it has been found that , with respect to surgical instruments configured to apply energy to tissue to treat tissue , proper spacing between the energy radiating portion or portions and other components of the instrument helps facilitate optimal performance of the instrument . with respect to microwave ablation probes in particular , is has been found that proper axial spacing between the distal radiating portion and the trocar helps ensure optimal performance of the microwave ablation probe . more specifically , it has been found that variation in the axial distance may result in a sub - optimal ablation shape , an irregular ablation zone , and / or degraded ablation performance . the various embodiments of probes and methods of manufacturing probes described in detail hereinbelow are configured to help eliminate this variation in axial distance , thereby facilitating optimal performance of the probe . turning now to fig1 - 7 , a microwave ablation system provided in accordance with the present disclosure is shown generally identified by reference numeral 10 . microwave ablation system 10 includes a microwave antenna probe 12 configured to couple to a microwave generator ( not shown ) via a flexible coaxial cable 16 . although the present disclosure is shown and described with reference to a microwave ablation system 10 , the present disclosure is equally applicable for use in determining and / or setting a particular distance between components of any suitable energy - based surgical instrument . for the purposes herein , microwave ablation system 10 is generally described . with continuing reference to fig1 - 7 , microwave antenna probe 12 generally includes an antenna assembly 20 , an outer jacket and trocar assembly 70 and a connection hub 80 . antenna assembly 20 defines a longitudinal axis “ x - x ” and includes a radiating section that defines a dipole configuration , e . g ., the radiating section includes a feed gap 43 and proximal and distal radiating portions 42 , 44 . a feedline 30 extends proximally from the radiating section into connection hub 80 , ultimately coupling to cable 16 via transition 60 to connect antenna assembly 20 to the generator ( not shown ) for supplying energy thereto . feedline 30 defines a coaxial configuration having an inner conductor 32 surrounded by an insulator 34 . insulator 34 , in turn , is surrounded by an outer conductor 36 , thus defining the coaxial configuration of feedline 30 . feedline 30 may be formed from a semi - rigid or flexible coaxial cable , although other configurations are also contemplated . as mentioned above , and with reference to fig2 , 4 , and 6 - 7 , the radiating section of antenna assembly 20 includes feed gap 43 , proximal radiating portion 42 , and distal radiating portion 44 . feed gap 43 is defined by the portion of inner conductor 32 and insulator 34 of feedline 30 that extends distally from outer conductor 36 , e . g ., outer conductor 36 may be stripped from the distal end of coaxial feedline 30 to define feed gap 43 . proximal radiating portion 42 is defined by the portion of feedline 30 disposed between the proximal end of feed gap 43 and the distal end of the choke 50 . distal radiating portion 44 is attached to feed gap 43 via any suitable process and extends distally therefrom . for example , as shown in fig7 , distal radiating portion 44 may be soldered to inner conductor 32 of feed gap 43 to establish electromechanical contact therebetween . antenna assembly 20 , as shown in fig2 , 4 , and 6 - 7 , further includes a choke or balun 50 disposed about feedline 30 . choke 50 includes an inner dielectric layer and an outer conductive layer . choke 50 may be a quarter - wavelength shorted choke that is shorted to outer conductor 36 of feedline 30 at the proximal end of choke 50 , although other configurations are contemplated . the dielectric layer of choke 50 may also be configured to extend distally beyond the conductor layer thereof towards the distal end of antenna assembly 20 . with additional reference to fig3 , as mentioned above , antenna assembly 20 includes a transition 60 from which feedline 30 extends . during assembly , the radiating section and feedline 30 of antenna assembly 20 are inserted through lumen 82 of connection hub 80 , while transition 60 is inserted into proximal port 83 of connection hub 80 sufficiently such that transition 60 is sealingly engaged within proximal port 83 of connection hub 80 via o - ring 62 . feedline 30 extends into transition 60 , wherein inner conductor 32 is coupled to an inner conductor ( not explicitly shown ) of coaxial cable 16 and outer conductor 36 is coupled to an outer conductor ( not explicitly shown ) of coaxial cable 16 , while maintaining the spacing therebetween via an insulator ( not explicitly shown ). cable 16 may be secured to feedline 30 within transition 60 via soldering , laser welding , or any other suitable process for establishing electromechanical contact therebetween . outer jacket and trocar assembly 70 , as best shown in fig1 - 3 and 5 , includes an outer jacket 72 configured to surround antenna assembly 20 , e . g ., proximal and distal radiating portions 42 , 44 , feed gap 43 , and feedline 30 , such that a coolant fluid may be circulated thereabout to maintain antenna assembly 20 in a relatively cooled state during use . more specifically , coolant fluid is pumped into lumen 82 of connection hub 80 to circulate through outer jacket 72 via supply tube 17 and inlet 87 and returns to the coolant fluid source ( not shown ) via outlet 89 and return tube 19 . a ferrule 74 is molded or otherwise engaged about outer jacket 72 towards the proximal end thereof to facilitate sealing engagement of the proximal end of outer jacket 72 within distal port 85 of connection hub 80 via o - ring 76 . connection hub 80 , as mentioned above defines a longitudinal lumen 82 that is configured to receive feedline 30 therethrough , while sealingly engaging outer jacket 72 within distal port 85 and transition 60 within proximal port 83 . connection hub 80 further includes an outlet fluid port 87 and an inlet fluid port 89 that are disposed in fluid communication with lumen 82 . outlet and inlet ports 87 , 89 are configured to receive tubes 17 , 19 ( see fig1 ), respectively , such that , as mentioned above , coolant fluid from a coolant fluid supply ( not shown ) may be circulated through connection hub 80 and outer jacket 72 . more specifically , an elastomeric ( or otherwise configured ) hub divider 81 a is sealingly engaged within lumen 82 of connection hub 80 to isolate the inlet and outlet portions of lumen 82 of connection hub 80 from one another . further , an inflow tube 81 b is coupled to hub divider 81 a and extends distally through outer jacket 72 . as such , coolant fluid may flow from tube 17 , through inlet port 87 , into the inlet portion of lumen 82 , and distally through inflow tube 81 b , ultimately returning proximally through outer jacket 72 ( exteriorly of inflow tube 81 b ), the outlet portion of lumen 82 , outlet port 89 , and into tube 19 . this configuration allows for the circulation of coolant fluid about antenna assembly 20 to maintain antenna assembly 20 in a relatively cooled state during use . the coolant fluid may be a liquid , gas , other flowable material , or combination thereof . with reference to fig4 - 5 , outer jacket and trocar assembly 70 further includes a trocar 90 defining a tapered distal end that terminates at a pointed distal tip 92 to facilitate insertion of microwave antenna probe 12 ( fig1 ) into tissue with minimal resistance , although other configurations may also be provided . trocar 90 may be formed from a variety of heat - resistant materials suitable for penetrating tissue , e . g ., metals ( stainless steel , for example ), various thermoplastic materials ( such as polytherimide , polyamide thermoplastic resins , etc . ), or any other suitable material . base 94 of trocar 90 is configured for insertion into the open distal end 78 of outer jacket 72 for sealing engagement therein via any suitable process , e . g ., using adhesives or via soldering . as such , trocar 90 , once engaged within distal end 78 of outer jacket 72 , sealingly encloses antenna assembly 20 ( see fig2 ) within outer jacket 72 and connection hub 80 , thus defining an internal chamber for circulation of coolant fluid through microwave antenna probe 12 ( fig1 ) and inhibiting coolant fluid from leaking out of microwave antenna probe 12 during use . trocar 90 may be secured to distal end 78 of outer jacket 72 prior to engagement of outer jacket and trocar assembly 70 to connection hub 80 and / or engagement of transition to connection hub 80 , or may be secured to distal end 78 of outer jacket 72 after each of these assembly steps are complete . referring again to fig1 - 7 , as mentioned above , it has been found that proper axial spacing between the distal end of distal radiating portion 44 of antenna assembly 20 and the proximal surface of base 94 of trocar 90 helps ensure optimal performance of microwave antenna probe 12 . in particular , it has been found that , if the axial distance between the distal end of distal radiating portion 44 of antenna assembly 20 and the proximal surface of base 94 of trocar 90 is too large , the ablation zone , or ablation shape may be sub - optimal and complete ablation may not be readily achieved . likewise , where the axial distance between the distal end of distal radiating portion 44 of antenna assembly 20 and the proximal surface of base 94 of trocar 90 is too small , or where distal radiating portion 44 and trocar 90 are in contact with one another , ablation performance may be degraded . the optimal axial spacing , e . g ., the target axial distance , between distal radiating portion 44 and trocar 90 depends on , among other things , the dimensions and configuration of microwave antenna probe 12 , and may be determined empirically , experimentally , or in any other suitable fashion . variation in the axial distance may result from : variation in the length