a two-band antenna structure, operating on two subwoofers. wide frequency bands, consisting of radiating elements forming a minimum diffraction sub-antenna operating in the upper sub-band, UHF, and a minimum diffraction sub-antenna operating in the low VHF sub-band, each element being in service or out of service separately depending on the sub-band operated. The antenna output is connected to the input of a narrow-band wideband low-noise amplifier having a high impedance input and an output equal to the impedance of the transmission line through which it is connected to the receiver, the low subband dedicated sub-antenna having low pass filters arranged to prevent operation of this element when the high subband element is in use.

Description

The present invention relates to the field of VHFUHF passive radars. It relates more particularly to the realization of high-bandwidth, omnidirectional VHF-UHF antennas and the networking of such antennas.

In the field of passive UHF radar development, an important area of research is to find ways to achieve, either in vertical polarization or in horizontal polarization, an antenna operating in reception in a wide frequency band, typically between 88 MHz and 860 MHz. This antenna should ideally have an omnidirectional radiation pattern in the horizontal plane and must retain this property when, networked, it is located in the vicinity of other antennas. In addition, its gain must be of the same order throughout the frequency band.

It is known that when it is desired to have a vertically polarized VHF or UHF antenna having an omnidirectional diagram, it is possible to use a vertical dipole. Similarly, it is known that when it is desired to have a horizontally polarized VHF or UHF antenna having an omnidirectional diagram, it is possible to use a "big wheel" or "turnstile" type antenna. These two types of antennas have the advantage of leading to simple embodiments However, used at resonance, these antennas have an incompatible narrow-band operation of the desired wide bandwidths and the requirement of corresponding gain constancy. Moreover, during networking, these antennas have the disadvantage of presenting a very degraded horizontal radiation pattern, due to the couplings present between antennas. In addition, the radiation patterns also change a lot in the frequency band.

An object of the invention is to provide a simple solution for making broadband antennas VHF-UHF, horizontal or vertical polarization, omnidirectional, and can be networked without undergoing alteration of their radiation pattern.

For this purpose the invention relates to a dual-band antenna, operating on two wide frequency sub-bands, a high UHF sub-band and a low VHF sub-band; consisting of radiating elements forming a minimally diffracting sub-antenna operating in the high subband, and a minimally diffractive sub-antenna operating in the low subband, each element being switched on or off separately as a function of the exploited sub-band. This antenna is connected to the input of a broad band low noise amplifier, placed directly at its output and having a high impedance input and an output equal to the impedance of the transmission line through which it is connected to the receiver . The sub-antenna dedicated to the low sub-band further comprises low-pass filters arranged to prevent the operation of this element when the element dedicated to the high subband is in use.

According to a particular embodiment of the invention, the antenna is a dipole formed of two vertical strands, each strand being formed of a central strand and an extension strand connected to the central strand by a low pass filter tuned on the low subband. The lengths of the central strand and the extension are defined so that in low band operation the antenna has two long strands of total length LB, less than half the wavelength of the signal received over the entire width of the low sub-band, and that in operation in high subband the antenna has two short strands of total length LH less than half the wavelength of the signal received over the entire width of the high subband.

According to a preferred variant of this embodiment, each extension strand is constituted by a set of vertical elementary strands, connected to each other by low-pass filters. The length of each elementary strand is defined such that, over the entire upper subband, it is less than 0.3X, 1. being the wavelength of the received signal. In this way, in operation in high sub-band, the extension strand does not re-radiate any signal.

According to another particular embodiment of the invention, the antenna 35 comprises two superposed "big wheel" sub-antennas, a sub-antenna whose dimensions are adapted for operation in the low subband and a sub-antenna. antenna whose dimensions are adapted for operation in the high subband. Each sub-antenna is coupled to the input of a low noise amplifier placed directly at the output of the sub-antenna. Each amplifier is configured to operate on a frequency band corresponding to the sub-band of the considered sub-antenna and has a high impedance input and an output whose impedance is equal to the impedance of the transmission line by which it is connected to the coupler which connects these two amplifiers with low noise to the receiver by a common line.

According to a preferred variant of this embodiment, the sub-antenna adapted to low-band operation comprises low-pass filters, arranged so that in high-sub-band operation, this sub-antenna does not radiates no signal.

