The cross-shaped fractal antenna design is simulated initially with FR4 substrate and after that simulated with synthesized lanthanum substituted Ba-Sr hexagonal ferrite substrate material in (6-12) GHz band. Then, the performance of intended antenna is analyzed for both substrates with the investigation of certain important parameters such as return loss, bandwidth, and gain. The return loss represents how much energy is going from the feeding point to the designed antenna. If there is impedance mismatch, then the signal can be reflected back from the feed point and will not be radiated by antenna. The return loss of less than -10 dB at a resonant frequency is admissible. The radiation pattern of antenna represents the directional strength of electromagnetic waves from the prototype antenna. It presents the radiation of signal strength by designed antenna corresponding to azimuth and elevation angles at a particular resonant frequency. It is presented as far-field radiation pattern for two main planes referred to as E plane (phi=90 deg) and H plane (phi=0 deg) at observed resonant frequencies [4]. Simulation results of the proposed antenna with FR-4 glass epoxy material and lanthanum substituted Ba-Sr hexagonal ferrite substrates are given below.
FR4 Glass Epoxy Material
FR4 is a flame-resistant (called flame retardant) composite material which is composed of woven fiberglass cloth by an epoxy resin binder. The material FR4 has dielectric loss value i.e., tan δ of 0.02 and permittivity of 4.4 [2]. The important parameters of designed cross-shaped antenna with FR4 substrate material are explained in this section. The performance of different parameters is observed in the (8-12) GHz frequency region. The simulation results of return loss, and bandwidth for all iterations using FR4 substrate material are given in Table no 2.
Table 2 Results of the simulated cross-shaped fractal antenna design using FR4 substrate material
Iteration
No
|
Resonant
frequency
(GHz)
|
Return loss
(dB)
|
Bandwidth
|
0
|
8.9710,
10.0599
|
-23.1478,
-12.0282
|
(9.17-8.6898) GHz
=480.2 MHz,
(10.17-9.8604) GHz
=309.6 MHz
|
1
|
9.3916
|
-30.7699
|
(9.82-8.9439) GHz
=876.1 MHz
|
2
|
9.1789
|
-21.4309
|
(9.6093-8.8005) GHz
=808.8 MHz
|
3
|
9.1789
|
-22.2827
|
(9.5750-8.7617) GHz
=813.3 MHz
|
Fig. 2 shows the comparison of simulated return loss for all iterations using FR4 substrate. It is perceived from the figure that two values of return loss -23.1478 dB and -12.0282 dB are examined at resonant frequencies 8.9710 GHz and 10.0599 GHz in iteration 0, also return loss of -30.7699 dB, -21.4309 dB and -22.2827 dB are obtained at resonant frequencies 9.3916 GHz, 9.1789 GHz, and 9.1789 GHz in iterations 1, 2 and 3 respectively. There is another specific parameter called impedance bandwidth which is a range of frequencies where antenna represents good impedance matching. It can be examined that antenna covers frequency regions 8.6898-9.17 GHz and 9.8604- 10.17 GHz with corresponding bandwidths of 480.2 MHz and 309.6 MHz in iteration 0, also antenna covers frequency regions of 8.9439-9.82 GHz, 8.8005-9.6093 GHz, and 8.7617-9.5750 GHz having bandwidths of 876.1 MHz, 808.8 MHz and 813.3 MHz in iterations 1, 2 and 3 respectively.
It is observed that resonant frequency is reduced with increasing iterations from iteration 1 to iteration 3. Moreover next iterations are not preferred because there is no further reduction of resonant frequency. Additionally, complexity is increased in the design for the iteration 4 that makes it difficult to get it fabricate with required precision (the slot width of cross used in iteration 3 is already 0.35mm). Hence, only four iterations (iteration 0 to iteration 3) are included in the final design. Therefore, iteration 3 is considered to finalize the design of fractal antenna.
Lanthanum doped Ba-Sr Hexagonal Ferrite as a Substrate
Lanthanum substituted Ba-Sr hexagonal ferrite substrate is prepared based on solid-state reaction (standard ceramic) method. There are some steps that are to be followed to compose a substrate using this method. These are named as weighing, mixing, grinding, drying, pre-sintering, crushing of powder in a pestle mortar and sieving of powder using sieves with a mesh size of 220 B.S.S. Moreover, the filtered powder is mixed with PVA binder and then the pallet is prepared using a hydraulic press of 75 KN/m2 uniaxial pressure [29,30]. The final sintering is done at a temperature of 1100 oC. Then, some characterization techniques are used to investigate the structural, electric and magnetic properties of the designed substrate to observe its suitability in X-band.
The S-parameters are analyzed using VNA (Vector Network Analyszer) to find complex permittivity and complex permeability of material compositions in X-band.