of antenna assembly 20 , e . g ., due to variation in the attachment point of distal radiating portion 44 to inner conductor 32 and / or variation in the length or size of the individual components of antenna assembly 20 ; variation in the position of transition 60 and / or ferrule 74 relative to connection hub 82 and / or one another ; variation in the distance trocar 90 extends from ( or extends into ) outer jacket 72 once engaged to outer jacket 72 ; variation in the axial position of ferrule 74 relative to outer jacket 72 ; and / or other factors , e . g ., the particular manufacturing processes or materials used , the particular components used or relationship between the components , the configuration of the probe in general , etc . with reference to fig8 , in accordance with one embodiment of the present disclosure , as will be described in greater detail below , electrical discharge may be utilized to help achieve the target axial distance between distal radiating portion 44 and trocar 90 during assembly , thereby facilitating optimal performance of microwave ablation probe 12 . with continued reference to fig8 , during assembly , once transition 60 is engaged within proximal port 83 of connection hub 80 via o - ring 62 and once ferrule 74 is engaged within distal port 85 of connection hub 80 via o - ring 76 such that antenna assembly 20 , outer jacket 72 , and connection hub 80 are secured in fixed position relative to one another , base 94 of trocar 90 is inserted into the open distal end 78 of outer jacket 72 for sealing engagement therein via any suitable process , e . g ., using adhesives or via soldering . since , at this point , outer jacket 72 and distal radiating portion 44 have already been fixed in position relative to one another , it is trocar 90 that is moved into a particular axial position corresponding to the target axial distance between the distal end of distal radiating portion 44 and the proximal surface of base 94 of trocar 90 . once in position , trocar 90 may be adhered , soldered , or otherwise engaged to outer jacket 72 at the particular axial position such that the target axial distance is achieved . in order to determine when trocar 90 is properly positioned such that the target axial is achieved between trocar 90 and distal radiating portion 44 during assembly , trocar 90 is charged to a first voltage , while distal radiating portion 44 is charged to a second voltage that is different from the first voltage such that a pre - determined voltage differential is established between trocar 90 and distal radiating portion 44 . as will be described below , the particular voltage differential between trocar 90 and distal radiating portion 44 may be selected in accordance with the target axial distance between distal radiating portion 44 and trocar 90 , among other factors . further , although fig8 illustrates trocar 90 being charged to a greater voltage than distal radiating portion 44 such that trocar 90 is the anode , e . g ., relatively positive “+,” while distal radiating portion 44 is the cathode , e . g ., relatively negative “−,” this configuration may be reversed , e . g ., wherein trocar 90 is the cathode and distal radiating portion 44 is the anode . with the pre - determined voltage differential established between trocar 90 and distal radiating portion 44 , trocar 90 may be advanced proximally relative to outer jacket 72 ( and , thus distal radiating portion 44 ) such that base 94 of trocar 90 is inserted into the open distal end 78 of outer jacket 72 . while monitoring the respective voltages of trocar 90 and distal radiating portion 44 ( and / or the voltage differential therebetween ), trocar 90 is advanced further proximally relative to distal radiating portion 44 , e . g ., further into open distal end 78 of jacket 72 , until electrical discharge occurs between trocar 90 and distal radiating portion 44 , as evidenced by change in voltages of trocar 90 and distal radiating portion 44 or a decrease in the voltage differential therebetween . when discharge occurs , the target axial distance between the distal end of distal radiating portion 44 and the proximal surface of base 94 of trocar 90 has been achieved . more specifically , the pre - determined voltage differential is set in accordance with the target axial distance such that discharge occurs once trocar 90 and distal radiating portion 44 are spaced - apart by the target axial distance . once this target axial distance is achieved , e . g ., once discharge occurs , the supply of voltage to trocar 90 and distal radiating portion 44 may be terminated , and trocar 90 may be engaged to outer jacket 72 at that particular position , thereby establishing the target axial distance between trocar 90 and distal radiating portion 44 . as mentioned above , the voltage differential between trocar 90 and distal radiating portion 44 is set such that the distance between trocar 90 and distal radiating portion 44 at which discharge occurs corresponds to the target axial distance between trocar 