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In this paper, we provide a detailed three-dimensional numerical analysis of the optical properties of common and modified bowtie aperture antennas integrated onto a silicon waveguide platform, to discuss the influence of geometrical parameters on the electric field enhancement factor and waveguide transmission when such antennas are excited by the evanescent field of the Si waveguide mode. We demonstrate that waveguide transmission is severely affected by the interference between Si waveguide modes and surface plasmon polariton modes excited in the antenna, while the antenna's field enhancement factor is mainly determined by the localized surface plasmon resonance occurring in its nano-gap. These mechanisms lead to a mismatch between the wavelength at which the antenna's field enhancement factor is maximized, and the wavelength at which transmission through the Si waveguide is minimized, suggesting that in some multi-mode cases, the optical properties of integrated nanoantennas determined through direct measurement of Si waveguide transmission may be misleading. Methods for improving the electric field enhancement (such that it has a bigger modulation depth) that have minimal impact on the resonant wavelength, and for improving the shape and location of the corresponding hot spot of the bowtie aperture antennas, are also discussed and analyzed. We believe that this analysis will be helpful in design of on-chip bowtie-type optical antennas for surface enhanced Raman spectroscopy, near-field optical microscopy, high sensitivity detection, and plasmonic optical tweezers.