=Paper=
{{Paper
|id=Vol-2076/paper-08
|storemode=property
|title=Simulation of the Near-field of a Ferrite Antenna
|pdfUrl=https://ceur-ws.org/Vol-2076/paper-08.pdf
|volume=Vol-2076
|authors=Alexey A. Kalmykov,Kirill D. Shaidurov,Stanislav O. Polyakov
}}
==Simulation of the Near-field of a Ferrite Antenna==
https://ceur-ws.org/Vol-2076/paper-08.pdf
Simulation of the Near-field of a Ferrite Antenna
Alexey A. Kalmykov, Kirill D. Shaidurov, and Stanislav O. Polyakov
Ural Federal University named after the first President of Russia B.N.Yeltsin
Ekaterinburg, Russian Federation
k.d.shaidurov@ieee.org
WWW home page: http://urfu.ru
Abstract. The paper presents results of simulation of the near-field
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-field
boundary estimation and the near-field components visualization. The
conclusions were made on the applicability of this configuration of the
ferrite magnetic antenna in the area of ground penetrating radars.
Keywords: Ground penetrating radar, Near-field estimation, Ferrite
antenna
1 Introduction
The ground penetrating radar is aimed at finding and visualizing the objects in
concealment environments. The process of finding objects is often complicated
due to the nature of the underlying surface (snow, ice, asphalt). A powerful
masking reflection from the top edge of the underlying surface layer can com-
pletely suppress the reflection signal from the searched object because of the
limited receiver dynamic range.
The authors hypothesis is that with specific restrictions on the ground pene-
trating 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 ob-
jects made of iron, it is appropriate to use magnetic antennas, characterized by
predominance of the magnetic field over electrical in the near-field zone. Thus,
the hypothesis is that the use of magnetic antennas will allow one to receive the
magnetic component of the field, reflected from the searched object undermining
the influence of concealing effect reflect from the top layer of the concealment
environment with high dielectric permeability [1].
The purpose of this work is to examine the characteristics of the electromag-
netic field 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 field model-
ing in the near-field zone of the ferrite antenna. It also evaluates the long-range
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boundary of the radiation on the basis of the wave resistance approximation to
the value of 120 ohm. Analysis of the influence of the amount of the wire turns
on the antenna parameters is also given. Chapter 4 provides a brief discussion
of the results.
2 Model of the ferrite antenna
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 specified 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 defined
q p
d= d2outer − d2inner = 12.72 − 7.142 = 10.5(mm). (1)
Now the condition that the maximum efficiency 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
L = d · (25...30) = 10.5 · 25 = 263(mm). (2)
The specified 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 wind-
ing 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 field strengths because the coil is asymmetrical. Turning to the problem
of suppressing a direct coupling signal by constructing a differential circuit of
two or more receiving antennas [2], it is important to ensure the uniformity of
wave propagation in the direction of each receiving antenna. Such a task might
be solved by a modified model of a ferrite antenna with two in-phase coils con-
nected at a common point. The 3D-view of the ferrite antenna with in-phase
coils is shown in Fig. 1. It is seen that two coils are symmetric relative to the
center of the rod. They are connected at a common point, and the coil ends are
connected by an earth conductor (red line) and contact the power port at the
center. Also, the coil has the mentioned parameters turns, wire d and pitch.
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Fig. 1. 3D-view of a ferrite antenna model
Firstly, let us consider the input impedance of the antenna in the range 50-
100 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.
Fig. 2. Antenna input impedance: imaginary (red line) and real (blue line) parts
69
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.
Fig. 3. Azimuth plane (blue line) and elevation plane (red line) radiation patterns
3 Near-field estimation
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 influence of the number of coil turns on the character-
istics of the field in the near zone. For this purpose, we analyze the graphs of
intensity of the electric and magnetic fields along a five-meter-long line emanat-
ing from the geometric center of the antenna. The number of turns in each half
of the coil is two turns, five turns, or ten turns. Figure 5 shows that the curves
practically coincide. It can be concluded that within a single design in a fixed
frequency range, varying the number of turns of the coil significantly affects only
the input impedance of the antenna. Wherein, it should not be forgotten about
the different frequencies of a coil self-resonance.
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Fig. 4. Parasite resonance, depending on the number of wire turns at fixed fre-
quency 75MHz: imaginary (above) and real (bottom) components of the antenna input
impedance
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Fig. 5. Intensities of the electric (upper curves) and magnetic (lower curves) fields
depending on the number of wire turns, dB
There are different approaches to divide the radiation field of the antenna
into the near-field and far-field radiation zones, as well as there are different
criteria for determining the boundary of far-field radiation. One of criteria is to
consider the wave impedance, namely, the ratio between the intensities of electric
and magnetic components of the electromagnetic field. When an exact ratio is
established, then the intensity of an electric field is 120 times more than the
magnetic field 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-field zone (Fig. 6). It can be
seen that the wave impedance in the near-field zone has an inductive character,
i.e. the magnetic field prevails over the electric one. As the oscillations of this
curve settle around 377 Ohm, the presence of the far-field 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-field radiation boundary for a full-size antennas
λ
df = = 0.63(m). (3)
2·π
Finally, the isosurfaces of intensities of the electric field (Fig. 7) and magnetic
field (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 field
mapping is five by five meters, which is 1.25λ at an average wavelength equals
to 4 meters. The maximum linear size of the antenna is 263 mm, or 0.066λ.
The structure of the electric and magnetic fields forming in the near-field zone
is clearly visible. We can say that the structure of the magnetic field in the near
72
zone is more uniform and distinctly formed. This is one of the distinguishing
features of magnetic antennas in comparison with electrical ones.
Fig. 6. Estimation of the far-field radiation boundary by wave impedance, Ohm
4 Conclusion
The modeling results show that it is reasonable to design short-range VHF an-
tenna systems based on ferrite antennas. This will allow one to implement the
discussed advantages that are impossible to radio-vision systems with shorter
wavelength [3]. A small directivity and complicated impedance is a compromise
with an extremely large reduction in the size of the antenna (up to 0.09λ). The
boundary of the far-field radiation zone is at a range of about 1 meter according
to the simulation results. Thus, at ranges up to 1 meter, the magnetic field emit-
ted from the antenna dominates. In other words, surrounding objects and the
underlying surface differently affect on the GPR antenna in the cases of using
the magnetic or electric antennas.
Thus, the results of the research confirm the hypothesis of the authors that
the use of magnetic antennas can allow one to receive the magnetic component
of the field reflected from the searched object undermining the influence of con-
cealing effect reflecting 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 influence of the
underlying surface.
It should be noted that the ferrite antenna is an electrically small antenna
with all the inherent disadvantages [4]. It is necessary to carefully design the
matching circuits, since the impedance of the ferrite antenna has a large imagi-
nary part and must be accurately matched [5].
73
Fig. 7. Electric field intensity in the near-field zone, V/m
74
Fig. 8. Magnetic field intensity in the near-field zone, A/m
75
References
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practice of three-dimensional radio frequency visualization of objects. In: CriMiCo
2013 –2013 23rd International Crimean Conference Microwave and Telecommuni-
cation Technology, Conference Proceedings. 1169–1170 (2013)
3. Kalmykov, A. A., Shaidurov, K. D.: Evaluating the performance of several types
ofantennas in a holographic radar with continuous wave. In: CriMiCo 2014 –2014
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