RU2117368C1 - Georadar antenna - Google Patents

Abstract

FIELD: geophysics and geology; investigating subsurface layers of earth by sounding with electromagnetic pulses. SUBSTANCE: since georadar pulse propagates in medium with permittivity ε = 4-80, proposed antenna is made of conducting material in the form of paraboloid with radiator (resistor-loaded dipole) placed in its focus; internal space of paraboloid is filled with material having permittivity ε = 16-8 approaching that of surface being sounded. Antenna is filled with anisotropic material having maximal permittivity in direction parallel to dipole arms. Light-weight anisotropic material may be obtained from parallel-oriented ferroelectric ceramic needles. Under discussion is antenna design in the form of cylindrical paraboloid for operation in frequency range of 30-150 MHz. Directivity pattern of antenna is about 10-50 deg. EFFECT: improved noise immunity, narrow directivity pattern, and improved selectivity to polarization of radiated rays. 5 cl, 2 dwg

Description

 The invention relates to antenna devices and is intended for use in geophysics and geology in the study of radio sounding subsurface layers of the earth.

 In recent years, the study of subsurface layers of the earth has been finding wider application by sounding with short electromagnetic pulses with a central frequency of the spectrum of 10 MHz - 1 GHz. High-frequency georadars (300 MHz - 1 GHz) provide high spatial resolution, but due to the rapid increase in signal attenuation with a frequency, they have a small (units - first tens of meters) sounding depth. To achieve great sounding depths (tens to first hundreds of meters), it is necessary to use low frequencies from 10 MHz to 200 MHz, and the lower the frequency, the deeper layers are reachable for examination. In low-frequency GPRs, resistively-loaded dipole vibrators are usually used as antennas [1]. Since the electromagnetic wavelength at a frequency of 10 MHz is 30 m, the length of the dipole antennas is 15 m. The large dimensions of the antennas lead to many operational inconveniences. One of the main disadvantages of such antennas, especially when working near sources of man-made interference, is their low noise immunity. For high-frequency GPR shielding from interference antennas, due to the small size, does not present a difficult problem. At the same time, placing bulky low-frequency GPR antennas in the shielding casing is rather difficult, therefore it is necessary to reduce the size of the antennas in one way or another. A simple reduction in the size of the antenna is unacceptable due to a sharp (as the 4th degree of the length of the vibrator) decrease in antenna efficiency [2, 2.VII., P. 150].

 From the equations of electrodynamics it follows that in a medium with a dielectric constant ε the wavelength decreases n = √ ε times, where n is the refractive index of the medium. This circumstance is used in the solution of the patent [3] to reduce the size and increase the efficiency and directivity of the antenna of the ship’s gun guidance station. The dielectric rod antenna is made of materials with high permeability (ε = 16 - 232 on the axis of the rod and ε = 14 - 169 in the outer layer) and is capable of operating up to a frequency of 300 MHz.

 Especially for the purposes of subsurface radio sounding of the earth, the patent [4] proposes a dipole resistively loaded antenna at a frequency of 10 MHz, which is adopted as a prototype. The vibrator shoulders are enclosed in a double coaxial dielectric cylinder. The gap between the inner, which serves as the electrical insulation of the vibrator, and the outer cylinder is filled with saline. GPRs use short pulses, and during shock excitation in the antenna extremely undesirable vibrations from re-reflections of radiation occur, the so-called ringing of the antenna. Typically, to suppress ringing associated with the reflection of waves from the ends of the vibrator, resistors distributed according to a certain law are symmetrically integrated into each arm [1]. In this case, such a resistive load is the saline solution surrounding the vibrator. At the same time, saline filling leads to a reduction of approximately three times from 15 to 5 m of the physical length of the antenna. The obtained shortening coefficient is three times less than theoretically expected (ε ≈ 80 for the saline solution), and this discrepancy can be explained by the fact that the saline solution fills only a small volume adjacent to the vibrator.

