ARTICLE
Year : 2011 | Volume
: 57 | Issue : 5 | Page : 452--460
Frequency and Time Domain Characterization of a Novel Hexagonal-shaped Microstrip Antenna for Ultra Wideband Applications
Mithilesh Kumar, Ananjan Basu, Shiban K Koul
Centre for Applied Research in Electronics (CARE), Indian Institute of Technology, Delhi, Hauzkhas, New Delhi, India
Correspondence Address:
Mithilesh Kumar
Centre for Applied Research in Electronics (CARE), Indian Institute of Technology, Delhi, Hauzkhas, New Delhi
India
Abstract
This paper presents frequency- and time-domain characterization of a new hexagonal-shaped monopole antenna with small slots for ultra-wideband (UWB) applications. The antenna was simulated using commercial software tools (CST) microwave studio and Agilent ADS. Neltec make a soft substrate having a dielectric constant of 2.2 and a board thickness of 0.254 mm (10 mil) was used to fabricate the proposed antenna. Frequency-domain characterization using an automatic network analyzer showed return loss less than -10 dB over the entire UWB frequency band. To study the effect of various parameters on the antenna performance, the detailed parametric study was carried out. The radiation patterns and group delay characteristics were measured in an anechoic chamber. Using an Agilent DSO9000A oscilloscope, time-domain characterization of the antenna was carried out. The simulated and measured results in the time domain showed good agreement. It was observed that the proposed antenna shows reasonably good frequency characteristics and time-domain pulse distortion is small indicating usefulness of the antenna for UWB communication applications.
How to cite this article:
Kumar M, Basu A, Koul SK. Frequency and Time Domain Characterization of a Novel Hexagonal-shaped Microstrip Antenna for Ultra Wideband Applications.IETE J Res 2011;57:452-460
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How to cite this URL:
Kumar M, Basu A, Koul SK. Frequency and Time Domain Characterization of a Novel Hexagonal-shaped Microstrip Antenna for Ultra Wideband Applications. IETE J Res [serial online] 2011 [cited 2013 Jun 19 ];57:452-460
Available from: http://www.jr.ietejournals.org/text.asp?2011/57/5/452/90167 |
Full Text
1. Introduction
Ultra-wideband (UWB) technology in the 3.1 to 10.6 GHz has received considerable attention in recent years for high data-rate short distance communication. UWB communication offers a radically different approach to wireless communication compared to conventional narrow band systems. It is based on transmission of short pulses with relatively low energy, ultra-wide spectrum, low power spectral density, and acceptable interference with other users. This technology offers great promise to growing demand for low cost, low complexity, impulsive radio, and high- speed digital wireless home networks [1] . One of the challenges of a UWB system for wireless communications is the development of a broadband antenna. In the recent years, researcher's engineers and scientists have adopted different methods to solve UWB antenna problem.
Broadband antennas have been around for many decades and are being used extensively, especially for reception of signals. Traditional broadband antennas satisfy the requirements of commercial UWB systems in some cases, thereby permitting to use some of the existing designs. However, there is a significant difference between the proposed commercial UWB radio concepts with its frequency range of 3.1 to 10.6 GHz from the traditional wideband short-pulse applications, such as in radar. In addition, UWB antennas for portable consumer electronics and mobile communications applications have different requirements than those intended for fixed-point communications or broadband testing; consequently, many of conventional frequency independent and log-periodic design are not suitable. These antennas have practical applications in impulse radar, electromagnetic pulse (EMP) measurement, and various communication systems. Considerable work has been reported on pulse antennas and on the analysis and synthesis of the associated radiated fields and received signals. However, detailed investigations on wave shapes of radiated and received pulses are not available in the literature. The traditional parameters used to describe the performance of antenna may not be the most convenient description for UWB applications.
Generally, in UWB antenna design, both the frequency and time-domain responses should be taken into account. The frequency-domain response includes input impedance and radiation characteristics. The impedance bandwidth is measured in terms of return loss or voltage standing wave ratio (VSWR). Usually, the return loss should be less than -10 dB or VSWR < 2.1. Antennas having an impedance bandwidth narrower than the operating bandwidth tailors the spectrum of transmitted and received signals, acting as a band pass filter in the frequency domain, and reshape the radiated or received pulses in time domain.