According to another particular embodiment of the invention, the antenna comprises two perpendicular horizontal dipoles, each formed of two horizontal strands, whose signals are combined in phase quadrature. Each strand is formed of a central strand and an extension strand connected to the central strand by a low pass filter tuned to the lower subband. The lengths of the central strand and of the extension are defined so that, in low-band operation, the dipole considered has two strands long of total length LB, less than the half wavelength of the signal received over the entire width of the under low band, and that in operation in high subband the same dipole has two short strands of total length LH, less than half the wavelength of the signal received over the entire width of the high subband.

According to a preferred variant of this embodiment, each extension strand is constituted by a set of horizontal elementary strands connected to each other by low-pass filters. The length of each elementary strand being defined so that, over the entire upper subband, it is less than 0.3k, X being the wavelength of the received signal. In this way, in operation in high sub-band, the extension strand does not re-radiate any signal.

The characteristics and advantages of the invention will be better appreciated thanks to the description which follows, a description which sets forth the invention through particular embodiments taken as non-limiting examples and which is based on the appended figures, figures which represent:

FIG. 1 is a diagrammatic representation of a vertically polarized dipole antenna made in accordance with the invention; - Figure 2, the schematic representation of a "big wheel" type antenna, horizontally polarized, made according to the invention; FIG. 3 is a diagrammatic representation of a horizontally polarized "turnstile" antenna made in accordance with the invention; As stated above, the object of the invention is to propose a solution for producing a broadband VHF-UHF antenna, typically between 88 MHz and 860 MHz, having a gain varying little or nothing at all. frequency function, and whose omnidirectional diagram in the horizontal plane is not altered when the antenna is used in a network. A VHF-UHF antenna having such a bandwidth is a complex device that makes it difficult to obtain a constant behavior over the entire bandwidth explored. Therefore, the solution proposed by the invention, taking into account the bandwidth requirements, consists in producing an antenna that can operate alternately in two sub-bands, a high subband, UHF, and a sub-band. low, VHF, covering as much as possible the total bandwidth desired. For this purpose, a "dual-band" antenna consisting of two elements or "sub-antennas" operating alternately according to whether the antenna operates in the high or low sub-band is defined. To this antenna is associated a broadband amplifier or two amplifiers, similar gains, which is switched depending on the subband used, each amplifier covering one of the subbands.

In view of the requirements for maintaining the omnidirectional character of the antenna radiation pattern even in the case of network operation, the two sub-antennas constituting the antenna according to the invention are defined and arranged so as to constitute an antenna having the characteristics of a minimum diffraction antenna ("minimum scattering antenna" according to the English name). It is recalled here that a minimum diffraction antenna is an antenna for which there is a load such that no power is absorbed or reflected. For so-called "canonical" diffraction antennas this occurs when the load is an open circuit. In the context of the invention, therefore, the interposer and the associated receiver interpose a low-noise, broadband amplifier having a very high input impedance and an output impedance equal to the characteristic impedance of the line which transmits the signal received by the antenna of the low noise amplifier to the receiver. According to the invention, the low noise amplifier used here is a voltage amplifier, placed directly at the output of the antenna. The antenna thus constituted is an active VHF-UHF antenna.

We first look at a first embodiment of the antenna according to the invention, as shown in FIG. 1. The antenna of FIG. 1 is constituted on the basis of a conventional dipole antenna consisting of two conductive elements, or strands, vertical 11. Thus arranged the two vertical strands 11 constitute, in known manner, a vertically polarized antenna having an omnidirectional radiation pattern in the horizontal plane. Strands 11 having a total length LB corresponding to the length of a dipole adapted to the operation of the device in the low VHF sub-band, which extends for example from 88 MHZ to 240 MHz.

