A compact ultra-wideband double balun feeding network on a single layer PCB

An ultra-wideband (UWB) passive double balun for the frequency range from 3.2 GHz to 9.8 GHz realized on a single layer printed circuit board is presented. Each of the two baluns consists of a Wilkinson power divider and a 180° phase shifter. The intended combination of the baluns with a center-fed multi-arm antenna calls for the integration of an impedance transform from 50Ω to 75Ω and for a crossing between two microstrip lines by a copper bridge. Finally, the maximal dimensions of the layout are restricted to solely 50.8 mm by 50.8 mm by the application. To guarantee an adequate matching of the input ports, an analytical estimation of the worst case return loss is first performed, determining the maximum admissible reflection coefficients of the individually designed subcomponents. A prototype of the balun is fabricated on a Rogers RO4003 substrate using a laser-based PCB prototyping process. Key results include a return loss at the unbalanced port better than 10dB and a phase difference between the balanced ports between 165° and 205° in the operating frequency band.