Recently, non-contact type IC (Integrated Circuit) cards have been popular, and they have been introduced as common tickets to transportation facilities such as subways, buses, or ferries and for electronic money systems, etc. The data writing/reading into/from the IC cards are carried out in the non-contact style on the basis of the principle of electromagnetic induction.
That is, in the non-contact IC card system, electromagnetic wave is emitted from a loop antenna having a reader/writer for writing/reading data into/from an IC card, and the electromagnetic wave emitted is received by a loop antenna equipped in the IC card, whereby communications are made between the IC card and the reader/writer.
Further, in order to eliminate a load imposed on an IC card maintenance work, the IC card is provided with no battery, and thus it is designed to achieve required power from the electromagnetic wave received. Therefore, the IC card is required to receive electromagnetic wave as efficiently as possible.
In general, the IC card is equipped therein with a loop antenna 1 having two or more turns of wire formed in a rectangular shape which is as large as possible as shown in FIG. 1. The loop antenna 1 receives signals by resonating a resonance circuit with the carrier frequency of electromagnetic wave emitted from the reader/writer.
The IC card is basically used alone, however, there is a case where two IC cards are used together with being stacked. For example, when a user passes over a commuter pass zone and then gets off at a station out of the commuter pass zone, the user presents his/her commuter pass and an iO card (trademark) (it is assumed that each of the cards comprises an IC card) to a reader/writer while these cards are stacked in order to adjust the surcharge of the over-zone. At this time, the reader/writer or a non-contact IC card system connected to the reader/writer recognizes the zone of the commuter pass thus presented, and then carries out the processing of calculating the extra distance the user has ridden, calculating the surcharge based on the extra distance of the over-zone and adjusting the account on the iO card.
Not only when one IC card is presented, but also when plural IC cards are stacked, it is necessary that electromagnetic wave can be efficiently received so that each of the IC cards can operate desired communication operations.
For example, when two IC cards each having a loop antenna 1 having the construction shown in FIG. 1 are stacked with the IC card placed face up while being perfectly overlapped with each other in the view from the upper side (when the two IC cards are stacked so that the top side of one card faces the back side of the other card) as shown in FIG. 2, the linear portions L1 to L9 constituting the respective sides of rectangles of the conductor constituting the loop antenna 1-1 of one IC card are overlapped with the corresponding linear portions L11 to L19 of the loop antenna 1-2 of the other IC card, respectively. At this time, the resonance frequency of the resonance circuit of the loop antenna 1-1, 1-2 is reduced to the inverse of the square root of 2 (1/√2) of the resonance frequency when only one loop antenna 1 is provided (only one IC card is provided).
That is, when the IC cards are overlapped, coupling occurs between the resonance circuits thereof. Therefore, the resonance frequency is equal to 1/(2π√(LC)) in the case of one IC card, however, it is equal to 1/(2π√(L·(2C))=(1/√2)·1/(2π√(LC)) in the case of two stacked IC cards. That is, when IC cards are stacked, the resonance frequency of the resonance circuit of each IC card is displaced from the carrier frequency of the electromagnetic wave by 1/√2, and the reception efficiency is reduced.
However, for example when the carrier frequency of the electromagnetic wave emitted from the reader/writer is equal to 13.56 MHz, by setting the resonance frequency of the resonance circuit of each IC card to 17.5 MHz higher than the carrier frequency in advance, the electromagnetic wave can be received not only when only one IC card is used, but also even when two IC cards are used while they are stacked so as to be faced in the same direction.
When IC cards are stacked while one IC card is placed face up and the other IC card is placed face down as shown in FIG. 3 (two IC cards are stacked while both the top sides thereof or both the back sides thereof face each other), both the linear portion L1 of the loop antenna 1-1 and the linear portion L13 of the loop antenna 1-2, both the linear portion L2 of the loop antenna 1-1 and the linear portion L12 of the loop antenna 1-2 and both the linear portion L3 of the loop antenna 1-1 and the linear portion L11 of the loop antenna 1-2 are respectively basically overlapped with each other, however, the overlapping style is more complicated than that of FIG. 2.
The theoretical ground has not been necessarily clear, however, it has been discovered from at least experiment results that the resonance frequency is further reduced as compared with the case where IC cards are stacked as shown in FIG. 2. As described above, the method of setting the resonance frequency of the resonance circuit to a value higher than the carrier frequency in advance in consideration of the case where plural IC cards are stacked has such a problem that it can achieve a sufficient effect both when one IC card is used and when two IC cards are stacked so as to be faced in the same direction as shown in FIG. 2, however, it cannot achieve a sufficient effect when two IC cards are stacked so as to be faced in the opposite directions as shown in FIG. 3.
Further, there is a problem that when two IC cards are stacked, reception signals contain distortion.