Abstract: A variable tuning antenna includes a tuning circuit (1) and a radiation element (2), connected to each other in series. The tuning circuit (1) includes a first inductance element (11) and a parallel circuit (12) to each other in series, and the parallel circuit (12) includes a second inductance element (121) and a variable capacitance element (122) to each other in parallel. And the tuning circuit is set so that the combined reactance of the radiation element and the first inductance element and the combined reactance of the parallel circuit can be canceled by each other, and the parallel circuit does not resonate in a desired receiving frequency band, and moreover, the tuning circuit is formed so as to be tunable in the desired frequency band by varying the capacitance of the variable capacitance.

Abstract: A variable tuning antenna includes a tuning circuit (1) and a radiation element (2), connected to each other in series. The tuning circuit (1) includes a first inductance element (11) and a parallel circuit (12) to each other in series, and the parallel circuit (12) includes a second inductance element (121) and a variable capacitance element (122) to each other in parallel. And the tuning circuit is set so that the combined reactance of the radiation element and the first inductance element and the combined reactance of the parallel circuit can be canceled by each other, and the parallel circuit does not resonate in a desired receiving frequency band, and moreover, the tuning circuit is formed so as to be tunable in the desired frequency band by varying the capacitance of the variable capacitance.

Abstract: A baseband signal communication apparatus includes a transmission unit and a reception unit. The transmission unit superposes a subcarrier on one value of a baseband signal consisting of binary values, modulates a carrier with the baseband signal on which the subcarrier is superposed, and transmits the baseband signal as a transmission signal wave. The reception unit demodulates the baseband signal having the subcarrier superposed thereon by using a demodulator upon reception of the transmission signal wave, extracts the subcarrier from the demodulated signal by using a bandpass filter, and demodulates the baseband signal by causing a detector to detect an envelope of the subcarrier.

Abstract: A microwave responder resonates or does not resonate with a microwave transmitted from a radio transmitter, and radiates the resonated microwave as a response signal wave. Two microstrip lines are linearly arranged to be separated from each other at a predetermined interval. A diode which is switched between ON and OFF states in response to the response signal is interposed between two end portions of the microstrip lines separated by the predetermined interval, and a distance, between distal ends of the two linearly arranged microstrip lines, which includes an impedance defined by the ON or OFF state of the diode is determined to be 1/2 a wavelength of the microwave.

Abstract: A full duplex transponder system includes an interrogation apparatus and a transponder apparatus. The transponder apparatus includes a reception antenna for receiving an interrogation signal wave whose carrier wave is phase- or frequency-modulated with an interrogation signal, a demodulator for demodulating the interrogation signal from the interrogation signal wave received by the antenna, a rectifier for rectifying the carrier wave of the interrogation signal wave received by the antenna and outputting a DC power used as a drive power source of the transponder apparatus, a multiplier for generating a predetermined harmonic component from the carrier wave of the interrogation signal wave received by the antenna, a modulator for amplitude-modulating the harmonic component with a response signal, and a transmission antenna for transmitting a response signal wave output from the modulator.