Ka-band branch line coupler using high-performance air-buried microstrip lines

In this paper, a branch line coupler using air-buried microstrip lines (ABMSLs) has been monolithically fabricated for millimeter-wave applications. Because the ABMSL structure employs air-dielectric medium and ground conductor walls surrounding strip signal line, it is very effective in reducing a dielectric loss, as well as parasitic coupling between close transmission lines over high frequency range. The fabricated branch line coupler shows a coupling and transmission loss of 3.6 dB and 3.9 dB, respectively at the center frequency of 29 GHz. In the frequency range from 26 GHz to 32 GHz (20% bandwidth), it exhibits a return loss of better than 15 dB and isolation of better than 14 dB.