90 and distal radiating portion 44 . the particular relationship between the voltage differential and the distance at which discharge occurs may be determined experimentally , empirically , or in any other suitable fashion , such that an appropriate voltage differential may be established . further , the above - described process may be automated , e . g ., using robotics or other automated or semi - automated assembly processes , such that trocar 90 is advanced , e . g ., incrementally at a pre - determined step size or continuously at a pre - determine rate , until discharge occurs , whereby feedback as to the occurrence of discharge is provided to stop further movement of trocar 90 and maintain trocar 90 in position such that trocar 90 may be engaged to outer jacket 72 at that position , thereby achieving the target axial distance between trocar 90 and distal radiating portion 44 . any of the other embodiments described herein may similarly be incorporated into an automated or semi - automated feedback system , as described above . as opposed to using the occurrence of electrical discharge to determine when proper axial spacing between trocar 90 and distal radiating portion 44 has been achieved , a resistivity or conductivity sensor ( see , e . g ., sensor assembly 300 ( fig1 )), may be used in conjunction with the voltage differential between trocar 90 and distal radiating portion 44 to determine the axial spacing between trocar 90 and distal radiating portion 44 . more specifically , with trocar 90 and distal radiating portion 44 immersed in a conductive fluid , e . g ., a conductive gas or liquid , having a known conductivity or resistivity , and with the pre - determined voltage differential established between trocar 90 and distal radiating portion 44 , the known conductivity or resistivity of the fluid , along with the known voltage differential between trocar 90 and distal radiating portion 44 , can be used to determine the axial spacing between trocar 90 and distal radiating portion 44 based upon the sensed conductivity or resistivity between the charged components , e . g ., trocar 90 and distal radiation portion 44 . this is because the sensed conductivity or resistivity between the charged components is dependent upon the conductivity or resistivity of the fluid , the distance between the charged components , and the voltage differential between the charged components . thus , working backwards from the known conductivity or resistance of the fluid and the voltage differential between the charged components , the distance therebetween can be determined . a controller / processor ( see , e . g ., controller / processor 320 ( fig1 )) may be utilized in conjunction with the resistivity or conductivity sensor to determine and display or provide feedback as to the axial spacing between trocar 90 and distal radiating portion 44 . turning now to fig9 , in accordance with another embodiment of the present disclosure , a sensing member 200 may be utilized to achieve the target axial distance between trocar 90 and distal radiating portion 44 during assembly . sensing member 200 is disposed within lumen 82 of connection hub 80 and , more particularly , is disposed with outer jacket 72 . sensing member 200 is electrically coupled to a controller / processor 210 via one or more wires 220 . sensing member 200 may be permanently affixed within connection hub 80 and outer jacket 72 , or may be removably disposed therein such that , once trocar 90 is moved into position and engaged to distal end 78 of outer jacket 72 , sensing member 200 may be withdrawn from outer jacket 72 and connection hub 80 . further , sensing member 200 may be disposed at any suitable position within outer jacket 72 , e . g ., in a more distal position towards distal radiating portion 44 , within connection hub 80 ( as shown ), or at any suitable position therebetween . the particular positioning of sensing member 200 , as can be appreciated , may depend of the particular type of sensing member used . sensing member 200 may be in the form of an acoustic transmitter / receiver configured to emit , via the transmitter , a pre - determined acoustic signal distally into outer jacket 72 and to detect the acoustic response via the receiver . the acoustic response , which is correlated to the axial distance between the distal end of distal radiating portion 44 and the proximal surface of base 94 of trocar 90 , may thus be used to determine the axial distance between trocar 90 and radiating portion 44 . more specifically , the acoustic response detected by the receiver of the acoustic transmitter / receiver is transmitted to controller / processor 210 , which analyzes the response to determine the distance between trocar 90 and distal radiating portion 44 . alternatively , the response may be compared to a target response value , e . g ., the response value corresponding to the target axial distance between trocar 90 and distal radiating portion 44 , that is stored in controller / processor 210 . in such a configuration , the controller / processor 210 determines whether the trocar 90 is in proper position by determining whether or not the responses value match ( or are sufficiently similar ). if a match is determined , trocar 90 is in the proper position wherein trocar 90 and distal radiating portion 44 are spaced - apart by the target axial distance . comparison of the sensed response value to the target response value may also be used to determine whether trocar 90 and distal radiating portion 44 are too close or too far from one another , thus indicating the required direction of movement of trocar 90 to achieve the target axial distance between trocar 90 and distal radiating portion 44 . the particular relationship between the response values sensed by sensing member 200 and the axial distance between trocar 90 and distal radiating portion 44 may be determined experimentally , empirically , or in any other suitable fashion . further , similarly as described above with respect to the previous embodiment , sensing member 200 and controller / processor 210 may be incorporated into an automated , feedback - based system . other suitable sensing members , e . g ., piezoelectric sensors , optical sensors , or any other suitable sensor for determining the distance between trocar 90 and distal radiating portion 44 , may also be used . with reference to fig1 , in accordance with another embodiment of the present disclosure , a magnetic field “ m ,” multiple magnetic fields , changing magnetic field ( s ), etc ., may be selectively applied to microwave antenna probe 12 to determine the positioning of trocar 90 and distal radiating portion 44 relative to one another such that trocar 90 may be moved into position to achieve the target axial distance between trocar 90 and distal radiating portion 44 . more specifically , by analyzing the magnetic field ( s ) “ m ,” the various components of microwave antenna probe 12 and the relative positioning therebetween can be determined since the magnetic field ( s ) “ m ” exhibits different characteristics adjacent the different components , e . g ., adjacent trocar 90 and distal radiation portion 44 . accordingly , while monitoring the magnetic field ( s ) “ m ,” and , thus , while monitoring the axial spacing between trocar 90 and distal radiating portion 44 , trocar 90 may be moved into position to achieve the target axial distance between trocar 90 and distal radiating portion 44 . once this target axial distance is achieved , trocar 90 may be engaged to outer jacket 72 at the proper position to retain trocar 90 in fixed position relative to distal radiation portion 44 . an appropriate magnetic field ( s ) “ m ” may also be applied to retain trocar 90 in position during engagement of trocar 90 to outer jacket 72 , thereby obviating the need to mechanically retain trocar 90 in position . referring to fig1 , in accordance with another embodiment of the present disclosure , an external sensor assembly 300 may be utilized to achieve the target axial distance between distal radiating portion 44 and trocar 90 during assembly . external sensor assembly 300 is a contactless sensor disposed exteriorly of microwave antenna probe 12 and is configured to determine the axial distance between trocar 90 and distal radiating portion 44 without the need to contact either trocar 90 or distal radiating portion 44 ( or any other portion of microwave antenna probe 12 ). thus , trocar 90 may be moved into position in accordance with the feedback , e . g ., the axial distance between trocar 90 and distal radiating portion 44 , obtained from sensor assembly 300 without being inhibited by sensor assembly 300 . sensor assembly 300 may include cooperating components 310 , 340 , e . g ., an emitter 310 and a detector 340 , although the particular configuration of components 310 , 340 may depend on the particular type of sensor assembly used , that are coupled to a controller / processor 310 via one or more wires 320 , 350 , respectively . controller / processor 310 is configured to analyze the response from sensor components 310 , 340 to determine the axial spacing between trocar 90 and distal radiating portion 44 . sensor assembly 300 may be in the form of a capacitive proximity sensor , an inductive ( eddy current ) proximity sensor , a magnetic proximity sensor , or any other suitable external contactless sensor configured to emit a signal and receive a response for determining the axial spacing between trocar 90 and distal radiating portion 44 . more specifically , in use , after emission of a signal , application of an energy field , or other excitation by sensor component 310 and / or sensor component 340 , the response ( s ) received by sensor component 310 and / or sensor component 340 is sent to controller / processor 310 , which analyzes the response to determine the axial distance between trocar 90 and distal radiating portion 44 . using this feedback , trocar 90 may be accurately positioned relative to distal radiating portion 44 . for example , with respect to a