Adaptive Antenna for Handheld GPS Receivers 作者： A.S.C. Svendsen and I.J. Gupta 報告出處： Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May 天線工程 碩通二甲 M98S0107 涂紹桓

目錄 1. Abstract 2. INTRODUCTION 3. ANTENNA DESIGN 4. SIMULATED ANTENNA PERFORMANCE 5. ANTENNA IN ADAPTIVE MODE 6. EXPERIMENTAL ANTENNA 7. EFFECT OF APERTURE REDUCTION 8. SUMMARY AND CONCLUSION 9. 心得

Abstract Global positioning system (GPS) has become the backbone of many commercial and military systems. Because of the weak satellite signals used in GPS, the ability to mitigate interference is of pressing concern, in particular for military applications. State-of-the-art military GPS receivers use adaptive antennas for interference suppression.

INTRODUCTION In this paper, we present a novel antenna array that can be used in hand-held GPS receivers for interference suppression. The antenna uses spiral elements to achieve the required number of elements in the array, the dual-band radiation coverage, and to have a small aperture, small volume, and be lightweight.

INTRODUCTION The antenna and integrated feed parameters are optimized for the L1 and L2 band radiation coverage, and the size of the antenna and feed with four elements is 4”x4”x0.03” (10.16cm x 10.16cm x 0.762mm).

ANTENNA DESIGN An adaptive antenna for a handheld GPS receiver needs to possess a number of electrical and physical characteristics. The first electrical characteristic is three- dimensional interference suppression.

ANTENNA DESIGN To provide this, the antenna needs to have at least three elements. The second electrical characteristic is satellite signal reception. For this, the antenna needs to have a broad right-hand circularly polarized (RHCP) radiation pattern across a 24 MHz bandwidth at L1 (1575 MHz) and L2 (1227 MHz) frequency bands.

ANTENNA DESIGN The primary advantage of spirals is efficiently using the available space to achieve large bandwidth in terms of the input impedance and pattern [10, 11]. Consequently, we can have each element cover both L1 and L2. The number of elements can be increased without increasing the height, weight, or footprint of the antenna. For our preliminary design, the footprint is set to be 4”x4” (10.16cm x 10.16cm).

ANTENNA DESIGN To fit the elements in the footprint, we chose four square Archimedean spirals and placed them in a 2x2 arrangement. We increased the number of elements from three to four which leads to an increase in interference suppression performance. With the use of such small spiral elements, it is difficult to obtain satellite coverage at the desired frequency bands.

ANTENNA DESIGN In order to restore the coverage, each spiral element is allowed to use the entire aperture to radiate. This is done by shorting the elements together in the center of the array. The short allows the currents from one spiral to flow into the other spirals, thereby allowing each element to use the whole aperture to radiate.

ANTENNA DESIGN Figure 1. The 4”x4” spiral array is composed of four square Archimedean spirals arranged 2x2 with the center arms shorted together 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

ANTENNA DESIGN The balun converts an unbalanced coaxial cable feed to the balanced transmission line feed needed at the center of the spirals. The coaxial cables are placed in the corner of the footprint. Figure 2 shows the striplines (red) that connect the coaxial cables (orange) to the common conductor of the spirals (green) by vias with a dielectric substrate in between.

ANTENNA DESIGN Figure 2. The spiral array with feed is composed of the spiral elements (blue/green), substrate (clear), and balun (red). The coaxial cables (orange) are in the corners. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

ANTENNA DESIGN With this feeding arrangement, the striplines and spiral elements can be printed on the top and bottom of a circuit board, respectively. Therefore, the antenna with integrated feed is easy to construct. In addition, the antenna is planar and lightweight because the circuit board is very thin.

ANTENNA DESIGN 0.7mmStripline width of the balun 0.5mmGap width of the spirals 5mmArm width of the spirals 2Number of turns for the spirals 30mil (0.762mm)Substrate thickness Loss tangent 3.48Relative permittivity Rogers 4350Dielectric substrate TABLE I. PARAMETERS OF DESIGNED ANTENNA.

SIMULATED ANTENNA PERFORMANCE The S11 performance, radiation efficiency, and radiation patterns of the designed antenna were analyzed using HFSS software.

SIMULATED ANTENNA PERFORMANCE Figure 3 shows the S11 performance versus frequency when one element of the array is excited and other elements are terminated into 50 ohms load. The other elements have the same response because of the physical symmetry. There is an excellent match (S11 is less than -10 dB) over the 24 MHz bandwidth around the L1 and L2 carrier frequencies. Figure 3. Simulated |S11| versus frequency when one element of the 4”x4”. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

SIMULATED ANTENNA PERFORMANCE Figure 4 shows the radiation efficiency (power radiated divided by power accepted) versus frequency with one element of the array excited. The power radiated includes RHCP and LHCP radiation. The radiation efficiency is 85% and 91% at the L1 and L2 carrier frequencies, respectively. The radiation losses are due to the losses in the dielectric substrate. Figure 4. Simulated radiation efficiency versus frequency when one element of the 4”x4” antenna is excited. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

SIMULATED ANTENNA PERFORMANCE Figure 5 shows the upper hemisphere RHCP realized gain patterns at the L1 and L2 carrier frequencies with one element of the array excited. (The center of the plot is zenith, the edge is the horizon, and the azimuth angle is represented by going around the circle.) Because of the mutual coupling from the other elements, the element pattern is distorted, and a single element does not provide adequate upper hemispherical coverage in that the realized gain in certain angular regions is well below -5 dBi.

