High Bandwidth Electrically Small Antennas Through BFSK Direct Antenna Modulation

The demand for antenna systems with higher bandwidth and higher efficiency all within a smaller physical form factor have significantly increased in the past decade. Meeting such simultaneous requirements are difficult because as the famous Chu's limit shows, scaling the antenna size reduces the antenna's radiation bandwidth due to the cubic fold increase in the radiation quality factor. Such antennas are operated within a linear-time invariant framework. In this work, we utilize the concept of Direct Antenna Modulation (DAM), a technique used to employ time-variance into the antenna system. In this methodology, the current or voltage is directly modulated by a switch, conserving the energy within the high Q system per cycle, preventing required recharge time to steady-state. In doing so, the switch modulates an input signal, transmitting a high-bandwidth signal with an electrically small antenna. We develop a circuit model for a capacitor loaded loop antenna with a switch. The switch modulates the loop's current in order to transmit an input binary frequency-shift keying (BFSK)signal at 50 MHz. Using DAM, transmission of a high-bandwidth signal was achieved. Although the electrically small antenna itself is inefficient, we show that such radiators can in fact transmit high-bandwidth signals. We show that when compared to traditional electrically small antennas, we obtain a 45-efficiency bandwidth product improvement. This result proves that even in small platforms where available volumes are limited or where antenna sizes are scaled to small physical dimensions, high data rate transfers are possible through time-variant systems.