Simulation, Fabrication and Tests of a GPS Antenna on the Roof-Top of an Automobile

doi:10.4236/jemaa.2012.412071

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Journal of Electromagnetic Analysis and Applications, 2012, 4, 504-512

http://dx.doi.org/10.4236/jemaa.2012.412071 Published Online December 2012 (http://www.SciRP.org/journal/jemaa)

Simulation, Fabrication and Tests of a GPS Antenna on the

Roof-Top of an Automobile

Margarita Tecpoyotl-Torres, Jose G. Vera-Dimas, Gustavo Urquiza Beltrán, L. Cisneros-Villalobos,

Vladymyr Grimalsky, Svetlana V. Koshevaya

Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Mexico.

Email: [email protected]

Received October 4th, 2012; revised November 6th, 2012; accepted November 16th, 2012

ABSTRACT

It is very important for the car users to have several communication systems and many antennas, which respond to the

demands of aesthetics and efficiency. In this case, it is necessary to consider different types of materials and geometries.

Among the automobile systems, the navigation systems are found, which are formed by a Global Position System (GPS),

with a pre-charged base of maps and highways in order to locate the vehicle. For the reception of the GPS signals, cir-

cularly polarized patch antennas are used, which must be designed to operate at 1.57 GHz. This work is focused on the

analysis of a circular antenna performance, designed for GPS, by means of the simulation of the following cases: an-

tenna without radome, antenna with acrylic radome, and, finally, antenna on the roof of an automobile, considering the

effect of the complete chassis. A Basic, a Simplified and a More Simplified Advanced Car Models (BCM, SACM, and

MSACM) were used in order to analyze the chassis shape. The simulations were carried out with the software FEKO.

The fabrication and experimental and practical tests are also presented.

Keywords: Car Model; Circular Polarization; Patch Antennas; Rogers RT/Duroid 5880

1. Introduction

The evolution of the automobiles has corresponded to

several demands of the users. Especially in wireless com-

munication systems, the necessity of aesthetics and effi-

ciency has determined several proposals of antenna mo-

dels designed considering different types of materials and

geometries. The location of the antennas constitutes also

a very interesting analysis problem [1,2].

The automotive sector has shown its interest in the de-

velopment and implementation of navigation systems,

which are composed by a GPS and a pre-charged base of

maps and highways in order to locate the vehicles [3].

With the increase in automobile usage, accurately deter-

mining automobile location has become one of the grow-

ing priorities. The main advantages of the navigation sys-

tems are reduction in the time of journey, in consumption

of combustible, and in the emission of contaminant gases

[4]. Other application of the GPS technology is automatic

location of vehicles (AVL) [5,6], as the medium to de-

termine the geographic position of the vehicle and to

transmit this information to a point where it can be used

and exploited. This tool is extremely useful to the man-

agement of fleets of service vehicles, emergencies, con-

struction, public transportation, recovery stolen vehicles,

and public security [7].

The GPS antenna design has received a special atten-

tion. Toyota Central R & D Laboratories of Japan has de-

veloped an antenna for installation on the roof of auto-

mobile [2]. They have used a dual-feed, stub loaded sin-

gle patch to achieve circular polarization at the two fre-

quencies of the satellite.

On the other hand, the antenna radomes have received

a special attention, not only on the materials used, but

also in shapes. Motorola, one of the leading manufactur-

ers of automotive GPS systems, recommends the place-

ment of the GPS antennas at the roof, roofline or trunk

[8].

Additionally, several companies are dedicated to com-

mercialize navigation systems for vehicles that do not

account with the original device [9]. The antennas con-

tribute greatly to the total operation of the navigation

systems, and they can be designed in accordance to the

aesthetics and the requirements of the vehicle, in order to

obtain prototypes totally personalized.

Antennas, commonly used for circular polarization,

are based on corner-truncated square microstrips [10],

and on square or circular patches with one or two feed

points [2]. A circular polarization can, also, be obtained

from a single-point square or a circular patch.

