Dimensions and fractional bandwidth (
In ultra wideband (UWB) systems, extremely short pulses are used. These pulses can provide data with high bit rate. They usually occupy ultra wide band in the frequency domain. The spectrum of frequencies reserved for these systems is 3.1-10.6GHz. Examples of UWB signal applications are communications, radar and imaging systems (Aiello & Batra, 2006; Schantz, 2005).
Planar antennas are widely used in UWB systems because of their low cost of fabrication, low size, and simple structure. Some examples of conventional planar monopoles antennas with rectangular, triangular and circular geometries are presented in (Chen & Chia, 2006). One of the deficiencies of the rectangular monopole is its relatively small matching bandwidth which is about 80% (Ammann, 1999). This value is smaller than the full bandwidth of the UWB systems, which is 110% (the frequency range is 3.1-10.6 GHz).
Some techniques can be used to enlarge the bandwidth of planar monopole antenna. For this purpose, modifications of the ground plane and a T aperture in the geometry of the antenna were made (Hong et al., 2006). In (Valderas et al., 2006), a monopole planar antenna with a folded patch was used, and antennas with elliptical geometries were analyzed in (Abbosh & Bialkowski, 2008).
To improve the impedance bandwidth, a technique of combining microstrip and linear antennas with parasitic loops is used. This technique is based on minimizing the reactive energy stored in the near zone of the radiator. This is achieved because the two elements possess opposite average reactive energy.
In this work, we analyze four planar UWB antennas with cuts at the edges and parasitic loops. The antennas investigated are: a rectangular monopole with two loops, a rectangular monopole with four loops, a rectangular monopole with cuts at the edges, and a rectangular monopole with cuts at the edges and two parasitic loops. To enlarge the matching bandwidth, the dimensions of the antennas were optimized with cut-and try method. For the numerical analysis, some Method of Moments (MoM) codes were developed (Harrington, 1968). For comparison, some calculations were made using the software IE3D.
2. Geometries of the antennas
Fig. 1 shows the geometries of the proposed antennas. In this figure,
3. Mathematical model
3.1. Integral equation for the electric field
where (V/m) is the electric field radiated by a current density (A/m) on the conductors of the antenna. This current will appear when the antenna is fed by a coaxial cable connected at the point
3.2. Numerical solution by MoM
The numerical MoM solution of (1) presented in this section is explained by using example of the rectangular monopole antenna (Fig. 2). With minor modifications in the geometry, this model is used to analyze the proposed UWB antenna (Fig. 1).
The problem to be solved here is to find the current distribution in (1) when a given external electric field is falling on the antenna. This incident field represents the source of the problem. The conductors of the antenna are considered lossless. In this case, the boundary condition on
Fig. 3 shows geometrical details used in each current element of index
The left side of (7) means a voltage Δ
The ground plane is modeled by infinite and perfect conductor, therefore one can use the image theory. The coaxial cable is modelled by a delta gap Δ
The rectangular loops were described by striplines with one-dimensional current density. This current possesses component
4. Numerical results
Four MoM codes based on the model presented in previous section were developed in this work. With these computational programs, several simulations were made. Some of the geometrical parameters such as (
In all simulations with the developed codes, the discretization with square cells Δ
4.1. Input impedance and reflection coefficient
Figs. 4-5 show the results of the input impedance (
We see in Fig. 4 the effects of the loops on the resonant response of the input impedance of the antennas. In the cases 1 and 2 of Figs. 4(a) and (b) the resonance of the rectangular patch is near the frequency F=3.5GHz and the resonance of the loops is near the frequency F=6GHz, where for the case 2 (Fig. 4(b)) this resonance is more intense because in this case there are more loops (four) than the case 1 (two). The antennas that have cuts at the edges in the patch (Figs. 4(c) and (d)), the resonances of the patch and loops are similar to the other cases 1 and 2, but the resonance of the patch with cuts is a little bit greater.
The proximity of the patch and loops resonances in all these antennas produce a coupling of resonances, where the real part of the input impedance
From Fig. 5, one can observe that the conventional rectangular monopole antenna has a bandwidth near the 80%, which does not cover the whole frequencies of UWB systems. The antenna of case 1 possesses impedance matching (Γ<−10dB) in the bandwidth of 91%, and for the antenna of case 2, the bandwidth is 95%. The antennas of cases 3 and 4 with cuts (with and without loops) possess impedance matching in the range of frequencies of UWB systems (3.1-10.6 GHz). But only the antenna of case 4 with cuts at the edges and two loops has the reflection coefficient Γ<−15dB in the frequency range 3-7GHz (Fig. 5(d)). Thus, the antenna of case possesses a better impedance matching.
4.2. Current distribution
This section presents the modal current distributions of the proposed antennas in some frequencies, calculated with the developed codes. Fig. 6 shows the distributions of the superficial currents calculated with the developed MoM codes on the patch of the antenna case 2 (Fig. 1(b)) with four loops at the frequency F=4.7GHz. This figure presents the rectangular components
Fig. 7 presents examples of current distribution on the surface of the antenna in case 4 with cuts and two loops at the frequencies F=4 and 8GHz obtained by the MoM code. These current distributions are similar to that of the resonant modes with
4.3. Radiation diagrams
Figs. 8 and 9 present the radiation diagrams of the antennas for cases 2 and 3 with two and four loops respectively. These diagrams were calculated in the middle frequency of the band. The results showed were calculated with the MoM codes and with the software IE3D. A good agreement of the results is observed.
The vertical diagrams at the planes
Fig. 10 shows the radiation diagrams of the antenna case 4 with two loops for the frequencies F=2.5, 5.0, 7.5 and 10.0 GHz. The results were calculated by the MoM code and by the software IE3D. One can see a good agreement between them. These figures present the radiation diagrams in the planes
In the vertical plane, the variation of the diagrams with the frequency is a function of the current distribution in the patch of the antenna, where for low frequency the distribution is near the resonant mode of
We presented in this work four antennas with good input matching and radiation diagrams for applications in UWB systems. The antennas are planar monopoles with cuts at the edges and parasitic loops. The analysis of these antennas was made by the developed MoM codes and by the software IE3D. The results obtained by these programs have a good agreement. From the presented results, one can note that the loops improve the input matching, and the cuts at the edges enlarge the bandwidth. The effect of the loops is to introduce a new resonance near the patch’s resonance, so that the coupling of these resonances enlarges the input matching of the antenna. The antenna that presented the best input matching was the one with cuts at the edges and two loops, where the bandwidth covers all the range of frequencies of UWB systems (3.1-10.6 GHz). We also observed that the radiation diagrams of these antennas are a function of the frequency and the antenna’s geometry. One proposal for future work is the analysis of antennas with cuts at the edges and four parasitic loops placed symmetrically with respect to the plane xy in order to improve the radiation diagram of the antenna.
This work was supported by the Brazilian agency CNPq.