parasitic inductance and capacitance (see self-resonant frequency) of the antenna wire and the shelf steel limit the range of tuned frequency of any antenna circuit. a simple loop of speaker wire about 100 ft (30 m) in diameter maybe tuned to resonate at 131 khz with a simple external capacitor. a loop of only a 1-inch (25 mm) may be also tuned to resonate at 131 khz. at 30 mhz, however, you might be able to tune the 1 inch (25 mm) antenna, but not the 100 ft (30 m) antenna, and not the shelf. at 30 mhz the largest tunable loop is about . rubee's frequency is low on purpose so that it can nearly always re-tune to compensate for the parasitic inductance and capacitance despite use in harsh environments like steel shelves (see roche et al. 2007). back to the shelf example—the rubee installation actually tunes the steel in the shelf, and the shelf itself becomes the antenna - the shelf becomes part of the resonate circuit and the h signal gets stronger near the shelf. for frequencies over 1 mhz it's not possible to incorporate most things you find in a warehouse, office building or factory as part of the antenna.