Ultra-Wideband Conformal Low-Profile Four-Arm Unidirectional Traveling-Wave Antenna With A Simple Feed

CROSS-REFERENCE TO RELATED APPLICATION

0001This application claims priority to copending U.S. provisional application entitled, “Ultra-Wide Conformal Low-Profile Four-Arm Unidirectional Traveling-Wave Antenna with a Simple Feed,” having Ser. No. 61/469,409, filed Mar. 30, 2011, which is entirely incorporated herein by reference.

TECHNICAL FIELD

0002The present invention is generally related to radio-frequency antennas and, more particularly, ultra-wideband low-profile multi-arm unidirectional traveling-wave (TW) antennas for conformal mounting on platforms.

BACKGROUND

0003The traveling-wave (TW) antenna is a class of ultra-wideband platform-compatible low-profile antennas, including the spiral-mode microstrip (SMM) antennas and miniaturized slow-wave (SW) antenna, among others. The SMM antenna was discussed in publications (Wang, J. J. H. and V. K. Tripp, “Design of Multioctave Spiral-Mode Microstrip Antennas,” <i>IEEE Trans. Ant. Prop., </i>March 1991; and Wang, J. J. H., “The Spiral as a Traveling Wave Structure for Broadband Antenna Applications,” <i>Electromagnetics, </i>20-40, July-August 2000) and U.S. patents (U.S. Pat. No. 5,313,216, issued in 1994; U.S. Pat. No. 5,453,752, issued in 1995; U.S. Pat. No. 5,589,842, issued in 1996; U.S. Pat. No. 5,621,422, issued in 1997; U.S. Pat. No. 7,545,335 B1, issued in 2009) which are incorporated herein by reference. The SW antenna is a subset of the TW antenna with its size miniaturized by the SW technique (U.S. Pat. No. 6,137,453 issued in 2000, which is incorporated herein by reference). These thin planar antennas generally consist of an ultra-wideband planar radiator in the form of a multi-arm spiral, sinuous structure, or other frequency-independent geometries, among which the most widely used is the two-arm spiral antenna, having a unidirectional radiation pattern.

0004The unidirectional radiation pattern is due to mode-1 of TW modes; presence of other TW modes, 0, 2, 3, 4, etc. would distort the radiation pattern. Because of the lack of full symmetry, the commonly used two-arm unidirectional spiral radiator cannot achieve a high degree of mode purity, thus is limited in radiation pattern performance. For applications requiring high-quality radiation patterns, such as the GNSS (Global Navigation Satellite System) receive antenna or elements in planar phased arrays, a four-arm spiral radiator in the SMM antenna was more desirable (e.g., Wang and Triplett, “High-Performance Universal GNSS Antenna Based on GNSS Antenna Technology,” <i>IEEE </i>2007 <i>International Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, </i>Hangzhou, China, 14-17 Aug. 2007 which is incorporated herein by reference).

0005Unfortunately, to realize the potential of the four-arm SMM antennas, or the cavity-loaded spiral antenna, a high-quality four-terminal feed is needed to provide equal amplitude and relative phases of 0°, 90°, 180°, 270°, respectively. Such a complex feed, which uses a number of hybrids, power dividers, couplers, matrices, etc. leads to enormous escalation in cost and reduction in gain/efficiency as compared with the two-arm version. Additionally, the complexity and size of such a four-arm feed pose a serious difficulty in its physical implementation in GNSS and array antennas. Disclosed are various embodiments for a method in which these 4-arm unidirectional TW antennas are fed with a mechanism using a single balun that is generally smaller, much simpler, and thus much less costly, feed. The geometric symmetry of the new approach can also lead to a more accurate feed and thus improve the high performance of the four-arm version further above the two-arm version, at a low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

0006<figref idref="DRAWINGS">FIG. 1A</figref> depicts, in top view, an ultra-wideband low-profile 4-arm unidirectional traveling-wave antenna fed by a simple balun with a mode suppressor.

0007<figref idref="DRAWINGS">FIG. 1B</figref> depicts, in side view, the ultra-wideband low-profile 4-arm unidirectional traveling-wave antenna of <figref idref="DRAWINGS">FIG. 1A</figref>.

0008<figref idref="DRAWINGS">FIG. 2A</figref> shows top view of the feed region for the ultra-wideband low-profile 4-arm traveling-wave antenna in <figref idref="DRAWINGS">FIG. 1A</figref>.

