Schantz Bottom Fed Planar Elliptical Uwb Antennas (Ieee Uwbst

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BOTTOM FED PLANAR ELLIPTICAL UWB ANTENNAS Hans Gregory Schantz ([email protected]) Next-RF, Inc. 4811 Cove Creek Drive Brownsboro, AL 35741 ABSTRACT This paper describes “bottom fed” planar elliptical dipoles. These antennas are electrically small ultrawideband (UWB) dipoles with bandwidths on the order of 3:1 or better. Return loss is typically –14 dB, and boresight gain is nominally 3 dBi. Bottom fed planar elliptical dipole antennas are well suited for commercial applications.

1. INTRODUCTION Antennas for ultra-wideband (UWB) use must meet demanding performance specifications. They must be well-matched and efficient to take best advantage of parsimonious spectral limits. Ideally, UWB antennas should be non-dispersive or dispersive in a controlled fashion that is amenable to compensation. For a wide variety of applications, an omni-directional response is highly desirable. Commercial operation imposes additional constraints. A mass market UWB antenna must be small and inexpensive, yet must not compromise on performance. Planar elliptical dipoles perform well in commercial applications [1]. They allow matching with return loss on the order of –15 dB or better. Despite their planar form factor, they also exhibit near omni-directional dipole-like patterns over better than a 3:1 span in frequency. Planar elliptical dipoles elements are as small as 0.14λ at their lowest frequency of operation. These antennas also offer radiation efficiencies in excess of 90% in band [2]. Planar elliptical dipoles can be implemented on printed circuit board substrates making them inexpensive and readily manufacturable [3]. “Center fed” planar elliptical dipoles have a significant disadvantage, however. The feed region of a center fed planar elliptical dipole lies in the heart of the most intense near fields surrounding the antenna. Any feed structure is vulnerable to coupling with these intense near fields. Coupling is liable to directly distort the antenna pattern due to blockage and indirectly distort the antenna pattern due to undesired cable currents.

Figure 1: Prototype “bottom-fed” planar elliptical dipole antenna, front (to the left) and back (to the right). The aim of this paper is to present a solution to these problems: a means for “bottom feeding” planar elliptical dipoles [4]. In addition, this bottom-fed planar elliptical dipole incorporates a balun transformer to help improve matching and reduce undesired cable currents [5].

2. OPERATION Figure 1 depicts a prototype bottom fed UWB planar dipole antenna designed to radiate and receive UWB impulses with frequency content from about 2.5 GHz to about 6 GHz. This antenna is an early version of the Time Domain Corporation’s “BroadSpec P200” antenna. A coaxial feed line delivers a UWB impulse to a balun transformer at the base of the antenna. The balun transformer allows the inherently unbalanced coaxial line (+ signal & ground) to be connected to an inherently balanced dipole structure (+ signal & – signal). This helps avoid spurious currents on the sheath of the coaxial feed line that could cause distortions in the antenna pattern and undesired variations in overall system performance. The balun transformer divides the bottom radiating element into two sections as it connects to a slotline between the top and bottom radiating elements. The balun transformer allows the 50 Ω impedance of the coaxial feedline to be precisely matched to the somewhat higher impedance of

the slotline. This slotline guides energy between the top and bottom radiating elements, decoupling it cleanly from the antenna so that it can radiate away with minimal reflection. The process works in reverse when the antenna is receiving. The radiating elements collect energy and the slotline guides it into the balun transformer. The balun transformer couples this energy to the coaxial line. The coaxial line then guides the energy into the RF front end where the energy is made available to the receiver. Ideally, antennas like this could be integrated onto the same board as a complete UWB radio.

