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I have a dual-linear patch antenna and need to generate both RHCP and LHCP outputs. What are the practical design options for a 90° hybrid coupler to interface with it?

 My antenna has two isolated ports for vertical and horizontal polarization. I need to feed it with a 90° hybrid coupler to produce simultaneous RHCP and LHCP outputs for polarization diversity. The coupler must cover the 5G n78 band (3.3–3.8 GHz) with good amplitude/phase balance and isolation. I'm weighing a microstrip branch-line coupler (simple but large bandwidth?), a Lange coupler (wider band but requires bonding wires), or a multilayer overlay design. Which offers the best trade-off in bandwidth, size, and fabrication complexity for this application?

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By Prithvi Patel Answered 1 year ago

 For your 15% bandwidth requirement (3.3–3.8 GHz), a conventional branch-line coupler is insufficient; its bandwidth is typically 10-15% at best, and phase balance degrades at the edges. A Lange coupler is the classic solution, offering an octave bandwidth (>50%) with excellent phase balance, but it requires precise bonding wires over gaps, which adds fabrication risk and cost. My recommended choice for this frequency and integration level is a stripline or broadside-coupled multilayer coupler. It offers wide bandwidth (easily 30-40%), excellent isolation, and no bonding wires, as the coupling is achieved through overlapping traces on different layers. It’s more complex to layout but is reproducible and robust. For a quick prototype, a three-section branch-line design can be pushed to ~20% bandwidth with careful optimization. 

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