Broadband microstrip directional coupler

Sinusoidally modulated coupling functions are considered for high bypass equal ripple characteristics with the lowest achievable peak-to-peak ripple. Design characteristics of a P/4 microstrip coupler with nominal coupling of 20 dB with + or -6 percent ripple characteristics. The periodicity is set at 12 cm, associated with a cut-off frequency of 875 MHz. Performance data from two microstrip circuits manufactured according to the design specifications demonstrate that the coupling features follow theoretical predictions in the 1-7 Ghz range, where the ripple stayed within 1 dB. A directional coupler in particular displays a 2.25 dB ripple in the 1-10 Ghz range, better than both uniform and exponential line couplers.