Micro-strip antennas have been studied extensively because of the attractive advantages of low profile, ease of integration with active devises, low cost of production and light weight. Because of these merits, forms of the micro-strip patch antenna have been utilized in many applications such as in mobile communication base stations, space-borne satellite communication systems and even mobile communication handset terminals.
Despite these features, micro-strip patch antennas suffer from several inherent
disadvantages of this technology in its pure form namely, they have small bandwidth
and relatively poor radiation efficiency resulting from surface wave excitation
and conductor and dielectric losses. Also to accurately predict, the performance
of this form of radiator in particular, its input impedance nature, typically
a full-wave computationally intensive numerical analysis is required (Xiao
et al., 2006; Yang et al., 2001; Godara,
In order to improve the bandwidth characteristics of micro-strip antenna, different
techniques have been proposed such as using stacked patches, slot coupling and
parasitic patches on the same plane (Yu and Zhang, 2003).
A general law for antennas states that the lowest achievable quality factor
of an antenna is inversely related to the antenna volume. This implies that
the absolute bandwidth increases with increasing patch substrate height (h/λo>0.1)
since, the bandwidth is in inverse proportion to the quality factor. However,
this technique introduces various problems. A thicker substrate will support
surface waves which will deteriorate the radiation pattern as well as reduce
the radiation efficiency.
Also, problems with the feeding technique of the antenna appear. The patch width has similar influence on the bandwidth like the substrate thickness. This comes from the fact that by increasing one of these parameters the volume of the antenna is increased.
Another important substrate parameter that influences the bandwidth is the
permittivity. The bandwidth of a patch antenna increases with decreasing substrate
permittivity (Bahl and Bhartia, 1980). By using low dielectric
constant and thick substrate (h/λo>0.1), the bandwidth can be reached
Another technique for improving the bandwidth is the parasitic technique. There are two configurations of the parasitic geometry: the coplanar geometry and the stacked geometry. The coplanar geometry consist of many patches incorporate coplanar on the dielectric substrate and they are coupled to the main patch (only one patch has been excited). For stacked geometry, the patch radiators are employed one above the other with intervening dielectric layers. This allows two or more resonant patches to share a common aperture area. The stacked patch configuration has the following advantages:
||It does not increase the surface area of the element
||It can be used in array configurations without the danger
of creating grating lobes. Its radiation patterns remain relatively constant
over the operating frequency band
The drawbacks of stacked patch configuration are:
||It has a large number of parameters which make the design
and optimization process is very complex
||It requires >1 substrate layer to support the patches
Applications of micro-strip antenna in present day communication systems require
smaller antenna size in order to meet the miniaturization requirements of radio-frequency
RF units (Xiao et al., 2006). This study introduces
the design and analysis of different (regular and irregular) slots coupling
configurations of micro-strip patch antenna and their effects on the bandwidth
MATERIALS AND METHODS
Design considerations of the proposed antennas: The transmission line model can be considered for the calculations of the rectangular micro-strip patch antenna length.
The length (L) should be slightly <λ/2 where λ is the wavelength in the dielectric medium and it is equal to:
εθ is slightly <εΥ and thats
because the fringing fields around the periphery of the patch are not confined
in the dielectric substrate but are also spread in the air as shown in Fig.
1 and it can be approximately calculated as (Kumar and
Ray, 2003; Grag et al., 2001):
||The patch substrate height
The radiation edge (W) is usually chosen such that it lies within the range
L<W<2 L for the efficient radiation, the ratio W/L = 1.5 gives good performance
according to the side lobes appearance (Mohammad et al.,
|| E-field distribution for rectangular micro-strip antenna
showing the fringing field
The effective length for rectangular micro-strip antenna which caused by fringing
effect is measured to be:
where, Δl is the extension along the length and it is calculated as (Mohammad
et al., 2006):
and then the effective length for the TMmn mode could be calculated
from (Kumar and Ray, 2003):
The resonance frequency for the TMmn mode could be calculated from:
Where εΥ is the relative permittivity. Assuming that:
||The required resonance frequency is 1.23 GHz
||Relative permittivity (εΥ) is 2.62
For this assumptions and according to the above analysis, the patch dimensions
of 76x121.125 mm (LW) can be printed on a ground substrate of thickness (h)
4 mm (~1.6% of the working wavelength) and size 30.4x45.6 cm with good impedance
matching which can be achieved by chosen a proper feed position.
The normalized patterns in the E-plane (Eθ in Φ = 0°
plane) and the H-plane (EΦ in Φ = 90° plane) are given
by Hammer et al. (1979):
Design and simulation of the proposed antenna: This study propose and design certain configuration to enhance the bandwidth without increasing the antenna size by using thin substrate (about 1.6% of the working wavelength), the design will be based on loading number of slots with different shapes into the rectangular patch of the micro-strip antenna.
The proposed micro-strip patch antenna has been designed and simulated using Microwave Office simulation package. The procedure for the design and simulation process can be summarized as follows.