The radiation for short-duration UWB signals from an antenna is significantly different compared with the radiation produced by long-duration narrowband signals. These differences are mostly due to the characteristics of the radiation of the signal from the antenna aperture, the time-domain attributes of the radiation field, the amplitude of the radiation field, the spatial attributes of the radiation field, and the properties of side radiation.
In UWB systems, antennas are significant pulse-shaping filters. Any distortion of the signal in the frequency domain (which is a normal filtering operation) causes distortion of the transmitted pulse shape, therefore increasing the complexity of the detection mechanism at the receiver. The antenna's characteristics should be flat in the UWB frequency range. For wireless communications application, the antenna should have omni-directional radiation pattern. It should radiate the pulse with minimal distortion and minimal late-time ringing. It should also be possible to integrate the antenna with the generator. It is also desirable that the UWB antennas be mounted on a dielectric substrate that will serve as a protective mechanical shield and at the same time radiate through it. Other major constraint is the FCC mask [1] . In fact, the antenna designer and RF engineer should cooperate to ensure that the overall UWB communication device meets a desired spectral mask. UWB antenna should be an integral part of the system and not stand-alone element. This is an important issue in the implementation of UWB technology for wireless communication application.
In the recently published papers [2],[3],[4],[5],[6] , it has been shown that certain design guidelines can be followed to build UWB antennas. Several broadband monopole configurations, such as circular, square, elliptical, pentagonal, and hexagonal, have been reported in the literature [7],[8],[9],[10]. However, these are not planar structures as the ground planes are perpendicular to the radiators. The frequency domain parameters such as return loss, group delay, radiation pattern, and gain are presented in the literature for some of these antennas [11],[12],[13],[14] and the time domain-parameters are reported [15],[16],[17] . Complete characterization in terms of frequency and time domain keeping in view the received pulse shape and spectrum of UWB pulse is rarely available in the literature.
This paper describes a method for complete characterizing of UWB antenna system. A passive microstrip-fed hexagonal UWB antenna is designed and fabricated using conventional microwave integrated circuit (MIC) techniques. Studies on this monopole antenna are presented in both frequency and time domain. Antennas response in transmit mode, and transmission between two identical antennas is investigated. These methodologies are verified through simulation and experimentation.
In Section II, the antenna design procedure is described and the effect of various design parameters on the antenna performance is presented. In Section III, experimental results are compared with simulation results to verify the antenna performance in frequency domain. In Section IV, the time-domain characteristic of monopoles is evaluated. The overall performance of the antenna system developed is analyzed. Antennas response in both transmit and receive modes are investigated. Further, the received signal waveforms are assessed by the pulse fidelity. A conclusion of the work presented is reported in Section V.
2. Antenna Design and Optimization
Structure of the proposed antenna configuration for UWB application is shown in [Figure 1]. The hexagonal patch antenna is printed on a Neltec substrate of thickness h=0.254 mm and relative permittivity εr = 2.2. The substrate size of the proposed antenna is 50.8 mm × 60.0 mm. The size of the ground plane is chosen to be rectangular and has dimension 50.8 mm × 20.0 mm. A microstrip line that couples the energy to the antenna constitutes the feed-line. The microstrip line is designed to be of 50 Ω impedance that translates to a width W= 0.745 mm. Two small rectangular slots of the dimension 0.8 mm × 1.0 mm are etched out on the patch. The dual rectangular slot dimensions, their position on the patch, the tapered connection between the patch and the feed line, and partial ground plane dimension are optimized using CST Microwave Studio, to maximize impedance bandwidth.{Figure 1}
The geometric parameters of this structure are adjusted to tune the return loss and the bandwidth characteristics over wide range of frequency. The final optimized geometry including dimensions of the antenna is shown in [Figure 1].
