High power, broadband, coaxial to microstrip bi-directional coupler in the VHF/UHF band

Abstract

Broadband couplers consist usually of multiple coupled lines that are each a quarter wave-length long. Herein, in contrast to conventional methods, a short length of a microstrip line is coupled to a coaxial line. The length of the coupled microstrip line is small compared to the wavelength at the lowest operating frequency. The coupling is weak and the microstrip line is λ/8 at the highest operating frequency. The proposed structure resembles a pair of coupled lines with a linear coupling response. The coupling exhibits a slope of 6 dB/octave over multi-octave bandwidth. The microstrip line is connected to a lumped element compensating network at the coupled port. The compensating network is a low-pass circuit with a slope of −6 dB/octave. The power handling of the coupler is high due to the coaxial line and the weakly coupled microstrip line. Electromagnetic simulations and analytical formulations are presented. The broadband coupler handles nearly 1 kW of input power. Due to the weak coupling of the microstrip-coaxial coupler, the lumped element circuit needs to handle <1.5 W. A high-power coupler has been fabricated for the frequency range of 30 to 500 MHz. It exhibits a coupling response of 56 ± 0.4 dB.