Remotely located tuning circuits for multi-frequency, multi-puropse induction antennae in downhole tools

Abstract

A unified circuit without using a relay is proposed to resonate an antenna at more than two frequencies. The use of such analog IC switches are common place for transceiver applications in high RF frequencies (500 MHz and above). However, the use of selectively tuned antennas has for the most part been accomplished by the use of latching relays or fixed value components. The IC switch allows for not only the functionality of a single antenna to be either a transmitter and/or a receiver, but also allows, by adding additional switches to select any number of frequencies to resonate the antenna. This frequency resonation could be at fixed frequency points or sweep in either linear or logarithmic steps. The component selection of capacitors for either method would be controlled by a DSP or other microcontroller device. In one embodiment, an antenna for a downhole measurement tool is provided with a tuning circuit couple by a transformer to at least one antenna. The tuning circuit is disposed entirely within and protected by the hollow body of the tool and selectively tunes the antenna to at least three discrete frequencies. Advantageously, no structural modification is made on the outer surface of the tool to accommodate the tuning circuit. In one embodiment, a switching circuit enables a single antenna to function as both a transmitter and a receiver.

1 . An antenna system for a downhole measurement tool, comprising:
 a substantially hollow cylindrical body;   at least one antenna exposed upon the periphery of said cylindrical body;   a tuning circuit, coupled by means of a transformer to said at least one antenna;   wherein said tuning circuit is disposed entirely within and protected by said hollow cylindrical body.   
     
     
         2 . An antenna system for a downhole measurement tool, comprising:
 a substantially hollow cylindrical body;   at least one antenna exposed upon the periphery of said cylindrical body and separated from said cylindrical body by placement of a ferromagnetic material;   a tuning circuit, coupled by means of a transformer to said at least one antenna;   wherein said tuning circuit selectively tunes said antenna to at least three separate frequencies;   and wherein said tuning circuit disposed within said hollow cylindrical body such that no structural modification is made on the outer surface of said hollow cylindrical body to accommodate said tuning circuit.   
     
     
         3 . An antenna system in accordance with  claim 2 , wherein no junction box is formed on the outer surface of said hollow cylindrical body to accommodate said tuning circuit. 
     
     
         4 . An antenna system for a downhole measurement tool, comprising:
 an antenna comprising at least one antenna loop;   a switching circuit having a first state and a second state;   wherein when said switching circuit is in said first state, said antenna functions as a transmitter antenna and when said switching circuit is in said second state, said antenna functions as a receiving antenna.   
     
     
         5 . An antenna system in accordance with  claim 4 , further comprising control circuitry for controlling said switching circuit. 
     
     
         6 . An antenna system in accordance with  claim 5 , further comprising a tuning circuit selectively tuning said antenna to be resonant at least two frequencies; 
     
     
         7 . An antenna system in accordance with  claim 5 , further comprising a tuning circuit selectively tuning said antenna to be resonant at least three frequencies. 
     
     
         8 . An antenna system in accordance with  claim 5 , further comprising a tuning circuit selectively tuning said antenna to be resonant at a multiple of swept single frequencies over a given dynamic range. 
     
     
         9 . An antenna system in accordance with  claim 7 , wherein said tuning circuit does not include switches or relays. 
     
     
         10 . An antenna system for a downhole measurement tool, comprising:
 an antenna comprising at least one antenna loop;   a transmitter driver for generating a transmit signal;   a tuning circuit, coupled to said transmitter to tune said transmit signal to at least two resonant frequencies;   a transformer, coupled to said tuning circuit and adapted to couple said tuned transmit signal to one end of a connecting cable;   wherein said antenna is coupled to another end of said connecting cable.   
     
     
         11 . An antenna system in accordance with  claim 9 , wherein said connecting cable is a shielded twin-axial cable. 
     
     
         12 . An antenna system in accordance with  claim 9 , wherein said connecting cable is a coaxial cable. 
     
     
         13 . An antenna system in accordance with  claim 9 , wherein said connecting cable is a shielded twisted pair cable.