High Efficiency Bandwidth VHF Electrically Small Antennas Through Direct Antenna Modulation

The increasing need to improve the bandwidth and efficiency in antennas is a continuous topic of research in RF systems. However, when it comes to physical platforms that are electrically small relative to the desired frequency of operation, balancing miniaturization, bandwidth, and efficiency is challenging and often counterintuitive. To overcome these challenges, this article proposes utilizing a direct antenna modulated (DAM) electrically small antenna (ESA). Here, we propose using efficiency–bandwidth (EB) product in theoretically analyzing and in measuring the improvement of DAM over a conventional capacitively loaded loop antenna (CLLA). To quantify the benefits of DAM, a prototyped antenna with DAM ( $0.0275\lambda \times 0.0251\lambda$ ) was designed and it successfully transmits a binary-frequency shift keying (BFSK) signal centered at 75 MHz. Though transistor parasitics reduce the possible EB product improvement, $\boldsymbol {\beta }$ , of DAM, measurements show the key importance of maintaining proper switching synchronization to achieve system bandwidths beyond the antenna’s impedance bandwidth. The DAM system achieves significant measured EB product improvement, ${\beta }$ , up to 5 over bit rates of 700 kb/s–7 Mb/s.