3. Frequency Domain Characteristics
By selecting the optimized dimensions of the antenna shown in [Figure 1], simulations are carried out using CST Microwave Studio to determine the antenna return loss bandwidth, radiation patterns, group delay, and the insertion loss between the antennas. [Figure 2] shows the variation of the measured and simulated return loss of the proposed antenna as function of frequency. As observed the 10-dB return loss bandwidth is more than 7.5 GHz. The measured results show reasonable agreement with the simulated result at low frequencies. However, the simulated results display larger bandwidth at higher frequencies. This could be due to varying dielectric constant and dissipation factor of the Neltec soft substrate as a function of frequency. It should also be noted that simulation was carried without any RF feeding cables. In the actual measurements, a 50 Ω SMA connector is connected to the end of the feeding strip and grounded to the edge of the ground plane. Furthermore, RF cable from the vector network analyzer is connected to the SMA connector to excite the antenna. In small-antenna measurements, the RF cable usually affects the performance of the antenna under test (AUT) greatly. From the comparison given in [Figure 2], it is evident that the presence of RF cable hardly affects the lower edge frequencies around 3 GHz. This implies that the design is less dependent on the ground plane in terms of impedance matching. This feature makes the printed antenna design flexible and suitable for practical application where the antenna is to be integrated into various circuits or devices.{Figure 2}
In a practical UWB communication system, the transmit-receive antenna system can be considered as a two-port network. The transfer function H(ω) can be measured in terms of S 21 when the source and the load are matched to the antenna input and output, respectively. This implies that the measurable parameter S 21 or H(ω) is able to integrate all the important system parameters in terms of gain, impedance matching, polarization matching, path loss, and the phase delay. Therefore, it can be used to assess the performance of UWB antenna systems and other antenna systems whose performance is frequency-dependent.
In the measurement of H(ω), the orientations of the transmit and receive antennas are shown in [Figure 3]. The transmission between the two identical proposed antennas is examined in the anechoic chamber. [Figure 3] shows a pair of antenna with a variable separation positioned in parallel and face-to-face. [Figure 4](a) shows the simulated │S 21│ at D=150 and 300 mm and [Figure 4] (b) shows the measured │S 21│for the distances D=150 300, and 600 mm. As observed, the measured │S 21│varies as a function of D. The differences between the simulated and measured patterns are mainly considered as fabrication tolerances, measurement errors, and mesh size used in modeling of the antenna. From [Figure 2] and [Figure 4], it is evident that the 10-dB return loss bandwidth of the antenna covers the whole UWB band. However, as observed the 10-dB return loss bandwidth for transmission case ranges only from 2 to 6.5 GHz, partially covering the lower range of the UWB band. The bandwidth of the system gain is still broad and covers part of the UWB band that is widely used in high-speed/short-range mobile communication devices.{Figure 3}{Figure 4}
The insertion loss │S 21│ between two antennas is measured in anechoic chamber for two cases. In the first case, both antennas are of the same type (designed UWB hexagonal monopole antennas) and in the second case, the UWB hexagonal monopole antennas is used at the transmitting end and a horn antenna at the receiving antenna. It was observed that measured results obtained in the two cases are similar.
The UWB communication systems require a small size antenna with flat group delay characteristics, so that the high and low frequency signal components arrive at the receiver simultaneously. The shape of the incoming waveforms that excite the antenna changes due to distributed reactances of the antenna. Group delay, an important parameter in the UWB antenna design, represents the degree of distortion of the transmitted and received pulse signal. The measured group delay characteristics are plotted in [Figure 5]. It is observed from the plot that the group delay variation is less than 0.5 ns. This kind of antenna is reasonably suitable to transmit and receive very short-time duration pulses of electromagnetic energy.{Figure 5}
As the radiation patterns are of interest for UWB application, we have studied the radiation characteristics of the fabricated antenna. The E-plane and H-plane radiation patterns are measured at frequencies of 4, 7, and 10 GHz. The radiation patterns were measured in an anechoic chamber. [Figure 6] shows the setup. The simulated and measured E-plane and H-plane radiation patterns are plotted in [Figure 7]. It is evident from the figure that the radiations are almost omni-directional, which is unlike a typical planar monopole/dipole antenna. Such omni-directional radiation performance makes these antennas suitable for application in mobile devices. We can see that at low frequencies, the radiation is primarily broad-side, while at higher frequencies much of the radiation goes parallel to the substrate.{Figure 6}{Figure 7}
This is confirmed by making S 21 measurements between a UWB antenna and a broad-band (0.8-18 GHz) standard gain horn separated by 0.5 m. The transmission characteristic for the broad-side direction from the UWB antenna pointing toward the horn is shown in [Figure 8](a). For the case of the horn located in a direction tangential to the substrate, the characteristics are shown in [Figure 8](b).{Figure 8}
Feed-line is parallel to horn E-field in both cases.