Simulation, Fabrication and Tests of a GPS Antenna on the Roof-Top of an Automobile 505

In this research, the design of a circular patch antenna

for GPS is presented. The interest is focused on the

simulation of the antenna, due to the benefits to know the

performance of the complete system instead of only to

simulate the single antenna. The analyzed cases are: an-

tenna without radome, antenna with an acrylic radome,

and the antenna on the chassis of the vehicle (considering

three car models, Basic, Simplified and More Simplified

Advanced Car Models (BCM, SACM and MSACM), in

order to analyze also the influence of the chassis shape).

A similar analysis can be found in [11].

The software used for simulations is FEKO.

The circular antenna is designed at 1.57 GHz, consi-

dering a unique feed point and a substrate of RT/Duroid

5880.

The content of this paper is organized as follows. In

Section 2, the principles of the circular antenna design

are mentioned. The simulations considering the antenna

without and with acrylic radome are shown in Sections 3

and 4. The simulations of the antenna considering three

car models are presented in Section 5. In Section 6, the

fabrication process in briefly described. The simulated,

experimental and practical results are presented in Sec-

tion 7 and are discussed in Section 8. Finally, in Section

9, some concluding remarks are given.

2. Individual Patch Antenna Design

A simple approximation [12] is used for the design of the

circular patch antenna:

0reff

, with





, (1)

where r is the patch radius, λg is wavelength of group, c

is the speed of light in vacuum, εr = 2.2, the dielectric

permittivity of RT/duroid 5880 and f0 = 1.57 GHz is the

operation frequency.

Equation (1) is very similar to the used equation for

ring resonators [13]. The calculated antenna sizes of the

antenna implemented on RT/Duroid 5880, with a thick-

ness h = 0.00317 m are: the patch radius is of 0.0419 m

and the length of the square substrate is Lg = 6*h + 2*r =

0.1028 m.

A coaxial type feed is used. The location of the feed

point is determined in accordance to the impedance

matching. In this case, the feed point is located near to

the center of the antenna.

3. Simulation of the Individual Patch

Antenna Design

Two cases were used: linear and circular polarizations

(Figure 1). The gain of the circular antenna with linear

polarization has a maximum value of 7.1 dB.

To observe the performance of the antenna on the cir-

cular polarization, it is not necessary to modify the estab-

lished values of the antenna, it is enough to select in the

main menu, the component, and the values, considering

the scale that we want to observe. With the selection of

the circular polarization, Figure 1(b) was obtained. In

this case, the gain value is considerably reduced (at 4.2

dB) in comparison to the linear polarization gain.

4. Simulation of Antenna with Radome

After to design the patch antenna, it is also important to

design the radome, because it prevents the early physical

deterioration of the antenna, the copper oxidation, and

affects lightly the performance of the antenna. It allows

an easy manipulation of the antenna by the user. As a

first approximation, a simple radome design of low cost,

which does not affect the radiation pattern of the antenna,

was chosen.

The selected material to implement the radome is

acrylic, with a thickness of 0.0047 m, due to its availabi-

lity in the market. ABS (Acrylonitrile-Butadiene-Styrene)

is widely used for protective housing for electronic

(a)

(b)

Figure 1. Gain of the circular patch antenna (RT/duroid

5880, and h = 0.00317 m) considering (a) linear polarization

and (b) circular polarization.

Simulation, Fabrication and Tests of a GPS Antenna on the Roof-Top of an Automobile

506

equipment and computers, but it is more difficult to ob-

tain and to manipulate for us. In [14], it is shown the ef-

fect of various materials on the radiation pattern of this

antenna. In Figure 2, top and cross section views of the

square radome are presented, where the circular patch

antenna can be appreciated. The highest values of the

antenna gain with radome under linear polarization (6.9

dB), and with the circular polarization (4.1 dB). They are

lightly smaller compared to the values obtained without

the radome.

The simulation results of the antenna gain with acrylic

radome, considering linear and circular polarization are

presented in Figure 3.