capacitive proximity sensor , capacitance , e . g ., capacitance sensed by sensor assembly 300 , can be used to determine the axial distance between trocar 90 and distal radiating portion 44 since capacitance is dependent upon the distance between the components , e . g ., trocar 90 and distal radiating portion 44 . with respect to an inductive proximity sensor , since inductance is likewise dependent on distance , an inductive proximity sensor can be used to determine the axial distance between trocar 90 and distal radiating portion 44 , the difference being that capacitive proximity sensors utilize electrical capacitance , while inductive proximity sensors utilized magnetic inductance . referring in general to fig1 - 11 , the above - described embodiments provide for accurate placement of trocar 90 relative to distal radiating portion 44 despite variation among the individual components of microwave antenna probe 12 or the engagements therebetween , e . g ., without the need to rely on the accuracy of the dimensions , positioning , or engagement of the other components . that is , placement of trocar 90 relative to distal radiating portion 44 as described above is accomplished irrespective of variation among the components , e . g ., variations in length , relative positioning , and / or configuration of one or more of the components . accordingly , accurate positioning of trocar 90 relative to distal radiating portion 44 can be readily achieved , thereby helping to ensure optimal performance . although the assembly of microwave ablation probe 12 is described above wherein trocar 90 is engaged to outer jacket 72 once transition 60 has been engaged within proximal port 83 of connection hub 80 via o - ring 62 and engagement of ferrule 74 within distal port 85 of connection hub 80 via o - ring 76 , it is also contemplated that above - described assembly methods may similarly be performed wherein the engagement of transition 60 to connection hub 80 or the engagement of outer jacket 72 to connection hub 80 is performed once the other components are fixed relative to one another . for example , with trocar 90 engaged to outer jacket 72 and transition 60 engaged within proximal port 83 of connection hub 80 , any of the above - described embodiments may be utilized to guide the positioning and engagement of ferrule 74 within distal port 85 of connection hub 80 such that the target axial spacing between trocar 90 and distal radiating portion 44 is achieved . likewise , with trocar 90 engaged to outer jacket 72 and ferrule 74 engaged within distal port 85 of connection hub 80 , any of the above - described embodiments may be utilized to the guide the positioning and engagement of transition 60 within proximal port 83 of connection hub 80 such that the target axial spacing between trocar 90 and distal radiating portion 44 is achieved . although the various embodiments above are described with respect to determining the spacing between trocar 90 and distal radiating portion 44 during assembly of trocar 90 to outer jacket 72 , it is also contemplated that any or all of the above - described embodiments may be used to record , mark , or otherwise note the proper positioning of trocar 90 ( and / or the other components of microwave ablation probe 12 ) such that , upon subsequent assembly , trocar 90 ( and / or the other components ) may be positioned in accordance with the recorded , marked , or otherwise noted position previously obtained . as such , proper positioning of trocar 90 ( and / or the other components ) can be readily achieved . alternatively or additionally , the above - described embodiments may be utilized for quality control , e . g ., to ensure that the target axial distance between trocar 90 and distal radiating portion 44 has been achieved once assembly has been completed . from the foregoing and with reference to the various figure drawings , those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same . while several embodiments of the disclosure have been shown in the drawings , it is not intended that the disclosure be limited thereto , as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise . therefore , the above description should not be construed as limiting , but merely as exemplifications of particular embodiments . those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto .
a method of manufacturing a surgical instrument includes charging a first component to a first voltage , charging a second component to a second voltage such that a pre - determined voltage differential is established between the first and second components , axially moving at least one of the first and second components relative to the other , monitoring an electrical characteristic to determine whether an axial distance between the first and second components is equal to a target axial distance , and retaining the first and second components in fixed position relative to one another once the axial distance between the first and second components is equal to the target axial distance .