Ultrawideband Vivaldi Antenna for DVB-T, WLAN, and WiMAX Applications

Compact Vivaldi patch antenna with a parasitic meander line is presented in this paper. A PIN diode switch is used to connect and disconnect ultrahigh frequency band (UHF) with ultrawide bandwidth (UWB). The operating frequencies can be switched among different services, depending on the switching states (ON/OFF) to add the lower band when required. This antenna is suitable for portable DVB-T which extended from 450MHz to 850MHz receiver applications and theWLAN (Wireless Local Area Network) IEEE 802.11b,g (5.1–5.8) GHz frequency bands and WiMAX band (3.3–3.8) GHz. The measured reflection coefficient of the proposed antenna is compared with the simulated one; good agreement is observed. Also, simulated radiation pattern of the antenna is presented.All simulations are carried out using the EMcommercial simulator, high frequency structure simulator (HFSS) ver.13.


Introduction
The rapid developments in broadband wireless communications and the great number of commercial and military applications necessitate new types of antennas which can support higher bit rates.4G mobile terminals are gaining immense popularity thanks to their advanced and userfriendly features.Also, designing new compact antennas with good characteristics applied for 3G mobile terminals is quite necessary although number of similar studies had been conducted so far [1].According to the federal communication commission (FCC) UWB rulings the signal is recognized as UWB if the signal frequency band is from 3.1 GHz to 10.6 GHz or the signal bandwidth is more than 500 MHz [2].
An exponentially tapered slot antenna (ETSA) suitable for packaging and integration with other components is introduced and proposed for the UWB range with gain more than 5 dBi and reflection coefficient below −6 dB which represents VSWR <3 for the whole frequency range.The ETSA is a variation of the Vivaldi antenna, with the outer edge exponentially tapered, and it was introduced for the first time in [3].The design and performance characteristics of the ETSA are investigated in [4].
Microstrip antennas have some attractive advantages such as small volume, very low-profile, light weight, easy fabrication, and constant directional radiation patterns, which have been widely used in designing miniaturized antennas.Since the development of tapered slot technology, many traditional antennas could be made into corresponding printed antennas, such as printed Vivaldi antennas [5] and printed log-periodic dipole antennas [6]. Figure 1 shows the geometry of the proposed antenna.
The Vivaldi antenna also demonstrates good performance, such as wide band and symmetric E-and H-plane beam patterns.The antenna was first proposed in [7].Since then some advances have been made.In designs, fullwave numerical techniques and simulation tools are used to analyze the antenna.In applications, Vivaldi arrays were built for radar applications [8], wireless communications applications [9], and dual polarization applications [10].Those multifunction applications are the current trends.The linearly polarized antenna shows good performance: reflection coefficient better than <−6 dB and gain between 2.53 and 3.48 dBi over the operating bandwidth in the DVB-T band and about 4.4 dBi in the WLAN band.This antenna has an end-fire radiation pattern when the parasitic element  is disconnected and omniradiation pattern when the parasitic element is connected [11].
In this paper, a low-profile Vivaldi with parasitic meander line either connected or disconnected for added extra lower band is presented.The impedance bandwidth of the proposed antenna is ultrawide, with dimensions 0.52  × 0.6  (where   is the wavelength at the lowest free space frequency) which is smaller compared with 0.762  × 0.7  in [12].PIN diode is used to switch the lower frequency band in addition to the high frequency band.This antenna operates over LTE bands, GSM1800 MHz/GSM1900 MHz, Bluetooth 2.4 GHz, WI-MAX frequency range (3-4) GHz, and W-LAN frequency range (5-6) GHz.

