A Novel Continuous Direct Antenna Modulation System Through Varactor Diode Tuning

In this paper, we propose and evaluate a direct antenna modulation (DAM) system. We attempt to bypass the upper bound on efficiency bandwidth product (EBP) for an electrically small antenna, which is related to the lower bound on antenna Q established by Wheeler and Chu. We do this by using a time-varying system, breaking the time-invariance assumed in the derivation of these bounds. There is thus potential to see an efficiency bandwidth product that exceeds the limitation for an antenna of a given size. The proposed antenna is based on a top-loaded monopole antenna, and then utilizes varactor diodes in order to continuously vary the resonance frequency of the antenna as a function of time. We use a frequency modulated (FM) input signal that is synchronous to the tuning of the antenna resonance, matching the instantaneous frequency of the input signal to the current resonance of the antenna, thus enabling transmission of an arbitrary FM signal. We evaluate the performance of the antenna through use of full wave solver simulations, equivalent circuit model simulations, and prototype measurements. We demonstrate through both simulation and physical measurements that time variation in the antenna synchronized with the input signal improves the efficiency bandwidth product of the antenna in comparison to the linear time-invariant case.