1. Technological Field
The present disclosure relates generally to an antenna apparatus for use in electronic devices such as e.g., wireless or portable radio devices, and more particularly in one exemplary aspect to a short-range antenna apparatus for use within e.g., a metal device or a device with a metallic surface, and methods of utilizing the same.
2. Description of Related Technology
Antennas are commonly found in most modern radio devices, such as mobile computers, portable navigation devices, mobile phones, smartphones, personal digital assistants (PDAs), or other personal communication devices (PCD). One area of increasing interest relates to short-range radio frequency interfaces, such as so-called “near field communication” (NFC) and radio frequency identification (RFID) systems. These interfaces are used to wirelessly transfer information, whether in an active mode or passive mode (or combinations thereof) between two entities, such as when one “bumps” their NEC-enabled smartphone against an in-store reader to complete a purchase transaction. Typically, antennas in such short-range interfaces comprise a planar radiating element (e.g., a loop) with a ground plane that is generally parallel to the planar radiating element. The term “near-field” refers generally to the fact the energy radiated by the antenna (or readable by the antenna) is stored in a spatially proximate region to the face of the antenna.
A typical NFC antenna may be for instance a planar loop with 3-5 rounds or loops of conductive trace, wherein the ground plane is generally parallel to the planar loop. This type of antenna requires a typical area on the order of 40×20 mm for adequate performance. However, in many metal cover devices, such a comparatively large metal-free area is not usually available within the cover. For instance, the available metal-free area may be as little as 5 mm in width, making such prior art loop-type antenna impractical.
When the X- or Y-dimension (i.e., planar dimension) of the aforementioned prior art antenna loop is decreased (e.g., to try to fit it within the aforementioned 5 mm-wide space), the parallel loop traces with opposite currents are disposed increasingly closer to each other. This reduced spacing causes cancellation of net current in the loop's generated near-field. This causes a decrease in loop inductance and so-called “Q” value or quality factor (in effect, a measure of the antenna's bandwidth and energy stored in the near field).
Some attempts to work around the foregoing limitations have included for example, increasing the amount of plastic or other dielectric material area in the metal cover, so as to allow for the larger form factor loop. However, one salient drawback of this approach is the decreased metal area, which leads to reduced mechanical strength and a visually less attractive device. Many device manufacturers will simply not tolerate the sacrifice in strength and aesthetics; the NFC antenna must accommodate the design and aesthetic dictated by the host device, and not the inverse.
Another prior art approach includes the use of a slot in the metal back cover of the device to excite image currents in the metal cover. However, this approach also produces less-than-optimal results, particularly with respect to producing a low inductance and Q-value, and hence poor antenna performance.
Accordingly, there is a salient need for a short-range antenna solution for use with, for example, a portable radio device having a small form factor metal body and/or external metallic surface that provides for the desired level of antenna performance, yet which is also compatible with the spatial and other constraints imposed by the use of a metallic cover.