The paper presents results of simulation of the near- eld radiation zone of ferrite antenna. It describes the HFSS model of the ferrite antenna consisting of a ferrite core, several turns of wire, and an excitation port. The paper also shows results of the antenna far- eld boundary estimation and the near- eld components visualization. The conclusions were made on the applicability of this con guration of the ferrite magnetic antenna in the area of ground penetrating radars.
The ground penetrating radar is aimed at nding and visualizing the objects in concealment environments. The process of nding objects is often complicated due to the nature of the underlying surface (snow, ice, asphalt). A powerful masking re ection from the top edge of the underlying surface layer can completely suppress the re ection signal from the searched object because of the limited receiver dynamic range.
The authors hypothesis is that with speci c restrictions on the ground
penetrating radar, namely, the search and visualization of objects with high magnetic
permeability, such as iron products, the following solution of the raised problem
is possible. Due to the highly expressed magnetic properties of the searched
objects made of iron, it is appropriate to use magnetic antennas, characterized by
predominance of the magnetic eld over electrical in the near- eld zone. Thus,
the hypothesis is that the use of magnetic antennas will allow one to receive the
magnetic component of the eld, re ected from the searched object undermining
the in uence of concealing e ect re ect from the top layer of the concealment
environment with high dielectric permeability [
The purpose of this work is to examine the characteristics of the electromagnetic eld in the near zone of the magnetic antenna as a result of the ANSYS HFSS simulation. It is proposed to develop and use the ferrite antenna model as a magnetic antenna. The article is organized as follows. Chapter 2 examines the ferrite antenna model created in the ANSYS HFSS program. It also analyzes the input impedance of antenna. Chapter 3 shows the results of the eld modeling in the near- eld zone of the ferrite antenna. It also evaluates the long-range 2
The basis of the ferrite antenna is a ferrite core in the form of a solid rod or set of rings. Taking into account the frequency range used, the ferrite material must meet the broadband requirements. It also should have low losses. Therefore, on the basis of the minimum loss requirement in the speci ed frequency range (up to 100 MHz), we have chosen the ferrite ring Amidon FT-50-68. The size of each ring is 12.7 x 7.14 x 4.78 mm. The admissible frequency range is from 1 MHz to 150 MHz.
On the basis of known equations, the equivalent diameter of the ferrite rod is de ned d = q 2 douter dinner = p12:72 2 7:142 = 10:5(mm):
Now the condition that the maximum e ciency of the ferrite antenna on the rod is achieved with a ratio of its length to the diameter being about 25{30, the required rod length can be calculated boundary of the radiation on the basis of the wave resistance approximation to the value of 120 ohm. Analysis of the in uence of the amount of the wire turns on the antenna parameters is also given. Chapter 4 provides a brief discussion of the results.
L = d (25:::30) = 10:5 25 = 263(mm):
The speci ed ferrite rod has a wire winding with the following parameters: the number of coil turns (turns), the diameter of the wire in millimeters (wire d ), and the pitch of the coil winding in times (pitch). Ferrite rod with wire winding represents an inductor with ferrite core, or a ferrite antenna from another point of view.
The widely used design of a ferrite antenna in the form of a simple
winding of the wire on the rod has a number of disadvantages. In particular, the
power supply terminals of the coil are spaced apart from each other, causing
a loop forming when connected. There is irregularity in the phase distribution
of the eld strengths because the coil is asymmetrical. Turning to the problem
of suppressing a direct coupling signal by constructing a di erential circuit of
two or more receiving antennas [
Firstly, let us consider the input impedance of the antenna in the range 50100 MHz (Fig. 2). In this case antenna parameters are the following: the number of wire turns is eight, the wire diameter is 1.5 mm, and the winding step is four wire diameters.
