Aluminum (Al) has prominent material and plasmonic properties in the ultraviolet (UV) spectral range. However, the large losses of plasmon antennas with multicrystal Al is the bottleneck for plasmonic applications. Here, the plasmon properties of single-crystal and multicrystal Al nanostructures are compared. In the platform of bulk single-crystal Al, spatially and spectrally resolved cathodoluminescence (CL) spectroscopy is used to excite and image the plasmonic modes of I-shaped and cross-shaped nanoridge antennas. The evolution and coupling of plasmon modes at the nanoscale are clearly observed in these antennas. Plasmon modes for I-shaped antennas can be exactly understood from the standing waves of the propagating plasmons in Al nanoridge waveguides. Moreover, the plasmon response of cross-shaped antennas is determined by the two different standing wave paths. The experimental results agree well with full wave electromagnetic simulations. Our results lay a foundation for the design of more complex and efficient optical antennas with single-crystal Al, which have great potential applications in UV plasmonics, such as fluorescence enhancement or biosensing.