IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 12, DECEMBER 2005
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Side-Fed Bifilar Helix Antenna Muhammad Amin and Robert Cahill
Abstract—A side-fed bifilar is shown to generate a similar radiation pattern as a dipole antenna, but the structure has a significantly reduced axial length. Simulated and measured results show that the helix turn angle can be used to control the ratio of the orthogonal linear field components and the input impedance. Index Terms—Dual mode antenna, low profile antenna, omnidirectional antenna.
I. INTRODUCTION
Q
UADRIFILAR helix antennas (QHAs) are often used to provide the broad beam circularly polarized patterns which are required for satellite communications [1]. The QHA consists of four helical conductors which are excited in phase quadrature at the feed point which is located at the center of the top radials [2]. A feature of future communication systems is the need to integrate both satellite and terrestrial services into a single radiating structure [3], [4]. Therefore, the antenna must be able to generate a circularly polarized cardioid shaped pattern in addition to a linearly polarized monopole beam which is necessary to provide operation in terrestrial systems [4]. Mounting a monopole coaxially within the QHA can provide dual mode operation [5], however a more compact structure could be produced if the terrestrial mode antenna can be accommodated within the envelope of the quadrifilar. In this letter, we show that this can be achieved by repositioning the feed point of a bifilar antenna to the center of the helix. Unlike a monopole antenna it is not necessary to employ a ground plane and the structure is more compact than a dipole which generates a similar radiation pattern. Furthermore simulated and experimental results are used to show that the helix turn angle can be used to control the ratio of the vertical and horizontal field components, the resonant frequency and the input impedance of the radiating structure. II. ANTENNA CONFIGURATION
The polarization type and pattern shape of a QHA is determined by the phase relationship between the radiating elements [3]. For space mode operation, these must be fed in phase quadrature [1] at the feed point which is located at the midpoint of either the top or bottom radials [2]. However to achieve the toroidal pattern which is required for terrestrial applications, it
Manuscript received June 14, 2005; revised September 16, 2005. This work was supported in part by a scholarship from the Ministry of Science and Technology, Pakistan. The review of this letter was arranged by Associate Editor A. Weisshaar. The authors are with the Institute of Electronics, Communications and Information Technology, Queen’s University Belfast, Belfast BT3 9DT, U.K. (e-mail:
[email protected];
[email protected]). Digital Object Identifier 10.1109/LMWC.2005.859945
Fig. 1. Schematic diagram of side-fed bifilar loop with turn angle 180 showing infinite balun feed arrangement.
Fig. 2. Photograph of side-fed bifilar loops with turn angle 218 and 180 showing infinite balun feed arrangement.
is necessary to excite the four monofilars with signals that are equal in amplitude and phase [3]. A similar monopole radiation pattern can also be generated by a bifilar when the two half loops are energised with identical signals at the center of the top radial. However this phase relationship cannot be obtained from infinite and folded balun feed arrangements which are often used to excite wire antenna structures [6]. For these driven configurations it is however possible to provide the required signal excitation by repositioning the feed point to the side of a half wavelength bifilar as shown in Figs. 1 and 2. In this structure, the current flows in the same direction in the two half loops and therefore at the top and bottom radials the current flows either toward or away from the midpoint. The fields along the axis of the bifilar are therefore cancelled and an omnidirectional linearly polarized pattern is generated in the azimuth plane.
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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 12, DECEMBER 2005
Fig. 5. Predicted and measured relative power plots in azimuth and elevation planes for side-fed bifilar with turn angle 180 . Fig. 3. Predicted and measured resonant frequency and input resistance of side-fed bifilar helix antenna versus turn angle.
Fig. 6. Predicted and measured relative power plots in azimuth and elevation planes for side-fed bifilar with turn angle 230 .
Fig. 4. Predicted and measured percentage bandwidth and ratio of vertical to horizontal gain of side-fed bifilar helix antenna versus turn angle.
