Miniaturized designs often require the integration of multiple devices into a common, compact structure. With this in mind, the authors pursue an active antenna design with integrated oscillators with a nominal operating frequency of 2.45 GHz. Voltage series feedback is used to amplify the unstable regions of the active device while also maximizing input and output reflections. The design includes a stripline feedwire antenna as an output component of an unstable active device with a coupling effect between the antenna and the neglected active RF circuitry. The output power at the input of the antenna is optimized with constraints that are affected by specific phase noise and harmonic levels. In order to evaluate the active antenna oscillation characteristics without interference from the radiation characteristics, the calibrated sensor is placed at the radiation edge of the antenna, which has the highest voltage. As shown, the target design specifications are met after the oscillation frequency adjustment is achieved.
The oscillator-type active microstrip antenna integrates an active device with a microstrip antenna to generate steady-state oscillations. The oscillator uses the negative-impedance characteristics of active devices to convert DC power to RF power. An integrated version of this active antenna has been developed for sensor applications at low power levels. Further research has managed to overcome the power limitations of this solid state source design due to the combination of space power technology. The oscillator includes an active device that incorporates a microstrip antenna that is both a load that determines the oscillation frequency and a device that generates RF power to the space radiation. Proper selection of the operating point of the active device is important for performance.
For an oscillator-type active microstrip antenna, the active device may be a two-terminal device, such as an IMPATT device and a Gunn diode, or a three-terminal device such as a metal-epitaxial-semiconductor field effect transistor (MESFET), high. Electron mobility transistor (HEMT), and heterojunction-bipolar transistor (HBT) devices. In general, each type of solid source has advantages and disadvantages. The two-terminal device is suitable for high-power applications with millimeter-wave frequencies, but with low DC-to-RF conversion efficiency, careful attention must be paid to heat dissipation in circuit and system design. On the other hand, three-terminal devices can provide high DC to RF conversion efficiency and low noise figure, but reduce power consumption levels.
Microstrip antennas have the advantages of moderate size, small form factor, and planar shape, resulting in low production costs. The planar structure is also suitable for integrating related electronic circuits, such as in the form of active antennas. This paper reports an experiment developed for local wireless local area networks (WLANs) and Bluetooth active transmit antennas. The antenna is an oscillator-type microstrip source antenna operating near 2.45 GHz, which is connected to a two-terminal unstable active device. The active device is directly integrated with the rectangular patch antenna, except for one that introduces a short microstrip line between the antenna input port and the active device for measurement. In general, feeder loss is considered negligible during this design process, but it is included in this paper.
All wiring antennas and oscillator design steps are performed in parallel. The radiation effects of the antenna feed are introduced next to the antenna, and the input impedance variation at the feed serves as an input parameter to the oscillator design. Voltage series feedback is used to maximize the dynamic range of the oscillator output and to ensure that it remains in the most unstable region of the active device to meet the oscillation conditions.
The antenna is considered to be a single-ended input (it is also considered to have two or more input ports), and all related results are converted to the RF circuit simulator in the frequency band of interest. However, the coupling effect between the antenna and other RF circuit components such as matching devices and DC feeders is ignored. The current simulator is used to implement the design to predict the required oscillation frequency and then optimize. Thereafter, nonlinear simulations are implemented to predict oscillation conditions, phase noise, and power performance.
Simulation and analysis of antenna characteristics including feeder and oscillating circuits was performed using Agilent Technologies' Advanced Design System (ADS) design software tools. 10 It should be noted that the antenna is modeled using the Momentum software package, which is included in the ADS. The oscillation frequency is fine-tuned and controlled by inserting a capacitor into the drain pin of the GaAs MESFET active device to meet the design goals (see table below). It has been observed that the obtained oscillating frequency range deviates from the maximum of the 2.45 GHz center frequency by approximately 6.87%, with low phase noise and acceptable output power.
The sensor correction factor is used to determine the frequency and forward power measured by the antenna input port, which is estimated when the antenna and oscillator circuits are truncated. The sensor is a small 3&TImes; 5 mm path that is placed on the edge of the antenna to produce the highest voltage. The distance between the sensor and the edge of the antenna is optimized to not affect the input return loss of the antenna port, and the linear condition of the correction factor is also satisfied. It was found that a distance of 2 mm (adjusted experimentally) was required for a coupling of approximately -22.6 dB near the resonant frequency of the stripline between the sensor and the antenna. The sensor path is also connected to ground through a 50-O load to improve the output matching of the sensing circuit. The second pin connects the sensing path to the coaxial probe on the back of the board, which connects the sensor output to the spectrum analyzer. The included 50-O resistor ensures that the sensor is functioning correctly and also ensures that the sensor's output connector appears as a relatively well-matched source. This will reduce the errors that can be caused by connecting it to a poorly matched power meter or spectrum analyzer. First, when the antenna is cut off from the active RF circuit, the correction factor is measured: then, reconnected to measure the output power of the oscillator.
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