A new microwave exposure system for biological experiments with well-defined exposure conditions and improved control of the exposure parameters consisting of variable frequency power source, coaxial to waveguide transition, matching network and single-mode resonant cavity with movable shorting plunger was fabricated and characterized. The introduction of a biological sample into a resonant cavity has a large impact on its field configuration and port impedance. As such, the properties, geometry and position of the biological sample become a part of the electrical properties of the microwave circuit. With that change, the electrical properties of the resonant cavity, such as impedance, quality factor and resonant frequency, also change. In this study, an appropriate coupling system with effective power transfer and an algorithm to tuning and coupling of resonant cavity in resonance before and after the introduction of biological sample have been proposed. This procedure will lead to a known dose distribution within the biological sample and allow a better comparison with other studies. Coupling of the electromagnetic energy into a resonant cavity was experimentally investigated. Graphical representation of cavity impedance in case of undercoupled, critically coupled and overcoupled cavity has been presented. Critical coupling of an empty resonant cavity has been accomplished at voltage standing wave ratio (VSWR) 1.01, at resonance frequencies 900 and 947.5 MHz. Critical coupling with the introduction of a biological sample has been accomplished at VSWR ≤ 1.07 for frequency bandwidth 1 MHz and VSWR ≤ 1.5 for frequency bandwidth up to 5 MHz with central frequency 947.5 MHz.