Dual-Frequency Ring Patch Antennas for GPS

Shoichi Kumagai, Yasuhiro Kazama, and Naohisa GotoR & D Department, Japan Radio Co., Ltd. Takushoku University

E-mail: kazamadlqab.jrc.co.jp

Abstract- Global Positioning System (GPS) is widely used forcar-navigation system all over the world. At present, GPS systememploys only the frequency of 1.57542 GHz (LI) in the civilianuse. Recently, to improve the positioning accuracy, GPSmodernization enterprise is scheduled and executing. For thefinal stage new GPS satellites which will be launched in 2014,additional two new frequency signal transmitters that are for thefrequency of 1.22760 GHz (L2) and 1.17645 GHz (L5) will beequipped. Prior to the final stage, the first stage new GPSsatellites will be launched in July, 2005. This satellite equips thetransmitters for the frequencies of 1.57542 GHz (LI) and 1.22760GHz (L2). In this paper, we propose compact, small, and low-profile dual-frequency operation microstrip antennas for thisfirst stage new GPS system.

I. INTRODUCTIONGPS spreads widely all over the world, particularly for the

use in car-navigation system. At present, GPS system uses onlythe frequency of 1.57542 GHz (LI) in the civilian use. Thepositioning accuracy of the current GPS system is fromapproximately 5 meters to 20 meters using correction data [1].To improve this accuracy, GPS modernization enterprise [2] isscheduled and executing. The goal of new GPS satellites equipadditional two new frequency signal transmitters that are forthe frequency of 1.22760 GHz (L2) and 1.17645 GHz (L5).Prior to the goal, the first stage satellites which equip thetransmitters for the LI and L2 frequencies will be launched onJuly 29th, 2005. While in Europe, Galileo system which issimilar to GPS system, is under development.

As dual-frequency-band operation microstrip antenna forGPS applications, many techniques are reported. One of thetechniques is used for aperture-coupled stacked microstripantenna [3], [4]. The aperture-coupled microstrip antenna hassome advantages such as wide bandwidth and noncontactingfeed transition. However, the feeding structure of this type ofmicrostrip antenna is complicated in fabrication, in addition tothe increase of antenna noise temperature due to feeding circuitloss. This antenna noise temperature degrades the receivingperformance of GPS. A single layer and single feed microstripantenna is also proposed [5]. This antenna has switchable slotsand radiates circularly polarized wave. The disadvantage of thisantenna lies in the complicated feeding structure whichemploys either pin diodes or MEMS switches to realize dualband operation [6]. These switches also cause high antennanoise temperature owing to feed loss. A more effective designis proposed in [7] where shorted annular patch antenna isdemonstrated.

In this paper, we propose a stacked antenna which is acombination of open type annular patch antenna with shorttype annular patch antenna. The proposed antenna is a compact,small sized, and low-profile dual-frequency operationmicrostrip antenna in the use of the first stage new GPS system.The designed antenna has been fabricated and tested. Somenumerical results and measured performances of the designedantenna are presented.

II. OPEN TYPE ANNULAR-RING MICROSTRIP ANTENNASShort type annular patch antennas are prominent for self-

diplexing characteristics. To obtain dual frequency antennas byusing this short type antenna, it is natural to stack two this typeantennas. Usually, in stacked antennas, low frequency antennais arranged under high frequency antenna to avoid the blockageof large diameter low frequency antenna. Another reason whythe high frequency antenna has to be above the low frequencyantenna is that it is easy to design multi-frequency-bandantennas by stacking some same type antennas. However, thisstacked antenna technique brings the antenna size to be large.

To overcome this weakness, in this paper, we attempt tocombine two different type annular patch antennas, that is thecombination of short type with open type annular-ring antennas.

So far, eigenvalues of short type annular-ring antenna arecalculated and reported [8], [9]. Using the short type antenna,some efforts are also paid for GPS applications [7], becausethis antenna type is prominent for self-diplexing characteristics.However, the paper on the open type eigenvalues can not befound in our findings. Then, we calculate the eigenvalues onthe open type antenna by solving the following equation forthat type of antenna fed by TM1I mode.

