a compact dual-band gps antenna design

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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013 245

A Compact Dual-Band GPS Antenna DesignMing Chen, Student Member, IEEE, and Chi-Chih Chen, Senior Member, IEEE

AbstractThis letter discusses a small slot-loaded, prox-

imity-fed patch antenna designed for GPS operation at L1(1575 MHz) and L2 (1227 MHz) bands. High-dielectric substrate

and meandered slotsare employed to reduce theantenna size downto 25.4 mm in diameter and 11.27 mm in thickness. The thicknessis important for achieving the wide bandwidth ( MHz) insupport of modern GPS coding schemes. The dual-band coverageis achieved by utilizing the patch mode in L2 band and slotmode in L1 band. This design features additional slot stubs for

independently tuning the L1 frequency. The right-hand circularlypolarized (RHCP) field property is achieved by connecting twoproximity probes to a small surface-mount 0 90 hybrid chip.Simulated and measured antenna performance will be presented.This compact GPS antenna design is suitable for small GPS

antenna arrays and portable GPS devices.

Index TermsCircular polarization, dual-band, GPS, patchantenna, proximity feeding.

I. INTRODUCTION

W ITH the deployment of several major global navigationsatellite systems (GNSS) such as GPS, GLONASS,Galileo, and Beidou [1], [2], many more frequency bands will

be available for global positioning applications. In addition,

the clustering of many neighboring GNSS channels requires

better coding schemes for these satellite signals [3], [4]. These

advanced coding schemes often need a wider bandwidth.

Therefore, future GNSS and GPS antennas will need to be able

to receive more GNSS channels and have wider channel band-

widths. Most existing commercial small L1/L2 GNSS/GPS

antennas have relative narrow bandwidth (10 MHz), and thus

are not adequate for supporting advanced GPS codes. Some

GNSS/GPS antennas adopting wideband designs such as

bowtie dipoles or spiral antennas have good bandwidth, but are

relatively large in size [5], [6]. Dual-band GPS antennas not

only can operate in two frequency bands, but can also provide

more reliable and accurate positions when used with proper

GPS receivers that combine information received at both fre-

quencies [7][9]. Therefore, the dual-band GPS antenna design

presented in this letter attempts to address size, bandwidth,

manufacturability, and scalability, i.e., easily redesigned forother frequencies.

Manuscript received October 11, 2012; revised November 28, 2012 and Jan-uary 28, 2013; accepted February 06, 2013. Date of publication February 20,2013; date of current version March 14, 2013. This work was performed undera subcontract from Applied EM, supported by the US Air Force under SBIRTopic A073-72, Miniature GPS Antenna Arrays Using Novel Materials. Dis-tribution is unlimited for public release (Air Force Public Release ApprovalNumber: 88ABW-2013-0727).

The authors are with the ElectroScience Laboratory, Electrical and ComputerEngineering Department, The Ohio State University, Columbus, OH 43212USA (e-mail: [email protected]).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LAWP.2013.2247972

Fig. 1. Geometry of the new compact L1/L2 GPS antenna.

Existing compact dual-band circularly polarized (CP) patch

antennas include stacked-patch designs [10], [11], specially

shaped patches [12], and slot-loaded patch [13]. However,these designs are still not small enough ( mm). In addition,

the stacked-patch design is not easy to manufacture due to its

internal probe and bonding issue at the metaldielectric and di-

electricdielectric interfaces. Zhou et al. proposed a proximity

probe design that requires only external probes conveniently

located on the side of the antenna [14].

In this letter, we present a compact dual-band (L1: 1575 MHz,

and L2: 1227 MHz) GPS antenna design (see Fig. 1, patent

pending) that is only 25.4 mm in diameter and 11.27 mm in

thickness [15]. The size of the antenna is only about in

L2 band. Unlike conventional stacked-patch designs, this de-

sign does not have an internal conducting patch. The dual-bandcoverage is achieved by operating the patch mode in L2 band

and slot mode in L1 band. The low-loss, high-dielectric sub-

strate and the meandered-slot designs are employed to increase

the antennas electrical size. This design also adopts external

proximity probes [14]. The combination of the above features

greatly improves its manufacturability and reliability. In addi-

tion, this design utilizes a small 0 90 hybrid chip (Mini-cir-

cuit QCN-19) to reduce the size of the feeding network and

achieve good right-hand CP (RHCP) performance over a wider

frequency range. Later, the application of this new GPS antenna

design in a four-element array without suffering performance

degradation due to mutual coupling will also be demonstrated.

