Measurements of the active properties of a Dayem micro-bridge at X-band frequencies is described. The bridge was mounted in a microwave cavity designed to match the bridge properly and the microwave output from the cavity was detected using a sensitive X-band spectrometer. Microwave power was detected from the freely running Josephson oscillations with the bridge dc-biased to emit at the receiver frequency and the maximum power extracted was of order 10-12W when the receiver was tuned to the cavity resonance. With an external rf-excitation of the bridge the amplitude of either the first or second harmonic response was measured. On the basis of analogue computer simulations an equivalent circuit was obtained describing the bridge coupled to the cavity. The large self inductance of the background film adjacent to the bridge was found to play a major role in explaining our results.