5. Simulation of Three Car Models of

Antenna

First of all the antenna was placed on the roof of a com-

plete vehicle chassis, with the aim of observing the per-

formance of the circular patch antenna in a more real

environment. At this stage, it is necessary to design a ve-

hicle with similar dimensions to a real one. Therefore, we

decided to use a 3D drawing software that allow us to ex-

port the corresponding design in a file with extension *.x_t

(parasolid files), which can be imported by CADFEKO,

and after that, to realize the simulation of the complete

system.

We have chosen Autodesk Inventor to design the chas-

sis of the vehicle. Autodesk Inventor is a design program

with a friendly environment. The implemented car design

only consists of metal layers; without doors, and other

not relevant details, such as crystals, because the antenna

is not closely interacting with one of them.

The width of the chassis is 1.4 m, the length is 4.17 m,

and the height is 1.08 m. The roof-top has an area of 1

m × 1.2 m (Figure 4).

(a)

(b)

Figure 2. (a) Top and (b) cross section view of the radome

containing the circular patch antenna.

(a)

(b)

Figure 3. 3D radiation pattern of the circular antenna in-

side the acrylic radome, with (a) linear and (b) circular po-

larizations.

Figure 4. View of the BCM system implemented in CAD-

FEKO, with the patch antenna, with radome, located on the

rear part of the roof-top.

The next step is in following. With the vehicle design,

it is necessary to implement both the antenna system, and

the metallic chassis of the vehicle in CADFEKO, for its

subsequent simulation. In the corresponding simulations,

the antenna was located on different places on the chassis.

The radiation patterns of the antenna gain with linear and

circular polarization are shown in Figure 5, where it can

be observed that the values of the gain reach a maximum

Simulation, Fabrication and Tests of a GPS Antenna on the Roof-Top of an Automobile 507

(a)

(b)

Figure 5. Gain pattern of circular patch antenna with

acrylic radome, on the rear part of the roof-top of the vehi-

cle chassis, with (a) linear and (b) circular polarization.

gain of 7.6 dB and of 5.2 dB for linear and circular po-

larizations, respectively.

The last case included Simplified Advanced Car Mo-

del (SACM). The model car corresponds to an AUDI R8

[15] (Figure 6(a)). Due to computer limitations, it was

necessary to simulate a SACM (Figure 6(b)) with maxi-

mum width of 2.01 m, maximum length of 4.64 m, and a

height of 1.08 m.

The roof-top has a maximum and a minimum width of

1.33 m and 1.19 m; and a maximum and a minimum

length of 1.48 m and 1.28 m.

The maximum gain obtained considering a SACM cor-

responds to the case of the antenna located on the central

and on the front part of the roof-top (Figures 7(a) and

(b)); and on the trunk (Figure 7(d)).

Finally, considering the MSACM (Figure 8), the maxi-

mum gain obtained (10 dB) corresponds to the case of the

antenna located on the central part of the roof-top (Fig-

ure 9(a)) and on the trunk (Figure 9(d)).

(a)

(b)

Figure 6. (a) View of an Audi R8, and (b) SACM system

implemented in CADFEKO, with the antenna located on

the central part of the roof-top.

6. Fabrication Process

The antenna was fabricated using a PROTOMAT S-42

machine. After the pattern transfer and the drilling for the

feeding point, the soldering of the required connectors is

realized (Figure 10(a)). For laboratory tests, BNC fe-

male connectors were used. For experimental case, a

female MCX connector was used to couple the antenna

to a GPS development kit, in order to compare its per-

formance with the original antenna of the kit (Figure

10(b)).

7. Analysis of the Results

At first, it is necessary to analyze the simulation results.

In Table 1, a resume of the obtained values of simulation

considering the cases of Sections 3 and 4 is presented. As

it can be observed, the differences between linear and cir-

cular polarization results are remarkable.

Simulations considering the car models are also rea-

lized for the antenna with radome. Table 2 shows the cor-

responding results, where it can be observed that the gain

value depends on the system where it is interacting. In

the BCM, the simulation results show a bigger gain value

at the central part of the roof-top. This fact can be attri-

buted to the high symmetry in the car model. The effect

of the smooth lines of the chassis contributes notably to

the increment of the antenna gain. In the case of the an-

tenna located at the front part of the roof-top in the

Simulation, Fabrication and Tests of a GPS Antenna on the Roof-Top of an Automobile

508

(a)

(b)

(c)

(d)

Figure 7. Gain of the GPS antenna located on (a) the center;

(b) frontal part; and (c) rear part of the roof-top, respec-

tively and (d) on the trunk, using SACM.

