InductorsMobile Phones -Antenna-

Recent wireless terminals are equipped with multiple wireless communication systems, and multi-functionalization is progressing as represented by the smart phone. Since the frequency bands used in each of the wireless communication systems differ, it is necessary to prepare separate antennas for each system. In order to install multiple antennas in the limited space of mobile devices, miniaturization is required. The antenna is designed according to the wave length of the frequency to be used, therefore as the frequency becomes lower the wave length becomes longer, and the antenna also becomes larger.

Particularly, since UHF bands (470-800MHz) are used in antennas for MDTV(ISDB-T/CMMB/DVB-H) , it is difficult to miniaturize the antenna. As a method of miniaturization, adjustment of the resonance frequency by the inductor can be mentioned; however, it is necessary to select components taking into consideration the deterioration of the antenna characteristics. Antennas which cover the 700 MHz band or multiple bands have also been required by LTE which has already been started. Particularly, as a technology that is attracting attention, there is a switching method to adjust the frequency using a switch to change 1 antenna element. Even with this switching method, there are cases where a lumped constant (inductor and capacitor) is used to adjust the frequency along with the switch.

This paper introduces examples of selecting inductors for frequency variable chip dielectric antennas for MDTV. The frequency variable chip dielectric antenna covers the bands of 470-800MHz over 50% of the fractional bandwidth, by adjusting the resonance frequency of the antenna. Capacity variable diodes are used for the variable frequency, to change the frequency for each channel. This antenna was built into a mobile device using the LQW series to realize the miniaturization.

When using inductors on antennas for the purpose of adjusting the frequency, it is necessary to select an inductor with a high Q which can maintain a low insertion loss, in order to prevent the deterioration of the radiation characteristics of the antenna. Also, it is preferable to use an inductor with less tolerance in order to suppress the variation in the resonance frequency. Furthermore, an abundant lineup of constant values will also become a selection factor. This is because the antenna is easily affected by the peripheral environment, and it is necessary to make fine adjustments of the frequency for each change of specification in the terminal.

The affects on the antenna characteristics by the Q of the inductor, and the variation of the frequency caused by the tolerance are described in the following section, using examples of applications on variable frequency chip dielectric antenna.

Figures 3 and 4 show the frequency characteristics of Q and R by the types of inductors indicated in SimSurfing. As an example, Figures 3 and 4 show the results of a 10nH inductor. Compared with the LQG series, it is clear that the inductors of the LQW series have high Q characteristics and low insertion loss.

MDTV is assigned with 6-8MHz bands per channel. Antenna characteristics which cover such a band range are required when viewing and listening. There are various tolerances and tolerances as factors of changing the resonance frequency of antennas, such as the tolerance of the electrode to be used as the radiating element, the shifting when mounting to the board, and the variation when assembling to the terminal. In consideration of these factors, antenna designs which can secure the necessary bands are required. Therefore, it is necessary to select the type of inductor to be used with less tolerance.

Figures 6 and 7 show the return loss waveform of the variable frequency chip dielectric antenna when the tolerance of the inductor is ±2% and ±5%. When an inductor with a ±2% tolerance is used, the necessary band can be covered even though the resonance frequency varies; however, when an inductor with a ±5% tolerance is used, the resonance frequency will shift out of the necessary band, which becomes a concern regarding the deterioration of the radiation characteristics.

he first requirement of an antenna is to have excellent radiation efficiency. In order to realize this, it is necessary to carefully select an inductor as mentioned above.
Also, the characteristics of the recent miniaturized antennas that can be built into the terminals, change depending on the conditions of the device (size of device, location of antenna, layout of peripheral parts, etc.) to be mounted.

For this reason, the antenna characteristics change according to the changes of the peripheral environment in each prototype phase, such as the first prototype and second prototype. In such a case, it is unfeasible to reproduce an antenna each time. Therefore, by effectively utilizing inductors from an abundant lineup as an adjustment element, such changes can be handled more flexibly.
The demand for flexible antenna designs will continue into the future, such as switching between multiple bands in LTE, WLAN and etc., or suppressing the coupling between antennas of the MIMO system, therefore it is necessary to effectively utilize the lumped constant.