It is clear from the results that the bandwidth is actually not fully evident from S 11 plots alone. It actually depends on the direction of transmission. We have thus reported an improved antenna that gives wide bandwidth suitable for UWB applications irrespective of direction of transmission.
4. Time Domain Characteristics
In addition to return loss, group delay, gain, and radiation pattern characteristics, time-domain impulse response is another important criterion to determine the performance of UWB antenna. The UWB pulse generator circuit reported [18] used as a reference pulse generator. In the UWB pulse generator circuit, the load is replaced by the antenna measured S-parameter data and simulated using Agilent ADS software. The input pulse train given to the transmitter and the received output UWB pulses are shown in [Figure 9](a) and the zoomed view of output-received pulse is shown in [Figure 9](b). The frequency spectrum of output single UWB pulse is shown in [Figure 10]. We performed time-domain measurements in an anechoic chamber, using the experimental setup shown in [Figure 11]. A reference pulse is applied to the terminals of one of the antenna that operates as a transmitting antenna. The other antenna operates as a receiving antenna and is connected to a high speed oscilloscope (Agilent Infiniium DSO 9000A). The antennas are separated through a distance D of 0.5 m. The received pulse shape is compared with the reference pulse shape in order to know the distortion due to transmitting and receiving antennas. The source impulse applied to the transmitting antenna as reported [18] is shown in [Figure 12]. The generated pulse shape and time domain response of the generated pulse is depicted in [Figure 12](a) and (b), respectively.{Figure 9}{Figure 10}{Figure 11}{Figure 12}
The received UWB pulses shape and its zoomed view for D=500 mm are shown in [Figure 13] (a) and (b), respectively. The frequency spectrum of received single UWB pulse is shown in [Figure 14].{Figure 13}{Figure 14}
For completeness, we mention here that by spectrum, we mean the Fourier transform of the waveform, defined in this case by the following equation:
[INLINE:1]
Its magnitude is plotted in dBm using the following expression:
[INLINE:2]
the factor of 2 being used to account for the power in negative frequencies.
For an oscilloscope sampling rate of N samples/sec, and a data set of P points, T obviously comes out to (P/N) sec. N and P are displayed with the plots below.
In characterizing the antenna for transient radiation behavior, not all the energy is transmitted when the pulse (transmit) arrives at the transmit antenna due to imperfect match at low frequencies. This means that a part of the signal is reflected back. This reflected pulse travels back through the cable to the generator. Because the generator is short-circuited after firing the pulse, this reflected pulse is then re-reflected back to the antenna and is partially transmitted into the channel and the process repeats. Therefore, the pulse bounces between the UWB generator and UWB transmit antenna. To minimize ringing between the circuit and the antenna, a 10-dB attenuator was inserted between them. For completeness, the middle trace in [Figure 14] shows the digital signal generator output that triggers the UWB pulse generation [18] .
It is interesting to note that while the spectrum has been severely changed (which we can easily correlated with [Figure 8](a)), the time-domain pulse shape is maintained except for small ringing. One problem evident from the spectrum is that if frequencies below 3 GHz are used, it would be necessary to use filters to avoid conflict with other systems using these frequencies.
5. Conclusion
In this paper, we have carried out frequency and time domain characterization of a novel hexagonal monopole antenna for UWB wireless applications. The proposed structure offers a wider impedance bandwidth, broader radiation patterns, and an improved time-domain behavior. The transmitted pulse shape is analyzed and compared with the reference pulse feeding the transmit antenna. The pulse shape and their spectrum can be improved by using appropriate wave-shaping network. The proposed hexagonal monopole antenna is well suited for UWB application.
6. Acknowledgment
The authors wish to acknowledge the assistance of Agilent Technologies in providing high-speed digital signal analyzer Model DSA91304A.
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