Mixtenna: A Self-Biased Nonlinear Patch Antenna for Passive Third-Harmonic Radiation

TL;DR

This work tackles passive harmonic control in compact antennas by embedding a self-biased nonlinear load—a back-to-back Schottky diode clipper—into a rectangular patch antenna. A SPICE-assisted time-domain framework models the diode as a power-dependent nonlinear load and guides the design of a two-stage matching network to maximize third-harmonic generation while preserving fundamental performance. The authors validate the concept with simulations and measurements, achieving up to approximately $25\%$ conversion efficiency for the third harmonic at $f_0=925$ MHz, with $3f_0=2775$ MHz radiation that is directive thanks to a width-optimized radiator. This passive approach eliminates external biasing and demonstrates a viable path to frequency-agile, spectrum-efficient, compact radiators for multifunction wireless systems.

Abstract

A nonlinear rectangular patch antenna (RPA) is presented in which back-to-back Schottky diodes are embedded at high-field regions to enable passive, bias-free harmonic generation. The self-biased diodes introduce a power-dependent impedance that drives efficient frequency up-conversion and selective third-harmonic radiation. A tailored matching network enhances third-harmonic excitation and coupling while preserving radiation efficiency at the fundamental frequency. Analytical modeling combined with SPICE-assisted full-wave time-domain simulations predicts strong odd-harmonic content, and measurements on RPA prototypes employing SMS7630 diodes confirm these results. Simulated and measured S-parameters and far-field patterns at 925 MHz and 2.775 GHz show excellent agreement. The demonstrated approach establishes nonlinear loading as an effective mechanism for passive harmonic control in compact radiators, enabling frequency-agile and spectrum-efficient antenna systems.

Figures (16)

• Figure 1: A graphic explanation showing a harmonic signal exciting a non linear device. The output signal is modified by the I-V curve relations and produces a harmonic-rich signal.
• Figure 2: (a) Anti-parallel diode pair connected to a sinusoidal source with internal resistance $R_{\mathrm{src}}$. (b) Schottky diode large-signal equivalent model.
• Figure 3: (a) Comparison between analytical model and SPICE simulation of the clipper circuit. We compare the total current, $I_{\text{total}}$, results for input powers of -10 dBm and -3 dBm. (b) FFT results of the total current signals from the analytical model simulation. The fundamental frequency, $f_0 = 925$ MHz, and its third harmonic at $f_3 = 2775$ MHz are clearly visible. The second harmonic is completely canceled.
• Figure 4: (a) Simulation model of the PCB coupon used for diode impedance characterization. (b) Fabricated PCB coupon used for diode impedance characterization. The SMS7630 back-to-back diode pair is soldered on the right open-ended edge. (c) Are the Real and Imaginary part of Zin parts results vs. frequency for Pin=-30dBm (small signal condition). (d) Are the Real and Imaginary part of Zin parts results vs. frequency for Pin=0dBm (large signal condition).
• Figure 5: (a) The cavity model and its parameters. (b) RPA theoretical normalized radiation pattern in the XY plane of figure \ref{['fig:cavity_model']}. The RPA parameters are: W=113.25mm, L=93.5mm, h=1.52mm, f0=925 MHz.