FM broadcast stations generate reverse intermodulation signals that cause communication problems between ATC controllers and pilots in the Air Traffic Control systems (ATC) inThailand. In this paper proposes a balanced RF power amplifier using the improvement 3-dB quadrature hybrid couplers to reduce the reverse intermodulation of FM broadcasting systems. The mathematics analysis of the balanced RF power amplifier for two closely located FM stations shows that can reduce the reverse intermodulation products. A reverse signal from nearby FM stations travels into two amplifiers of a balanced amplifier that create intermodulation signals. The quadrature hybrid coupler in a balanced RF amplifier to provide intermodulation signals of two amplifiers have a 180˚ out-of-phase which cause reducing intermodulation signals. In this paper designs 3-dB quadrature hybrid couplers for a proposed RF balanced amplifier. The low-cost PCBs are used to make prototype couplers. The proposed couplers are provided experimental results of return losses that are lower than -15 dB. The directed port has a coupling coefficient -3.3 dB and the coupled port has a coupling coefficient -3.4 dB. The prototype couplers are applied to a balanced amplifier that obtains a maximum output power 210 watts with 17.20 dB gain. A proposed balanced RF amplifier is measured the reverse intermodulation products of with a situation of two closely located FM broadcast stations to compare with a single-stage class-C amplifier. The experimentation demonstrates of the transmitter using a balanced RF power amplifier that can reduce an intermodulation product 18.47 dBc.

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