I present a PhD thesis with four primary contributions relevant to the commissioning of 21cm cosmology experiments, the first two being devoted to the characterization and mitigation of VHF-band radio frequency interference (RFI), and the last two being devoted to the characterization and mitigation of astrophysical foregrounds in radio interferometers, in the presence of mutual coupling between array elements. First, The SDR Pathfinder for Understanding Transient and Noise-level Interference in the Karoo (SPUTNIK) is presented. I describe how a low-cost RFI monitoring system, using solely consumer-off-the-shelf (COTS) components, directly contributes to the analysis efforts of a precision 21cm cosmology instrument. A SPUTNIK system overview is provided, as well as a generalized software-defined radio (SDR) internal calibration technique to achieve wideband, $\pm$1.5 dBm-level accuracy and a measured dynamic range of $\geq$40dB. Second, I present findings from a survey of the 50-240 MHz radio spectrum at various locations of the SKA-SA core site, the results of which informed the site selection for the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH), a global 21cm experiment.

Third, I derive a general formalism for interferometric visibilities, which considers first-order antenna-antenna coupling and assumes steady-state, incident radiation. I simulate such coupling features for non-polarized skies on a compact, redundantly-spaced array and present a phenomenological analysis of the coupling features. Contrary to previous studies, I find mutual coupling features manifest themselves at nonzero fringe rates. For all studied baseline lengths, baseline orientations, and local sidereal times (LSTs), coupling features appear at delays which are outside the foreground `wedge', which has been studied extensively and contains non-coupled astrophysical foreground features.

Fourth, I analyze HERA Phase II data for systematics related to mutual coupling between array elements. For all baselines and frequencies studied, I find coupling features to be consistent with predictions made from the semi-analytic model. To mitigate these effects, which if ignored could prevent HERA Phase II from placing competitive upper limits on (or making a detection of) the EoR signal, I apply a fringe-rate filter and baseline orientation de-selection strategy to both the observed and simulated data. While the noise floors of the observed power spectra are too high to directly compare improvements to simulation, through application of the mitigation strategy to simulated power spectra I find that, in both LST ranges analyzed, some $k-$ modes are consistent with having no coupling effects down to at least $\Delta 2=50[(mK)2h-3Mpc3k3/(2\pi 2)]$, a level consistent with fiducial models of the 21cm power spectrum.

The semi-analytic coupling model herein presented is not only helpful to the field of 21cm cosmology, but any study involving interferometric measurements, where coupling effects at the level of at least 1 part in $104$ could corrupt the scientific result. The model may be used to mitigate coupling systematics in existing radio interferometers and to design future arrays where the configuration of array elements inherently mitigates coupling effects at desired LSTs and angular resolutions.