Using radio-absorbing materials to protect ground penetrating radar from electromagnetic interference.

Using radio-absorbing materials to protect ground penetrating radar from electromagnetic interference..

GORBATENKO Olga Nikolaevna,
BIBIKOV Sergey Borisovich, Candidate of Physical and Mathematical Sciences

USE OF RADIO-ABSORBING — RADIO-SCATTERING MATERIALS FOR PROTECTING GEORADAR FROM ELECTROMAGNETIC INTERFERENCE

The problem of shielding the OKO-M1 type georadar in various electromagnetic conditions is considered. The issue of choosing a multilayer coating of optimal thickness, possessing the necessary level of reflection and absorption, is investigated.

Modern ground penetrating radar equipment widely uses various types of antennas, such as dipole antennas of the “butterfly” type. Structurally, the antenna is a dipole arm made in the form of two flat metal triangles (Fig. 1). In this case, both the emitting and receiving pairs of triangles are fixed on a single substrate, which is pulled directly along the ground surface. Such antennas provide a symmetrical radiation pattern, where in addition to the main lobes, there are also side lobes (Fig. 2). During actual operation of the device, scattering and reflection of radiation into the upper hemisphere (rear lobe) can significantly distort the picture of the received signal, therefore, the issue of shielding the ground penetrating radar in order to suppress radiation into the upper half-space is relevant.

Fig. 1. Diagram of a butterfly-type dipole antenna

Fig. 2. Directional pattern of a butterfly-type dipole antenna

In OKO type ground penetrating radars, shielding is provided on top with a special conductive and absorbing composite material [1]. Compared to unshielded dipole antennas, such a design allows for acceptable results due to partial suppression of the main regular interference during observation. But this shielding is not enough. Research has shown that it makes sense to use radio-absorbing materials (RAM) to increase shielding.

The purpose of this work is to study the possibility of using RPM to improve the characteristics and reliability of the OKO-M1 type ground penetrating radar with a central frequency of 400 MHz in various electromagnetic environments.

The problem of choosing an absorber of electromagnetic waves of the decimeter and meter ranges based on bulk conductive materials is that such structures, as a rule, provide the desired level of absorption at a thickness comparable to the wavelength, and have a significant height (up to 3…4 m) for the long-wave range. A coating consisting of such absorbers is a bulky structure and requires large areas for its operation during inspection, and also creates problems during transportation. It has been shown [5] that an absorber of electromagnetic waves with a minimum thickness for a given range can have the form of multilayer structures.

It is necessary to create an absorbing shielding coating based on the synthesis of a layered structure with the necessary electrophysical properties, for example, a given level of reflection and absorption in a certain frequency range. It is desirable that such a structure have a minimum thickness. The goal is to synthesize a thin coating with a minimum reflection coefficient. Obviously, by increasing the overall thickness of the coating, it is possible to obtain an arbitrarily small reflection coefficient in the selected frequency range (wavelengths).

It is known [3] that it is impossible to obtain a reflection coefficient on film coatings of less than -10 dB from traditional materials. Such coatings can be obtained, for example, by forming an ensemble of randomly located resistive fibers of finite length due to multiple reflections in the scattering absorbing material.

From here on in the paper we consider the material under study, which is an ensemble of randomly arranged resistive fibers of finite length. The problem of wave scattering by an ensemble of conductors does not have a correct solution [7]. Of interest is the assessment of the reflection coefficient (Kotr) and transmission coefficient (Ktrans) of this very inhomogeneous material. The proposed solution to the problem is based on the following assumptions and conclusions from previous works:

• the radius a of the fiber is less than the length l of the incident wave, ka <<1, where k = 2p/l;
• the random orientation of the fibers in the dielectric leads to a decrease in the reflected field by 3 times compared to the case of fiber orientation parallel to the electric field;
• the length of the fibers, their radius and conductivity are such that the reflection of currents between the ends of the fibers can be neglected.

The properties of the coating can be described by the following characteristics.

Firstly, the nature of the coating material. Secondly, the reflection coefficient of the coating and its angular dependence for different radiation polarization. Using the reflection coefficient to assess the effectiveness of the coating allows you to compare different types of coatings. It should be noted that the actual effectiveness of the coatings may differ from the measured values ​​of the reflection coefficient depending on the electromagnetic environment.

Fig. 3. Coating model:
Р пад(q ) – the power of the electromagnetic wave incident on the coating;
Рипт(q ) – the power of the electromagnetic wave reflected from the coating;
Ррасс(q ) – the power of the scattered electromagnetic wave;
Pprox(q ) – power of the electromagnetic wave transmitted through the coating;
q – angle of incidence of the electromagnetic wave

 Let's determine the reflection and transmission coefficients in the standard way, in accordance with the illustration in Fig. 3:

Kotr (q ) = Potr /Ppad; Kprox (q) = Pprox /Ppad. (1)

Note that in the given expressions the corresponding coefficients, as a rule, depend significantly on the angle of incidence q . However, in the case of using a radio-scattering — radio-absorbing material of the «Ternovnik» type, this dependence is expressed much less significantly due to the significant isotropy of the scattered radiation.