According to the invention, each conducting strand 11 is furthermore constituted of two conductive segments, a central segment 12 and a lateral segment 13, interconnected by a low-pass filter whose bandwidth covers the low operating sub-band of the antenna. The length of the central segments 12 is such that they both have a length LH corresponding to the length of a dipole suitable for operation in the UHF high subband, which extends for example from 470 MHz to 860 MHz. This configuration of the antenna wires 11 in two elements 12 and 13 interconnected by a low-pass filter 14 advantageously makes it possible to produce a single antenna adapted to the two sub-bands used. Indeed with respect to a low subband operation, the low-pass filter 14 behaves as a short circuit connecting the two elements 12 and 13 which constitute each strand 11. In this way in subband operation low, it has a dipole antenna of length LB adapted to the low sub-band and equivalent to the single dipole antenna shown schematically in Figure 1-b of Figure 1. Conversely, with respect to an operation in high sub-band, the low-pass filter 14 behaves like an open circuit. As a result, the elements 13 constituting each of the strands of the antenna are disconnected, so that in high-subband operation there is a dipole antenna of length LH adapted to the high subband and equivalent to the simple dipole antenna shown schematically in FIG. 1-a of FIG. 1. Advantageously, two antennas are produced with a single device.

According to the invention, the lengths LH and LB are defined so as to give the antenna both a broadband character and the properties of a minimum diffraction antenna, properties which allow the antenna to operate in a network. maintaining an omnidirectional radiation pattern. However, in order to keep an omnidirectional diagram network, it is necessary to suppress the effect of the couplings between antennas. We recall here that when a plane wave arrives on an antenna it creates currents on it. These currents produce a signal at the antenna output, a part of the received wave being re-radiated. It is this re-radiation that causes the coupling with the neighboring antennas. To reduce the coupling it is therefore important to reduce the re-radiation. For a "minimum diffraction" antenna in particular, no re-radiation occurs when the antenna is loaded on an open circuit (infinite impedance). In the exemplary embodiment of the antenna according to the invention illustrated in FIG. 1, the "minimum diffraction" antenna is produced by charging the dipole output with a high impedance (-300 ohms) and limiting its total length LB, or LH at a value less than 0.4 in the entire frequency band considered. Thus, for an antenna according to the invention the operation in a low sub-band which extends for example from 88 MHz to 240 MHz leads to choose a total length LB less than 500 mm. Similarly, operation in a high sub-band which extends for example from 470 MHz to 860 MHz leads to choose a total length LH less than 140 mm. This constraint leads to the realization of a dipole of reduced length with respect to the half wavelength a, / 2 of the received signal. However, in known manner, the operation of a radiating dipole is optimal, when the length of the strands is close to a quarter of the wavelength. Likewise, it is known that when used at resonance, these antennas exhibit an incompatible narrowband operation of the desired broadband and the equivalent gain requirement. It is also known that, by accepting a certain degradation in performance, it is possible to deviate slightly from the resonance in order to operate the antenna over a given bandwidth. This is particularly the case if one chooses optimum lengths of LH / 2 and LB / 2 strands, substantially equal to one quarter of the wavelength at the center of the frequency band of the subband considered. Thus, on a high sub-band extending for example from 470 MHZ to 860 MHz, the central frequency is 665 MHz. At this frequency, the optimum length LH of the dipole is equal to about 225 mm. Similarly, on a low sub-band extending for example from 88 MHz to 240 MHz the center frequency is 164 MHz. At this frequency, the optimum length LB of the dipole is equal to about 900 mm. It can therefore be seen that, in order to satisfy the requirement of maintaining, in a network operation of its omnidirectional character, the dipole antenna according to the invention, has a total length, LH or LB, depending on the sub-band considered, less than optimal length. According to the invention, this limitation of length is compensated for by the use of a broadband amplifier 17 placed at the output of the antenna. The amplifier used here is configured to have a low noise factor, as well as a high input impedance whose nature is determined to provide the desired noise factor and an output impedance of equal value. the characteristic impedance of the line 18 which connects it to the receiver circuit 19 associated with the antenna according to the invention, a 50 Ohm line for example. The dipole antenna according to the invention thus described constitutes, theoretically, because of the limited length of its strands, and the combination of a low-noise, broadband amplifier and having the impedance characteristics described above, a minimal diffraction antenna. However, in practice this result is completely obtained only in the case of a low subband operation. Indeed, in a high sub-band operation, the lateral segments 13 which are disconnected from the antenna circuit because of the low-pass filters 14 may, in certain cases of arrangement of the low and high subbands. , have lengths sufficient for currents to be created on their surface. They then start to radiate and can couple to neighboring antennas and degrade the diagram of the latter. Therefore, in a preferred embodiment, illustrated in Figure 1, the lateral segments 13 are themselves constituted by conductive elements 15 interconnected by low-pass filters 16, the length of each element being defined so that it does not re-radiate any wave operating in high subband. The low-pass filters 16 are of the same nature and have the same bandwidth characteristics as the filters 14 connecting the central segment 12 of each strand to the corresponding lateral segment 13. According to the invention, the length of the elements 15 is less than 0.3 k, λ being the wavelength of a signal whose frequency is in the high subband. The number of elements 15 and low pass filters is here a function of the value of the upper frequency of the high subband used. Thus, if the high subband considered extends from 470 MHZ to 860 MHz, the length of each element 15 is less than 105 mm, so that each lateral segment 13 having a length of about 305 mm, it is thus divided into two or three elements according to the electrical length represented by the low-pass filters 16 added. Side segments are thus produced which have the advantageous property of not re-radiating the signal, or at least of re-radiating a weak signal, during the operation of the antenna in the high sub-band, whatever this sub-band . The dotted curves 111, 112 and 113 of FIG. 1 graphically illustrate the result obtained. The curve 112 represents in an imaged manner the intensity of the current re-radiated by the central segments during a high subband operation, whereas the curves 113 and 111 respectively represent the intensity of the current re-radiated by the segments. when they are not subdivided into elements 15 and when they are subdivided. It is thus found that the subdivision of the lateral segments advantageously results in a decrease in the re-radiated intensity which becomes identically low for each conductive element constituting the strands 11 of the dipole.