Complex permittivity; εr = ε' - ε'' (2)
Complex permeability; μr = μ' - jμ'' (3)
εꞌ, µꞌ, εꞌꞌ, and µꞌꞌ are called as permittivity, permeability, dielectric loss, and magnetic loss [29].
Moreover, tangent loss and magnetic loss are expressed as
tanδ ε = ε''/ε' (4)
tanδ μ = μ''/μ' (5)
εr, µr, tanδε,and tanδμ are required parameters of ferrite substrate which are used to increase performance of fractal antenna design. Figs. 3 and 4 represent variations of ferrite parameters with respect to frequency. Because of variations of these parameters versus frequency is achieved as desired so we used this ferrite material as antenna substrate in the present research based on our earlier research.
Comparison of simulated antenna using Fr4 Epoxy substrate material and lanthanum substituted Ba-Sr hexagonal ferrite substrate material
After the preparation of lanthanum doped Ba-Sr hexagonal ferrite, it is used as a substrate of proposed antenna. This proposed design is simulated with ferrite substrate and its results are analyzed. It is observed that the simulated antenna using ferrite substrate covers the frequency region of 7.8702-9.44 GHz with lower cut-off frequency (-10 dB) existing below the X-band region. Also, the antenna covers another frequency region i.e. 9.68-9.7746 GHz at resonant frequency 9.74 GHz, therefore antenna is simulated in frequency region 6-12 GHz to observe its behavior in this frequency range. Then, it covers one more frequency region 6.2969-6.4 GHz at resonant frequency of 6.36 GHz. Further, various performance parameters of the proposed antenna are compared for both substrate materials in frequency region 6-12 GHz, as represented by Table 3. It is cleared from Table 3 that the proposed antenna designed with FR-4 epoxy resonates at frequency 9.1789 GHz and provides return loss, bandwidth, and gain of -22.2827 dB, 813.3 MHz, and 5.7928 dB respectively. On the contrary, the proposed antenna designed with lanthanum doped Ba-Sr hexagonal ferrite substrate material resonates at 6.36 GHz, 9GHz, and 9.74 GHz and provides return loss of -21.8962 dB, -21.0406 dB, and -11.1134 dB. It also provides bandwidths of 103.1 MHz, 1.5698 GHz, and 101.1 MHz along with gain of 9.2927 dB, 4.4306 dB, and 2.0587 dB at resonant frequencies 6.36 GHz, 9 GHz, and 9.74 GHz respectively.
Table 3 Comparison of simulated design using FR4 epoxy and Lanthanum doped Ba-Sr hexagonal ferrite substrate
materials
FR4 Epoxy
|
Lanthanum doped Ba-Sr hexagonal ferrite as a substrate
|
Resonant Frequency
(GHz)
|
Return loss
(dB)
|
Bandwidth
|
Gain
(dB)
|
Resonant Frequency
(GHz)
|
Return loss
(dB)
|
Bandwidth
|
Gain
(dB)
|
9.1789
|
-22.2827
|
(9.5750-8.7617) GHz
=813.3 MHz
|
5.7928
|
6.36
|
-21.8962
|
(6.4-6.2969) GHz
=103.1 MHz
|
9.2927
|
9
|
-21.0406
|
(9.44-7.8702) GHz
=1.5698 GHz
|
4.4306
|
9.74
|
-11.1134
|
(9.7746-9.68) GHz
=101.1 MHz
|
2.0587
|
Fig. 5 represents a comparison of return loss using both substrate materials. Fig. 6 represents 3D polar plot gain of the designed antenna using FR4 substrate at resonant frequency 9.1789 GHz, Where figs. 7(a)-(c) represent 3D polar plots of designed antenna using ferrite substrate at resonant frequencies 6.36 GHz, 9 GHz, and 9.74 GHz respectively.
Also, the radiation pattern is one of the specific parameter of antenna, which represents power directing capacity of an antenna in a particular direction. Figure 8 (a) represents E plane (phi=90 deg)/H plane (phi=0 deg) simulated normalized far field radiation patterns of designed antenna using Ferrite substrate at 6.36 GHz which are directional approximately toward 330o-30o, fig. 8 (b) represents E/H plane radiation pattern at 9 GHz, where E plane is directional near 30o and H plane shows omnidirectional radiation pattern and fig. 8 (c) represents E/H plane radiation patterns at 9.74 GHz where E plane is directional pattern towards 30o including some distortions and H plane shows bidirectional characteristics nearly towards 90o and 270o. Thus E plane represent directional pattern at frequencies 6.36 GHz, 9 GHz and 9.74 GHz with slight distortions at high frequencies and H plane depicts directional characteristics at low frequency and bidirectional characteristics at high frequency.