A directional coupler using the microstrip lines for large RF power measurement applications such as the plasma processing is proposed. This coupler consists only of a planer edge coupled microstrip line, which is about 1/300 long of wavelength in coupled section at 13.56MHz. The transmission model analysis is employed for analyzing and designing of this coupler and shows stable operation can be performed in the varying of the load impedance. The lumped circuit equivalent model is also proposed for a convenient analysis. Experimental results of the coupler are 20dB of directivity and the 50dB of coupling factor at the design frequency. These are good agree with the theoretical results.
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