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: [email protected]++ 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 (−jX d ) of a small dipole by the reactive reactance (jX L ) 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 X L is increased, so X d must be increased. Accordingly, H/λ is decreased so as to increase X d . 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.
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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
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