Parameters selection and design: According to the analysis mentioned bofore, the antenna parameters can be shown as follows:
||Frequency of operation = 1.23 GHz
||Patch dimensions (L x W) = 76x121.125 mm
||Substrate of thickness (h) = 4 mm (~1.6% of the working wavelength)
||Relative permittivity (εΥ) = 2.62
||Substrate size = 30.4x45.6 cm
||The probe feed is positioned at (xf = 0, yf = -2.13 cm) with
(0.0) is the center of the patch
Regular slots configuration analysis: The geometrical parameters of
the slot play an important role to control the behavior of the proposed antenna.
The effect of these parameters on the antenna performance (return loss and input
impedance) of the rectangular micro-strip antenna has been discussed on the
basis of simulated results by MW office simulation package. The first step is
to propose three antenna designs with 1-3 slots as shown in Fig.
2a-c simultaneously and their behavior simulated in order
to study the antenna performance. In order to optimize, the numbers and positions
of the regular slots, different number of slots have been proposed and simulated
with different positions as shown in Fig. 3. The results of
this simulation show that a good result was obtained by loading three slots
of width w1, w2 and w3 and lengths of l1-l3,
respectively into the patch of the antenna as shown in Fig. 4.
The determination of the optimum numbers and positions of the slots permit to
go further in the optimization process, the next step is to estimate the optimum
dimensions of each slot, this process can be started by testing the effect of
changing the slot length (l1) for certain value of slot width (w1)
on the bandwidth, the results of this simulation is shown in Table
1, then testing the effect of changing (w1) for certain value
of (l1), the results of this simulation is shown in Table
2. The analysis of these tests leads to estimate the optimum dimensions
of each slot.
Irregular slots configuration analysis: In order to optimize, the slot
shape of the patch antenna, slots with different irregular shapes have been
loaded to the rectangular patch of the antenna as shown in Fig.
||Antenna configurations with 1-3 slots
||Different number of slots with different positions to optimize
the best bandwidth
||Configuration of the proposed micro-strip antenna with three
|| Performance of the proposed antenna verses slots length (l1)
|| Performance of the proposed antenna verses slots width (w1)
|** The two modes excited with separated resonance frequencies
||Slots with different irregular shaped loaded to the patch
of the antenna
|| Configuration of the antenna with irregular slots
The simulation results for different shapes configuration show that the proposed
configuration shown in Fig. 6 gives the optimum performance;
the proposed design gives bandwidth enhancement of about 5 times the bandwidth
of the unslotted antenna.
RESULTS AND DISCUSSION
Analysis of the antenna with the configurations shown in Fig. 7. Figure 7 shows that adding slots into the patch of the antenna create a new resonance frequency and by adding more slots the two resonance regions
|| Return loss for antennas with 1-3 slots
|| Simulated return loss of the antennas parameters
come close to each other yielding an enhanced bandwidth. Analysis of the results shown in Table 1 and 2, lead to estimate the optimum dimensions of the slots according to the designed central frequency and the required bandwidth.
Table 1 shows that the central frequency can be controlled by changing the slot length while Table 2 shows that the bandwidth can be controlled by changing the slot width.
Figure 8 and 9 show that maximum bandwidth
which can be obtained for the structure shown in Fig. 4 is
about 180 MHz which represent a ratio of 14.5% of the central frequency. This
ratio gives an enhancement ratio of about 4.5 compared with the unslotted micro-strip
patch antenna bandwidth is 40 MHz which represent a ratio of 3% of the central
|| Return loss of the antenna parameters
|| E-plane and H-plane, a) For the unslotted antenna and b):
For antenna 3 at the central frequency 1.25 GHz
Figure 10 shows a reduction in the radiation beamwidth for
the slotted micro-strip antenna (beamwidth = 71.1°) compared with that of
unslotted antenna (beamwidth = 75.9°).
Figure 11 shows the bandwidth for the structure shown in Fig. 6 is approximately 200 MHz, this value represent a ratio of 16.2% of the central frequency which gives an enhancement of about 5 times compared with the unslotted micro-strip antenna. Figure 12 shows that there is no any deformation in the radiation pattern and there's only a reduction in the beamwidth of about 4.7% with a small shift in the E-plane from the zenith (φ = 0°).
||Return loss of the proposed antenna with the structure
||E-plane and H-plane, a) For the unslotted antenna and b) For
proposed bandwidth enhanced antenna at 1.23 Ghz
This study introduces the design and simulation of an enhanced micro-strip
patch antenna. Different slots, positions and shapes of the slots has been proposed
and examined. Simulation results show that an enhancement in the bandwidth of
about 5 times (16.2% of the central frequency) that of the unslotted micro-strip
antenna can be achieved without any deformation in the radiation pattern for
h = 1.6% of the working wavelength. This results is better than that obtained
by other researchers Jia-Yi and Wong (2000) presented
a bandwidth of 4.6% of the central frequency for h = 1% of the working wavelength
and Yu and Zhang (2003) presented, a bandwidth of 6.8%
of the central frequency for h = 3.7% of the working wavelength.