{xJo (x) - J1 (x)}{rx No (rx)- N1 (rx)}- {rx Jo (rx) - J1(rx)} {x No (x) - N1 (x)}I = 0

where x = ka = 2m/Ag, and r = b/a: a is outer radius ofannular patch antenna, b is inner radius of the antenna.

The results are shown in Fig. 1. In this figure, the short typevalues are also indicated by solving the following equation forshort type.

N1(rx){x Jo (x) - Ji (X)}- l (rx) {x No (x) - N1 (x)} = 0 (2)

0-7803-9206-X/05/$20.00 ©2005 IEEE

601

(1)

where J0 and J, are Bessel function, and No and N, areNeumann function.

In this proposed antenna design, b1,a, = 0.58 and b21a2 =0.32 are chosen. For the short type 1.5 GHz-band antenna,k1.5Ga2=2.38, on the other hand, klj2Gal=1.28 in the case ofopentype 1.2 GHz-band antenna. Therefore, the diameter of 1.2GHz-band antenna is smaller than that of 1.5 GHz-bandantenna. From this fact, the low frequency annular-ring antennais arranged above the high frequency antenna (see Fig. 4). Thisantenna arrangement allows the proposed antenna to becompact and low-profile.

111. PROPOSED MICROSTRIP ANTENNA STRUCTURE

Figure 2 shows a proposed dual-frequency operationmicrostrip antenna and figure 3 is a photograph of thefabricated antenna. This antenna consists of a ground plane, 1.5GHz-band annular-ring microstrip antenna, and 1.2 GHz-bandannular-ring microstrip antenna. In this figure, a, is outerradius of 1.2 GHz-band (low frequency band) antenna, and a2is outer radius of the 1.5 GHz-band (high frequency band)antenna. On the other hand, b, shows inner radius of 1.5 GHz-band antenna, and b2 shows inner radius of 1.2 GHz-bandantenna. The inner part of 1.5 GHz-band antenna is shorted to aground plane (short type), and the inner part of 1.2 GHz-bandantenna is kept to be opened (open type). Each antenna is fedby different ports (see Fig. 4) using most commonly usedfeeding methods for microstrip patch antenna, that is coaxialprobe. The coaxial probes are directly connected to feed lineswhich is for impedance matching. Each annular-ring microstripantenna is fed through the feed lines. The advantage of the useof directly connected coaxial probe is low feeding loss, whichprevents the degradation of the receiving performance of GPSfrom the increase of antenna noise temperature. Anotheradvantage of the coaxial probe feed is ease of fabrication. Eachannular-ring microstrip antenna [10] is etched on a dielectricsubstrate, and is piled up each other. In this case, the thicknessof the substrates is 4.0 mm for 1.2 GHz-band antenna and 1.6mm for 1.5 GHz-band antenna. Both the substrates have thedielectric constant Sr = 2.55.

IV. RESULTSA prototype antenna has been fabricated and tested. The

calculated return loss characteristics of the antenna is shown inFig. 5. In this figure, at the frequency of 1.2 GHz (L2 band)and 1.5 GHz (LI band), over -25 dB return loss characteristicsare obtained. This return loss characteristics are sufficient forGPS applications. The measured return loss characteristics areshown in Fig.6. In comparison with the calculated data shownin Fig. 5, good agreement can be seen. Figure 7 shows thetypical example of the calculated radiation patterns of theproposed antenna. It is found that the radiation patterns of 1.2GHz is almost same as that of 1.5 GHz. The calculated antennagain of 1.5 GHz is 7.5 dBi, and that of 1.2 GHz is 6.3 dBi. Also,the calculated axial ratio of these antenna is 0.3 dB at 1.2 GHz,and 0.2 dB at 1.5 GHz. Figure 8 shows the measured results of

the radiation patterns of the antenna. As you can see theradiation patterns in Fig. 7 and Fig. 8, good agreement betweenthe measured and the calculated results is obtained. From theseresults, it is apparent that the proposed antenna has thesufficient characteristics for GPS applications.

