a method and apparatus for determining accurately , both during and after insertion , the full length position of catheters , tubes and placement guidewires within biological tissue is disclosed including a transmitter / detector unit having an alternating current radio - frequency transmitter with antenna and a radio signal transmitter in the form of a continuous or segmented fine wire receiving antenna situated along the full length of the catheter , guidewire , and implanted port assemblies . the guidewire apparatus also includes a tip antenna . these antennae are connected by a removable clip to a wide - band rf detector circuit , situated within the transmitter / detector unit . the rf transmitter / detector circuit provides a voltage output that is a direct function of the relative spatial proximity of the transmitting antenna . this proximity is displayed visually to the operator using a sequential linear led array whose sensitivity is controlled by a gain control knob . an audible tone is also produced by a miniature speaker , and whose tone frequency is varied as a direct function of the transmitter proximity . also shown is a method and apparatus for locating the full length position of implanted port mounted catheters and a method and apparatus for simultaneous infusion into an implanted port assembly and detecting the full length position of the port mounted catheter .