Tests of various operating modes of the OKO-M1 georadar were conducted using a number of radio-shielding and radio-scattering materials of various types.

First of all, the possibility of using metallized fabrics with high shielding factors (about 60 dB) was investigated. However, it turned out that the use of such materials with a high reflection factor led to parasitic reflections inside the shielded contour and, as a consequence, to significant profile distortions.

Therefore, a bulk material with finite conductivity and performing the function of both antenna shielding and partial absorption of EMI was proposed as an electromagnetic screen. This type of RPM based on polyurethane foam really reduced the effect of interference without deteriorating the profile pattern. But this type of RPM turned out to be unacceptable from the point of view of its use in real measurements due to the existing limitations of operational properties.

The third type of materials were multilayer RPMs based on radio-absorbing — radio-scattering carpet-type modules “Ternovnik”, which had acceptable operational properties, stability of parameters under various climatic conditions and ease of installation (Table 1).

Table 1. Characteristics of materials of the “Ternovnik” type

The base that determines the performance characteristics of the materials “Blackthorn” is a processed polyethylene terephthalate film with a metallized coating. The material is characterized by the surface resistance of the metallized film, the type of sprayed metal and the thickness of the polymer base for spraying.

A module of such a structure consists of a network base, into which elements are woven (a film in the form of ribbons, dissected at the edges and twisted into a spiral), which are radially diverging from the axis of the eyelashes (villi) (Fig. 4).

Fig. 4. The design of the carpet-type material “Blackthorn”

To match free space, each layer of material must have an effective resistance that decreases gradually from the inner layer to the outer layer. It is necessary to determine the value of the surface resistance for the metallized film, the types of the deposited metal and the polymer base for deposition, in order to achieve a suitable dependence of the decrease in the “effective” wave resistance of the layers as they move away from the antenna.

For the production of the “Blackthorn” type material, polyethylene terephthalate film with a thickness of 20 and 50 microns is used, metallized with aluminum with a surface resistance of 5…50 Ohm and stainless steel with a resistance of 50-400 Ohm, depending on the required reflection coefficient of the coating. The coating is used both outdoors and indoors, at an ambient temperature of -60 to +60° C.

Let's consider in more detail the interaction of the signal with the “antenna — coating” system.

The interaction of ground penetrating radar radiation with the environment, including the coating, is illustrated in a simplified manner in Fig. 5. The incident electromagnetic wave is partially reflected in the direction of the receiving antenna, partially scattered by the coating in different directions from the antenna, partially absorbed by the coating and partially passes through the coating.

Fig. 5. Interaction of ground penetrating radar radiation with the environment, including the coating

 For the device to operate efficiently, it is necessary to reduce the interference power Р’пом, the power of “parasitic” radiation Р’изл and the radiation power from the coating Р’отр.

The reduction in the intensity of external interference is due to a single passage through the coating. Therefore, it is possible to use thin, highly effective shielding materials.

The effect of re-reflection is reduced by passing through the screen twice. However, in this case, problems arise with interference re-reflections from the shielding coating, which interfere with the normal operation of the receiving antenna. Therefore, the screen material, in addition to sufficient shielding, should also ensure a reduction in its own reflection coefficient. This problem can be solved by using a gradient distribution of the RPM resistance from the inner to the outer layer. According to the results of our studies, it was advisable to use radio-absorbing and radio-scattering materials of the Ternovnik type as layers. The effective conductivity of the layers increases with distance from the transmitting antenna, thereby ensuring matching of the wave resistance and, consequently, a reduction in the reflection coefficient from the screen.

As a result of these studies, a cover was developed, which is assembled from four layers of combined (radio-absorbing and radio-scattering) carpet-type material Ternovnik-MO-20 with different values ​​of surface resistance of the original film.

It turned out that it is advisable to place the layers of the interference volume coating at a certain distance (~10 cm) from the antenna surface in order to eliminate sharp reflection from the boundary between the antenna and the “Blackthorn”. The necessary distance is provided by the shape of the box to which the coating layers are attached.

An examination of known solutions of the wave equation for a flat-layered inhomogeneous medium [4] shows that absorbing dielectric layers with a linear, quadratic and exponential dependence of the ohmic resistance of the layer on the thickness have a minimum thickness of d > 0.35…0.5l, where l is the operating wavelength. The most optimal of such dependencies turned out to be exponential at d > 0.25l [2], on the basis of which many designs of absorbers of electromagnetic waves were developed [5].

Pili 45 mm long and 0.8…1.2 mm wide form a carpet-type coating of average thickness 5 mm. Total thickness of coating consisting of 4 layers of “Ternovnik-MO-20” is 20 cm (d і l/4 = 75/4 = 18.75 cm).

