The protection circuit in wideband high-power radio frequency(RF)transmission sys-tems requires accurate RF signals collected in real time.The wideband high-power coupler,as the key device of the protection circuit,coupling's its flatness and directionality play a key role in the accuracy of the RF signal collected by the protection circuit.The directional coupler with coaxial structure has minimal insertion loss,and has minimal influence on the passing power after access-ing the RF channel.Broadband high flatness directional coupling is realized by using a transmis-sion-transformer resonant loop in the fitted network.The measured results show that,in the high-power directional coupler above 5 000 W in 80～1 000 MHz band,when the coupling is 50 dB,the flatness±0.3 dB,and the directionality＞20 dB,as well as the voltage standing wave ratio(VSWR)of the coupling port ≤1.25 and the insertion loss of the main channel ≤0.1 dB.The main channel VSWR ≤1.10.
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