Design and implementation of VHF-UHF antenna with non-foster matching circuit

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2018
Aşcı, Cihan
Matching networks are widely used in the antenna transmitter and receiver applications and thus they are an essential part of the RF system. Conventional passive matching networks are very broadly used for matching an antenna for a narrow band of frequencies; however, achieving a broad bandwidth characteristics for electrically–small antennas (ESAs) is not possible with the use of passive matching circuits. ESAs possess a large input reactance and the electrical size of the antenna element is very small compared to the wavelength of operation. Non–Foster matching networks can overcome the limitations of passive matching networks and the problems due to electrical size of ESAs. The design and implementation of non–Foster matching networks realized by transistor negative impedance converters is presented in this thesis. Non–Foster matching provides a reactive cancellation in the antenna impedance, and thus, overcoming the gain-bandwidth limitation accomplishes a broadband operation. The non–Foster matching network is designed for an electrically–small monopole antenna in the VHF/UHF band. The overall performance is evaluated between 100 MHz and 900 MHz for different input power levels. The designed non–Foster circuit can provide impedance matching up to a bandwidth of 600 MHz. Between 100 MHz and 900 MHz, it is observed that the imaginary part of the input impedance has been significantly reduced. The designed non–Foster circuit provides improvement not only for return loss but also antenna gain. The gain measurements are taken in between 150 MHz and 500 MHz where 5 to 12 dB gain improvement is recorded. In addition to the non–Foster matching, a reactively–loaded monopole antenna whose physical dimensions are different is also designed using a genetic algorithm (GA) as a comparative study. Among three different reactive loading designs, for the third design for the thin monopole antenna, an antenna gain better than –10 dB is obtained in between 200 MHz and 430 MHz. Two reactively–loaded antenna designs have roughly 200 MHz bandwidth and for one other antenna design, 290 MHz bandwidth is achieved.

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Citation Formats
C. Aşcı, “Design and implementation of VHF-UHF antenna with non-foster matching circuit,” M.S. - Master of Science, Middle East Technical University, 2018.