In this work, two novel oscillating antennas were proposed and experimentally tested. The first oscillating antenna was realized by integrating a negative resistance transistor oscillator and a line impedance transformer inside a rectangular opening in a rectangular patch antenna. The line impedance transformer, connected between the oscillator and the patch, allowed suitable impedance matching for oscillation start-up and stable operation of the oscillating antenna. The changes of the operating frequency and of the radiated power with dc bias voltage were measured. The change of the operating frequency was very linear. The measured co-polarization radiation patterns in E and H planes were symmetrical. The measured cross-polarization levels in both planes were low. This oscillating antenna showed clean spectrum and stable operation in the whole bias voltage tuning range. Two of these antennas were integrated in arrays in E and H planes. Symmetrical beam scanning was obtained for both arrays by adjusting the dc bias voltages of the oscillating antennas. The obtained efficiency of spatial power combining was high. The second oscillating antenna was constructed by embedding a pushpull oscillator inside a rectangular opening in a rectangular patch antenna. Suitable impedance matching between the oscillator and the patch was achieved by choosing the dimensions of the rectangular opening. The patch antenna was excited in two symmetrical points on the line of symmetry along the resonant dimension. By using the push-pull oscillators with two transistors the power, radiated by the antenna, was increased. A method for suppressing undesired operating modes of the oscillating antenna was proposed and experimentally verified. The measured change of the operating frequency with bias voltage was linear. In the whole bias voltage tuning range the antenna showed clean spectrum and stable operation. The measured co-polarization radiation patterns in E and H planes were not affected by the modifications in the patch and by the oscillator circuit. Symmetrical antenna excitation and symmetrical design of the push-pull oscillator resulted in low cross-polarization levels in both planes. This antenna was integrated in a two-element array in H plane and in two-, three-, and four-element arrays in E plane. All four arrays showed stable operation and symmetrical beam scanning was obtained by adjusting the bias voltages of the oscillating antennas. All arrays had clean spectrum and showed very good efficiency of spatial power combining. The theoretical analysis, calculated and measured results confirmed that both proposed oscillating antennas maintained small dimensions and that their radiation properties were not affected by the modifications of the patch antennas and by the oscillator circuits.