Introducing an example of inductor selection for a variable frequency chip dielectric antenna for MDTV

In recent years, radio terminals have been equipped with a plurality of radio communication systems, and the multi-function represented by smart phones has been continuously developed. Since the frequency bands used by various radio communication systems are different, the antennas must be prepared separately. In order to assemble a plurality of antennas in a limited space in a mobile phone terminal, the antenna is required to be miniaturized. The antenna is designed according to the frequency wavelength used, so the lower the frequency, the longer the wavelength and the larger the antenna.

In particular, the antenna for MDTV (ISDB-T/CMMB/DVB-H) is difficult to achieve miniaturization because the UHF band (470-800 MHz) is used. It is possible to achieve miniaturization by adjusting the resonance frequency with an inductor, but it is necessary to select a part that takes into consideration the deterioration of the antenna characteristics. In addition, LTE, which has already begun to use, also requires the antenna to cover the 700 MHz band and multiple wave bands. One particularly striking technique is the switching method, which switches one antenna unit with a switch or the like to change the frequency. This switching method sometimes uses a lumped constant (inductor, capacitor) to adjust the frequency together with the switch.

This article will introduce an example of inductor selection for a variable frequency chip dielectric antenna for MDTV. The variable frequency chip dielectric antenna changes the resonant frequency of the antenna, covering the 470-800 MHz band that exceeds 50% of the relative bandwidth. Use a varactor to change the frequency and switch the frequency by channel. Since the antenna is miniaturized, the LQW series can be used and built in the mobile phone terminal.

Figure 1: Appearance of Antenna

Figure 2: Return Loss Curve

Method of selecting an inductor for an antenna

When an inductor is used for an antenna for the purpose of adjusting the frequency, in order to avoid deterioration of the radiation characteristics of the antenna, it is necessary to select an inductor that can increase the Q value and reduce the insertion loss. In addition, in order to reduce the standard deviation of the resonant frequency, the smaller the deviation of the inductor, the better. In addition, having a rich variety of constant values ​​is also a factor of choice. The antenna is susceptible to the external environment and must be carefully adjusted each time the internal specifications of the terminal are changed.

The next item describes the application of the variable frequency chip dielectric antenna to the influence of the Q value of the capacitor on the antenna characteristics and the frequency standard deviation caused by the deviation.

Radiation characteristics of antennas of various inductors

Figures 3 and 4 show the Q and R frequency characteristics of various inductors mounted in SimSurfing. Figures 3 and 4 illustrate the results of a 10nH inductor. It can be seen that compared with the LQG series, the LQW series is an inductor with high Q-value characteristics and low insertion loss.

Figure 3: Q characteristics for 10nH

Figure 4: R characteristics for 10nH

Fig. 5 shows the radiation efficiency of the antenna when the type of the inductor of the inverter chip dielectric antenna is changed. Since the inverter chip dielectric antenna employs multiple inductors, an efficiency difference of about 1 dB will result. The application of the LQW series minimizes the deterioration of radiation efficiency. The larger the inductance value used, the more the ESR (Equivalent Series Resistance) of the inductor increases, so the type of inductor must also be carefully selected.

* Changed the results of multiple inductor types. Figure 5: Antenna Radiation Efficiency

Frequency variation caused by the deviation of the inductor

Each channel of the MDTV is allocated a 6-8 MHz band. When viewing audio, it is necessary to cover the antenna characteristics of this band. The main reasons for the variation of the resonant frequency of the antenna are various tolerances and deviations, as well as the tolerance of the electrode-like radiating element, the misalignment when assembled onto the printed circuit board, and the standard deviation when inserted into the terminal. It is required to consider the above factors when designing the antenna to ensure the necessary frequency bands. Therefore, the inductor to be used must also be selected with a smaller deviation.

6 and 7 show the reflection loss waveform of the inverter chip dielectric antenna when the inductor deviation is ±2% and ±5%. If ±2% of the inductor is used, the necessary frequency band can be covered even if the resonant frequency changes; when ±5% of the inductor is used, the resonant frequency will move outside the necessary frequency band, and the radiation characteristics will deteriorate.

Figure 6: Return Loss for ±2% Tolerance

Figure 7: Return Loss for ±5% Tolerance

Figure 8 shows the radiation efficiency characteristics obtained by simulation. For Mkr1, assuming that the inductor deviation is ±2%, the radiation efficiency deterioration amount is about 0.8.4 dB; if it is ±5%, it will deteriorate by about 0.37 dB. This is an example of when the inductor used is always a product that reaches the deviation limit. In order to improve the radiation characteristics of the antenna, it is preferable to select an inductor with a small deviation in advance.

Figure 8: Antenna Radiation Efficiency

to sum up

The antenna first requires good radiation efficiency. For this reason, as described above, the inductor to be used must be carefully selected.

In addition, the characteristics of the nearest small antenna built in the terminal will vary depending on the condition of the device to be mounted (the size of the device, the location of the antenna, the configuration of peripheral components, etc.).

Therefore, in the first trial, the second trial, and each trial phase, the antenna characteristics will vary according to changes in the external environment. In this case, it is unrealistic to re-make the antenna each time, and it is possible to effectively utilize the inductors of various types supplied as adjustment elements to flexibly cope with such changes. In the future, the antenna design is also required to be more flexible, and it is possible to switch multiple frequency bands using LTE, WLAN, etc., and suppress the inter-antenna connection of the MIMO system, and effectively utilize the lumped constant.