 The technical task to be solved is the creation of an effective, compact antenna for ground penetrating radars, shielded from interference, selective to radiation polarization and with a relatively narrow radiation pattern.

 This is achieved by the fact that the antenna emitter, made in the form of a resistively-loaded dipole vibrator, is placed in the focus of the reflector, which is a paraboloid (rotation or cylindrical) of conductive material. To solve the problem of reducing the dimensions of the antenna, the internal cavity of the paraboloid should be filled with a material with the greatest permittivity ε. But at the same time, to reduce the ringing of the antenna, it is necessary to choose a filler with a dielectric constant close to that of the soil.

где
ε_c - проницаемость смеси;
ε₁ и ε₂ - проницаемости первой и второй среды;
p - часть объема, занятая первой средой;
u - число, зависящее от формы и ориентации частиц этой среды.According to the proposed solution, liquids (water ε = 80, methanol ε = 34, ethanol ε = 25), solid dielectrics (titanium dioxide ε = 100), ferroelectrics (ε = 1000 - 10000), and also mixtures of various dielectrics. There is an empirical formula for determining the permeability of a mixture of two dielectrics [5, 6.12, p. 400]

Where
ε _c is the permeability of the mixture;
ε ₁ and ε ₂ - permeability of the first and second medium;
p is the part of the volume occupied by the first medium;
u is a number depending on the shape and orientation of the particles of this medium.

If particles with ε ₁ distributed in a medium with ε ₂ have a spherical shape, then u = 2. If the first medium consists of elongated particles oriented along the direction of the electric field, then, depending on the degree of elongation, the number u varies within 2 <u <∞. In the case u _ → ∞ (more precisely, u >> ε ₁ ), formula (1) takes the form
ε _c = pε ₁ + (1-p) ε ₂ (2)
If the electric field is directed perpendicular to the direction of elongation of the particles, then 0 <u <2, and as u ---> 0, expression (2) can be written as
1 / ε _c = p1 / ε ₁ + (1-p) 1 / ε ₂ (3)
Note that formulas (2) and (3) correspond to well-known formulas for parallel and serial connection of capacities. Thus, the dielectric of oriented elongated particles is substantially anisotropic, and when filling the internal cavity of the reflector with such a mixture, the selectivity to the polarization of the antenna radiation increases. The radiation of the dipole vibrator to one degree or another (depending on the shape of the vibrator) is partially polarized so that the electric vector is mainly directed parallel to the shoulders of the vibrator. Therefore, the orientation of the elongated particles of the dielectric should also be parallel to the shoulders of the vibrator. For radiation with orthogonal polarization, the antenna is ineffective, since the permeability of the filler in this direction is minimal (ε = 1).

= 16 и ε// = 25, в то время как поперек направления вытянутости ε₁ = 1 в соответствии с формулой (3). Для исключения зависимости проницаемости от частоты размеры капсул выбирались из условия малости их по сравнению с длиной волны излучения. Недостатком заполнителя с водяными капсулами является его большой вес. В этом плане большие преимущества сулит использование в качестве заполнителя внутренней полости отражателя антенны частиц сегнетоэлектриков, размещенных в пористом материале. Как следует из формул (1-3), для получения больших значений диэлектрической проницаемости смеси форма частиц сегнетоэлектрика должна быть такова, чтобы выполнялось условие u>> ε₁, чего не так просто добиться из-за очень больших значений ε₁ . Если соблюдено выполнение условия u>> ε₁ , то проницаемость смеси можно рассчитать по формуле (2).The validity of formulas (2) and (3) was checked on samples of polyethylene capsules with water mounted in a foamy material. In accordance with formula (2) at p = 0.2 and p = 0.32 and the degree of elongation l / d = 10, where l is the length and d is the diameter of the volume of water in the capsule, the dielectric constant in the direction of elongation was respectively

= 16 and ε // = 25, while across the direction of elongation ε ₁ = 1 in accordance with formula (3). To exclude the dependence of permeability on frequency, the size of the capsules was chosen from the condition of their smallness compared to the radiation wavelength. The disadvantage of a filler with water capsules is its high weight. In this regard, the use of ferroelectric particles placed in a porous material as a filler of the internal cavity of the antenna reflector of the antenna promises great advantages. As follows from formulas (1-3), to obtain large values of the dielectric constant of the mixture, the shape of the ferroelectric particles must be such that the condition u >> ε _{1 is} satisfied, which is not so easy to achieve because of the very large values of ε ₁ . If the condition u >> ε ₁ is satisfied, then the permeability of the mixture can be calculated by the formula (2).