SIMULATED ANTENNA PERFORMANCE Figure 5. Simulated RHCP realized gain for 4”x4” array at L1 (a) and L2 (b) carrier frequencies when one element is excited. (a)(b)(a)(b) 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

SIMULATED ANTENNA PERFORMANCE Figure 6 shows the new L1 and L2 carrier frequency upper hemisphere RHCP realized gain patterns. The distortion and nulls are eliminated and a broad beam is centered on zenith with azimuthal symmetry. The peak realized gain is 2.27 dBi for L1 and 0.9 dBi for L2. Furthermore, the RHCP realized gain in the upper hemisphere is above -4 dBi for both L1 and L2 carrier frequencies. Therefore, good coverage is obtained at the desired frequency bands.

SIMULATED ANTENNA PERFORMANCE (a)(b) Figure 6. Simulated RHCP realized gain for 4”x4” array at L1 (a) and L2 (b) carrier frequencies when four elements are excited using (1). (a)(b)(a)(b) 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

ANTENNA IN ADAPTIVE MODE In order to suppress interference, the spiral antenna is used with a space time adaptive processor (STAP). Figure 7 shows a STAP block diagram. There are L channels with each output being summed and the total sent to the receiver. The components of each channel are an antenna element, RF front end, and FIR filter. The RF front end, Gl(f) for the lth element, consists of LNAs, bandpass filters, mixers, and A/D converters.

ANTENNA IN ADAPTIVE MODE Figure 7. Space time adaptive processor block diagram. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

ANTENNA IN ADAPTIVE MODE Figure 8 shows the percent angular region versus SINR with no interferers and one interferer at the L1 and L2 carrier frequencies. The important conclusion is that at L1 and L2 significant portions of the sky are still available in the presence of one interferer.

ANTENNA IN ADAPTIVE MODE At -35 dB SINR, L1 has 75% and L2 has 60% of the sky available. Therefore, the antenna provides 3D interference suppression to enable satellite signal reception. Figure 8. Percent angular region versus SINR is shown with no interferers and one CW interferer at the L1 and L2 carreir frequencies. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

EXPERIMENTAL ANTENNA (a)(b) Figure 9. (a). Top view of the antenna with stripline feeds. (b). Bottom view of the antenna with spiral elements. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

EXPERIMENTAL ANTENNA Figure 10. The spiral array is setup for measurement with coaxial cables, a plastic holder, and SMA connectors. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

EXPERIMENTAL ANTENNA Figure 11. |S11| versus frequency for the simulated and experimental antennas. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

EXPERIMENTAL ANTENNA Figure 12. RHCP realized gain versus elevation at L1 and L2 for the simulated and experimental antennas. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

EFFECT OF APERTURE REDUCTION The original aperture for the antenna is reduced from 4”x4” to 3”x3”, but the spiral element structure, element arrangement, and feed structure are kept the same. The parameters of Table I were re-optimized for S11 less than -10 dB in a 24 MHz bandwidth centered around the L1 and L2 carrier frequencies. Table II contains the optimized parameters for the smaller aperture antenna.

EFFECT OF APERTURE REDUCTION TABLE II. PARAMETERS OF DESIGNED 3”X3” ANTENNA. 0.25mm Stripline width of the balun 1.1mm Gap width of the spirals 3.25mm Arm width of the spirals 2 Number of turns for the spirals 20mil (0.508mm) Substrate thickness Loss tangent 3.48 Relative permittivity Rogers 4350Dielectric substrate

EFFECT OF APERTURE REDUCTION Figure 13. Simulated |S11| versus frequency antenna when one element of the 3”x3” antenna is excited. Figure 14. Simulated radiation efficiency versus frequency when one element of the 3”x3” antenna is excited. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

EFFECT OF APERTURE REDUCTION (a)(b) 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

EFFECT OF APERTURE REDUCTION Figure 16. Percent angular region versus SINR of the 3”x3” antenna with no interferers and a single interferer at the L1 and L2 carrier frequencies. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

EFFECT OF APERTURE REDUCTION Figure 17. Percent angular region versus SINR of the 3”x3” and 4”x4” antennas for a single CW interferer at L1 the and L2 carrier frequencies. 資料來源 :A.S.C. Svendsen and I.J. Gupta. Adaptive Antenna for Handheld GPS Receivers. Position Location and Navigation Symposium (PLANS), 2010 IEEE/ION 4-6 May

SUMMARY AND CONCLUSION The antenna has a 4”x4” aperture, and consists of four spiral elements. Using the spiral elements, one can obtain the upper hemispherical coverage at L1 as well as L2 GPS frequency bands. The S11 performance, radiation efficiency, and radiation patterns of the antenna are excellent in the L1 and L2 bands. Using STAP, the antenna was shown to have good interference suppression capabilities.

SUMMARY AND CONCLUSION it was found that the reduction in aperture causes a large degradation in the antenna performance. This happens because of the decrease in directivity or resolution of the antenna. Therefore, we do not recommend reducing the aperture of the adaptive antenna array far below a 4”x4” footprint.

心得 這個天線的優點：厚度薄，重量輕，採 用平板型天線，從此報告中可將這個規格 當做一個標準，當低於此規格時天線的功 能就會降低出錯。 缺點：天線尺寸太大，無法適用於小型 GPS ，小於該尺寸的 GPS 都不適用。