A Novel Continuous Direct Antenna Modulation System Through Varactor Diode Tuning

In this article, we propose and evaluate a direct antenna modulation (DAM) system. We attempt to bypass the upper bound on an efficiency bandwidth product (EBP) for an electrically small antenna (ESA), 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 EBP 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 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 the transmission of an arbitrary FM signal. We evaluate the performance of the antenna through the 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 EBP of the antenna in comparison to the linear time-invariant (LTI) case.