Compact Water-based Microstrip Filters

Implementation of Microstrip circuits at low radio frequencies presents challenges due to the longer wavelengths, resulting in increased form factor. This can be solved by using a higher dielectric constant substrate. In this paper, we propose a novel approach of implementing microstrip circuits on a composite multilayer substrate based on water having a higher effective dielectric constant $(\epsilon_{\mathrm{e}ff}=26)$. We demonstrate the practical implementation of microstrip circuits on a composite multilayer substrate, by designing a 5th order Stepped Impedance Low Pass Filter with cut-off frequency at 350 MHz. The overall reduction in size of the filter designed on composite substrate is demonstrated by comparing it with a filter designed on a single layer dielectric medium (FR-4). Filters simulated on both the substrates have been optimized for identical electrical specifications, and prototypes have been built and measured to validate the design concept. Measurement and simulation results indicate that filter designed on the composite substrate is 2.5 times smaller in length than the filter designed on a single layer substrate. We propose to use this concept for designing compact microstrip circuits at low frequencies.