We evaluate spectral efficiency limits of low power portable communication devices which are only constrained by fixed transceiver aperture G "subset&equal" real³ for mounting antennas. A new notion of antenna-space capacity of G naturally emerges from analysis of the achievable spectral efficiency. Antenna-space capacity measures the maximum growth of spectral efficiency that antenna configuration can achieve, given arbitrary number of antennas. Thus, the notion of antenna-space capacity also has a practical utility as a benchmark for design of actual antenna configurations. As a concrete example, we compute the antenna-space capacity of a circular geometry, where antennas can only be placed on the circumference of a circle. We show that uniform antenna placement on the circle is suboptimal antenna configuration for any fixed number of antennas, but comes arbitrarily close to optimum as the number of antennas grows unboundedly.