Figure 8. View of the MSACM system implemented, with

the antenna located on the central part of the roof-top, in

CADFEKO.

Table 1. Gain of the antenna, in dB.

Features Linear polarization Circular polarization

Without radome 7.1 4.2

With radome 6.9 4.1

Table 2. Gain on circular polarization of the antenna with

different car models .

Antenna location Model Gain (dB)

Central part, roof-top BCM 7.1

Front part, roof-top BCM 5.33

Rear part, roof-top BCM 5.2

On trunk BCM 7.06

Central part, roof-top SACM 10

Front part, roof-top SACM 10

Rear part, roof-top SACM 8

On trunk SACM 10

Central part, roof-top MSACM 10

Front part, roof-top MSACM 8

Rear part, roof-top MSACM 8

On trunk MSACM 10

MSACM, the decreasing of the gain value compared to

the case of the SACM could be attributed to the absence

of the hood. On the trunk, the values are almost as high

as the case of the central part of the roof-top.

The simulation results of the S21 parameter of the an-

tenna with acrylic radome is shown in Figure 11, where

also the experimental S21 parameter is displayed.

Experimental Results

Transmission-reception tests were realized using a signal

Simulation, Fabrication and Tests of a GPS Antenna on the Roof-Top of an Automobile 509

(a)

(b)

(c)

(d)

Figure 9. Gain of the GPS antenna located on (a) the center,

(b) frontal part, and (c) rear part of the roof-top, respec-

tively and (d) on the trunk, using MSACM.

(a) (b)

Figure 10. Front (a) and rear view (b) of the patch circu-

lantenna.

Figure 11. S21 simulated and experimental parameter of

the GPS antennas with acrylic radome.

synthesized generator (AGILENT 83732B and 8563EC,

respectively). The frequency range from 1.3 GHz up to

1.8 GHz was considered to observe the antenna behavior

using a constant power generated by the transmitter. The

antenna separation used was 7 cm. In Figure 12, the re-

ceived power in the spectrum analyzer is shown. The

oscillations could be attributed to the use of the connec-

tion converters to couple the laboratory equipment, and

especially by the noise given by the coaxial cable.

Practical results are in following. For the test of the

antenna prototype as a replacement antenna, a GPS de-

velopment kit was used, which accounts with a GPS de-

velopment card (ER-102-J, SiRF Star-II), and the

NMEAgent data (this software shows the current loca-

tion). The response of the antenna prototype with circular

polarization can be analyzed in this case. Three meas-

urement points were established at CIICAp parking (Fig-

ure 13, Table 3). Results are also compared with a Gar-

min GPS.

For the test of the antenna prototype in movement, it

was located on the rear roof-top of a Cavalier car model

Simulation, Fabrication and Tests of a GPS Antenna on the Roof-Top of an Automobile

510

Figure 12. Power reception (S21 experimental parameter).

Figure 13. Three measurement points at CIICAp parking.

Table 3. Measured position at 3 points in the CIICAp par-

king.

With Garmin GPS

Location Latitude (N) Longitude (W)

Point 1 18˚58'55.6'' 99˚14'1.4''

Point 2 18˚58'54.05'' 99˚14'0.48''

Point 3 18˚58'53.85'' 99˚14'1.6''

With GPS kit (with its original antenna)

Point 1 18˚58'54.52'' 99˚14'0.39''

Point 2 18˚58'54.13'' 99˚14'0.54''

Point 3 18˚58'53.95'' 99˚14'1.7''

With GPS kit (with the replacement antenna)

Point 1 18˚58'55.55'' 99˚14'1.46''

Point 2 18˚58'54.21'' 99˚14'0.51''

Point 3 18˚58'53.87'' 99˚14'1.63''

95. The trajectory is shown in Figure 14, and the meas-

ured positions (marked with arrows), in Table 4.

