1. Field of the Invention
The present invention relates to a high-frequency coupler and a communication device that perform large-volume data transmission in proximity through a weak UWB (Ultra Wide Band) communication method using a high-frequency wide-band, and particularly to a high-frequency coupler and a communication device that secure a communication range in the transverse direction in weak UWB communication using electric field coupling.
2. Description of the Related Art
Non-contact communication has been widely used as a medium for authentication information, electronic money, or other value information. For example, NFC (Near Field Communication) developed by Sony and Philips adopts the RFID (Radio-Frequency Identification) standard that defines the specification of NFC communication devices (reader and writer) that can communicate with IC cards of Type A, Type B, and FeliCa conforming with ISO/IEC 14443, uses 13.56 MHz band, and enables proximity type (0 to 10 cm or shorter: Proximity) non-contact bidirectional communication in a way of electromagnetic induction. In addition, in recent years, additional application of such non-contact communication systems can be found in large-volume data transmission such as downloading and streaming of moving images, music, or the like. Such large-volume data transmission can also be implemented by the operations of a single user, preferably completed within the same access time as used by the existing authentication or billing process, and therefore it is necessary to increase the communication rate.
The general RFID standard uses the 13.56 MHz band, is for proximity type (0 to 10 cm or shorter: Proximity) non-contact bidirectional communication adopting the main principle of electromagnetic induction, and employs a communication rate of about 106 kbps to 424 kbps. On the other hand, TransferJet (for example, refer to Japanese Patent No. 4345849 and www.transferjet.org/en/index.html) that uses weak UWB signals can be exemplified as a proximity wireless transfer technology applicable to high-speed communication. The proximity wireless transfer technology (TransferJet) is basically a method for transmitting signals by using the action of electric field coupling, and a high-frequency coupler of such a communication device includes a communication circuit unit that processes high-frequency signals, a coupling electrode that is arranged in a certain height apart from the ground, and a resonating unit that supplies high-frequency signals to the coupling electrode efficiently.
The proximity wireless transfer using the weak UWB has a communication distance of about 2 to 3 cm, only about as wide both in the longitudinal and transverse directions, is without polarized waves, and has a communication range in the shape of a substantially hemisphere dome. For that reason, it is necessary to activate electric field coupling effectively by facing the coupling electrodes appropriately to each other between communication devices for performing data transmission.
If a functioning unit of proximity wireless transfer is manufactured in a small size, the function will be suitable for incorporation, and can be mounted in various kinds of information equipment, for example, personal computers, mobile phones, or the like. However, if the size of a coupling electrode in a high-frequency coupler is reduced, there is a problem that the communication range diminishes particularly in the transverse direction. For example, if a target point, which indicates a spot where a high-frequency coupler is embedded, is marked on the housing surface of information equipment, a user may conduct an alignment aimed toward the target point. However, if the communication range of the transverse direction is narrow, a target point may be obscured by the shadow of the other equipment when they are adjacent, resulting that the target point is aligned while shifted from the center thereof in the transverse direction.
In order to improve usability in practical use of the proximity wireless transfer function, it is necessary to extend the communication range in the transverse direction. However, if the size of a coupling electrode in a high-frequency coupler is simply increased, a standing wave occurs on the surface of a coupling electrode. Then, since the electric fields of both directions are cancelled at a neighboring portion where the amplitude of the standing wave travels in opposite directions, places having the electric field with high intensity and low intensity appear. The place having the electric field with low intensity becomes a dead-point (null point) in which fine effect of electric field coupling is not easily obtained, even when the coupling electrode of a communication partner is contacted.