Several antenna designs have been made in order to obtain a novel electrically small 3D UHF spherical antenna (
The RFID sensor network (RSN) is an important wireless networking technology which belongs to Wireless Personal Area Network (WPAN). It forms a new research area, which solicited the interest of both the industry and the research community. This new technology is the result of the integration of the radio frequency identification (RFID) technology [
The RSN architectures are possible architectures of the integrated RFID and WSN; according to research works [
Integration of RFID tags with sensors.
Integration of RFID tags with sensors WSN nodes.
Integration of RFID readers with WSN sensor nodes.
Mix of RFID and WSN.
In RFID, WSN, and RSN applications, the antenna is a fundamental and an essential element in the wireless communication between devices. There are many types of antennas operating at UHF frequencies located below the GHz, used in tags with or without sensor integrated, sensor nodes, and tags with sensor nodes integrated, such as the planar antennas [
In the RFID system, an unread tag can cause a significant economic loss or threat to security [
The use of multiple reader antennas [
The use of tags with two dipoles orthogonally placed [
Regarding the WSN and RSN systems, we find the problem of intermittent communication between network nodes due to the use of dipole or monopole antennas, because the radiation pattern of the two antennas has zeros along its wire axis. In the zero direction, the information does not attain the node and the retransmission of information may cause a significant loss of energy, which leads to a reduction of the node lifetime.
So, the optimal solution of these problems is to use an antenna that produces a quasiisotropic radiation pattern; that is, a radiation covers all directions of space (
In previous works, the authors in [
Our goal in this work will focus on the contribution of the development of RFID, WSN, and RSN technologies based on several UHF 3D spherical antenna designs in order to obtain an electrically small 3D spherical antenna with a quasiisotropic radiation and an opening angle equal to 360° used for tags with or without integrated sensors, sensor nodes, and tags with integrated sensor nodes. These latest devices can communicate only at the frequencies that belong to the allocated bands to industrial, scientific, and medical applications; these bands are called ISM bands. So, we have chosen the 915 MHz frequency as the operating one of the antenna which belongs to the ISM band [902–928 MHz].
Note that the design and the simulation were performed using the 3D electromagnetic simulator, HFSS (High Frequency Structure Simulator) which is based on the finite element method (FEM). The use of this simulator has helped us study the geometrical parameters of the antenna to determine the effect of each parameter and to determine an optimal value for each. The simulation results are compared by the CST Microwave Studio simulator (Computer Systems Technology) which is based on the finite integration technique (FIT). A slight difference between the results obtained by the two simulators is due to the difference between the numerical method of each simulator with the simulation step and the mesh used during the simulation.
In this section, we will design several antennas in order to find a very small antenna that operates approximately at 915 MHz and produces quasiisotropic radiation with an opening angle of 360° in all directions of space. The starting point for these designs is a halfwave planar dipole antenna of total length
Structure of the halfwave dipole.
We have chosen this antenna because it has omnidirectional radiation pattern with zeros along its axis wire even though its electrical size
In this design, we will wrap the dipole around the surface of a sphere as illustrated in Figure
Structure of the spherical 3D antenna (
The used sphere is a Styrofoam sphere of dielectric constant close to the air (
The return loss
Dimensions of the Tmatch (Figure
Parameters 



Values (mm)  2.8  21.8 
Return loss
Structure of the 3D spherical antenna (Figure
The new
Return loss
Structure of the 3D spherical antenna with Tmatch: (a) front view and (b) back view.
Table
Dimensions of the 3D spherical antenna (Figure
Parameters 





Values (mm)  26.34  156.64  6.89  30.66 
Return loss
An antenna is electrically small when
New dimensions of the 3D spherical antenna (Figure
Parameters 





Values (mm)  25.18  156.64  6.88  30.54 
Return loss
From the results, the antenna resonates at a frequency lower than the desired frequency (915 MHz) as before. So when we fix
Final dimensions of the 3D spherical antenna (Figure
Parameters 





Values (mm)  23.9  148.94  6.86  30.39 
Return loss
From the results of simulation of Figure
through HFSS, the
through CST, the
As these results shown, the antenna resonates approximately at 915 MHz, with a lower
Regarding the radiation pattern, Figure
Radiation pattern (
Radiation patterns (
So, the radiation pattern becomes nearly isotropic because the two arms of the dipole are wound in the same direction of current along the axes
Current directions along the two arms of the 3D spherical antenna (Table
The antenna gain is also simulated, the maximum gain is 1.52 dBi, and the minimal is −0.68 dBi. Therefore, the gain is reduced to 2.2 dBi which is less than 3 dBi, which means that we have an opening angle of the antenna equal to 360° in all directions of space.
We can compare the antenna of this work with the antenna of the work presented in [
Comparison between this work and the work [
Antenna  Present  [ 


23.9  26 

0.458  0.498 
Resonant frequency desired (MHz)  915  915 
Resonance frequency obtained (MHz)  914.4  930 

1.52  1.81 

−0.68  −8.07 

2.2  9.88 
Opening angle  360°  <360° 
From Table
If we continue the decrease of
In this design, we have reduced the electrical size of the antenna above by folding the arms in the opposite directions (Figure
(a) Structure of the 3D spherical antenna with folded arms and (b) current direction along the folded arms.
We will decrease the radius
Dimensions of each antenna (Figure
Parameters  Values  