8

I.)

enP

7

6

5

4

3

2

1

0

-ShortType - /orte

-Open Typeil/

III

t - - - - - - - - -_

.___

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8bla

Fig. 1. Eigenvalues of the annular-ring microstrip antenna.

I .5GHz-band Annular-Ring I .2GHz-bandMicrostrip Antenna Annular-Ring\ ~N~Microstrip Antenna

DielectricSubstrate

V//Z//A

Fig. 2. Geometry of proposed dual-frequency operationmicrostrip antenna.

602

/121

-

0 I

-5 -

-10-

-15 -

2025-20-- --

-30

-35 . L L_

-401.1 1.2 1.3 1.4 1.5

Frequency [GHz]

Fig. 3. Out Side view of the fabricated antenna. Fig. 5. Calculated return loss characteristics of proposedantenna.

1.2GHz-bandAnnular-Ring Antenna

1 .5GHz-bandAnnular-Ring Antenna

l ;Ground Plane

Short-Wall

(1.2GHz-band)

Feed Point(1.5GHz-band)

Fig. 4. Antenna arrangement of proposed antenna.

0

-5

X -10a

-15

0

._j -20

E_E -25

X -30

-35

-401.1 1.2 1.3 1.4 1.5 1.6 1.7

Frequency [GHz]

Fig. 6. Measured return loss characteristics of proposedantenna.

603

1.6 1.7

-

-

0-RHCP-1 HCP

90 --90

+180

(a) 1.2GHz-band (a) 1.2GHz-band

-RHCP0 - LHt

)S4-9~~~~-0i+

+ 180

(b) 1.5GHz-band

Fig. 7. Example of the calculated radiation patterns.

0 -

90

- RHCP-LHCP

±180

(b) 1.5GHz-band

Fig. 8. Example of the measured radiation patterns.

V. CONCLUSIONDual-frequency operation microstrip antenna is proposed

for New GPS system use. By calculating the eigenvalues ofshort and open type annular-ring microstrip antennas, lowfrequency antenna can be positioned above high frequencyantenna. As a result, a compact, small, and low-profile antennacan be obtained. Good antenna characteristics for GPSapplications are achieved.

604

0- RHCP-LHCP

90

±180

90

ICP-

on

nn

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"Galileo:Lmpact on road transport," 11th. World Congress on ITSNagoya, Aichi, Japan, Nov. 2004.

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[4] Y. Lee, M. Kirchner, and S. Ganguly, "Multiband L5 Capable GPSAntenna with Reduced Backlobes," ION GNSS 17th InternationalTechnical Meeting of the Satellite Division, pp. 1523-1530, Sep. 2004.

[5] F. Yang, and Y. Rahamat-Samii, "A Single Layer Dual Band CircularlyPolarized Microstrip Antenna for GPS Applications," IEEE Antennasand Propagation Society International Symposium, Vol. 4, pp. 720-723,Jun. 2002.

[6] F. Yang, and Y. Rahamat-Samii, "Switchable Dual-band CircularlyPolarized Patch Antenna with Single Feed," Electronics Lett., Vol. 37,pp. 1002-1003, Aug. 2001.

[7] L. Boccia, G. Amendola, and G. D. Massa, " A Dual FrequencyMicrostrip Patch Antenna for GPS Applications," IEEE AntennasWireless Propagat. Lett., Vol. 3, pp. 157-160, 2004.

[8] W. C. Chew, "A Broad-Band Annular-Ring Microstrip Antenna," IEEETrans. Antennas Propagat., Vol. AP-30, pp. 918-922, Sep. 1982.

[9] M. Ogawa, T. Watanabe, K. Nishikawa, and Y. Minami, "Radial linemicrostrip array antenna using film substarate for mobile BS antennasystem," Proceedings of the 1998 IEICE General Conference, B-1-153,Mar. 1998

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