Communication system

1. A communication system provided with a transmitter for applying a transmission signal to a transmission medium as an electric field, and a receiver for receiving the transmission signal by detecting the electric field via the transmission medium, wherein the receiver includes:

a reception electrode located opposite the transmission medium;

a resonance circuit provided on the circuit substrate, the resonance circuit being formed of a resonance inductor and a resonance capacitor;

a coupling capacitor for capacitively coupling the reception electrode and the resonance circuit, the coupling capacitor having a first electrode at the reception electrode side formed on the circuit substrate;

a demodulation circuit side wiring formed on the circuit substrate; and

a ground side wiring formed on the circuit substrate;

wherein the coupling capacitor is formed by a capacitive coupling between the first electrode at the reception electrode side and the resonance inductor, and has a capacitance which is equal to or smaller than a value of a capacitance between the receiver and the transmission medium when the receiver moves away from the transmission medium up to a maximum distance within which the receiver can perform communication,

and wherein the demodulation circuit side wiring and the ground side wiring are formed such that a capacitance between the first electrode and the demodulation circuit wiring becomes larger than the capacitance between the first electrode and the ground side wiring.

2. The communication system according to claim 1 , wherein the coupling capacitor is of variable capacitance type.

3. The communication system according to claim 1 , wherein the resonance circuit includes plural resonance inductors capacitively coupled with the reception electrode.

4. The communication system according to claim 3 , wherein each of the plural resonance inductors includes a switching member.

5. The communication system according to claim 3 , wherein the resonance circuit includes the plural resonance capacitor, and each of the resonance inductors and/or the resonance capacitors has a switching member.

6. The communication system according to claim 1 , wherein:

the resonance inductor is located at a position opposite the first electrode at the reception electrode side.

7. The communication system according to claim 1 , wherein the resonance circuit has a standard resonance frequency set with respect to a standard capacitance between the reception electrode and the transmission medium corresponding to a predetermined standard distance between the receiver and the transmission medium,

and wherein the capacitance of the coupling capacitor is set such that a signal strength of the transmission signal at the standard resonance frequency received by the receiver at the maximum distance from the communication medium is about a half of a signal strength of the transmission signal at the standard resonance frequency received by the receiver at the standard distance from the communication medium.

8. A communication system comprising:

a transmitter for applying a transmission signal to a transmission medium as an electric field; and

a receiver for receiving the transmission signal by detecting the electric field via the transmission medium, the receiver including:

a reception electrode facing the transmission medium;

a resonance circuit formed on the circuit substrate, the resonance circuit including a resonance inductor and a resonance capacitor;

a coupling capacitor for capacitively coupling the reception electrode and the resonance circuit, the coupling capacitor having a first electrode at the reception electrode side formed on the circuit substrate;

a demodulation circuit side wiring formed on the circuit substrate;

a ground side wiring formed on the circuit substrate;

wherein the communication system has a maximum distance between the reception electrode and the transmission medium within which the receiver is capable of communicating,

wherein the coupling capacitor has a capacitance which is equal to or smaller than a capacitance between the reception electrode and the transmission medium at the maximum distance,

and wherein the demodulation circuit side wiring and the ground side wiring are formed such that a capacitance between the first electrode and the demodulation circuit wiring becomes larger than the capacitance between the first electrode and the ground side wiring.

9. The communication system according to claim 8 , wherein the resonance circuit has a standard resonance frequency set with respect to a standard capacitance between the reception electrode and the transmission medium corresponding to a predetermined standard distance between the receiver and the transmission medium,

and wherein the capacitance of the coupling capacitor is set such that a signal strength of the transmission signal at the standard resonance frequency received by the receiver at the maximum distance from the communication medium is about a half of a signal strength of the transmission signal at the standard resonance frequency received by the receiver at the standard distance from the communication medium.