Designing Antennas For UWB Systems

Ultrawideband (UWB) technology offers several advantages over conventional communications methods. For example, UWB systems feature information bandwidths of 1 GHz and more while being able to share spectrum with other applications without causing interference. UWB systems use narrow pulses to transmit data. Since no carrier frequencies are involved, the transmitter (Tx) and receiver (Rx) hardware can be made very simple. Still, a challenge lies in the development of an antenna capable of handling these high-speed pulse trains. UWB antennas must cover multiple-octave bandwidths in order to transmit pulses that are of the order of a nanosecond in duration. Since data may be contained in the shape of the UWB pulse, antenna pulse distortion must be kept to a minimum.

The Vivaldi antenna offers great promise for UWB applications. This planar design is suitable both for radar-like and communications applications. First conceptualized in 1979 as a wideband antenna,¹ the initial designs were balanced structures and therefore had to be fed by a wideband balun transformer. The primary disadvantage of this architecture is that the balun must provide good performance over the entire bandwidth of the transmitted signal, significantly increasing the implementation costs. The Vivaldi antenna is still in use today in its original form for broadband microwave and electronic-countermeasure (ECM) applications. Newer designs² involve the use of double-sided and stripline versions employing innovative techniques to eliminate the balun.

The Vivaldi antenna can preserve the shape of transmitted UWB pulses, ensuring error-free, high-data-rate communications. To understand why, Fig. 1a compares the time-domain S₂₁ response for a Gaussian monocycle input when fed to a log-periodic dipole antenna (a classical broadband antenna structure) and a Vivaldi antenna. In the log-periodic antenna, the smallest antenna element radiates the highest-frequency component while the largest antenna element radiates the lowest-frequency component after the pulse has had time to propagate to the far end of the antenna. The use of these resonant elements, however, results in an antenna, which is dispersive in the time domain. The dispersion results in difficulties distinguishing individual multipath signalsa well-known advantage of UWBat the Rx, due to broadening of the pulses resulting in significant overlap. Figure 1b shows the time-domain S₂₁ response of a Vivaldi antenna. This antenna produces a near-perfect Gaussian doublet in response to the Gaussian monocycle input, (i.e., the first-order derivative³), and has a greater efficiency than the log-periodic dipole antenna.

What follows is meant to guide the reader through an engineering analysis of the Vivaldi antenna, attempting to explain how the different aspects of the antenna affect its performance, as well as provide a prototyping methodology to design and fabricate the antenna. Vivaldi antennas have been designed for diverse purposes from ranging to communications within the FCC approved limits.⁴ The performance of several antenna designs were evaluated in an anechoic chamber, and the effects of several different designs will be reviewed. The Vivaldi antenna is composed primarily of three different structures: a microstrip feed, a paired-strip middle section, and the radiating section. The design of the microstrip and radiating sections have a critical impact on the antenna performance, while the paired strip serves primarily as a transition region. Adjustments to the designs presented here can be made without causing a substantial loss in overall performance. Figure 2 shows an image of the overall Vivaldi antenna.

Early Vivaldi designs⁵ used longitudinal tapered slots on semirigid coaxial cable for the antenna feed. Although covering several octaves in bandwidth, this method is difficult to implement since the taper (100:1) which would have to be cut into the shield of the coaxial cable. A more elegant feed structure is a simple microstrip transmission line. The width of the microstrip is designed for a 50-Ω characteristic impedance for the type and thickness of dielectric material, using standard formulae.⁶

In the current design, the microstrip transmission line gradually tapers to a paired-strip transmission line. The transition region is responsible for connecting the highly capacitive feed structure to the inductive radiating section, and decouples the microstrip structure from the radiating portion of the antenna. It was empirically discovered that the transition region should be three to five wavelengths long to prevent a sharp discontinuity (and the resulting pulse distortion) between the feed and radiating regions. In addition, a properly designed transition region will convert the unbalanced feed into a balanced structure that can then be connected to the radiating region of the antenna. A detailed analysis and discussion of the paired strip structure can be found in ref. 7. The equation helps calculate the characteristic impedance of the paired strip as a function of width, dielectric constant, and thickness of substrate:

SEE EQUATION BELOW

where:

Z₀ = the characteristic impedance,
e_r = the dielectric constant of the substrate,
e₀ = the characteristic impedance of free space (377 Ω),
a = the width of the paired line 0.5, and
b = the thickness of the dielectric substrate 0.5.

