Absolute Flux Density Measurement and Associated Instrumentation for Radio Astronomy below 100 MHz
Tillman, Richard Henry
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This dissertation reports new measurements of the absolute flux densities of the brightest astrophysical sources visible from the northern hemisphere with O[10%] accuracy between 30-78 MHz. These measurements provide additional confidence in the existing understanding of the flux density spectra of these sources in this frequency range. This dissertation also reports new measurements of the antenna temperature due to the diffuse Galactic background between 30-78 MHz, addressing a paucity of existing measurements in this band. These measurements are relevant especially in the context of contemporary interest in radio astronomy and 21 cm cosmology in this frequency range. A new active antenna system and measurement technique were developed to facilitate these measurements. The antennas are simple, thin dipoles, allowing for accurate characterization. Amplification is preceded by notch filters to mitigate interference induced non-linearity. Previous efforts have used well matched antennas. The narrowband antennas and notch filters on the front end create large, frequency varying impedance mismatch that must be accounted for, and we demonstrate how this can be done. We present a novel in situ technique that uses the antenna temperature measurements to improve the calibration of the antennas and internal noise sources.
- Doctoral Dissertations