Patent Application
20140300523
Radio frequency (RF) based systems are used for communicating with various types of access controls, logging systems, cashless payment systems as well as transferring data between units such as pictures between cameras in cell phones. The basis of these systems is the RF electronics unit and an antenna. The antenna generates a signal for communication with the card or other device. This antenna is typically a few inches on a side. Because of the small size relative to the operating frequency, these antennas are tuned with a matching network and the coil is a small number of turns of wire or turns on a printed circuit board.
A problem with using these antennas is that they need to be kept away from metal. Typical recommendations are that the antennas be kept at least four inches from a metal surface. Unfortunately, this is not practical for many systems and an alternate way of dealing with metal is needed. This is the object of the current invention.
There are ways of overcoming mounting issues which fall into the categories of nonmetal housings and ferrite shields. Some of the deficiencies of these approaches are that security requirements may make it difficult to employ them. A plastic housing and antenna mount is vulnerable to an attack which would render it useless. There is a similar problem with ferrite shields. The ferrite shields are placed behind the antenna and in front of the metal mounting surface. This then requires an additional nonmetallic cover over the antenna and it has the same issues as for the plastic cover. These limits may not be an issue in, for example, cell phone applications where the back cover of the phone protects the antenna but it is an issue in industrial and exterior applications.
The deleterious effect of the metal is that it carries electrical current generated from the antenna. This current loads the antenna and reduces or stops the radiation of the desired signal which needs to get to the device to be communicated with. The mechanism employed by the ferrite shielding is to divert the RF field away from the metal and thereby eliminate the generated currents in the metal. Since it is desirable to have the antenna in the plane of the metal surface or below it, an alternate approach is needed.
The present invention stops the current from flowing through the metal plate by providing a slot from the center opening to the edge of the plate thus preventing the circulating currents which load the antenna. In this manner, the effect of the metal plate can be minimized almost completely.
Persons of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons.
With the structure as described, an appropriately shaped antenna can be placed in the opening and the antenna will perform well even in the presence of other metal. When there is a nonmetallic mounting structure, the antenna can be placed behind the plate and a rectangular antenna will perform well even behind the circular opening.
If a ferrite shielding box surrounds the sides and back of the antenna, the slotted metal plate will allow good performance of the antenna even with metal enclosures such as outlet boxes.
With the ferrite shielding, the antenna can be placed on a solid metal plate and then covered with the slotted metal plate. This provides good performance and excellent protection to the antenna since it is behind the metal.
In the embodiments of
The various illustrative non-limiting embodiments of the invention shown herein have been disclosed in connection with an enclosure or mounting box. Persons of ordinary skill in the art will appreciate that an enclosure is not necessary for practicing the invention. Actually, a plastic enclosure is the same as no enclosure. The assembly could simply be fastened to a wall without employing a mounting box or enclosure. In other embodiments, the assembly could be built into a piece of equipment without a separate enclosure for the antenna.
While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.
This application claims the benefit of U.S. Provisional Application No. 61/810,231, filed Apr. 9, 2013, the disclosure of which is hereby incorporated in its entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61810231 | Apr 2013 | US |