III. SIMULATED PERFORMANCE The Numerical Electromagnetic Code (NEC V1.6) [7] was used to simulate the impedance and radiation patterns of a bifilar with a wire diameter, antenna radius and loop length of 2.2, 55.2, and 340 mm, respectively. For turn angles in the range 0 to 238 the axial length of the structure decreases from 114.7 mm to 0.8 mm. Fig. 3 depicts the computed resonant frequency of the antenna which varies between 995 MHz and 723 MHz. The input impedance of the antenna is shown to be proportional to the axial length of the structure at resonance, and for a turn angle of 218 the resistance at the input port is approximately 50 (Fig. 3). 2 refFig. 4 shows the computed bandwidth for a erenced to the input resistance at the resonant frequency of the
antennas which is shown in Fig. 3. For helix turn angles less than 230 , the bandwidth exceeds the 3% value which is obtainable for a half turn wavelength top fed QHA [6]. For a zero turn loop antenna the fields are predominantly vertically polarized and Fig. 4 shows that for helix turn angles up to 150 the crosspolar levels throughout the coverage region, excluding the region around pattern nulls, are generally lower than 10 dB. Beyond this range the horizontal component increases and for the shortest bifilar, with turn angle 238 the antenna is essentially horizontally polarized. The computed beamwidth in the elevation plane varies between 82 and 98 which is close to the 78 value which is obtained from a half wavelength dipole. In this plane the variation in the peak gain of the forward and backlobe is predicted to be less than 1.4 dB for a zero turn angle loop, and 1 dB for structures with a turn angle greater than 150 . The predicted azimuth and elevation patterns for half turn bifilar antenna are depicted in Fig. 5 at 963 MHz. Similarly the radiation patterns of a 230 bifilar are plotted at resonance in Fig. 6, where it is shown that almost identical vertically and horizontally polarized dipole radiation patterns are generated.
AMIN AND CAHILL: SIDE-FED BIFILAR HELIX ANTENNA
IV. MEASURED RESULTS Experimental models of eight side-fed bifilars with helix turn angles of 0 30 60 90 , 180 218 230 , and 235 were constructed using 1.2-mm diameter semirigid cables. In addition, scaled models of the latter four antennas were constructed to increase the frequency range (for VSWR 2) of the structures beyond the 950-MHz lower limit of the Scientific Atlanta 1780 pattern measurement system. The monofilars were shaped by winding the conductors around a cylindrical former and each helix was then glued to three sets of spokes or discs which were supported by a cylindrical plastic rod. The individual antennas were excited at the center of the helix using an infinite balun arrangement [1], as shown in Fig. 2. Swept frequency impedance and VSWR measurements were performed over the frequency range 0.5–1.5 GHz. Close agreement is shown in Fig. 3 between the measured and simulated resonant frequency of the bifilars and the corresponding input impedance values. The measured resonant frequency of the antennas was found to be approximately 9% lower than the NEC predictions because the computer model can not account for the effect of dielectric material that was used to support the radiating elements. Similarly in Fig. 4 the measured bandwidth variation and the ratio of the vertical and horizontal field components in the azimuth plane are also shown to be similar to the results that were generated by the NEC model. The radiation patterns of all eight bifilars were measured however only two of these are presented here for brevity. The measured azimuth radiation patterns of the half turn bifilar are plotted in Fig. 5 where the gain variation for the vertically polarized pattern is shown to be less than 2 dB and the average crosspolar isolation approximately 10 dB. In the elevation plane the shape of the orthogonally polarized patterns are in excellent agreement with the numerical results, and similarly in Fig. 6, where the measured radiation patterns of the 230
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turn angle bifilar are depicted. For this antenna the measured gain variation between the vertically and horizontally polarized toroidal shaped patterns was found to be less than 1 dB. V. CONCLUSION We have demonstrated that a linearly polarized toroidal radiation pattern can be obtained by exciting a half wavelength bifilar at the center of the helical section of the structure. The helix turn angle can be used to provide a simple method of impedance matching or by varying this parameter the ratio of the vertical and horizontal polarized fields can be controlled. The results also show that over a narrow range of helix angles between 225 and 235 , the antenna can be employed for applications which require dual linear polarization. Furthermore, the separate signal formats which are required for satellite and terrestrial based systems could potentially be generated by integrating a compact side fed bifilar within the envelope of a quadrature fed QHA. REFERENCES [1] R. Cahill, I. Cartmell, G. V. Dooren, K. Clibbon, and C. Sillence, “Performance of shaped beam quadrifilar antennas on the METOP spacecraft,” Proc. Inst. Elect. Eng., vol. 145, no. 1, pp. 19–24, 1998. [2] C. Kilgus, “Resonant quadrifilar helix,” IEEE Trans. Antennas Propag., vol. 17, no. 3, pp. 349–351, May 1969. [3] M. Barnard and S. McLaughlin, “Reconfigurable terminals for mobile communication systems,” Electron. Commun. J., pp. 281–284, 2000. [4] A. Petros, I. Zafar, and S. Licul, “Reviewing SDARS antenna requirements,” Microw. RF, pp. 51–62, 2003. [5] C. D. McCarrick, “A Compact monopole/quadrifilar helix antenna for S-band terrestrial/satellite applications,” Microw. J., vol. 44, no. 5, pp. 330–334, May 2001. [6] M. Amin and R. Cahill, “Bandwidth limitation of two port fed and self phased quadrifilar helix antennas,” Microw. Opt. Technol. Lett., vol. 46, no. 1, pp. 11–15, 2005. [7] NEC-Win Professional V1.6, Nittany Scientific, Inc., 2003.
