Fig. 3. Measured and simulated free space S11 of the proposed GPS antenna

III. ANTENNA PERFORMANCE IN FREE SPACE

The free space performance of the proposed antenna ismeasured in an anechoic chamber (with a sleeve balun for1575 MHz) and simulated using a FDTD-based numericalsolver [4]. As shown in Figure 3, the antenna is tuned toresonate at 1575 MHz, it is observed that the measurementresult exhibits a larger bandwidth than the simulated result, thisis due to the difference in metal and dielectric loss betweensimulation and measurement and the imperfect choking ofcoaxial cable. The measured total efficiency at 1575 MHzis found to be 53.7% while the simulated total efficiency is60.8% and the simulated upper hemisphere radiated efficiencyis 39.3% (with the red cover side facing the sky) .

The free space 3D gain pattern is shown in Figure 4, thepeak gain is found to be 1.69 dBi. It is observed that theantenna pattern exhibits a main lobe in the negative z direction(on the red cover side). One might assume that based onthe free space antenna pattern, the device should be placedwith the green cover facing the user body since there is anull in the radiation pattern towards the positive z direction.

Fig. 4. Simulated free space 3D gain of the GPS antenna at 1575 MHz,normalized to 1.69 dBi peak gain.

Fig. 5. Simulated (left) and measured (right) free space 2D far field (totalelectric field) polar plots at 1575 MHz, (a) XZ plane, (b) YZ plane and (c)XY plane.

Such assumption will be proven wrong in the next section.In Figure 5, the simulated and measured 2D far field pattern(total electric field) plots are presented. It is observed that afairly good agreement is achieved between the simulated andthe measured far field patterns. Note that due to the differencesin axis assignment, there is either a 90 degree or a 180 degreerotation for the measurement results when compared to thesimulated ones. Based on simulation and measurement results,the free space far field characteristics of the proposed GPSantenna is acquired.

IV. BODY EFFECTS ANALYSIS THROUGH NUMERICALSIMULATIONS

To analyze the effects of body loading to the proposed GPSantenna, the tracking device is simulated with a flat torsophantom and a dog phantom. A keep out distance of 8 mmbetween the phantom body and the device is applied to accountfor the space occupied by belt and buckle. The dielectric

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Fig. 6. Simulated antenna S11 under various phantom loading conditions.

property of the phantom follows the muscle tissue of thehuman body and is given as 53.8 for the relative permmittivityand 1.22 S/m for the conductivity [5] at 1575 MHz. Thedimension of the flat phantom block is 400 mm 400 mm 100 mm while the dimension of the dog phantom [6] is910 mm 260 mm 700 mm (length width height). Itshould be noted that the body loading environment simulatedin this study does not account for the heterogeneity of a userbody, i.e., a simplified loading environment is approximated.Two device orientations are investigated, scenario A is with thegreen cover side facing the sky while scenario B is with thered cover side facing the sky (the main lobe direction for thefree space condition). The antenna S11 resulted from differentphantom types and device orientations are plotted in Figure 6.As shown, the presence of a body phantom causes a shift inthe antenna resonance frequency. It is observed that the flattorso phantom and the dog phantom result in similar antenna

Fig. 7. Simulated 3D gain of the GPS antenna at 1575 MHz for flat phantomscenario A, normalized to 1.16 dBi peak gain.

Fig. 8. Simulated 3D gain of the GPS antenna at 1575 MHz for flat phantomscenario B, normalized to 0.45 dBi peak gain.

input impedances. Based on the S11 plots, the difference inthe antenna mismatch at 1575MHz is not significant amongthe investigated body loading conditions. As shown in Figure7 and 8, the far field gain patterns for the flat torso phantomappear to be fairly similar. However, the antenna peak gain ismuch higher in scenario A than in scenario B. This suggeststhat the device orientation in scenario B, which is optimumin free space, is no longer the preferred configuration underbody loading condition.

The 3D gain patterns for the dog phantom are presentedin Figure 9 and 10. A similarity is observed between the flattorso case and the dog phantom case: the device orientationscenario A provides higher antenna gain than scenario B. Thesimulation results of various antenna performance parameters

Fig. 9. Simulated 3D gain of the GPS antenna at 1575 MHz for dog phantomscenario A, normalized to 3.01 dBi peak gain.

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Fig. 10. Simulated 3D gain of the GPS antenna at 1575 MHz for dogphantom scenario B, normalized to 1.08 dBi peak gain.

(gain, total efficiency, upper hemisphere radiated efficiencyand partial GPS radiated efficiency [7]) due to body loadingeffects are shown in Table 1. The summarized results confirmthat the GPS antenna exhibits better performance in scenarioA than in scenario B, for both flat torso and dog phantomloading conditions. This implies that the device orientation forthe best GPS antenna performance in body loading conditioncannot be assumed based on the free space radiation pattern.As shown in Figure 7 to 10, the antenna radiation patternis more directive for the dog phantom loading than the flattorso loading. The differences in the respective 2D radiationpatterns can be observed in Figure 11. It is noted that for theestimation of total efficiency, a flat torso can be used as ageneric loading scenario. On the other hand, the antenna gainpattern and peak gain value are highly correlated to the actualshape of the phantom body.

TABLE ISIMULATED GAIN, TOTAL EFFICIENCY (TE), UPPER HEMISPHERE

RADIATED EFFICIENCY (UHRE) AND PARTIAL GPS RADIATEDEFFICIENCY (PGRE) VALUES AT 1575 MHZ

Gain (dBi) TE(%) UHRE (%) PGRE (%)Free Space 1.69 60.8 39.3 48.8

Flat Torso A 1.16 18.7 18.3 18.6Flar Torso B 0.45 14.3 13.4 14.1Dog Back A 3.01 20.5 18.8 19.9Dog Back B 1.08 14.5 12.5 13.9

V. CONCLUSION

The investigation results show that for a wearable GPStracking device, the device orientation for the best free spaceupper hemisphere radiation/reception performance might notcorrespond to that of the body loading condition, i.e., for abody-worn monopole-type GPS antenna with small ground

Fig. 11. Simulated 2D gain of the GPS antenna at 1575 MHz for (a) flattorso scenario A and (b) dog phantom scenario A.

plane, the body loading effects must be investigated in additionto its free space radiation performance to ensure an optimumGPS signal reception.

REFERENCES[1] S. Manoharan, On GPS Tracking of Mobile Devices, Networking and

Services, 2009 Fifth International Conference on, pp. 415-418, 2009.[2] M. Z. Azad and M. Ali, A Miniature Implanted Inverted-F Antenna for

GPS Application, Antennas and Propagation, IEEE Transactions on,vol. 57, pp. 1854 -1858, 2009.

[3] M. U. Rehman, Y. Gao, X. Chen, C. G. Parini and Z. Ying, Effectsof Human Body Interference on the Performance of a GPS Antenna,Antennas and Propagation, 2007. EuCAP 2007. The Second EuropeanConference on, 2007.

[4] SEMCAD X: Full Wave Electromagnetic/Thermal Simulation Platform.http://www.semcad.com.

[5] S. Gabriel, R. W. Lau and C. Gabriel, The Dielectric Properties ofBiological Tissues: Parametric models for the dielectric spectrum oftissues, Phys. Med. Biol., vol. 41, no.11, 1996, pp.2271-2293.

[6] Poser: Complete 3D Figure Design & Animation.http://poser.smithmicro.com.

[7] CTIA Test Plan for Mobile Station Over the Air Performance, Revision3.1, December 2010 (draft).

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