Exponential resistance distribution can be implemented in the form of an absorbing structure consisting of n layers of “Ternovnik-MO-20” material of equal thickness [6] with resistance gradually decreasing from the inner layer to the outer one (Fig. 6).

Surface ohmic resistance of the film of the first inner layer is comparable with resistance of free space (Z0 = 377 Ohm) and is Z1 = 400 Ohm.

Each subsequent layer has a resistance that is half that of each previous layer [2], i.e.

Z2 = 1/2 Z1 = 200 Ohm,
Z3 = 1/2 Z2 = 100 Ohm,
Z4 = 1/2 Z3 = 50 Ohm.

Fig. 6. Distribution of resistance by coating thickness

As a result of the research, the following model of a ground penetrating radar with a cover made of RPM was implemented. The base of the structure is a ski made of vinyl plastic (polyvinyl chloride). A polypropylene box with an attached four-layer “Blackthorn” is put on the antenna and secured to the ski using elastic straps (photo 1).

a) inside;

b) outside
Photo 1. Construction of the OKO-M1 ground penetrating radar with an RPM cover:

The cover was tested in flat terrain near trees. The effect of the trunk and branches of the birch tree, shown in photo 2 on the right, on the resulting profile was studied. Fig. 7, 8 shows one of the fragments of the ground penetrating radar profile with and without the cover. The profile taken without the cover (Fig. 7) shows the intersection of the synchronization axes of interference waves of various shapes.

Photo 2. Radar tests of the prototype

From the birch trunk located to the side of the observation line, the phase axes have a rectilinear shape, from the tree branches the air reflections have the form of a diffracted wave. In Fig. 8 it is clearly seen that there is no multiple reflection from the birch trunk, the intensity of the air waves-interference from the tree branches is much less.

Fig. 7. Fragment of the ground penetrating radar profile made near
trees without a cover (a shielded 400 MHz antenna was used)

Fig. 8. The same profile fragment made with a cover

To determine the efficiency of the coating, it is necessary to examine the signal shape in the profile in detail, using the tool called “Sight” in the GeoScan32 program (Fig. 9, 10). In this window, you can observe the oscillograms of any signal entering the profile and determine its amplitude at any point. A marker corresponding to the position of the sight appears on the profile image in the main window of the GeoScan32 program. By selecting two characteristic points on one route, you can track the change in the amplitude of both the useful signal and the interference. The sights indicate the signal amplitude in conventional units. You can evaluate the efficiency of the cover in decibels by comparing the values ​​of the amplitudes of the useful signal and interference when the georadar is operating with and without a cover.

Fig. 9 shows that the attenuation of the interference signal according to the oscillogram data is 125.67/4, i.e. 31.4 times the wave intensity, or 29.9 dB.

Fig. 9. Determining the amplitude of the harmful signal in conventional units
on profiles with a cover (top) and without a cover (bottom)

Fig. 10 shows a similar profile and oscillogram of the route for a ground penetrating radar with and without a cover. The amplification of the useful signal in the case of using a cover is clearly visible. An assessment of the amplification value shows that the useful signal has increased by 960/342.33 = 2.8 times, or by 5.5 dB.

Fig. 10. Determining the amplitude of the useful signal in conventional units
on profiles with a cover (top) and without a cover (bottom)

For an integral assessment of the interference suppression level, seven different routes of one profile were analyzed using the above method. To exclude the influence of any other factors on the result, tests of the cover were conducted outdoors, far from buildings, where there are no objects. The survey was carried out in a continuous time mode. The antenna was installed motionless on the ground. To create external interference, we carry a metal object past the georadar. In such conditions, two profiles were recorded: with a cover and without a cover (Fig. 11). Comparison of the profile patterns shows that the use of a shielding cover reduces the contribution of the parasitic signal reflected from the metal object (in this case, a shovel) located above the georadar. This is due, firstly, to the shielding effect of the cover and, secondly, to its radio-absorbing and radio-scattering properties.

Table 2 shows the results of assessing the reflection coefficient of seven different routes.

Table 2. Estimation of the reflection coefficient of the cover

The average reflectance value is 16 dB.

Fig. 11. Testing the case without any interference other than a metal object brought in from outside

The studies confirmed the efficiency of using multilayer radio-absorbing — radio-scattering materials to protect the ground penetrating radar from external electromagnetic interference and to reduce the impact of parasitic reflections from objects in the upper hemisphere of the device. Based on theoretical assumptions, the OKO-M1 ground penetrating radar system with a cover based on radio-absorbing — radio-scattering materials of the “Ternovnik” type was manufactured and tested in real conditions.

The cover showed acceptable results both in urban and natural conditions (plain-steppe). The use of a cover of this type allows suppressing the main regular interference during observations and obtaining much higher-quality field material.

At the same time, conditions were identified under which the limitations on the operation of the ground penetrating radar are not completely eliminated by using a cover of the considered design, in particular, when used in closed basements with metal reinforcement. To solve this problem, further development of the cover design is required, in particular, the addition of an additional shielding layer (layers).

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