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Electromagnetic coupling between a coaxial line and a stripline is studied using the generalised version of Bethe's aperture coupling theory. The equivalent electric and magnetic dipole moments accounting for the coupling of one line with the other through the aperture are determined using a novel but simple and physically illuminating application of the image theory. The analytical expression obtained for the angle between the aperture coupled coaxial line and stripline for optimum directivity is verified using a specially designed mechanical housing, which facilitates the reproducible rotation of the stripline on the coaxial line. Using a coaxial line of characteristic impedance 60 ¿¿ and stripline of 50 ¿¿, the design value of the coupling 60 dB is shown to be achieved experimentally at a frequency of 2.45 GHz with the main line power ranging from 3 to 300 W. This quasi-TEM-mode analysis gives results which agree with the experimental values very closely.