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Measurements of the complex permittivity of various concentrations of microbubbles in ethylene glycol liquid phantom have been carried out. A cavity perturbation technique using custom rectangular waveguide cavities, which are sensitive to small changes in the permittivity of the perturber, has been employed. Three different frequencies within the ultra-wideband (UWB) frequency spectrum have been used for the experiments. The results show that the concentration of the air filled microbubbles required to achieve a dielectric contrast as little as 2% exceeds the recommended dosage used in clinical ultrasound applications, by more than two orders of magnitude.