Small-folded, Printed Quadrifilar Helix Antenna for GPS Applications

Josh RABEMANANTSOA and Ala SHARAIHA

IETR (Institut d’Electronique et des Télécommunications de Rennes), University of Rennes , UEB Campus de Beaulieu - Bat. 11D, 263 Av. Général Leclerc, 35042 Rennes Cedex, France

josh.rabemanantsoa@univ-rennes1.fr, ala.sharaiha@univ-rennes1.fr

Abstract—A new approach to designing a small printed quadrifilar helical antenna (PQHA) is presented. The printed wire is spiral in shape. The entire system is designed using Ansoft HFSS. A size reduction of 43% is obtained compared to a conventional PQHA. A dual band antenna working in L1/L2 GPS application is realized with a good gain and a good circular polarisation. Good agreement with the experimental results is obtained.

Keywords— Miniaturization, circular polarization, folded antenna, printed quadrifilar helical antenna, dual-band

I. INTRODUCTION In recent years, Printed Quadrifilar Helical Antennas (PQHAs) have been receiving widespread attention in many applications, such as mobile communication and GPS, due to their many salient features, including: low cost, light weight, hemispherical coverage, and good circular polarization. Meandering techniques have been used by many authors to reduce the size of wire antennas, such as loop and monopole antennas [1-4]. They consist of using continuously folded wire antennas, which tend to resonate at much lower frequencies than an ordinary antenna of equal length. Several solutions have been reported in the literature to reduce the axial length of the PQHA working in a single band such as using an integrated feeding network [5], introducing a dielectric rod inside the helix [6], employing variable pitch angles on the wires [7] and meandering line technique [8] and [9]. In this paper, we investigated the meander line technique to reduce the size of the conventional PQHA (Fig. 1a) while maintaining a suitable radiation pattern and obtaining dual band, or even multiband behaviour. The axial length of a conventional PQHA is miniaturized by about 43% with meandering and by turning the helix arms to form square spirals (fig1b). The total wire length of the Spiral-Folded PQHA (called S-FPQHA) is adjusted such that it exhibits the two separate resonant frequencies for L1/L2 GPS application. Details of the proposed S-FPQHA are described, and the simulated as well as experimental results of the antenna are presented and discussed.

II. ANTENNA CONFIGURATION AND ANALYSIS The planar unwrapped representation of the S-FPQHA, as

well as the conventional PQHA, is illustrated in Fig.1. The four arms of these antennas were printed onto a thin dielectric substrate wrapped around a cylindrical support.

(a)

Fig.1. the unwrapped PQHA (a) S-FPQHA (b)

The radius of these antennas is equal to 18mm, with a constant pitch angle of 50° and an arm width of 2mm. These antennas were modeled using HFSS code [10].

The effect of folding the arm in several portions to create spiral arms is studied first. We consider a conventional PQHA (called ref-PQHA), with an axial length of 127 mm and an

arm length of 166mm, corresponding to 4

3 λ at the resonant

frequency of 1.26 GHz. The second resonant frequency F2 = 2.2 GHz goes with an arm length of

45 λ and F3=

47 λ . If we

maintain the axial length at 72 mm in correspondence to a height reduction of 43% and we fold the arms as shown in Figure 2a, Figure 2b, and Figure 2c, the resonance frequencies F1, F2, and F3 will decrease as shown in Figure 3 and can be summarized in Table 1 since we increased the arm length. We note that with longer antenna length, impedance matching decreases. Moreover, by changing the arms segment width and the distance between the segments in the folded arm, it is possible to obtain a good impedance matching.

