Development of a Very Small Normal Mode Helical

Antenna for the Tire Pressure Sensor of TPMS

Nguyen Quoc Dinh+, Naobumi Michishita

+, Yoshihide Yamada

+, Koji Nakatani

++

+Department of Electrical and Electronic Engineering, National Defense Academy

1-10-20 Hashirimizu Yokosuka 239-8686 Japan, e-mail: g47098@nda.ac.jp ++

The Yokohama Rubber Co.Ltd, Hiratsuka-shi, 254-8601, Japan

1. Introduction In order to support driving safety, a tire pressure monitoring system (TPMS) has been introduced

in U.S.A., and Europe. In Japan, an AIRwatch system has been developed [1] and commercialized

[2]. In the AIRwatch system, the antenna gain of a tire pressure sensor should be increased for the

battery power saving. Here, a normal mode helical antenna (NMHA) is considered as a promising

candidate by taking into account the circumstances surrounded by metallic objects. Previously, the

small NMHA was studied for RFID system at 900 MHz band [3]. A very small antenna for the

TPMS at 315 MHz is required.

In this paper, a very small NMHA of about 0.01 wavelength size is studied. First of all, an outline

of the TPMS and the installation environment of the antenna are explained. Then, the usefulness of

the NMHA is shown. Next, a design method of very small NMHA is explained. Here, the important

technical subject is the simple impedance matching method. The impedance matching is achieved

by a simple tap feed. Finally, calculated results of the 0.012 wavelength NMHA are compared with

the measured results. Moreover, radiation characteristics of the NMHA in proximity to a metal plate

are studied.

2. The AIRwatch System and Design Concept of a Tire Antenna The commercial AIRwatch system is shown in Fig. 1 [2]. The system consists of four transmitters

connected to air pressure sensors mounted in the tires, an on-board receiver unit for dashboard

mounting, and a receiving antenna (film antenna) on the windshield. Each sensor uses the FSK

scheme to modulate a 315 MHz continuous wave by tire pressure data. The modulated wave is

transmitted from a small loop antenna in the sensor. The receiving antenna collects all four

transmitted waves. Pressure levels are displayed on the on-board receiver unit. As for tire antenna

circumstances, the antenna is surrounded by metallic objects such as the tire wheel and the tire belt.

The tire belt composed of thin mesh wires is placed in the tire rubber for reinforcing the tire. So,

antennas having equivalent magnetic sources are preferable. Now, the main target of the present

AIRwatch system is the private car. So, the antenna gain of −30 dBd in the fundamental equivalent

magnetic source of the small loop antenna is acceptable.

In order to achieve effective battery power saving, high gain antenna working in a equivalent

magnetic source is requested. The typical antenna is the NMHA as shown in Fig. 2. The NMHA is

expressed as electrically equivalent small dipole and small loops. And the small loops are

considered as a small magnetic dipole. When the NMHA is surrounded by metallic objects, even

though the radiation from the small dipole is suppressed, the radiation from the small magnetic

dipole is enhanced. So, the NMHA effectively works in the tire.

3. Self Resonance Condition of NMHA In designing NMHA, cancellation of the capacitive reactance (−jXd) of a small dipole by the

reactive reactance (jXL) of small loops becomes the fundamental condition. This condition is called

the self resonance condition. This condition is determined by the structural parameters of antenna

length (H), antenna diameter (D) and number of turns (N). Relations of H/λ, D/λ and N can be

found out through electromagnetic simulations of antenna input impedances. Here, the wire

diameter of 0.55mm is used. Structural conditions for self resonances are shown in Fig. 3. When N

becomes large, because XL is increased, so Xd must be increased. Accordingly, H/λ is decreased so

as to increase Xd. At N = 10, electrical performances are ensured through measurement as shown by

black circles. Simulated and measured antenna input impedance of H/λ = 0.012 is shown in Fig. 4.

It is observed that the self resonant condition is satisfied at 315 MHz. Moreover, good agreement of

measured and simulated results is ensured. The simulation conditions are shown in Table 1. A

commercial electromagnetic simulator named FEKO is employed. Simulation scheme is the Method

of Moment. So, the antenna wire is divided into very small segment which size is 1/300 wavelength.

In order to estimate antenna gain, values of the radiation resistance (Rr) and the ohmic resistance

(Rl) are important. In Table 2, Rr, Rl and Rin = Rr + Rl (antenna input resistance) at the self resonance

point are shown. Here, antenna efficiency η can be calculated by the η = Rr/Rin equation. At H/λ of

0.012, η = −8.4 dB is expected.

4. Radiation Characteristics In the case of very small NMHA, whether the simulated radiation characteristics can be achieve in

an actual antenna is very important. Here, the problem is to archive impedance matching between

the very small resistance value (0.97 Ω) and the feed cable resistance (50 Ω). A convenient tap

structure shown in Fig. 5 is employed. The antenna structure used in measurements is shown in Fig.

