A Design Method of an Ultra-Wideband and Easy-to-Array Magic-T: A 6-14 GHz Scaled Model for a mm/submm Camera

TL;DR

This paper presents a novel design method for an ultra-wideband, easy-to-array Magic-T suitable for millimeter and submillimeter wave imaging, using a scaled model to optimize performance and manufacturability.

Contribution

A new design approach removing traditional constraints, enabling a compact, wideband Magic-T with optimized impedance and minimized discontinuity effects for practical applications.

Findings

Achieved a compact CPCL width of ~180 μm, much smaller than traditional solutions.

Demonstrated a Magic-T with return loss < -20 dB across 6.1-14.1 GHz.

Optimized line lengths for minimal amplitude and phase imbalance.

Abstract

We established a design method for a Magic-T with a single-layer dielectric/metal structure suitable for both wideband and multi-element applications for millimeter and submillimeter wave imaging observations. The design method was applied to a Magic-T with a coupled-line, stubs, and single-stage impedance transformers in a frequency-scaled model (6-14 GHz) that is relatively easy to demonstrate through manufacturing and evaluation. The major problem is that using the conventional perfect matching condition for a coupled-line alone produces an impractically large width coplanar coupled-line (CPCL) to satisfy the desired bandwidth ratio. In our study, by removing this constraint and optimizing impedances utilizing a circuit simulator with high computation speed, we found a solution with a $\sim$ 180 $\rm \mu$m wide CPCL, which is approximately an order of magnitude smaller than the conventional analytical solution. Furthermore, considering the effect of transition discontinuities in the transmission lines, we optimized the line length and obtained a design solution with return loss < -20 dB, amplitude imbalance < 0.1 dB, and phase imbalance < 0.5$^\circ$ from 6.1 GHz to 14.1 GHz.