(a) (b) (c) Fig. 2. the unwrapped spiral folded helix (a) S1-FPQHA, (b) S2-FPQHA and

(c) S3-FPQHA

Arm Length

F1 (GHz)

F2 (GHz)

F3 (GHz)

Ref-PQHA 166 mm 1,26 2,2 3,2 S1-FPQHA 1,03 1,85 2,2 S2-FPQHA 0,7 1,26 1,85 S3-FPQHA 0.59 0.95 1.42

Table :Resonant frequencies and antennas arm length

0.5 0.59 0.75 0.95 1.03 1.26 1.42 1.5

-25

-20

-15

-10

-5

0

Freq [Ghz]

Ret

urn

Loss

[dB

]

S1-FPQHA

S2-FPQHA

S3-FPQHA

Ref-PQHA

(a)

1.5 1.85 2.2 2.5 2.93 3.2 3.5-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

Freq [Ghz]

Ret

urn

Loss

[dB

]

S1-FPQHA

S2-FPQHA

S3-FPQHA

Ref-PQHA

(b)

Fig. 3. Simulated return loss versus frequency of (a) Ref_PQHA, (b) S1-FPQHA, (c) S2-FPQHA and (d) S3-FPQHA

Regarding radiation patterns, the S-FPQHA displays almost

identical characteristics to the radiation pattern of the PQHA at F1. For the higher frequencies the radiation pattern of the S-FPQHA remains in an axial mode, whereas the conventional PQHA will have a normal mode radiation patterns, as shown in Fig.4.

-160 -120 -80 -40 0 40 80 120 160-35

-30

-25

-20

-15

-10

-5

0

5

Dire

ctiv

ity (

dB)

F1 F2 F3

Angle Theta (deg)

_____ RHCP - - - - - LCHP

-160 -120 -80 -40 0 40 80 120 160-35

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-20

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-10

-5

0

5

Dire

ctiv

ity (

dB)

F1 F2 F3

Angle Theta (deg)

_____ RHCP - - - - - LCHP

(a) (b)

-160 -120 -80 -40 0 40 80 120 160-35

-30

-25

-20

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-10

-5

0

5

Dire

ctiv

ity (

dB)

F1 F2 F3

Angle Theta (deg)

_____ RHCP - - - - - LCHP

-160 -120 -80 -40 0 40 80 120 160-35

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-5

0

5

Dire

ctiv

ity (

dB)

F1 F2 F3

Angle Theta (deg)

_____ RHCP - - - - - LCHP

(c) (d)

Fig. 4. Comparison of radiation pattern between F1, F2 and F3 for (a) Ref_PQHA, (b) S1-FPQHA, (c) S2-FPQHA and (d) S3-FPQHA

III. EXPERIMENTAL RESULTS In order to validate the previous method, a single prototype

of S-FPQHA-working in L1/L2 bands for GPS application has been realized. The length of the arms as well as segments arms width have been optimised to obtain a good impedance matching and the desired frequencies.

This antenna is printed on a thin Neltec substrate of relative permittivity εr = 2, 2 and a thickness of 0,127 mm. The antenna dimensions are listed in Table 1. The final configuration of the antenna, assembled on a ground plane of radius R = 25mm, is shown in Fig.5.

S-FPQHA-L1/L2

Axial length (mm) Lax 78 Arm length (mm) Le 283

Radius (mm) R 18 Pitch angle (° ) α 50

Arm width (mm) wa 2 Arm width “g”(mm) Wg 12

N 0.75 Arm segment spacing

(mm) 2

Table 1: Geometrical parameters of S-FQPHA-L1L2.

Fig. 5. The unwrapped spiral folded helix and a picture of the

prototype

In Figure 6, the measured return loss versus frequency of the S-FPQHA is presented, as well as the corresponding Smith Chart. The bandwidth of the antenna is about 2% to 3 % in which is sufficient for each GPS frequencies band (GPS L1/L2).

1 1.1 1.22 1.4 1.58 1.83 2-18.15

-14.4

-12.9

-10.9-10

-5

0

Freq [Ghz]

Ret

urn

Loss

[dB

]

MEASURED SIMULATED

(a)

5.002.001.000.500.20

5.00

-5.00

2.00

-2.00

1.00

-1.00

0.50

-0.50

0.20

-0.20

0.00-0.000

10

20

30

40

50

60

708090100

110

120

130

140

150

160

170

180

-170

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-130

-120

-110-100 -90 -80

-70

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-10

SIMULATED

MEASURED

(c)

Fig. 6. S-FPQHA measured and simulated (a)return loss (b) VSWR and (c) the corresponding Smith Chart versus frequency

We can note three resonant frequencies: F1=1.22 GHz, F2=1.58 GHz, and F3=1.83 GHz.

