A Multi-Octave Matched Quarterwave Microstrip Taper

For impedance matching, quarterwave transformers as well as tapers are in use. The first one is most popular : for a 2 : 1 impedance ratio, its reflection does not exceed 20 per cent over an octave-wide frequency band. Although usually much longer, tapers are mainly used if low reflection over multi-octave bands is needed. The taperlength can be reduced to about a quarterwave at the lowest frequency by using a circular taper provided with transverse slots. The residual reflection has been matched below 10 percent by "selective" matching and this way we were able to realize a 2-12 GHz taper for an impedance ratio of 2. Now, a compact 2-way microstrip Wilkinson hybrid-with an isolation of at least 20 dB- could easily be made over this frequency band. Quartz substrates of 1.5 mm thick have been used throughout.