The experiments conducted on a T-1000 capacitor ferroceramics (ε = 1300 - 1500, an operating temperature range of (-40) - (85) ^o C, a change in permeability in the operating temperature range of ± 10%) showed that formula (2) is valid if the degree of elongation of ferroceramic particles l / d> 50. The use of such ferroceramic needles in comparison with water capsules with equal values of ε gives a gain in weight of 2.5 - 10 times depending on the density of the used ferroceramics (the density of various grades of ferroceramics is in the range of 3.5 - 7 g / cm ² ). An anisotropic mixture can be made, for example, as follows.

 On thin sheets of foamy material, parallel-oriented nests for ferroceramic needles are squeezed out, the nests are filled with needles, then filler is collected in the form of a packet of sheets glued with foamy sealant.

 From the theory of antennas it is known [2] that the more open (the surface bounded by the edge of the paraboloid) of the antenna, the narrower the radiation pattern. For reasons of convenience in operation, it is desirable that the antenna dimensions do not exceed ≈ 1.5 m. At high frequencies, the size of the emitter is much smaller than this value (≈ 8 cm at a frequency of 200 MHz when filling with material with ε = 81), and you can choose as a reflector paraboloid of rotation. For low frequencies, the size of the emitter in one coordinate becomes comparable with the size of the reflector (0.8 m at a frequency of 20 MHz at ε = 81), therefore, the reflector is made in the form of a cylindrical paraboloid. In order to shield from interference, the open ends of the cylindrical paraboloid should be closed with walls of conductive material.

 In FIG. 1 is a schematic view of a low-frequency antenna, where a is the length of the aperture of the paraboloid, b is the width at the base, h is the height, f is the focal length of the paraboloid, 1 is the shoulders of a resistively loaded dipole vibrator, 2 is a reflector made of a conductive material in the form of a cylindrical paraboloid, 3 - inclined end walls of conductive material, 4 - slot to prevent secondary exposure of vibrators, 5 - direct radiation absorber.

In FIG. 2 shows the reflection loss R from the earth’s surface depending on the dielectric constant ε _{g of the} soil: curve 1 - for an unfilled antenna with ε _a = 1; curve 2 - for an antenna filled with material with ε _a = 16; curve 3 - for an antenna with ε _a = 81.

