BW enhancement Using Stacked Configuration or Parasitic Coupling

Stacked configurations are possible with aperture coupled feeding, proximity feeding and co-axial feeding. Probe feeding technique is re-emerging in variety of antenna system due to its robust nature. It provides good isolation between feed network and radiating elements and due to direct contact with the radiator reduces dielectric layer misalignment difficulties. It also yields good front to back ratio which is very important where multiple arrays are located back-to-back in closed proximity. Therefore stacked configurations with probe –fed have been considered. The combination of low dielectric constant and high dielectric constant can yield good impedance behavior. The broadest bandwidth can be achieved when the first-order mode on the lower patch is considerably greater in magnitude than corresponding mode on the top patch or in other words the top patch is loosely coupled. For this the substrate of lower patch should have higher dielectric constant than the upper substrate.

fig: Parasitically Coupled Coaxial fed Microstrip Antenna

Fig: Multilayer Stacked Configuration

BW Enhancement using Proximity Coupling

Proximity coupling is basically a feeding mechanism which gives the highest bandwidth and low spurious radiation. In general, we use two substrates of different thicknesses and permittivity. A short tuning stub is connected in shunt with feedline at some distance from edge of the patch. The fabrication of such an antenna is difficult. Length of feeding stub and the width-to-line ratio of the patch is used to control the match.

BW Enhancement by Impedance Matching Network

The method is unique in that it does not alter the radiating element itself. Instead, a reactive matching network is added to compensate for the rapid frequency variations of the input impedance. The validity of the technique is based upon the relative frequency insensitivity of the radiation pattern and gain characteristics as compared to the resonant behavior of the input impedance. Thus what happens is a matching network is so designed which alters its impedance in such a way that it acts as a parallel resonant circuit over a range of frequencies.

Fig:i) Basic form of impedance matching ii)Comparison of structure of impedance matched antenna and reference antenna