8. Discussion

Simulations of antenna performance were realized con-

sidering linear and circular polarizations, but for the ex-

perimental tests, the available components and equip-

ment only allow us to check the response under the linear

case. Transmission-reception test shows a peak of re-

sponse in the range of GPS signals (1575.42 MHz).

In practical tests, the replacement antenna showed sat-

isfactory results (Table 3). The measurements in fixed

points in comparison with the Garmin GPS are nearest

using the kit with the prototype antenna. In movement, as

it can be observed in Figure 14, the measured points,

except for the corresponding to the CIICAp parking, are

located on the university circuit, without invade build-

ings, or traffic islands, that means that the measurements

Figure 14. Arrows indicate the place where the measure-

ments were realized. Balloons indicate the points where

Google Maps display photographs on line.

Table 4. Measured position of several points on the univer-

sity circuit.

Location Latitude (N) Longitude (W)

4 18˚58'50.52'' 99˚14'2.35''

5 18˚58'50.25'' 99˚14'5.45''

6 18˚58'44.05'' 99˚14'9.68''

7 18˚58'48.64'' 99˚14'14.87''

8 18˚58'53.03'' 99˚14'21.27''

9 18˚58'55.65'' 99˚14'25.59''

10 18˚58'59.95'' 99˚14'19.08''

11 18˚58'59.16'' 99˚14'12.13''

12 18˚58'57.51'' 99˚14'5.12''

Simulation, Fabrication and Tests of a GPS Antenna on the Roof-Top of an Automobile 511

are useful and precise.

It is necessary to mention the following differences

between the cars used in simulations and in practical tests:

the model, the chassis material (in simulation, it is a per-

fect conductor, and in the practical tests, it was covered

with red painting); and the sizes are lightly different. In

experimental and practical tests, the antenna performance

remains as it was expected.

The symmetry influences the gain value and the pat-

tern shape, showing in all cases a bigger gain in the cen-

tral part of the roof-top.

9. Conclusions

Due to the presence of changes in the gain values of the

antenna according to the system where it is immersed, it

is desirable to perform the simulation considering the

complete system elements, in order to know a more real

behavior. However, it requires the combination of tech-

nological factors in order to perform these simulations

and higher computer capabilities.

Surely, the antenna performance can be improved. In

our BCM, our purpose was only to show a procedure of

analysis based exclusively on the simulation results, en-

riching the corresponding environment, implementing a

complete system.

In SACM and MSACM, in spite of the simplified form,

it can be observed that the presence of smooth lines in-

creases the gain value, compared to the BCM, due to

losses reduction. The differences between SACM and

MSACM, are basically in the gain corresponding to the

case of the antennal located in the front part of the

roof-top, due to the absence of hood in the last case,

which produced a lower gain value. The modeling detail

could be even further improved, but this would increase

the computer memory requirements accordingly.

In reception-transmission tests, the prototype shows an

enough bandwidth to operate at the GPS frequency range.

It was also probed that the maximum peak of response

was very near to the design frequency (1.57542 GHz).

The lightly variation can be attributed to the tolerance of

the fabrication process.

The variations in the reflection characteristics and the

pattern distortion of the antenna using an acrylic radome

were minimal. Additionally, it was possible to appreciate

that the chassis contributes to increase the antenna gain.

From the average values of the experimental tests us-

ing the GPS kit in fixed points, it can be concluded that

the prototype has an adequate response for its using as a

replacement antenna, although the original antenna has a

low noise amplifier.

In practical measurements on a vehicle, the results

were also satisfactory, because all measured points cor-

respond to the university circuit.

Some improvements could be implemented, such as,

the antenna size considering alternative materials or geo-

metries. The radome also can be improved using liquid

acrylic or epoxy resin. The car model also can be im-

proved, with the consequent increment of computer re-

sources.

10. Acknowledgements

Authors want to thank to EM Software & Systems (USA)

Inc., for FEKO license, and to Rogers Co. for the mate-

rial supplied. J. G. Vera-Dimas expresses his sincere thanks

to CONACyT for the postgraduate scholarship under grant

270210/219230.

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