  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.