The far-field radiation patterns of magnitude and phase for linear polarizations were measured in an anechoic chamber and combined to give the circular polarization are shown in Figure 7 at frequencies of 1.2, 1.54, and 1.8 GHz.

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180-35

-30

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-5

0

Angle Theta (deg)

Dire

ctiv

ity (

dB)

RHCPLHCP

SIMULATED

MEASURED

(a)

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180-35

-30

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-5

0

Angle Theta (deg)

Dire

ctiv

ity (

dB)

RHCPLHCP

SIMULATED

MEASURED

(b)

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180-35

-30

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-5

0

Angle Theta (deg)

Dire

ctiv

ity (

dB)

RHCPLHCP

SIMULATED

MEASURED

(c)

Fig. 7 Measured and simulated radiation pattern in circular polarization of S-FQPHA-L1L2 at (a) F= 1.2 GHz, (b) F= 1.54 GHz and (c) F= 1.8 GHz

We can note that the radiation patterns remain stable with frequency. The measured maximum gain is greater than 1.5 dBic in the three frequencies bands, as shown in Figure 8.

Wg

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2

-10

-5

0

5

Freq (GHz)

Gai

n (d

B)

Fig. 8. Measured maximum gain versus frequency of S-FPQHA-L1L2

The performances of this antenna are resumed in Table 2. Name S-FPQHA-L1L2

Frequency (GHz)

1.2 1.54 1.8 2.22

Gain (dB) 4.6 1.4 2.5 1.1 BP (%) 2.61 3.05 2.97 2.36

Beam width at -3dB (°)

124 176 166 164

Table. 2. Measured results of S-FPQHA-L1L2 and simulated results of Ref-PQHA-L1L2

IV. CONCLUSION In this paper, a new Spiral-Folded, Printed Quadrifilar

Helical Antenna design is presented. A prototype of the S-FPQHA operating in an L-band is realized. It provides a 43% reduction in size for the axial length of the QHA structure, to satisfy space limitations of a mobile satellite terminal, with good performances. These properties make the antenna a good candidate to cover the dual frequency bands of global

positioning systems . Although the design results in a slight reduction in efficiency, an increase in operating earnings and improved stability of radiation patterns can be obtained at an even higher frequency. The resonant frequency of the S-FPQHA is also analysed and compared with the calculated resonant frequency of the PQHA. The result shows similar characteristics.

REFERENCES [1] R.C. Fenwick, “A new class of electrically Small antennas,” IEEE

Trans. on Antennas and Propag., vol. 13, pp. 379-383, 1965. [2] J. Rashed and C. Tai, “A new class of resonant antennas,”

IEEETrans. on Antennas and Propag., vol. 39, pp. 1028–1031, 1991.

[3] H. Nakano, H. Tagami, A. Yoshizawa, and J. Yanauchi, “Shortening ratios of modified dipoles,” IEEE Trans. Antennas Propagat., vol. AP-32, pp. 385–386, Apr. 1984. [4] S. Best, “On the performance properties of the koch fractal and

other bent wire monopoles,” IEEE Trans. on Antennas and Propag., vol. 51, pp. 1028–1031, 2003.

[5] A. Sharaiha, C.Terret, and J.P.Blot, « Printed resonant helix antenna with integrated feeding network, “Electronic Letters, vol.33, p.265-257, 1997 [6] B.Desplanches, A. Sharahia and C.Terret, “Parametrical study of printed quadrifilar helical antennas with central dielectric rods,” Microwave Opt. and Tech. Lett., vol.20, pp.932-933, 1999. [7] J. Louvigné and A. Sharahia, “Synthetis of printed quadrifilar helical antenna,” Electronic letters, vol.37, pp.271-272, 2001. [8] J.Rabemanantsoa, M.G.Imbabe, Ala Sharaiha, “Size Reduction of Printed Quadrifilar Helical Antenna, OHD, September 2007, Valence. [9] Daniel K.C.D. Chew, Simon R. Saunders, “Meander Line Technique for Size Reduction of Quadrifilar Helix Antenna”, IEEE, Antennas and wireless Propagation Letters. Vol. 1, pp 109-111, 2002. [10] HFSS v11, Ansoft, www.ansoft.com