Toward a Spectrally Efficient FSK Direct Antenna Modulation

Antennas in platforms where the physical dimensions are significantly smaller than the wavelength of the frequency of interest are limited to narrowband operations. A technique that enables transmission of such signals through narrowband antennas is called Direct Antenna Modulation (DAM). DAM utilizes switching circuitry to directly modulate the antenna at its maximum energy moments synchronized with the input RF signal. In this paper, we propose a method of attaining higher-order, N, frequency shift keyed, NFSK, modulation through a high Quality-factor electrically small antenna to achieve higher bit rates and better spectral efficiency within an instantaneous bandwidth. A DAM antenna transmitting a frequency shift keyed signal using four carrier frequencies, 4FSK, was designed, prototyped, and measured. Its electrical size is $0.018\lambda \times 0.02\lambda$ . A pseudorandom 8-bit and 16-bit sequence were transmitted through the 4FSK DAM antenna and received in the far-field. The results show that such a topology provides a roadmap in attaining even higher modulation orders, thus, higher spectral efficiency.