Antenna Design and Configuration
Figure 1 shows the geometry of the proposed Vivaldi antenna.According to [13,14], the design parameters are dielectric constant and thickness of the substrate, conductor width, and the gap width of the slot.The Vivaldi is described by exponential function of the tapered slot-line as given in ( 1) and ( 2): where   is the distance from the center line of the slot to the inner slot line.  determines the line slot width at the feed port,   specifies how the slot widens, and these values are given in (2).The feed port is located across the narrow slot, where a 50 Ω coaxial line is connected; the location of the feed point is also important for impedance matching.This section describes the design steps of the proposed Vivaldi antenna shown in Figure 1.For each step of the design, typical starting and ending values will be given.There are two influences in the optimized antenna design.
The first is the ground plane dimensions and the second is the feeding network dimensions.Exponential curvature can be changed with the value of parameter exponential rate (  ).First is the beginning of the taper, the second is the wide end of the taper.On both places, a reflection of the traveling wave is likely to occur.In the case of the beginning of the taper; reflection occurs with the initial change of the slot line width.Therefore, smoother taper in the throat length (th) minimizes the reflection there.Reflection at the wide end of the taper is connected to the fin termination and cannot be completely avoided.Changing exponential rate does not influence the wide end reflection significantly.The effect of balloon slot diameter , backwall , and open end width  Open are studied and optimized to improve the impedance bandwidth as shown in Figure 2.
Figure 2(a) shows that as the slot diameter increases the antenna bandwidth increases, reduces the lower frequency and improves the antenna matching; this is due to improve the impedance transition from 50 Ω to air impedance while, by increasing the back-wall width , increases the antenna reflection coefficient and reduces the bandwidth at high frequency.However, the width of open end decreases the antenna bandwidth and antenna matching.After optimizing the dimensions of the conventional Vivaldi antenna a parasitic meander line with different length is added and the position of this parasitic element is optimized under the antenna radiator to add lower frequency bands as shown in Figure 3. Figure 3 shows that the effect of the parasitic element location   is small on higher bandwidth and improves the bandwidth at lower frequency.The optimized dimensions are shown in Table 1.
The geometrical dimensions of the parasitic meandered shaped element are with width 1 mm and separation 1 mm with longest meandered length 24 mm and shortest meandered length 10 mm.Ideal switch model is firstly used to imitate PIN diode switches with the open (OFF) and close (ON) states to switch the antenna from 450 MHz to 2000 MHz in     addition to the basic bandwidth, which is extended from 2.5 GHz to 10 GHz with total 75% size reduction as compared to the original Vivaldi size.The open (OFF) and close (ON) states of the switches are simulated in the absence or presence of a metal pad with the area  sw ×  sw = 0.3 × 0.9 mm 2 , respectively [15][16][17].This is approximately the same area of a real PIN diode switch.Then the practical model of the PIN diode HPND-4005 is used as forward biased with 0.7 V and 10 mA.It exhibits an ohmic resistance of 3 Ω and intrinsic capacitance of 0.1 pF for forward bias while exhibits 2.7 K ohm and 9 pF at 0 V. 10 pF capacitors are used to isolate the RF signal from the DC and RF choke coil MCL 50-10000 MHz.PIN diode is added as shown in Figure 1(b) (red) to add a lower band which is extended from 450 MHz to 2000 MHz with main impedance bandwidth which extended from 2.5 GHz to 10 GHz as shown in Figure 4.

Measured Results and Discussion
A prototype has been fabricated using photolithographic technique and its reflection coefficient was measured using vector network analyzer ES8719.It was simulated using HFSS electromagnetic simulator based on the finite element method.The substrate material is FR4 with dielectric constant 4.7, dielectric height = 1.6 mm, and loss tangent tan  = 0.02.The total area of the antenna is 40 × 44 mm 2 (0.52  × 0.6  ).By carefully tuning, optimized design parameters are obtained.Commercial FR4 was used because it resists high heat and has excellent chemical and mechanical properties.The current distribution at resonant frequencies 500 MHz, 2 GHz, and 5.2 GHz is shown in Figure 5.It is clear that very wide-band impedance matching is achieved.The far-field radiation patterns are simulated; the normalized patterns are tabulated at resonant frequencies of 500 MHz, 900 MHz, 1.8 GHz, 2.5 GHz, 5.2 GHz, and 10 GHz in Figure 7.It can be seen that at low frequency band, the patterns are much similar to that of a dipole.Because the effective region is located at the top of the antenna, the bottom part has little effects on the radiation.At medium and high frequency bands, the patterns behave more directive.Because the effective region is moved to the middle part, the bottom part acts as a reflector.The comparison between measured and simulated radiation efficiency is shown in Table 2, which varies from 65% to 95% with average value of 80%.

Conclusion
A Vivaldi antenna with parasitic meander shape for DVPT and UWB applications has been presented in this paper.The antenna bandwidth extends from 450 MHz to 12 GHz with | 11 | < −6 dB.The PIN diode is connected to add and remove the UHF band which extends from 450 to 1.5 GHz frequency.The proposed antenna is compact with 0.52  × 0.6  mm 2 and there is very good agreement between measured and simulated results.The antenna gain varies from 1.5 dBi to 5.5 dBi over the operating band with 80% average radiation efficiency.The proposed antenna can radiate omnidirectional patterns at all the operating frequency bands which makes it suitable for use in wireless communication applications.

2 International
Journal of Antennas and Propagation

Figure 6 :
Figure 6: (a) The photo of the fabricated antenna, (b) comparison of simulated and measured | 11 | with PIN (ON) mode, and (c) with PIN (OFF) mode.

Figure 7 :
Figure 7: The simulated 2D radiation pattern at different resonant frequencies,  Φ solid line and   dash line.

Figure 6 (
a) shows a photo of fabricated antenna, while | 11 | of both PIN diode modes (ON/OFF) is shown in Figures 6(b) and 6(c).The experimental results reasonably agree with the simulated ones and the impendence bandwidth is extended from 450 MHz to 12 GHz.

Table 1 :
The antenna dimensions (in mm).

Table 2 :
The simulated antenna parameters at resonant frequencies of three turns meander shaped-slot (SW/ON/OFF modes).