A Compact High Data Rate 4-FSK HF Transmitter through Direct Antenna Modulation

Achieving high-bandwidth RF performance in electrically small and compact volumes remains a continuous challenge. The difficulty lies in the inability of electrically small antennas to radiate high-bandwidth RF signals efficiently because of their narrow bandwidth performance resulting from their high radiation Q-factors. Consequently, the RF signal is heavily distorted and/or reflected back to the source, while at the same time, the antenna suffers from minimal antenna gain. In this paper, we propose using a capacitively loaded loop antenna (CLLA) that utilizes Direct Antenna Modulation (DAM), enabling radiation of a high-bandwidth high data rate quadrature frequency shift-keyed (4-FSK) signal between 27 MHz and 39 MHz. The CLLA was integrated with two pairs of high figure-of-merit semiconductor transistors, modulating the RF signal to produce 4-FSK. The antenna ( $0.009\lambda\times. 013\lambda$ ) was prototyped and measured, showing a high-data rate 4-FSK transmission of 1.58 Mbps, beyond the inherent narrow bandwidth of the antenna. The design achieves superior high-efficiency bandwidth performance, enabling compact RF communication in electrically small volumes.