II. COMPACTL1/L2 GPS ANTENNADESIGN

A. Antenna Structure and Operational Principles

As shown in Fig. 1, the proposed antenna is composed of

a single slot-loaded conducting patch design on top of two

stackeddielectric layers. The top layer includes the slot-loaded

patch design that is fabricated on a Rogers TMM10i board

( mm, ) using standard

printed circuit board (PCB) fabrication processes. The bottom

substrate is a high-dielectric ceramic puck ( mm,

). The two substrates are bonded

together using ECCOSTOCK dielectric paste to

1536-1225/$31.00 2013 IEEE


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246 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 2. Equivalent current magnitude on slotted patch at L1 (1575 MHz) andL2 (1227 MHz) bands.

avoid air gaps and low-dielectric bonding layer formed by

common glues, both causing detuning of resonant frequencies.

This new design is also mechanically superior to conventional

stacked-patch designs where the presence of the middle con-

ducting patch weakens the bonding between the top and bottom

layers when the patch size is relative compared to the diameter

of the dielectric layers. Two conducting strips (width mm,height mm) on the side serve as proximity feeds for the

new design. The bottom ends of these strips are connected to

the outputs of a 0 90 hybrid to obtain RHCP property. These

two feeding strips are located in the middle of two adjacent

longer meandered slots at 90 azimuth angle from each other.

Fig. 2 shows the computed magnitude of equivalent currents

on the patch at 1227 MHz (left) and 1575 MHz (right). It shows

that resonant current distribution occupies the entire patch in L2

mode and is mostly concentrated around the meandered slots in

L1 mode. The meandered slots, the center circular hole, and the

high-dielectric substrate help to establish L2 mode resonance

within the physically small antenna volume. The concentrationoffields only around slots in L1 band also makes it possible to

tune the L1 frequency independently by adjusting the length

of the inner tuning stubs, as will be discussed shortly.

B. Design Procedures

The design procedure of the proposed compact dual-band

patch antenna begins with selecting the diameter based on

physical constraints and the two desired resonant frequencies of

a specific application such as GPS. The three-step design pro-

cedure is discussed as follows.

Step 1: The first step is to determine the dielectric constant

and thickness of the two stacked dielectric materials according

to the desired lower resonant frequency. The effective dielectric

constant of two stacked dielectric layers can be estimated

using a double-layer parallel-plate capacitor model that gives

(1)

where are the dielectric constant and thickness

of top and bottom dielectric layers, respectively. The resonant

frequency of the lowest mode can then be estimated from [16]

(2)

Note that, in practice, and are limited by available PCB

materials. By properly choosing , and , one canfirst

Fig. 3. Input impedance curves for the antenna in Fig. 1 as the meandering slotlength varied from 9 to 10 mm.

Fig. 4. Input impedance curves for the antenna in Fig. 1 as the meandering slotwidth varied from 0.5 to 0.7 mm.

Fig. 5. Input impedance curves for the antenna in Fig. 1 as the tuning stublength varied from 0.2 to 1.5 mm.

produce the patch-mode resonance close to the desired lower

frequency band. The total thickness can be deter-

mined according to the bandwidth requirement of the specific

application.

Step 2: The second step is to design the length and width

of the meandering slots (see Fig. 1) to tune the resonant fre-

quency of the lower mode. Figs. 3 and 4 plot simulated input

impedance as a function of slot length and width , respec-

tively. Fig. 3 shows that increasing the slot length from 9 to

10 mm effectively lowers the resonant frequency of both low-

and high-frequency modes. As is shown in Fig. 4, changing the

slot width from 0.51 to 0.76 mm shifts the higher resonant

frequency from 1.48 to 1.6 GHz, but only shifts the lower reso-

nant frequency slightly. Therefore, the low-frequency mode can


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CHEN AND CHEN: COMPACT DUAL-BAND GPS ANTENNA DESIGN 247

Fig. 6. (a) Feeding circuit and (b) the GPS antenna element.

Fig. 7. (a) Measured return loss ( ) and insertion loss ( ) and(b) phase difference of the testing board.

be first tuned to the desire frequency by changing slot length and

width.

Step 3: The last step is to tune the resonant frequency of

the higher mode independently by adjusting the length of the

inner tuning stubs. Fig. 5 shows that changing the length ofthe inner stub from 0.2 to 1.5 mm shifts the higher resonant

frequency from 1.57 to 1.51 GHz without affecting the lower

resonant mode.

