Abstract
An improved low-profile printed antipodal drop-shaped dipole antenna for wide-band wireless applications is presented. The proposed radiating structure is integrated with a planar metal reflector useful to enhance antenna gain up to 5.5 dBi and the front-to-back ratio up to 21 dB. The geometry of reflector, feeding line, and dipole arms is optimized in order to achieve a broad operating bandwidth useful to meet the requirements of modern wireless communication protocols. Furthermore, the particular shape of the metal reflector and the adoption of a thin low-permittivity dielectric substrate result in a low distortion of the radiated field and a limited back radiation which makes the antenna suitable for UWB applications as well. These features, together with the low profile and the limited occupation area, make the antenna well adapted to mobile terminals as well as radio base stations. A locally conformal FDTD numerical procedure has been adopted to design and analyse the radiating structure, while a SEM technique has been employed to highlight the field perturbation caused by the antenna reflector as well as to extract the characteristics underlying the transient behaviour of the antenna. The experimental measurements performed on an antenna prototype are found to be in good agreement with the numerical computations.
1. Introduction
Wide-band antennas, having low profile and reduced manufacturing costs, are finding widespread interest in the industrial and the scientific community [1–18]. This interest is dictated by the numerous applications such as those concerning the wireless communications [1–14], the automotive transportation [19–21], the remote screening of biological activities [22–24], as well as the analysis of the ground subsurface [25, 26], and through-the-wall radar imaging. Moreover, they may be used to realize UWB systems for short-range and indoor wireless communications characterized by low transmission-power consumption and high immunity to multipath interferences [27]. In addition, the growing demand for portable and/or wearable devices is further expanding the application ranges of such antennas [15–18]. In this context, antennas designed to support UWB signals, that is, such as to ensure a low distortion of the radiated signals, can be used in both wide-band wireless applications and pulsed radio communications. These design goals cannot be achieved with all classes of broadband antennas, or by using antennas realized with thick dielectric substrates having high permittivity, they being potentially responsible for dispersive phenomena and, in turn, by a significant signal distortion [28].
To overcome these limitations a low-profile printed drop-shaped dipole antenna for wide-band and UWB communications has been recently introduced in the literature [4]. This antenna is characterized by a reduced occupation area, a large operating bandwidth, a limited signal distortion, and a good rejection of the spurious backward radiation in the direction of the feeding line, where the sensitive electronic circuitry adopted to process the RF signal is typically located. Even though said antenna exhibits excellent electromagnetic performance, in some applications it may be useful to employ an antenna having a higher gain along the broadside direction, as well as a more limited emission in the direction of the feeding line. To this end, in this paper a low-profile drop-shaped antenna provided with a planar metal reflector is presented. This antenna, which is intended for wireless applications, is useful to meet the demanding requirements of different communication standards, such as WLAN/WiMAX and UWB protocols. Similarly to the drop-shaped dipole antenna presented in [4], the proposed antenna can be used in stand-alone or array configuration.
The paper is organized in five sections. In Section 2, the electromagnetic performances of the proposed antenna are analysed; in Section 3 the radiation characteristics of the antenna are presented, while in Section 4 the time-domain characteristics are derived and discussed in detail. Finally, some concluding remarks are given in Section 5.
2. Full-Wave Analysis of the Drop-Shaped Antenna Equipped with a Planar Metal Reflector
The geometry of the proposed antenna is sketched in Figure 1, while its physical prototype is shown in Figure 2. The antenna consists of a printed dipole whose radiating arms have a drop-like shape similar to that proposed in [4], whereas the ground plane is modified in order to play the role of a planar reflector, whose geometry is defined by means of arcs of ellipses having main axes and centres as shown in Figure 1.
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Two design formulas, describing the profiles of the feeding line and of the reflector in terms of simple parameters, have been adopted to simplify the design of the antenna. The following analytical expression, introduced in [4] to model the feeding line profile, has been employed:where the parameters , are the width of the initial and final sections of the tapered feeding line of length having tapering parameter . On the other side of the board (see Figure 1(b)), a suitable reflector, whose boundary profile is the combination of arcs of ellipses, is printed. In particular, its geometry is described by the following expression:where the parameters , identify the semiaxes and , the centres of the ellipse describing the reflector boundary, while , and , describe the tapering of the feeding line protruding from the ground plane.
In (2), denotes the point where the two curves match. To guarantee the continuity of the first derivative of function (2) at the matching point , the following relations between the above parameters have to be employed:In (3),where and identify the point where the curve describing the contour of reflector (2) is joined with the feeding line profile described in (1). All the parameters describing the geometry of the reflector are shown in Figure 3 for the sake of clarity.
Notice that, thanks to the full parameterization of the geometrical profile of the antenna, an automatic design procedure can be usefully adopted in order to ensure a proper Ω impedance matching over a wide frequency range.
