You could add a segment at the end of the line that has intrinsic impedance of 200 , and is exactly 4 long at the desired frequency ( ). Note that 200 is, logarithmically, halfway in between 50 and 800 . This strategy produces a perfect match at , meaning that the VSWR=1 in the 50 line. Plot the VSWR in the 50 line as a function of frequency, from 0.8 to 1.2 . What's important to note is that the length in meters is fixed by the design, so as the frequency changes, the segment is no longer 4 in length