C. Compact RHCP Feeding Network

The feeding circuitry for the new miniature GPS antenna ele-

ment is shown in Fig. 6(a) on a 1.27-mm-thick FR4 board

. Fig. 6(b) shows that the antenna is inserted into a tightlyfit

circular hole that is cut out of the FR4 board. The bottom of

the antenna and feeding circuitry shares the same ground. Two

equal-length microstrip lines with characteristic impedance of

50 connect the outputs of a commercial broadband 0 90

chip hybrid to the bottom of the two antenna probes. The hybrid

performance was verified usinga separate test board as shown in

Fig. 8. (a) Single-element GPS antenna configuration. (b) Four-element GPSantenna array configuration.

Fig. 9. Comparison between the measured and simulated broadside RHCP andLHCP gain.

TABLE I

PATCHANTENNA DESIGN PARAMETERS

Fig. 7(a). The measured reflection coefficient shows less

than 20 dB from 1.1 to 1.7 GHz, and the transmission coeffi-

cients ( ) are approximately 3.2 dB, very close to the

desired 3 dB from a half-power divider, within the frequency

range of interest. The measured phase difference between thetwo output ports varies monotonically from 88 at 1.227 GHz

to 90 at 1.575 GHz [Fig. 7(b)], which is very acceptable for CP

operation.

III. MEASUREMENTRESULTS

Table I summarizes the optimized design parameters

of the final L1/L2 GPS antenna element design. Fig. 8(a)

shows a fabricated antenna element was then mounted on a

117.2 117.2 mm FR4 board containing the feeding circuitry.

The simulated and measured broadside gain for the

Fig. 8(a) configuration are plotted in Fig. 9, which shows

an excellent agreement. The RHCP antenna gain is around

3.2 dBi at 1.227 GHz and 3.5 dBi at 1.575 GHz. The

RHCP-to-left-hand-CP (LHCP) isolation is 20 dB at L2


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248 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 10. Radiation pattern comparison between four-element array and singleelementat L2 andL1 band: (a) four-element pattern at L2 band; (b)four-elementpattern at L1 band; (c) single-element pattern at L2 band; (d) single-elementpattern at L1 band.

band and 15 dB at L1 band. The axial ratio is found to be 1.3 dB

at 1.227 GHz and 1.9 dB at 1.575 GHz. The 3-dB bandwidth of

lower mode is 45 MHz from 1200 to 1245 MHz, and high modeis 50 MHz from 1545 to 1595 MHz at zenith. Such bandwidths

are sufficient to support modern coding schemes such as P/Y

and M code [3], [4].

It is well known that mutual coupling between closely spaced

antenna array elements often affects impedance matching condi-

tion, resonant frequency, and radiation pattern of the element. To

examine these effects, a four-element GPS array was assembled

as shown in Fig. 8(b).The distance between adjacent elementsis

62.5 mm. The measured elevation patterns at center frequencies

of L1 and L2 bands are shown in Fig. 10(a) and (b) by operating

the Element 1 only with other three elements terminated with

50- loads. For comparison, the measured patterns for the pre-vious single-element configuration shown in Fig. 8(a) are also

included in Fig. 10 (c) and (d). These comparisons show that the

sky coverage and broadside gain level between the single-el-

ement and four-element configurations are quite similar. The

maximum gain for the four-element array is 3.3 dBi at L2 and

3.9 dBi at L1, which is fairly close to the single-element gain

performance. Notice that these patterns are slightly tilted and not

completely symmetric due to the finite ground-plane scattering

effect. Reasonable RHCP-to-LHCP isolation is also preserved

in the four-element array. Therefore, we concluded that the mu-

tual coupling impact on antenna performance is not significant

compared to a single element for this antenna design. Also, the

measurement data are in good agreement with the simulation

data.

IV. CONCLUSION

A novel compact GPS antenna design was presented for

operating at 1.227 GHz with 45-MHz 3-dB bandwidth and

1.575 GHz with 50-MHz 3-dB bandwidth at zenith. High-di-

electric substrate and meandering slots were shown to be able

to miniaturize the antenna size down to 25.4 mm in diameter

without the feeding network. The footprint of a single elementwith the feeding network is about 25.4 40.6 mm . Simulation

results indicated that 90% radiation efficiency is achieved

by using the low-loss dielectric materials in this design. A

three-step design procedure of this new antenna design was

discussed and can be used to design for different operating

frequencies. The RHCP feeding circuitry was implemented

using a small 0 90 hybrid chip that provides desired power

splitting and stable quadrature phase difference at its two out-

puts. The measured gain and pattern data validated simulated

performance and showed wide RHCP sky coverage and more

than 15 dB of RHCP-to-LHCP isolation at both L1 and L2

bands. We also demonstrated an application of this antennadesign in a compact four-element array configuration.

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a compact dual-band gps antenna design

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