A Cavity Balun for High-Power Shortwave Transmission

Compared with conventional systems, high power puts forward more requirements on the balun: the materials and structures should be more reliable, able to withstand high power, and have extremely high transmission efficiency. Based on the electromagnetic field theory and the visual optimization method by CST MW Studio, a parallel-connected coaxial cavity balun is proposed in this article that can realize the balance–unbalance conversion in almost the entire shortwave frequency band (4.4–27 MHz). For further improvement of the transmission efficiency and the reduction of voltage standing wave ratio (VSWR), an antenna tuning unit (ATU) was added to the unbalanced port of the cavity, which lowered the VSWR from 1.5 to 1.1. The theoretical value of the breakdown power of the balun is also calculated, and it is much larger than the transmit power. The results of the scaled mode measurement are consistent with the simulation results: the amplitude imbalance of the balanced port is within ±0.5 dB, the phase difference is 180° ± 5°, and the withstand power is at least three times the transmit power, which fully demonstrates that the proposed balun is suitable for high-power shortwave transmitting systems.