In order to evaluate the performance of the proposed drop-shaped dipole antenna integrated with a metal reflector, the same geometrical dimensions and dielectric material adopted in [4] for the substrate have been employed. This means that the radiating element is printed on a dielectric substrate (CuClad217) having dimensions mm and mm, relative dielectric permittivity , at 10 GHz, and thickness mm, approximately equal to about 0.6% of the free-space wavelength at the working frequency of GHz. Since the radiative properties of the antenna are strongly affected by the geometry of the feeding line and of the metal reflector, a thorough investigation has been carried out in order to ensure a proper Ω impedance matching.
So, after confirming the suitability of the feeding line profile (1) with , and , and as indicated in [4], to cover the UWB band, a particular attention has been devoted to the design of the metal reflector. To this end an extensive parametric analysis, useful to optimize the radiative characteristics of the antenna, has been carried out by means of a full-wave locally conformal finite-difference time-domain (FDTD) numerical technique [3]. In Table 1, the optimized values of all the parameters defining the antenna geometry, computed using the mentioned FDTD numerical technique, are reported.
By way of example, in Figures 4–7 the frequency behaviour of the magnitude of the antenna input reflection coefficient , obtained when the geometrical reflector parameters , , , and change, is reported. From the analysis of these figures it appears that the parameter , which identifies the distance of the antenna radiating arms from the edge of the metal reflector, has a significant impact on the antenna impedance matching contrary to the others geometrical parameters , , and . In fact, variations in the order of about one millimetre around the optimum value can impair the frequency behaviour of the antenna, increasing the value of the lower operative frequency or reducing the dB margin of the reflection coefficient.
Finally, Figure 8 shows the frequency behaviour of the reflection coefficient of the drop-shaped printed dipole antenna equipped with a metal reflector, as modelled by using the mentioned FDTD technique and as measured on the antenna prototype shown in Figure 2.
As it appears in Figure 8, the proposed antenna features a bandwidth larger than 10 GHz at 10 dB return-loss level (VSWR < 2 : 1) and a bandwidth of about 12 GHz at 6 dB return-loss level (VSWR < 3 : 1); both results are similar to those observed for the antenna without reflector [4]. A good agreement between the FDTD numerical results and the experimental measurements can be noticed.
3. The Radiation Characteristics of the Antenna
Using the computed electromagnetic field values, the spatial distribution of the surface current density excited on the reflector and on the dipole arms, as well as the real part of the Poynting vector, and the radiation patterns (see Figure 9) have been evaluated at the working frequencies of 5 GHz, 6 GHz, and 7 GHz, both for the antenna with and without metal reflector. Moreover, for the sake of completeness, the frequency behaviour of the maximum antenna gain and of the front-to-back ratio is reported in Figure 10.
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The maps relevant to the surface current densities, not reported here for the sake of brevity, show that the surface currents are mainly confined along the edges of the reflector and of the radiating arms of the antenna and exhibit a global maximum in proximity of the excitation region. Although the behaviour of the currents excited on the radiating arms of the antenna is similar to the case presented in [4], the presence of the metal reflector determines an enhancement of the quasi-traveling wave behaviour of the radiated electromagnetic field already at low frequencies, namely, at 4 GHz. This particular behaviour leads to an increment of the field level radiated along the broadside direction which reaches its maximum effectiveness at about 6 GHz, where the antenna gain exhibits an enhancement of about 4 dB along the desired direction and, at the same time, a reduction of about 10 dB in the backward direction.
Starting from the frequency of about 8 GHz the field focusing effect along the broadside direction tends to weaken due to the formation of antenna beams pointing in directions different from that of the boresight even though the reflector ensures, anyway, a reduction of the backward radiation level in the direction of the feeding line.
These results are confirmed by the analysis of the spatial distribution of the real part of the Poynting vector computed in the -plane at the air-dielectric interface (see Figure 11). The computed vector maps show that the electromagnetic energy is guided along the profile of the drop-shaped radiating arms and of the antenna reflector. This results in a larger radiation level at the boresight, as well as in a reduced energy emission in the backward direction. This important feature, often overlooked in antenna design, can be exploited to reduce the electromagnetic coupling with the RF circuitry hosted in the same board [29]. The peak gain, the main-lobe direction, and the antenna beamwidth in the -plane at different working frequencies are listed in Table 2. The reported values confirm the good performance of the proposed antenna.
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4. Time-Domain Antenna Behaviour
The results presented in previous sections have shown that an integrated reflector can improve the performance of the drop-shaped antenna in terms of gain and front-to-back ratio in a wide frequency range. Since these characteristics result from the specific field distribution determined by the reflector, an unavoidable alteration of the antenna response to the radiated pulse signals is expected in comparison to the antenna element without reflector. To this end, in this section, an analysis of the time-domain antenna response is performed in order to determine the impact of the reflector. This survey may be useful in the development of a new class of low-profile antennas aimed at the transmission/reception of UWB signals.