The paired-strip section is slowly tapered away on each side to develop into the radiating sections of the antenna. The taper into the radiating section of the antenna is the most critical aspect of the design. The taper should be as gradual and as smooth as possible to avoid significant discontinuities at higher frequencies, which will cause reflections resulting in a distorted pulse-shape.

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Two types of curvatures were investigated for the taper: an elliptical transition and a spline transition. The elliptical transition uses two-quarter ellipses with the same-length minor axis but different-length major axes to form the shape of the radiators. The spline transition follows the same general shape as the ellipse, but attempts to achieve a better transition by varying the rate of the curvature as the spline progress, something that cannot be done with an ellipse. A spline appeared to provide the best time-domain response, although the elliptical taper provides equal magnitude electric (E) and magnetic (H) fields. Because good time-domain response is critical in a UWB system, the spline transition was of primary interest.

Using the basic parameters described above, a designer can come up with a suitable antenna using a standard computer-aided-design (CAD) package such as AutoCAD™. The antenna can also be simulated with the use of electromagnetic (EM) simulation tools, such as Sonnet™ from Sonnet Software (Liverpool, NY), by importing the design from the CAD package. For this article, eight antennas were designed; the major parameters are summarized in the table. The radiating region of Antenna 2 has an elliptical taper; all other antennas used a spline curvature. The spline curvature was created using AutoCAD's SPLINE function with a start point at the end of the paired strip and an endpoint at the end of the antenna. Antennas 7 and 8 were designed to cover the frequency range of 0.3 to 10 GHz and 3 to 10 GHz; all others were designed to cover 1 to 20 GHz. Antennas 1 through 6 had an overall size of 8 × 11.5 in. (20.32 × 29.21 cm), Antenna 7 was 16 × 23.5 in. (40.64 × 59.69 cm), and antenna 8 was 1 × 3 in. (2.54 × 7.62 cm).

To facilitate a rapid design cycle, the antennas were fabricated in-house using a Protomat^® 91s/Vs printed-circuit-board (PCB) milling/drilling machine from LPKF Laser and Electronics (Wilsonville, OR). For each antenna, the milling machine was used to mill the antenna outline on the copper-clad substrate. Commercially available adhesive-backed paper was then used to mask off the antenna element, and the remaining copper was chemically etched away. This in-house prototyping technique allowed us to go from initial design to finished antenna in about 1.5 h.

Each of the antennas was then evaluated in an anechoic chamber to determine the S-parameters both in the time and frequency domains. These measurements provided the time-domain impulse response (Fig. 1, for example) along with the antenna efficiency and return loss from 0.1 to 20 GHz. The measurements were conducted in the frequency domain using an 8510 network analyzer with the time-domain option from Agilent Technologies (Santa Rosa, CA) and then converted into the time-domain using an inverse Fast Fourier Transform (IFFT). Including testing, approximately 4 to 6 antennas could be designed and evaluated in a single workday, whereas EM simulations required up to several days for each antenna design.

In order to provide a complete study of the antennas, they have been categorized into two applications: communications and ranging/measurements. The communications-specific antennas were designed using RO4003C materials from Rogers Corp. (Rogers, CT). The design focused on optimizing performance in the 3.1-to-10.6-GHz range to satisfy the FCC First Note and Order specifications with regards to short-range communication systems. Additionally, these antennas are primarily intended for short-range mobile communications or distributed sensor networks and, as a result, a primary design criterion involved minimizing the overall size of the antenna while maintaining an acceptable level of performance.

