Electrical Separation and Fundamental Resonance of Differentially-Driven Microstrip Antennas

This paper studies the electrical separation and fundamental resonance of differentially-driven microstrip antennas with dual-probe feeds on both electrically thick substrate of high permittivity and electrically thin substrate of low permittivity. The electrical separation is defined as the ratio of the distance ξ of the dual-probe feeds to the free-space wavelength λ_o. It is found that the occurrence of resonance of the fundamental mode is related with the electrical separation of the dual-probe feeds. When the electrical separation ξ/λ_o ≥ ψ is satisfied, the resonance occurs. Otherwise, the resonance does not occur. It is shown that the empirical factor ψ is smaller for the electrically thicker substrate of higher permittivity than that for electrically thinner substrate of low permittivity and is smaller for the circular patch than that for the rectangular patch. To validate the relationship of the occurrence of fundamental resonance with the electrical separation, several differentially-driven microstrip antennas were fabricated on the electrically thin substrate of the low permittivity and measured. It is observed that the simulated and measured results are in acceptable agreement for these differentially-driven microstrip antennas. Thus, the electrical separation condition derived in this paper should be very useful in guiding the design of differentially-driven microstrip antennas.