The present invention is directed to an automotive rearview mirror and, more specifically, to an automotive rearview mirror assembly that includes a helical antenna with a non-circular cross-section.
Various manufacturers of automotive rearview mirror assemblies, incorporating electrochromic (EC) mirror elements, have provided various receivers and antennas for those receivers within a housing of the mirror assembly. In general, loop, open wire and helical antennas with circular cross-sections have been implemented in automotive mirror assemblies incorporating EC mirror elements. Unfortunately, round helical antennas that are small enough to fit between a receiver printer circuit board (PCB) and a rear surface of an EC mirror element have generally been too small to provide adequate gain for many applications (e.g., tire pressure monitoring).
Further, loop antennas that are small enough to be useful in such applications generally have a high Q-factor, which makes them difficult to tune in a production environment and, in general, small loop antennas tend to have less gain than small helical antennas with a circular cross-section. Additionally, loop antennas experience transmission nulls when utilized in combination with receivers that receive signals from transmitters that rotate, e.g., tire transmitters. In general, the impedance of all antenna types are affected by EC mirror elements, which contain two or more highly conductive layers, when a given antenna is placed in close proximity to the EC mirror element.
What is needed is an antenna that is effective for receiving signals from transmitters that are rotating when the antenna is incorporated within an automotive mirror assembly that includes an electrochromic mirror element.
An embodiment of the present invention is directed to an automotive rearview mirror assembly that includes a mirror element, a housing for accepting and retaining the mirror element and a receiver circuit mounted on a printed circuit board (PCB) contained within the housing. The receiver circuit includes a helical antenna with a substantially non-circular cross-section, e.g., substantially one of a rectangular, an oval and an elliptical cross-section.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.