Typical excitation signals used to determine the suitability of wide-band antennas to transmit pulsed signals are those based on the higher order derivative of the Gaussian pulse. In this context, the antenna excitation signal has been selected to be the fifth derivative of the Gaussian pulsewhere defines the time at which the Gaussian pulse exhibits the peak value, whereas denotes the relevant variance.
The suitability of the antenna to work with impulsive signals has been verified by means of the analysis of the time-domain response and of the relative group delay, which properly quantifies the propagation properties of the so-called wave packet [28]. To this end, in Figure 12 the frequency behaviour of the relative group delay of the antenna with and without metal reflector in the frequency band 3–11 GHz is shown, while the corresponding time-domain responses are reported in Figure 13.
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Both these quantities have been evaluated at a distance of 30 cm from the centre of the dipole arms, under the assumption that the input signal, with parameters ps and ps, is characterized by a peak value of 1 V.
The analysis of the frequency behaviour of the relative group delay reported in Figure 12 reveals the presence of a peak at a frequency of about 10 GHz. This peak, however, does not jeopardize the overall time-domain behaviour of the antenna since it is located at the upper edge of the UWB band. In fact, the peak magnitude is below 1 ns and, accordingly, the group delay is compliant with the UWB requirements [30]. Such conclusion is confirmed by the time-domain response of the antenna shown in Figure 13.
From this figure it appears that the presence of the reflector does not affect negatively the quality of the radiated signal while allowing a reduction of the electromagnetic energy emitted in the direction of the feeding line. In particular, the level of radiated signals at the boresight is quite similar for both antenna configurations, while a 50% reduction of the peak-to-peak level of the signal radiated in the direction of the feeding line is observed in the reflector integrated antenna, making the use of such radiating structure more appealing when tackling coexistence issues with sensitive electronic devices sharing the same board.
Further information about the electromagnetic behaviour of the proposed antenna can be achieved by means of the Singularity Expansion Method (SEM) technique [31]. This technique, in fact, enables deriving information about the complex resonant processes taking place in the structure, allowing gaining a physical insight into the transient emission phenomena responsible for the structure behaviour. To this end, the radiative component of the electric field evaluated along the boresight direction is expressed as follows:where is the Laplace variable, is the current excited at the SMA connector, is the light velocity in vacuum, and is the number of the considered poles, while and are the complex pole and the residues related to the th resonant process taking place in the radiating structure. The corresponding space-time domain expressions are found by means of the inverse Laplace transform which leads to the following result:where the symbol denotes the convolution product, while denotes the Heaviside unit-step function. Equations (7)-(8) clearly show how the far-field waveforms are linked to the derivative of the current excitation signal . The poles and the residues in (7), evaluated along the boresight, by means of a modified pole/residue representation [32, 33], are reported in Figure 14. In particular, the number of poles, , is selected in such a way that the relative deviation, evaluated in norm, between the electric field radiated by the antenna, , and the relevant residue/pole expansion, , is smaller than a given threshold ; namely,In the proposed study, has been set equal to in order to ensure an adequate accuracy of the representation.
By comparing the poles configurations in the complex -plane (see Figure 14) for the two considered antennas, it appears that the reflector is responsible for a migration of some poles with respect to the antenna without reflector. These poles, linked with higher oscillation frequencies, result in a signal having slightly more pronounced undulations in the late-time response. This behaviour is confirmed by the antenna time-domain response reported in Figure 15.
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5. Conclusions
A low-profile printed drop-shaped dipole antenna integrated with a planar metal reflector has been presented. Parameterized equations describing the reflector and the feeding line profiles have been introduced to optimize and make the antenna design easy. A suitable shaping of the reflector and of the feeding line, as well as the adoption of a thin substrate having a low dielectric permittivity, has been selected in order to achieve a bandwidth larger than 10 GHz and meet, in this way, the demanding requirements of different communication standards, such as WLAN/WiMAX and UWB protocols. The analysis of the radiated flux power density, performed by means of a full-wave locally conformal FDTD numerical technique, has shown that the energy emission substantially takes place along the edges of the reflector, as well as along the dipole radiating arms. In particular, it has been observed that the quasi-traveling wave behaviour of the radiated field induced by the special shape of the radiating arms is enhanced by the presence of the metal reflector already at the lowest working frequency of the antenna. Furthermore, an increment of the antenna gain and of the front-to-back ratio, in the frequency range where the main wireless communication systems operate, often overlooked in antenna design, has been achieved. This important feature can be exploited to reduce parasitic coupling with radio frequency circuitry sharing the same board in wireless communication systems and radio base stations. Finally, the small variation of the relative delay spread caused by the reflector does not excessively affect the time-domain response of the antenna making the antenna suitable also for UWB applications. This characteristic is confirmed by the SEM analysis which shows a weak migration of some poles of the time-domain response of the antenna caused by the presence of the reflector. In conclusion, the low profile, the reduced dimensions, the wide-band behaviour, and the limited back radiation make the proposed antenna suitable to be easily integrated in mobile devices and radio base stations.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.