3. PERFORMANCE The prototype bottom fed antenna shown in Figure 1 has been optimized for use with ultra-wideband impulses whose spectral content ranges from about 2.5 GHz to about 6 GHz. The antenna radiates a 500 ps monocycle which becomes roughly a 1 ns waveform once it is received by a matched antenna. The pattern of the antenna is dipole-like: omni with variation no more than 3 dB in the horizontal or azimuthal plane, and a null along the vertical axis. Polarization is linear in the vertical direction. The antenna achieves a VSWR of about 1.5:1 across its operating band with reflection down about -14 dB. Because the elevation pattern of the antenna is a bit tighter than that for a conventional dipole, the antenna achieves a +3 dBi gain. This is about 1 dBi better than a conventional dipole. The phase response of the antenna is linear, so it transmits with minimal distortion. The efficiency of the antenna is on the order of about 90% or better. These characteristics are summarized in the table below, and examined at length in the following sections. Characteristic: Radiated Waveform Pattern Polarization Matching Gain Phase Response Efficiency

Specification: 500 ps monocycle Omni in azimuth to ± 1.5 dB Linear (vertical) VSWR ~1.5:1; |S11| ~ -14 dB Nominally ~ 3dBi Linear Nominally >90%

Table 1: Performance Summary for a Prototype Bottom Fed Planar Elliptical Dipole Antenna

3A. PERFORMANCE: WAVEFORM Impulse radio requires precise timing and clean, nontemporally dispersed waveforms to achieve high performance, accurate ranging, and crisp radar imaging. The antenna used in an impulse radio system must be able to radiate short pulses with minimal ringing. The bottom

fed planar elliptical dipole antenna was designed with this requirement in mind. When excited by a broadband impulse, this antenna radiates a monocycle-like impulse roughly 500 ps in duration. To verify this, a Picosecond Pulse Lab 4050B Pulser with a relatively flat response from 1-6 GHz was used to drive the prototype antenna [6]. A Farr Research Model TEM-2-50 TEM horn was used to receive the radiated impulse [7]. This horn antenna generates a voltage at its terminals that is directly proportional to the incident field. The signal was then captured by an HP 54750 sampling digitizing oscilloscope with a 12.4 GHz bandwidth. Figure 2 shows the measured radiated field. This radiated waveform becomes somewhat more elongated when received by a matched antenna.

Figure 2: Measured radiated field from a BroadSpec P200 antenna prototype excited by a UWB source.

3B. PERFORMANCE: PATTERN The bottom fed planar elliptical dipole antenna has a dipole-like pattern: omni (to within 3 dB). Peak gain for the antenna is about +3 dBi perpendicular to the face of the antenna, front and back. Gain is on the order of +0 dBi edge-on. The antenna has the usual dipole nulls along the axis of the antenna top and bottom. Thus, this antenna provides good coverage in the plane normal to the axis of the antenna, but may have difficulty achieving optimal range directly above or below the antenna. In most cases, an indirect path is likely to exist, however. In typical operation, the antenna is oriented with the long axis positioned vertically. This places the dipole nulls along the vertical axis, and provides best response in the equatorial, azimuthal, or horizontal plane. Ansoft HFSS (high frequency structure simulator) was used to model the prototype antenna and calculate its radiation pattern. Figure 3 provides a typical result. This view shows the antenna edge-on with the front face of the antenna to the right, and the back face of the antenna oriented to the left. Figure 4 shows the principal planes of the antenna, and Figures 5a-c show peak power patterns in the principal planes.

Figure 3: Gain pattern of a prototype antenna as calculated by Ansoft HFSS at 3.5 GHz.

Figure 4: Principal planes.

Figure 5b: Elevation plane (edge-on).

Figure 5a: Peak power pattern, azimuthal plane.

Figure 5c: Elevation plane (face).

3C. PERFORMANCE: MATCHING

3D. PERFORMANCE: GAIN

The bottom fed planar elliptical dipole antenna offers an excellent match to 50 Ω. The voltage standing wave ratio (VSWR) is 1.5:1 or better from just below 3.0 GHz to nearly 5.5 GHz. Reflections from the antenna are down about –14 dB across this same band, thus the antenna accepts 96% of the applied power over these frequencies. Figure 6 shows this matching as a function of frequency from 0-7 GHz in Figure 6. Although optimized for the 3.0-5.5 GHz band, the prototype antenna remains fairly well matched up to beyond 20 GHz with VSWR on the order of 2:1 to 3:1. Figure 7 displays matching from 020 GHz. In both cases, matching was measured using a 10 MHz-20 GHz Rohde & Schwartz Vector Network Analyzer Model ZVM.