As an example, consider the design of an antenna with a cylindrical paraboloid 2 (Fig. 1), designed to operate in the frequency range 30 - 150 MHz. The antenna has dimensions: the aperture length a = 1.5 m, the width at the base b = 1 m, the height h = 0.5 m. The aperture length a determines the width of the radiation pattern (according to the Rayleigh criterion), and if ε _a = 81 is selected, the width the diagram is ≈ 50 ^o at a frequency of 30 MHz (a / λ = 1.4, where λ is the radiation wavelength) and narrows to ≈ 10 ^o at a frequency of 150 MHz (a / λ = 7.5). In the orthogonal plane, the directivity is determined by dipole vibrator 1 [2, 1.VII, p.147], so the antenna has a knife-like radiation pattern. Since due to the large attenuation of the signal, the georadar is able to work only in the near zone, the divergence of ≈ 10 ^o does not too degrade the limiting spatial resolution determined by the working wavelength. To estimate the width of the radiation pattern, one can use the design patterns for various a / λ given in [2, 12.XVIII, p.493]. The antenna height h = 0.5 m is determined by the focal length f of the paraboloid. The size of the antenna in the transverse (along the generatrix of the cylinder) direction should be slightly larger than the length of the dipole vibrator (0.55 m at a frequency of 30 MHz). Based on this, the antenna width at the top of the paraboloid was chosen to be 0.75 m and at the base of the paraboloid b = 1 m. The ends of the paraboloid are shielded by conductive walls for screening from interference, and to reduce the sound of the antenna, the walls are made inclined so that the reflected radiation does not fall on the dipole vibrator . Also, to suppress ringing, a slit 4 is cut out at the top of the paraboloid, the size of which is larger than the size of the vibrator. When a short probe pulse is applied to the antenna emitter, a spherical wave arises, propagating directly from the emitter, and with a time delay, a plane wave formed by reflection from a paraboloid of a spherical wave. Instead of one, two pulses arise, and to suppress the first spurious pulse of direct radiation, an absorber 5 is placed under the emitter (Fig. 1), for example, from fibrous graphite [5, 12.5, p. 728]. The presence of an absorber is desirable, although not necessary, since the intensity of a spherical wave rapidly decreases with distance (as a square of the distance). With the selected dimensions of the cylindrical paraboloid, the weight of the antenna when filling with a mixture with ε _a = 81 ferroceramic needles with foam is ≈ 50 - 200 kg, depending on the brand of ferroceramics used. Filling the antenna with a mixture with ε _{a /} = 16 reduces the weight to ≈20 - 60 kg, but to maintain the width of the radiation pattern it is necessary to double the operating frequency of the georadar (60 - 300 MHz).

Значения проницаемости различных грунтов лежат в пределах ε_g = 4 - 80. Из выражения (4) можно получить, что при выборе диэлектрической проницаемости заполнителя антенны ε_a = 16 - 25 коэффициент отражения R≤0,1 почти для всех грунтов (R≈0,15 для грунтов с проницаемостью ε_g = 4 и ε_g = 81). На фиг. 2 для сравнения приведены потери на отражение излучения от земной поверхности незаполненной антенны (кривая 1), антенны с ε_a = 16 (кривая 2) и антенны с ε_a = 81 (кривая 3). Как видно из фиг. 2, потери на отражение излучения антенны с ε_a = 16 значительно меньше по сравнению с потерями незаполненной антенны. Антенна с ε_a = 81 имеет минимальные потери для грунтов с ε_g = 36 - 81. В тех случаях, когда важнее вопрос уменьшения габаритов антенны, имеет смысл использовать заполнитель с ε_a = 81. В отличие от обычных радаров, для которых рабочей средой является воздушное пространство, для георадаров распространение сигнала происходит в среде с ε_g = 4 - 80. Поэтому естественно и выгодно заполнить антенну средой с проницаемостью, близкой к проницаемости грунта. При выборе заполнителя с ε_a = 16 - 81 линейные размеры антенны уменьшаются соответственно в 4 - 9 раз по сравнению с незаполненной антенной, а объемные в 64 - 729 раз.In the operating position, the antenna is mounted on the ground surface, so the emitter is inside a closed cavity bounded by the surface of the paraboloid and the ground surface. The radiation reflected from the earth's surface is focused by the paraboloid onto the vibrator, leading to re-emission of the pulse, and instead of a single pulse, a sequence of pulses decreasing in amplitude occurs (the so-called ringing of the antenna). For GPR operation, it is important that the amplitude of subsequent pulses decays quickly, so the reflection of radiation from the earth's surface should be as small as possible. With normal burning of radiation, the reflection loss R at the boundary of two media with permittivities ε ₁ and ε _{2 are} determined by the expression [6]

