Transponder employing modulated backscatter microstrip double patch antenna

A method of identifying a remote object using an interrogator and a transponder located at the remote object comprising the steps of:

• transmitting an interrogating signal from the interrogator, the interrogating signal being at an RF frequency;
• receiving the interrogating signal at the transponder, the transponder comprising a microstrip patch antenna, the microstrip patch antenna comprising two one-half wavelength segments, at least one of which has an input line for receiving an external control signal associated with the remote object, connected together by a switch having a closed position and an open position, the microstrip patch antenna being full wavelength at the RF frequency;
• responding to the interrogating signal via a backscatter-modulated signal produced by the microstrip patch antenna, wherein when the switch is in the closed position, the two segments of the microstrip patch antenna are out-of-phase with each other, thereby producing a backscatter signal as seen by the reader, and when the switch is in the open position the two segments of the microstrip patch antenna act in concert, whereby a minimum backscatter signal is produced, the switch being opened and closed by a switching circuit in response to the external control signal associated with the remote object;
• and
• receiving the backscatter-modulated signal at the reader and demodulating the backscatter-modulated signal to produce an identifying signal associated with the remote object.

A transponder comprising:

• a modulator generating a control signal;
• and
• a patch antenna coupled to the modulator for emitting backscatter-modulated signals at a fundamental frequency in response to a received signal, the patch antenna comprising two one-half wavelength segments, at least one of which has an input line for receiving an external signal, connected together by a switch having a closed position and an open position, the antenna being full wavelength at the fundamental frequency, the two segments being out-of-phase with each other when the switch is in the closed position, whereby a backscatter signal is emitted in response to the received signal, and the two segments of the antenna acting in concert when the switch is in the open position, whereby a minimum backscatter signal is emitted in response to the received signal, and the two segments of the antenna acting in concert when the switch is in the open position, whereby a minimum backscatter signal is produced in response to the received signal;
• a switching circuit for actuating the switch between the open position and the closed position in response to the external signal;
• wherein the switch opens and closes in response to the control signal.