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The second type of antenna was designed for ranging/radar/channel sounding applications. In this case, the primary design criterion was an antenna that operated over several octaves in bandwidth (0.3 to 20 GHz). For UWB channel-sounding applications, it is highly desirable to decouple the antenna effects from the measured channel, in order to obtain the true impulse response of the channel. The use of classical broadband antennas, such as the log-periodic or Archimedean spiral complicates the decoupling process due to the distortion in the radiated pulse. Antennas such as the transverse-electromagnetic (TEM) horn and Vivaldi antenna impart a minimal amount of pulse distortion; however, the Vivaldi can be integrated onto the same circuit board as the Tx and Rx electronics, resulting in a more compact and portable measurement system.

The eight Vivaldi antennas were measured in the Virginia Tech anechoic chamber and an outdoor free-space antenna range from 50 MHz to 20 GHz (see table). Vivaldi antenna s fabricated on the R04003 and RT5880 substrates had the widest bandwidth, although the antennas fabricated on the FR4 material still performed adequately within the 3.1-to-10.6-GHz UWB spectrum. Antenna 1 was by far the best-performing design, with a bandwidth of over 18 GHz, and a gain of 10 dBi. Figure 3 shows the return loss (S₁₁) of Antenna 1, and Figures 4(a) and 4(b) show the azimuth and elevation patterns. Although the Vivaldi is a directional antenna, it has a very broad main beam in both the azimuth and elevation planes, and contains several relatively high sidelobes in the back of the antenna.

Antenna 8 was designed to be a compact antenna specifically for portable UWB applications such as laptop computers or distributed sensor networks. Even with its relatively small size, Antenna 8 performed reasonably well over most of the UWB spectrum, and could be easily integrated with Tx/Rx hardware. In short, the Vivaldi antenna is an excellent candidate for UWB applications. It offers good performance in terms of bandwidth, low pulse distortion, and ease of implementation. The antenna can be fabricated on commonly available PCB substrates can be integrated with other portions of the transceiver thus providing for an elegant and compact design.

ACKNOWLEDGEMENTS
The authors would like to express their appreciation to the following contributors: The Defense Advanced Research Projects Agency as part of the Networking in Extreme Environments (NETEX) program; Rogers Corp. for the substrates; Brian Moore at Ampel, Inc. (Elk Grove Village, IL) for etching Antenna 7; Jina Kim and Hyung-Jin Lee for assisting with the pattern measurements; and Nathan Cummings and Randall Nealy for the antenna S-parameter measurements.

REFERENCES

1. P.J. Gibson, "The Vivaldi Aerial," 9th European Microwave Conference, 1979.
2. J.D.S. Langley, P.S. Hall, and P. Newham, "Balanced antipodal Vivaldi antenna for wide bandwidth phased arrays," IEEE Proceedings on Microwave Antennas and Propagation, Vol. 143, No. 2, April 1996.
3. E. Funk, S. Saddow, L. Jasper, and A. Lee, "Time Coherent Ultra-Wideband Pulse Generation using Photoconductive Switching," Digest of the LEOS Summer Topical Meetings, pp. 55-56.
4. Federal Communications Commission (www.fcc.org), Authorization of Ultrawideband Technology, First Report and Order, Washington, DC, February 14, 2002.
5. P. Knott and A. Bell, "Coaxially-fed tapered slot antenna," Electronics Letters, Vol. 37, No. 18, August 2001.
6. K.C. Gupta, R. Garg, I. Bahl, and P. Bhartia, Microstrip Lines and Slotlines, 2nd Ed., Artech House, Norwood, MA, 1996.
7. B.C. Wadell, Transmission Line Design Handbook, Artech House, Norwood, MA, 1991.