 One of the main drawbacks of the image parameter design method in this , as in other , filters is that the impedance match at the terminations is not good . This usually requires that impedance matching sections are provided at the input and output . These usually take the form of multi @-@ section stepped impedance transformers . These add considerably to the overall length of the filter . A small improvement to matching can be had by starting and ending the filter on a half @-@ space instead of a full tooth or space . The lumped circuit equivalent of this is T @-@ half @-@ sections terminating the filter at either end . Starting and ending on a half @-@ tooth instead of a half space is the equivalent of Π @-@ half @-@ sections . 