22  20  18  16  14 

136.65  124.1  111.5  98.92  86.34 

5.9  8.4  15.6  24.47  35.46 

3.21  3.35  3.31  2.53  2.75 

16  10  10  9.8  10 

−36.52  −21.91  −22.6  −38.34  −21.63 
fr (MHz)  914.9  915.7  915.4  914.9  915.4 
Structure of 3D spherical antennas of different radius: (a)
Return loss
According to the results, all the antennas resonate at a frequency that is close to the desired frequency 915 MHz with different bandwidth which varies proportionally to the size of each antenna.
The radiation patterns of
Radiation patterns (
According to Figure
Radiation patterns (
From the results, we observe that the strong increase in
In this step, we have kept the radius of the previous antenna (
Dimensions of the 3D spherical antenna (Figure
Parameters 








Values (mm)  14  81.74  7.83  4  18.09  3.23  6 
(a) Structure of the 3D spherical antenna with meander line and (b) current direction along the
The
Return loss
The variation of the
Radiation patterns (
Radiation patterns (
The difference between the maximum and the minimum fields is 4.061 dB which is greater than 3 dB, which results in an antenna that has an aperture angle which is inferior to 360°. We will render the opening angle equal to 360° in all directions of space by the reduction in
Dimensions of each 3D spherical antenna (Figure
Antenna  1  2  3  4 


14  14  14  14 

80.94  80.14  79.34  78.54 

9.9  11.36  12.32  13.47 

4  4  4  4 

22.65  25.98  28.28  31.04 

3.23  3.23  3.23  3.23 

6  6  6  6 

−28.89  −42.38  −38.08  −23 
fr (MHz)  915  915.2  915  915 

3.892  3.382  3.044  2.856 
Opening angle (°)  <360  <360  <360  360 
Return loss
The
From the results of Table
Dimensions of each 3D spherical antenna (Figure
Antenna  1  2  3  4 


14  14  14  14 

77.96  77.36  76,36  75.36 

12.38  11.7  11.11  10.86 

4.5  5  5.5  6 

29.02  27.88  27.09  27.09 
Total length (mm)  163.75  160.52  157.76  157.26 

3.23  3.23  3.23  3.23 

6  6  6  6 

−28.09  −28.39  −26.44  −21.42 
fr (MHz)  915.3  914.9  915.1  915.1 

2.58  2.815  2.816  2.835 
Opening angle (°)  360  360  360  360 
Return loss
Radiation patterns (
We succeeded to reduce the total length of the dipole from 169.56 mm to 157.26 mm with the conservation of an opening angle equal to 360° in all the directions of space. The dipole total lengths of antenna 4 and antenna 3 are nearly equal which implies that we have achieved the limit of the reduction.
The electric size
Dimensions of the 3D spherical antenna (Figure
Parameters 








Values (mm)  12  64.76  12.21  5.5  30.48  2.3  6 
From the simulation results of Figure
through HFSS, the
through CST, the
Return loss
From the results, the antenna resonates at a frequency near to 915 MHz with a level of
Regarding the antenna radiation parameters, Figure
Radiation patterns (
Radiation patterns (
The difference between the maximum and minimum field is 2.845 dB, which leads to an antenna producing a quasiisotropic radiation with an opening angle equal to 360° in all directions.
The electric size
Dimensions of the miniaturized 3D spherical antenna (Figure
Parameters 








Values (mm)  10  54,16  13.14  5  33  2.43  4.5 
From the results obtained by HFSS (Figure
Circuit parameters of the 3D spherical antenna (Table
The radiation parameters of this antenna are simulated at the frequency 915 MHz. Figure
Radiation patterns (
Radiation patterns (
The difference between the maximum and minimum field is 2.72 dB. So, we have an antenna that produces a quasiisotropic radiation with an opening angle equal to 360° in each side. The gain and the directivity of the antenna are also simulated; the maximum gain and directivity are, respectively, 1.264 dBi and 1.287 dBi which results in an efficiency of 98.21%.
The radius of the miniaturized 3D spherical antenna is equal to 10 mm which implies a dimension depending on the wavelength equal to
Regarding the polarization, Figure
Axial ratio (AR) in linear scale of the 3D spherical antenna (Table
To compare properly this work with the work of [
Comparison between this work and the previous work [
Antenna  Present  [ 

Radius (mm)  10  12.5 

0.1916  0.24 
Total length (mm)  150.44  216 
Resonant frequency desired (MHz)  915  911,25 
Resonant frequency obtained (MHz)  915.1  912 

−24.59  −15.9 
Radiation pattern  Quasiisotropic  Quasiisotropic 
Gain (dB)  1.264  0.75 
Opening angle (°)  360  <360 
From Table
These results are relatively optimal than those found in [
We can configure the antenna for many input impedance values
The main goal of this work is to design an electrically small antenna on condition that it operates at a frequency of about 915 MHz and produces a quasiisotropic radiation with an opening angle equal to 360° in all directions of space. For this reason, we have first designed a spherical 3D antenna based on a linear dipole and the very small electrical size which can be found is
The authors declare that they have no competing interests.