The prototype antenna has a nominal gain of about +3 dBi in the direction normal to either face of the antenna. Edge-on gain is on the order of about 0 dBi. These gain numbers are representative of antenna response from 2.5 GHz – 6.0 GHz. Gain was determined from the through response (S12) of a matched pair of antennas using a 10 MHz-20 GHz Rohde & Schwartz Vector Network Analyzer Model ZVM. Figure 8 shows gain for a variety of orientations from 0-7 GHz, and Figure 9 depicts gain from 0-20 GHz. Note that even when the nulls of the antenna were aligned, a gain of about –6 dBi was still obtained. This is likely due to indirect propagation paths between the antennas under test, and represents a worst case scenario.

Figure 6: Matching 0-7 GHz.

Figure 8: Gain 0-7 GHz.

Figure 7: Matching 0-20 GHz.

Figure 9: Gain 0-20 GHz.

3E. PERFORMANCE: PHASE The bottom fed planar elliptical dipole antenna offers a very linear phase response. Linearity in phase as a function of frequency means that all frequency components of a signal have the same delay. Thus, a linear phase antenna transmits short pulse and ultrawideband waveforms without distortion. The phase linearity between a matched pair of prototype antennas was evaluated using a 10 MHz-20 GHz Rohde & Schwartz Vector Network Analyzer Model ZVM. The delay due to the path length between the antennas has been removed. The remaining phase response is due to the delay through the balun transformers and slotlines of the matched pair of antennas. A phase inversion is evident at around 7 GHz, but in-band, phase is remarkably linear. Figure 10 presents these results.

Figure 11: Efficiency 0-6 GHz.

4. CONCLUSION Bottom fed planar elliptical dipoles are well matched and radiation efficient. They are omni-directional and are thus well-suited for ad-hoc networks with arbitrary azimuthal orientations. Furthermore, these antennas are electrically small and inexpensive without compromising on performance. Thus, bottom fed planar elliptical dipoles are well suited for commercial applications.

5. ACKNOWLEDGEMENTS The work presented in this paper was performed while the author was employed by the Time Domain Corporation. The cooperation and assistance of the Time Domain Corporation are gratefully acknowledged. Figure 10: Phase response, 0-20 GHz.

3F. PERFORMANCE: EFFICIENCY As noted in the section on matching, the prototype antenna accepts about 96% of applied power in band. Since the antenna is constructed on a low-loss substrate, and because resistive loading is not employed, virtually all of the accepted energy radiates. Accurate measurement of antenna efficiency across ultra-wide bandwidths poses formidable challenges, since losses in the measurement fixture tend to be much greater than losses in the antenna itself. Figure 11 depicts the result of a measurement of antenna efficiency, but this result probably understates the true efficiency of the antenna. In any event, the prototype antenna very efficiently radiates and receives ultrawideband signals.

6. REFERENCES [1] H. Schantz, “Planar Elliptical Element UltraWideband Dipole Antennas,” IEEE APS 2002. [2] H. Schantz, “Radiation Efficiency of UWB Antennas,” IEEE UWBST 2002. [3] The Time Domain Corporation markets planar elliptical dipoles under the brand name “BroadSpec.” [4] Because these antennas are “bottom feeders” their original designation within the Time Domain Corporation was “catfish” antennas. [5] H. Schantz, “Apparatus for establishing signal coupling between a signal line and an antenna structure” U.S. Patent 6,512,488 (January 28, 2003). [6] Specifications for the PPL Model 4050B pulser are available at http://www.picosecond.com. [7] Specifications for the Farr Research Model TEM-2-50 TEM horn are available at http://www.farrresearch.com.