Novel 90° power-coupling concept based on broadside-coupled coplanar transmission lines for new VHF power transmitter family

Broadside-coupled coplanar transmission lines are employed as 90° power combiners. By optimising the geometry, tight coupling of up to -3dB at 98 MHz can be obtained, while preserving the high power handling capabilities of 2.5kW ≅ 1kV_pp. Furthermore, a novel variation of this topology, with a lateral offset between the coplanar lines, permits to obtain equal phase velocities for odd and even mode, thus allows to construct highly directional couplers (directivity>34dB). A new series of low-power VHF FM radio transmitters (500W, 1kW) is constructed around paralleling 250W RF power blocks using these couplers.