158 OEM4 Family Installation and Operation User Manual Rev 12Appendix D MultipathFigure 64: Illustration of Quadrifilar vs. Microstrip Patch AntennaeD.2.3 Antenna Ground PlanesNearby objects can influence the radiation pattern of an antenna. Thus, one of the roles of the antennaground plane is to create a stabilizing artificial environment on which the antenna rests and whichbecomes a part of the antenna structure and its resultant radiation pattern.A small ground plane (relative to one wavelength at the operating frequency) may have minimalstabilizing effect, whereas a large ground plane (multiple wavelengths in size) will have a highlystabilizing effect.Large ground planes also exhibit a shielding effect against RF signal reflections originating below theantenna’s radiation pattern horizon. This can be a very effective low angle shield when the antenna iselevated on a hill or other structure above other reflecting surfaces such as vehicles, railway tracks,soil with high moisture content, water bodies, etc.One of the drawbacks of a "flat plate" ground plane is that it gives a “hard boundary condition”, i.e.allowing electromagnetic waves to propagate along the ground plane and diffract strongly from itsedge. The “soft boundary” condition, on the other hand, will prevent the wave from propagating alongthe surface of the ground plane and thereby reducing the edge diffraction effects. As a result theantenna will exhibit a completely different radiation pattern. The “soft boundary” condition istypically achieved by a quarter wavelength deep, transversely corrugated ground plane surface(denoted as “choke ring ground plane”). When the depth of the corrugation (choke rings) is equal to aquarter wavelength, the surface wave vanishes, and the surface impedance becomes infinite and henceprovides the “soft boundary” condition for the electromagnetic field. This results in modifications tothe antenna radiation pattern that is characterized by low back lobe levels, no ripples in the main lobe,Quadrifilar ElementsRadome Antenna PatchDielectricPatch Ground PlaneQuadrifilar Helix Antenna Microstrip Patch Antenna