The permeability values of various soils are in the range ε _g = 4 - 80. From expression (4) it can be obtained that, when choosing the dielectric constant of the antenna filler ε _a = 16 - 25, the reflection coefficient R≤0.1 for almost all soils (R≈0 , 15 for soils with permeability ε _g = 4 and ε _g = 81). In FIG. 2 for comparison, the losses due to reflection of radiation from the earth's surface of an unfilled antenna (curve 1), antennas with ε _a = 16 (curve 2) and antennas with ε _a = 81 (curve 3) are shown. As can be seen from FIG. 2, the reflection loss of an antenna with ε _a = 16 is significantly less than the loss of an empty antenna. An antenna with ε _a = 81 has minimal losses for soils with ε _g = 36 - 81. In cases where it is more important to reduce the dimensions of the antenna, it makes sense to use a filler with ε _a = 81. Unlike conventional radars, for which the working environment is airspace, for GPR the signal propagates in a medium with ε _g = 4 - 80. Therefore, it is natural and beneficial to fill the antenna with a medium with a permeability close to that of the soil. When choosing a filler with ε _a = 16 - 81, the linear dimensions of the antenna decrease by 4 - 9 times, respectively, compared with an unfilled antenna, and the volume by 64 - 729 times.

 Consider briefly the antenna for georadar in action. A transmitting and similar in design receiving radar antenna is installed nearby on the Earth’s surface, at a distance of 2 - 4 m. The transmitting antenna emits a short probe pulse to the subsurface layers of the Earth, and then the waves reflected from various inhomogeneities are captured by the receiving antenna. The GPR output signal is a time sequence of pulses reflected from subsurface inhomogeneities. The issue of interpreting the output signal is the most difficult, still unsatisfactorily resolved problem. The task is to reconstruct the picture of the subsurface structure from a set of temporary sequences of pulses obtained by radiosounding from point to point in the studied area of the earth. But many factors, such as ringing of the antenna, spreading of the pulse, overlapping of pulses reflected from different inhomogeneities, various interference, greatly complicate the problem of interpretation of subsurface radio sounding data. Using the proposed antenna removes a number of difficulties: a narrow radiation pattern suppresses interference whose sources are outside the lobes of the diagram; increases the output signal with equal probe pulse power; significantly reduces the probability of impulses reflected from various inhomogeneities, since the reflected signal comes only from a small, irradiated by the transmitting antenna portion of the subsurface structure. The increased selectivity to polarization of the radiation of the antenna filled with ferroceramic needles makes it possible to obtain additional information about the state of polarization of the output signal and determine from this information the physical parameters of subsurface layers, such as permittivity and conductivity.

Literature
1. Watts RD and Wright DL, Instruments and Methods. Systems for measuring thickness of temperate and polar ice from the ground or from the air. Journal of Glaciology, vol. 27, N. 97, 1981, p. 459 - 469.

 2. Eisenberg G. 3. Antennas of ultrashort waves. - M .: Svyazizdat, 1957.

 3. Krall A.D. and Syles A.M., Embedded Dielectric Rod Antenna, U.S. Patent 4.274.097, NCI 343/719.

 4. Sandler S. S., Broad Band Liquid Loaded Dipole Antenna. U.S. Patent 4.498.086, NCI 343/807.

 5. King P., Smith G. Antennas in material environments, in two books. M .: Mir, 1984.

 6. Kaliteevsky N.I. Wave optics. - M.: Higher School, 1978, p. 62.

Claims (5)

 1. An antenna for a georadar containing a radiator made in the form of a resistively-loaded dipole vibrator, characterized in that the radiator is placed in the focus of the reflector, which is a paraboloid of a conductive material, the inner cavity of which is filled with a material with a dielectric constant close to the dielectric constant of the probed surface in the direction parallel to the shoulders of the vibrator.  2. The antenna according to claim 1, characterized in that the material is an anisotropic material with a dielectric constant close to unity in a direction perpendicular to the shoulders of the vibrator.  3. The antenna according to claim 2, characterized in that the anisotropic material is a mixture of porous material and oriented elongated particles, the longitudinal dimensions of which are significantly less than the working wavelength of the GPR.  4. The antenna according to claim 3, characterized in that ferroelectric needles are used as oriented elongated particles.  5. The antenna according to claim 2 or 3, characterized in that for operation in the low frequency range, the reflector is a cylindrical paraboloid, the end walls of which are made inclined.

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