0009<figref idref="DRAWINGS">FIG. 2B</figref> shows side view of the feed region for the ultra-wideband low-profile 4-arm traveling-wave antenna in <figref idref="DRAWINGS">FIG. 1A</figref>.

0010<figref idref="DRAWINGS">FIG. 2C</figref> shows A-A′ cross-sectional view of the feed region for the ultra-wideband low-profile 4-arm traveling-wave antenna in <figref idref="DRAWINGS">FIG. 2A</figref>.

0011<figref idref="DRAWINGS">FIG. 2D</figref> shows B-B′ cross-sectional view of the feed region for the ultra-wideband low-profile 4-arm traveling-wave antenna in <figref idref="DRAWINGS">FIG. 2B</figref>.

0012<figref idref="DRAWINGS">FIG. 3A</figref> depicts a planar four-arm sinuous TW radiator.

0013<figref idref="DRAWINGS">FIG. 3B</figref> depicts a planar four-arm log-periodic TW radiator.

0014<figref idref="DRAWINGS">FIG. 4</figref> shows measured VSWR over 1-10 GHz for the unidirectional traveling-wave antenna in <figref idref="DRAWINGS">FIG. 1A</figref> and <figref idref="DRAWINGS">FIG. 1B</figref>.

0015<figref idref="DRAWINGS">FIG. 5</figref> shows typical measured elevation radiation patterns in two orthogonal linear polarizations over 1-10 GHz for the unidirectional traveling-wave antenna in <figref idref="DRAWINGS">FIG. 1A</figref> and <figref idref="DRAWINGS">FIG. 1B</figref>.

0016<figref idref="DRAWINGS">FIG. 6</figref> shows measured antenna gain in dBi over 1-10 GHz for the unidirectional traveling-wave antenna in <figref idref="DRAWINGS">FIG. 1A</figref> and <figref idref="DRAWINGS">FIG. 1B</figref>.

DETAILED DESCRIPTION OF THE INVENTION DISCLOSURE

0017<figref idref="DRAWINGS">FIGS. 1A and 1B</figref> depict the top and side views, respectively, of an ultra-wideband low-profile mode-1 4-arm traveling-wave (TW) antenna <b>10</b>, which is of the shape of a pillbox, preferably circular but can be of other polygonal cylindrical form symmetrical about its center axis z. The antenna <b>10</b> is comprised of a planar conducting plane <b>110</b>, a feed network <b>120</b>, a planar conducting plane <b>130</b>, a TW structure <b>140</b>, and a planar TW radiator ensemble <b>160</b>, stacked, one on top of the other, sequentially, as well as a feed ensemble <b>200</b>. The thickness of the antenna <b>10</b> is electrically small, generally less than 0.1 λ_L, where λ_Ldenotes the free-space wavelength at the lowest frequency of operation. The diameters of the planar TW radiator ensemble <b>160</b>, the TW structure <b>140</b>, and the feed network <b>120</b> are generally the same and preferably less than 0.4 λ_L. The diameter of the planar conducting plane <b>110</b> must be at least as large as that of the TW structure <b>140</b>.

0018The planar TW radiator ensemble <b>160</b> is excited by feed ensemble <b>200</b>, which is connected with a simple balun contained in the feed network <b>120</b>. Note that, for the convenience of illustrating the details of the configuration, we define a small region in antenna <b>10</b> that contains the feed ensemble <b>200</b> in the center, with their components designated numerically in <b>200</b>s. The periphery of feed ensemble <b>200</b> is somewhat arbitrary, defined for the convenience of illustration, not as a structurally exclusive region. In fact, the drawings in <figref idref="DRAWINGS">FIGS. 2A</figref>, <b>2</b>B, <b>2</b>C, and <b>2</b>D showing the details of the feed ensemble <b>200</b> exhibit some structural overlaps with the rest of antenna <b>10</b>. Practically, the regions inside and outside feed ensemble <b>200</b> are expected to be well integrated in manufacturing.

0019The TW antenna <b>10</b> is to be conformally mounted on the surface of a platform, which is generally curvilinear. As a practical matter, the antenna is often placed on a relatively flat area on the platform, and does not have to perfectly conform to the platform surface since the TW antenna has its own conducting ground surface. In practice, the conducting ground surface is generally chosen to be planar or part of a canonical shape, such as a cylinder, sphere, or cone that is easy and inexpensive to fabricate. In any case conducting surfaces <b>110</b> and <b>130</b>, as well as TW structure <b>140</b> and TW radiator ensemble <b>160</b>, share the same canonical shape and are all parallel to one another and symmetrical about the vertical center axis z.