6. The tap feed is connected to the 50 Ω coaxial cable equipped with a spertopf balun. The

simulated and measured input impedances are shown in Fig.7. In the simulation, the case of the

conductivity is infinite (σ = ∞) is obtained so as to obtain Rr. The measured and simulated results

agree very well. Simulated Rr and Rl are shown in Table 3. In the case of H/λ is 0.012, Rl becomes

about 8 times of Rr. Considering the ratio of Rl/Rr in Table 2 is about 5, ohmic resistance of the tap

increases the Rl value. In the case of the tap feed, η becomes −9.52 dB. Measured and simulated

radiation characteristics are shown in Fig. 8. Measured and simulated results agree very well. So,

achievement of simulated gain is ensured through experiments. In this case, the electrical current

source component (Eθ) and the magnetic current source component (Eφ) become almost the same.

Moreover, in order to study the metal plate proximity, the structure shown in Fig. 9 is simulated.

NMHA is placed on a metal plate with the separation of 2 mm. A rectangular metal plate of 1

wavelength size is employed. The antenna input impedance is shown in Fig. 10. The resonant

frequency is slightly shifted. Good impedance matching to 50 Ω feed cable is still maintained.

Radiation patterns at f = 315.13 MHz is shown in Fig. 11. Eφ component that corresponds to the

magnetic current source becomes very large. The antenna gain of −8.2 dBd is achieved. On the

other hand, Eθ component that corresponds to the electric current source becomes very small. The

level is decreased to less than −15 dBd. So, it is expected that the developed NMHA is effective for

battery power saving.

5. Conclusions A very small NMHA of 0.012 wavelength size is designed and electrical characteristics are

ensured through measurements.

⋅The self resonance conditions for the very small NMHA are clarified.

⋅The usefulness of the simple tap feed is shown.

⋅Antenna gain of −12.47 dBd is ensured.

⋅When the antenna is placed in proximity to the metal plate, antenna gain of −8.2 dBd is achieved.

6. References

[1] K.Tanoshita, K.Nakatani, and Y.Yamada, “Electric Field Simulations around a Car of the Tire

Pressure Monitoring System”, IEICE TRANS. COMMUN., vol.E90-B, N.9, pp.2416-2422,

Sep.2007.

[2] http://www.yokohamatire.jp/yrc/japan/tire/airwatch.html

[3] W.G.Hong, W.H.Jung, Y.Yamada and N.Michishita, “High performance Normal Mode

Helical Antenna for RFID Tags”, IEEE International Sympo. on Aut. and Prop., pp.6023-

6026, June 2007.

PC Xeon(R) 3GB

Memory 15.929 GB

Simulator FEKO (MoM)

Frequency 215MHz - 415MHz

Number of segment 1/300 wavelength

segments 127

memory usage 168.604 kB

cal. times 0.125 second

0.012 0.022

Rin 0.97 Ω 1.15 Ω

Rr 0.14 Ω 0.311 Ω

Rl 0.83 Ω 0.835 Ω

η -8.40 dB -5.66 dB

Fig.3 Self resonance condition

(magnetic current

source)

+

D

H N I I J

N=10

D small dipole small loop

(electric current

source)

d -jXd jXL

Fig.2 Equivalent structure of NMHA

On-board receiver unit

Receiving antenna

Built in air pressure sensor and a

small loop antenna

Fig.1 AIRwatch system configuration

Table.1 Simulation conditions (H/λ=0.01)

Fig.4 Input impedances (H/λ=0.012)

H/λ

Table.2 Radiation resistance and ohmic resistance

Fig.5 NMHA structure with tap

12.8 mm

(0.0134λ)

11.5 mm

(0.012λ) 44 mm

8.6 mm

Tap feed

315MHz 415MHz

215MHz 0.97Ω

f=315MHz

d=0.55mm

0

0.005

0.01

0.015

0.02

0.025

0.01 0.02 0.03 0.04 0.05

H/λ

D/λ

meas.

N=8

N=10

N=12

0.033λ 0.022λ 0.012λ

0.012 0.022

Rin 50.05 Ω 51.19 Ω

Rr 5.59 Ω 12.45 Ω

Rl 44.46 Ω 38.75 Ω

η -9.52 dB -6.14 dB

Fig.7 Input impedances (H/λ=0.012)

H/λ

Table.3 Radiation resistance and ohmic

resistance (with tap)

Fig.6 Experiment model

315MHz

σ=∞

314.8MHz

315.2MHz 5.59Ω

Fig.9 NMHA structure (H/λ=0.012)

(metal plate proximity)

2mm

Eθ

Eφ

-11.4dBd

-14.04dBd

-12.47dBd

-13.56dBd

meas. sim.

dBd

Eθ

Eφ

315.13MHz

(45.7Ω)

315MHz

Eφ

Eθ

-8.2dBd dBd

f=315.13MHz

Fig.11 Radiation pattern (H/λ=0.012)

(metal plate proximity)

Fig.10 Input impedances (H/λ=0.012)

(metal plate proximity)

Fig.8 Radiation pattern (H/λ=0.012)