with VSWR in medical applications
here to go to our main page on medical applications of microwaves
here to go to our main page on voltage standing wave ratio (VSWR)
New for September 2011!
Content provided by Eamon McErlean of Emblation
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
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.
here to go to learn more about medical microwaves