Problems with VSWR in Medical Applications

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New for September 2011! Content provided by Eamon McErlean of Emblation Microwave.

Medical microwave systems particularly suffer from the effect of VSWR, as the impedance presented to microwave sources in these applications is typically poor compared with industrial and communication applications and often varies with changes in the tissue's dielectric properties during treatments.

Mismatch in medical systems can create a number of problems, including mismatch loss which necessitates the use of high performance cabling to avoid overheating of patient contacting cables. Additionally, taking power measurements of forward and reverse power levels can have significant errors due to standing waves, and where couplers are used in a reflectometer arrangement mismatches can cause errors in finite coupler directivity. Couplers only do what they are supposed to if they see a matched impedance at all ports!

Another problem with VSWR in medical systems is the use of continuous wave (CW) signals such as those generated by magnetrons or solid state amplifiers operated at spot frequencies. Depending upon the frequency, a change in the phase length of any components can move the detector's reference through a VSWR ripple (for example from a peak to a null) - this often happens if a cable is swapped etc. This uncertainty could be misinterpreted by a power measurement system used in a safety critical function.

Unfortunately microwave medical devices are often not built to the same microwave performance standards as a state of the art network analyzer and these issues if not understood or addressed can combine to result in misinterpretation and unreliability in treatments where patient's lives are at stake.

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Author : Unknown Editor