engleski

Self-oscillating push-pull active integrated antenna

A new design of a push-pull oscillating patch antenna was developed in order to increase the power radiated by the single active antenna element. The patch was excited at two points with signals having 180° phase shift. To maintain small dimensions the oscillator was integrated in a rectangular opening placed symmetrically inside the patch. Full wave analysis was used to analyze and optimize the dimensions of the patch and of the opening. The opening dimensions were determined by few opposite requirements. The opening has to be large enough for embedding the two transistors, but also as small as possible to reduce the disturbance of the current distribution on the patch and assure the excitation of the antenna in TM01 mode. The choice of the antenna operating frequency (2.1 GHz) is influenced by application of packaged transistors for prototype fabrication. The antenna dimensions have to be large enough to accommodate packaged devices. In the considered case the patch and the push-pull oscillator are two-port networks and the input impedance at one of the ports depends on the impedance connected to the other. Therefore, in contrast to an active antenna with a single active device, the oscillator and the patch can not be analyzed separately and then just connected together. A system of two active elements may operate in odd and even modes on the design frequency and its harmonics. The coupling between the active elements of the push-pull oscillator, which will be integrated in the patch, is determined by the microstrip line connecting their bases. The reflection coefficient at the oscillator outputs was calculated for different coupling conditions. First, a half wave microstrip line was used. This configuration showed negative resistance, and thus possible oscillations, in four frequency bands between 0.5 and 5.5 GHz. By connecting a resistor from the center of the line to the ground plane the negative resistance in two bands was removed. To reduce the overall dimensions of the oscillator the coupling line and the destabilizing stubs were integrated in one. Thus, a shorter line giving suitable coupling coefficient phase and destabilizing inductive impedance at the transistor bases was used. When resistors were connected from the center of this line to the ground plane, the negative resistance occurred only in the frequency band of interest. The phase of the transmission coefficient has to be around 180° for operation in push-pull mode. This is satisfied around the resonant frequency of the TM01 mode on the patch. To verify the theoretical results an active rectangular patch antenna with push-pull oscillator was manufactured. Two bipolar transistors were embedded in a rectangular opening inside a rectangular patch. The transistors were placed along the patch resonant dimension. Common collector configuration was used because it allowed simpler biasing in comparison to common base configuration. Frequency tuning was obtained by changing transistor dc bias voltage. The dependence of the operating frequency and radiated power on bias voltage was measured. The available radiated power from the active antenna is nearly doubled in comparison to an oscillating patch with single transistor. The E- and H-plane radiation patterns were also measured. There were practically no changes in the radiation pattern shape and cross-polarization levels with changing the bias voltage. The cross-polarization levels were below -21 dB in E-plane and bellow -19 dB in H-plane for all angles. Throughout the tuning range this active antenna showed good spectral purity, stable operation, and good radiation properties.

antenna; active antenna; active integrated antenna; oscillator; push-pull oscillator

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