0020<figref idref="DRAWINGS">FIG. 2A</figref> shows a top view of the TW radiator ensemble <b>160</b> in the feed region. As shown in the side view and cross-sectional A-A′ view in <figref idref="DRAWINGS">FIGS. 2B and 2C</figref>, respectively, the TW radiator ensemble <b>160</b> consists of three thin layers: the TW radiator <b>161</b> in the center layer, the dielectric superstrate <b>163</b> and the dielectric substrate <b>162</b>. Note that the drawings in <figref idref="DRAWINGS">FIGS. 1A and 2A</figref> show embodiments in which the thickness of superstrate <b>163</b> vanishes and thus the TW radiator <b>161</b>, a four-arm Archimedean spiral in this case, is visible. Note that the diameter of feed ensemble <b>200</b> is arbitrarily selected for the convenience of illustration, and there is no structural discontinuity at the circular boundary.

0021In prior art, the four terminals of the spiral in mode-1 operation, designated as arms <b>181</b>, <b>182</b>, <b>183</b>, and <b>184</b>, respectively, are fed with excitations of equal amplitude and relative phases of, say, 0°, 90°, 180°, 270°, respectively and consistent with the sense of the polarization of the spiral. In this invention, one pair of opposite terminals <b>181</b> and <b>183</b> is excited with equal amplitude and relative phases of 0° and 180°, respectively, and the other pair of opposite terminals <b>182</b> and <b>184</b> is excited parasitically, by the feed ensemble <b>200</b>, as shown in A-A′ cross-sectional view in <figref idref="DRAWINGS">FIG. 2A</figref>. To ensure that the parasitic excitation of terminals <b>182</b> and <b>184</b>, without direct contact with the feed line, is proper, we employ a feed ensemble <b>200</b>, which comprises a twin-lead feed <b>210</b> and a mode suppressor <b>240</b>.

0022The twin-lead feed <b>210</b> has an impedance around 100 ohms, and is to be fine-tuned to match the impedance of the TW radiator ensemble <b>160</b> in the environment of TW structure <b>140</b> and mode suppressor <b>240</b> over the ultra-wide frequency band of operation. As shown in <figref idref="DRAWINGS">FIGS. 1B</figref>, <b>2</b>B and <b>2</b>C, the twin-lead feed <b>210</b> extends beyond the conducting ground plane <b>130</b> and then connects the two output terminals on the balanced side of a balun positioned in the feed network <b>120</b>, which is generally a stripline or microstrip printed circuit board enclosed by conducting ground planes <b>110</b> and <b>130</b> and side conducting walls. A balun is a device that connects an unbalanced transmission line on one side to a balanced transmission line on the other side, and also performs needed impedance matching (transformation) between the two sides. In the present embodiment, the balanced side of the balun is connected to the balanced twin-lead transmission line, and the unbalanced side of the balun is connected to a matching output circuit which leads to an unbalanced coaxial connector at the end of the feed network for connection with an external transmitter/receiver.

0023The mode suppressor <b>240</b> is a circular conducting tube having a small diameter, generally less than about 0.01 λ_L, to ensure smooth transition of TW propagation from twin-lead feed <b>210</b> and the TW radiator ensemble <b>160</b> (<figref idref="DRAWINGS">FIGS. 1B</figref>, <b>2</b>B and <b>2</b>C). The top of mode suppressor <b>240</b> is spaced at a distance S below the TW radiator ensemble <b>160</b> and its bottom joining the conducting ground plane <b>130</b>. The spacing S is small, less than about 0.01 λ_L, and is a tradeoff between smooth launching of mode-1 spiral mode in the TW radiator ensemble <b>160</b> and the suppression of higher-order modes in the wave propagation between the TW radiator ensemble <b>160</b> and the conducting ground plane <b>130</b>. <figref idref="DRAWINGS">FIG. 2B</figref> further reveals a B-B′ cross-sectional view of the feed ensemble <b>200</b> showing the twin-lead feed <b>210</b> and the mode suppressor <b>240</b> in the form of a conducting cylindrical tube.