It can be seen that the real part of the input impedance is from 1.8 to 6.3 Ohm, and the imaginary part is from 142 to 406 Ohm. Positive reactance indicates the \inductive"nature of the input impedance in a given frequency range. It is seen that the graph of the reactance at the upper frequency becomes hyperbolic instead of the linear. It indicates that there is undesirable parallel resonance at an excess of 100 MHz.
The cross-sections of the 3D-body of the radiation pattern by the azimuth plane at zero elevation and the angle plane at zero azimuth are shown in Fig. 3. In this case, the axis of the ferrite rod is directed along the X axis. The zero directional diagram in this direction is observed. It is seen that the greatest directionality of the antenna is 2.5 dB in the direction perpendicular to the direction of the axis of the ferrite rod. The directional pattern in this case has the form of a torus. It is known that any induction coil has a parasitic intercoil capacity, which results in a parasitic parallel resonance. Let us analyze the antenna input impedance at 75 MHz when the number of coils is changed from 3 to 20. As it is seen in Fig. 4, with the number of coils between 7 and 8, a vivid resonance occurs. A more precise resonance level can be achieved by tuning the frequency.
We will consider the in uence of the number of coil turns on the characteristics of the eld in the near zone. For this purpose, we analyze the graphs of intensity of the electric and magnetic elds along a ve-meter-long line emanating from the geometric center of the antenna. The number of turns in each half of the coil is two turns, ve turns, or ten turns. Figure 5 shows that the curves practically coincide. It can be concluded that within a single design in a xed frequency range, varying the number of turns of the coil signi cantly a ects only the input impedance of the antenna. Wherein, it should not be forgotten about the di erent frequencies of a coil self-resonance.
There are di erent approaches to divide the radiation eld of the antenna into the near- eld and far- eld radiation zones, as well as there are di erent criteria for determining the boundary of far- eld radiation. One of criteria is to consider the wave impedance, namely, the ratio between the intensities of electric and magnetic components of the electromagnetic eld. When an exact ratio is established, then the intensity of an electric eld is 120 times more than the magnetic eld intensity. On one may say that the electromagnetic wave front has formed, which propagates in the environment. Let us consider the character of the wave impedance of a ferrite antenna in the near- eld zone (Fig. 6). It can be seen that the wave impedance in the near- eld zone has an inductive character, i.e. the magnetic eld prevails over the electric one. As the oscillations of this curve settle around 377 Ohm, the presence of the far- eld radiation boundary at this distance can be predicted. It can be said that this boundary is located at the 1{meter distance in case of the frequency equals to 75 MHz (wavelength equals to 4 meters). This result, with an error, corresponds to a theoretical estimate of the far- eld radiation boundary for a full-size antennas df = 2 = 0:63(m): (3)
Finally, the isosurfaces of intensities of the electric eld (Fig. 7) and magnetic
eld (Fig. 8) in the azimuth plane of the antenna are shown, while the position of
the antenna is displayed with a red dot at the center. The visible area of the eld
mapping is ve by ve meters, which is 1.25 at an average wavelength equals
to 4 meters. The maximum linear size of the antenna is 263 mm, or 0.066 .
The structure of the electric and magnetic elds forming in the near- eld zone
is clearly visible. We can say that the structure of the magnetic eld in the near
zone is more uniform and distinctly formed. This is one of the distinguishing
features of magnetic antennas in comparison with electrical ones.
The modeling results show that it is reasonable to design short-range VHF
antenna systems based on ferrite antennas. This will allow one to implement the
discussed advantages that are impossible to radio-vision systems with shorter
wavelength [
Thus, the results of the research con rm the hypothesis of the authors that the use of magnetic antennas can allow one to receive the magnetic component of the eld re ected from the searched object undermining the in uence of concealing e ect re ecting from the top layer of the concealment environment with high dielectric permeability. This results could be used for further research on evaluating the possibility of designing the GPR with a reduced in uence of the underlying surface.
It should be noted that the ferrite antenna is an electrically small antenna
with all the inherent disadvantages [