We then turn to FIG. 2 which presents a second example of implementation of the antenna principle according to the invention. The antenna described here is a horizontally polarized, minimal diffraction broadband antenna having an omnidirectional radiation pattern in the horizontal plane. As in the previous example, the antenna illustrated in FIG. 2 is an antenna designed to operate in two distinct frequency bands, one high and one low. For this purpose it comprises two sub-antennas 21 and 22, mounted on an axial vertical support 25 and configured to provide optimum operation respectively in the low subband and the high subband. These sub-antennas 21 and 22 are separate "big wheel" type antennas having by construction a horizontal polarization axis. According to the invention, the dimensions of the elements in arcs of circles 27 or 28 constituting each sub-antenna 21 or 22 are defined so as to achieve the best compromise to constitute a global antenna having a constant gain in each subband and the advantageous properties of a minimal diffraction antenna. Each sub-antenna 21 or 22 is furthermore associated with a low-noise, wide-band 23 or 24 amplifier placed immediately at the output of the sub-antenna in question. According to the invention the amplifiers 23 and 24 have high input impedances and output impedances substantially equal to the impedance of the coaxial line which connect them to the receiver. A coupling device, not shown in the figure makes it possible to connect the output of each amplifier to the input of the receiver.

This gives a global antenna consisting of two antennas whose structures are completely independent here. To avoid the phenomenon of re-radiation of a signal by the low frequency antenna 21 when the antenna operates in high subband, the elements 28 are, in a manner analogous to that described for the example of the FIG. 1, divided into elementary segments whose length is determined so that an elementary segment does not re-radiate any signal when the antenna operates in a high sub-band. The elementary segments constituting the same element 28 are electrically connected to each other via low-pass filters 26. Thus, when the frequency of the wave received by the antenna according to the invention is in the high subband the elementary segments constituting the elements 28 of the low frequency sub-antenna 21 are disconnected from one another so that this sub-antenna is decommissioned and does not re-radiate any signal. In contrast, the high frequency antenna 22 whose dimensions are adapted to the high subband is operational and captures the signals. Conversely, when the frequency of the wave received by the antenna according to the invention is in the low sub-band, the elementary segments constituting the elements 28 of the low frequency sub-antenna 21 are connected to each other by the intermediate of the current transmitted by the filters 26 so that this sub-antenna is put into operation. The high frequency antenna 22 is then out of service because of its size.