0024As can be seen in <figref idref="DRAWINGS">FIG. 2D</figref>, the twin-lead feed <b>210</b> can be fabricated on a double-sided printed circuit board of a low-loss dielectric substrate <b>260</b>. Between the twin-lead feed <b>210</b> and the mode suppressor <b>240</b> is filled, in part or in whole, another low-loss dielectric which may or may not be the same as that of the printed circuit board of the twin-lead feed <b>210</b>. The feed ensemble <b>200</b> can be mass produced by planar printed-circuit-board (PCB) fabrication techniques, in which case the twin-lead feed <b>210</b> can start with two circular via holes, which are then metal-plated for integration with the TW radiator <b>161</b> (<figref idref="DRAWINGS">FIGS. 2B and 2C</figref>) and balun in the feed network <b>120</b>.

0025The TW radiator <b>161</b>, which is a four-arm Archimedean spiral as shown in <figref idref="DRAWINGS">FIG. 1A</figref>, is in general a planar multi-arm frequency-independent structure, most of which are of self-complementary geometry. For example, <figref idref="DRAWINGS">FIG. 3A</figref> depicts a planar four-arm sinuous TW radiator <b>361</b>, and <figref idref="DRAWINGS">FIG. 3B</figref> depicts a planar four-arm log-periodic TW radiator <b>461</b>. The spiral type radiator has inherently circularly polarization (CP) with a sense of right-hand CP (RHCP) or left-hand CP (LHCP) determined by the spiral windings being counterclockwise or clockwise for the convention of time-harmonic fields chosen—either exp(jωt) or exp(−jωt).

0026The sense of the circular polarization of the planar radiators in <figref idref="DRAWINGS">FIG. 3</figref> is rooted not only in the radiator per se but also in the way the four arms are fed, in the sequence of (0°, 90°, 180°, 270°) or (0°, −90°, −180°, −270°). When a non-spiral is employed as TW radiator <b>161</b> (<figref idref="DRAWINGS">FIGS. 3B and 3C</figref>) and fed with the present simple feed, it will radiate in linear polarization, which results from the combination of the RHCP and LHCP, in equal phase and amplitude, inherent in the radiator.

0027The TW structure <b>140</b> can be of a slow-wave (SW) type. The use of an SW structure can lead to reduction of phase velocity characterized by a slow-wave factor (SWF). The SWF is defined as the ratio of the phase velocity V_sof the TW to the speed of light c, given by the relationship

0000<br /><i>SWF=c/V</i>_s=λ_o/λ_s  (1)

0000where c is the speed of light, λ_ois the wavelength in free space, and λ_sis the wavelength of the slow-wave, at the operating frequency f_o. Note that the operating frequency remains the same both in free space and in the slow-wave antenna. The SWF indicates how much the TW antenna is reduced in a relevant linear dimension. For example, an SW antenna with an SWF of 2 means its linear dimension in the plane of SW propagation is reduced to ½ of that of a conventional TW antenna. Note that, for size reduction, it is much more effective to reduce the diameter, rather than the height, since the antenna size is proportional to the square of antenna diameter, but only linearly to the antenna height. Note also that in this disclosure, whenever TW is mentioned, the case of SW is generally included. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention.

Experimental Verification

0028Experimental verification of the principles of the invention has been carried out satisfactorily. Several breadboard models were designed, fabricated, and tested. Some measured data on one model is displayed here to demonstrate that the principles of this invention are valid, and that the imperfections in the performance are primarily due to the deficiencies of the balun employed.

0029<figref idref="DRAWINGS">FIG. 4</figref> shows measured VSWR over 1-10 GHz for a breadboard model of the unidirectional traveling-wave antenna in <figref idref="DRAWINGS">FIG. 1</figref> using a four-arm Archimedean spiral radiator. <figref idref="DRAWINGS">FIG. 5</figref> shows typical measured elevation radiation patterns in two orthogonal linear polarizations (E_θ and E_φ) over 1-10 GHz for this antenna. <figref idref="DRAWINGS">FIG. 6</figref> shows estimated antenna gain in dBi (primarily CP and based on combining measured gain in dBiL and axial ratio for two orthogonal linear polarizations) for this antenna over 1-10 GHz. These data are fairly good for a crude breadboard. Separate tests on the balun alone revealed that amplitude and phase errors in the balun (which is outside the scope of the present invention) are primarily the cause of the imperfections at certain frequencies in the feed output and, consequently, the exhibited performance of the antenna. Later models focused on narrower bandwidths, such as GNSS, for which the component and fabrication tolerances can be more easily met, exhibited greatly improved performance.