FIG. 3 shows a third example of implementation of the antenna principle according to the invention. The antenna described here is also a horizontally polarized, minimal diffraction broadband antenna having an omnidirectional radiation pattern in the horizontal plane. As in the previous examples, it is designed to operate in two distinct frequency bands, a high subband and a low subband. Of "turnstile antenna" type, it comprises two sub-antennas constituted by two horizontal dipoles 31 and 32, arranged in a horizontal plane perpendicular to each other and arranged to be in phase quadrature so that the radiation of the assembly thus formed is omnidirectional in the horizontal plane. The outputs of the two dipoles are connected by means of a coupling device to an amplifier 33 low noise and wide bandwidth. As in the previous examples, this amplifier 33 has a high input impedance and an output impedance substantially equal to the impedance of the coaxial line that connects it to the receiver. Each dipole has a structure similar to that of the dipole according to the invention shown in FIG. 1, with central segments whose length is defined so as to obtain a high frequency antenna capable of operating in the high subband and lateral segments. 37 formed of conductive elements 36, connected by low-pass filters 38, whose length is determined so that it does re-radiate any signal when the antenna operates in high subband. As in the example of FIG. 1, the conductive elements 36 constituting a strand of a dipole are electrically connected to one another and to the corresponding central element 35 via low-pass filters 34. -band in which it operates, the antenna according to the invention behaves as an antenna having only two dipoles formed central elements 35, or as an antenna having two dipoles formed central elements 35 and side 37.

Claims (7)

1. Dual-band antenna, operating over two wide frequency sub-bands, consisting of radiating elements forming a minimum diffraction sub-antenna operating in the upper UHF sub-band, and a minimally diffracting sub-antenna operating in the low VHF sub-band, each element being switched on or off separately according to the sub-band operated, characterized in that the antenna is connected to the input of a broadband low-noise amplifier ( 17), placed directly at its output and having a high impedance input and an output equal to the impedance of the transmission line (18) by which it is connected to the receiver (19), the sub-antenna dedicated to the sub-antenna low band with low pass filters (14) arranged to prevent the operation of this element when the element dedicated to the high subband is in use. 2. A vertically polarized antenna according to claim 1, characterized in that it comprises a dipole formed of two vertical strands (11), each strand being formed by a central strand (12) and an extension strand (13). ) connected to the central strand by a low-pass filter (14) tuned to the low sub-band, the lengths of the central strand (12) and the extension (13) being defined so that in low-band operation the antenna has two long strands of total length LB, less than the half wavelength of the received signal over the entire width of the low subband, and that, in operation in high subband the antenna has two short strands of total length LH, less than half the wavelength of the received signal over the entire width of the high subband. Vertical polarization antenna according to claim 2, characterized in that each extension strand (13) is constituted by a set of vertical elementary strands (15) connected to one another by low-pass filters (16), the length of each elementary strand being defined so that, over the entire upper subband, this is less than 0.31, 1 being the wavelength of the received signal, so that in operation in high sub-band, the extension strand (13) does not re-radiate any signal. 4. A horizontally polarized antenna according to claim 1, characterized in that it comprises two sub-antennas (21, 22) superimposed "big wheel" type, a sub-antenna (21) whose dimensions are adapted for operation in the low sub-band and a sub-antenna (22) whose dimensions are adapted for operation in the high sub-band, each sub-antenna being coupled to the input of a low-noise amplifier (23, 24). ) placed directly at the output of the sub-antenna, configured to operate on a frequency band corresponding to the sub-band of the considered sub-antenna and having a high impedance input and an output equal to the impedance of the sub-antenna. transmission line by which it is connected to the coupler which connects the two low noise amplifiers to the receiver by a common line. 20 Antenna according to claim 4, characterized in that the sub-antenna (21) adapted for low-subband operation comprises low-pass filters (26), arranged so that in high-subband operation. , this sub-antenna (21) does not reray any signal. 25 6. A horizontally polarized antenna according to claim 1, characterized in that it comprises two perpendicular horizontal dipoles (31, 32), each formed of two horizontal strands, whose signals are combined in quadrature phase; each strand 30 being formed of a central strand (35) and an extension strand (37) connected to the central strand by a low pass filter (34) tuned to the low subband, the lengths of the central strand (35) and the extension (37) being defined so that in low band operation the dipole considered has two long strands of total length LB, less than half the wavelength of the signal received over the entire width of the subband , and that in operation in high subband it has two short strands of total length LH, less than half the wavelength of the signal received over the entire width of the high subband. 7. Antenna according to claim 6, characterized in that each extension strand (37) is constituted by a set of horizontal strands (36), connected to each other by low-pass filters (38), the length of each elementary strand being defined such that, over the entire upper subband, it is less than 0.3k, being the wavelength of the received signal, so that in high subband operation, the extension strand does not re-radiate any signal.