Capacitive Coupled Extendable Antenna for Portable Communication Devices

1. Field of the Invention

The present invention relates generally to the field of antennas and more particularly to an extendable antenna for use with portable radio communication devices.

2. Description of the Prior Art

Extendable antennas are widely used for receiving and transmitting radio frequency signals in portable radio communication devices such as cordless telephones and cellular/PCS (Personal Communication Service) telephones. Generally, such extendable antennas operate in an extended position when the telephone is in the "talking" or "transmit and receive" mode, but must remain functional in a retracted position to allow the telephone to receive an incoming call while in the "stand-by" or "receive only" mode. The extendable antennas are typically implemented in a dual-antenna configuration, as shown in FIG. 1A and 1B, where a quarter-wavelength helical antenna element 12 and a quarter-wavelength whip antenna element 14 are separated from each other at an interval such that the whip antenna element 14 is operational when the antenna 10 is in the extended position while the helical antenna element 12 is operational when the antenna 10 is in the retracted position. The whip antenna element 14 and the helical antenna element 12 do not affect performance and operate independent of each other, with the whip antenna element 14 being active during the transmit and receive mode of operation and the helical antenna element 12 being operative for receiving the incoming signals during the stand-by mode of operation.

However, since such conventional antennas are operated in the retracted position only by an inherently less efficient quarter-wavelength helical antenna, they generally suffer from narrow operating bandwidth and low radiation efficiency in the retracted or stand-by mode of operation. Accordingly, the present invention aims to provide a capacitive coupled extendable antenna which is capable of operating with wider operating bandwidth and improved radiation efficiency while achieving more stabilized operating characteristics and mechanical reliability.

The present invention overcomes the preceding and other shortcomings of the prior art by providing an improved capacitive coupled extendable antenna comprising feeding means connected to the radio frequency circuit within a housing, quarter-wavelength helical antenna means disposed in electrically insulated relation with the feeding means for providing capacitive coupling therebetween, and quarter-wavelength whip antenna means movable between an extended position and an retracted position through the helical antenna means. The whip antenna means is directly coupled to the feeding means in the extended position and capacitively coupled to the feeding means in the retracted position. The whip antenna means in the retracted position is short-circuited at a point of 90.degree. in electrical phase so as to form a .lambda./4 balun. In the preferred embodiment, the whip antenna means is enclosed within an insulator tube and adapted for longitudinal movement between the extended position and the retracted position through the cylindrical guide provided in the housing. The insulator tube provides capacitive coupling between the whip antenna means and the feeding means when the whip antenna means is in the retracted position. A stopper connected to the lower end of the whip antenna means prevents the whip antenna means from being fully removed from the housing and provides direct coupling between the whip antenna means and the feeding means when the whip antenna means is pulled out of the housing to the extended position.

With such an arrangement, the whip antenna means is operative through direct coupling with the feeding means when the whip antenna means is in the extended position and the helical antenna means is operative through capacitive coupling with the feeding means when the whip antenna means is in the retracted position. This capacitive coupling between the feeding means and the helical antenna means operates to compensate for the inherently deficient capacity component of the helical antenna means, thereby improving its operating characteristics. The performance of the helical antenna means is also stabilized since the whip antenna means in the retracted position forms a .lambda./4 balun. The resulting extendable antenna provides wider operating bandwidth, improved radiation efficiency, and more stabilized operating characteristics in the retracted mode of operation. Additionally, the present extendable antenna is simple in structure and mechanically reliable.

These and other features and advantages of this invention will become further apparent from the detailed description and accompanying drawing figures that follow. In the figures and description, numerals indicate the various features of the invention, like numerals referring to like features throughout both the drawings and the description.

FIGS. 1A and 1B are simplified views of a prior art extendable antenna shown in the extended position and in the retracted position, respectively.

FIG. 2 is a partially cut-away elevational view of an extendable antenna according to the present invention, shown in the retracted position.

FIG. 3 is a cross-sectional view of the extendable antenna according to the present invention, shown in the retracted position.

FIG. 4 is an enlarged cross-sectional view showing details of the extendable antenna in FIG. 3.

FIGS. 5A and 5B are simplified views of the extendable antenna according to the present invention, shown in the extended position and in the retracted position, respectively.

FIGS. 6A and 6B are simplified equivalent circuit diagrams of the extendable antenna according to the present invention, shown in the extended position and in the retracted position, respectively.

Referring now to FIGS. 2 and 3, the extendable antenna of the present invention includes a helical antenna 21, a whip antenna 31 enclosed within a cylindrical insulator tube 23, and an elongated cylindrical guide 25 having an upper end portion inserted within the helical antenna 21 for fixed engagement therewith. Within the interior of the guide 25, the insulator tube 23 is disposed for longitudinal movement between the extended position and the retracted position. A stopper 24 is provided at the lower end of the insulator tube 23 and connected to the lower end of the whip antenna 31. The whip antenna 31 is preferably made of highly elastic nickel-titanium alloy to improve the restorability and the mechanical reliability of the antenna. In the preferred embodiment, both the helical antenna 21 and the whip antenna 31 have an electrical length of substantially a quarter-wavelength.

Now referring to FIGS. 3 and 4, the helical antenna 21 includes a cover 41 having a hollow interior portion and an opening at both ends. At one end of the cover 41, a metallic sleeve 46 is partially inserted along a portion of the interior of the cover 41 for fixed engagement therewith. The sleeve 46 is adapted to act as a feed point of the antenna and has an outer-threaded portion 46a for fixedly mounting the sleeve 46 onto the separate telephone housing (not shown) . A plate spring 47 installed on the inner periphery of the sleeve 46 restraints the stopper 24 from further outward movement when the extendable antenna is pulled out of the telephone housing to the extended position, thereby preventing the whip antenna 31 from being fully removed through the helical antenna 21.

The helical antenna 21 further includes a helical coil 42 wound about a first insulator 43 at a predetermined coiling interval along the helical recess formed on the outer surface thereof. The first insulator 43 is attached at its lower end to a metallic base 44, which is connected to one end of the helical coil 42 for electrical coupling. The metallic base 44 is electrically insulated from the sleeve 46 by a second insulator 45 fixedly installed within the upper end portion of the sleeve 46. The first insulator 43 and the metallic base 44 are disposed centrally within the interior of the cover 41 and axially aligned with the sleeve 46, forming a longitudinally thorough hole 44a through which the whip antenna 31 is extended outward from and retracted inward into the telephone housing.

FIGS. 5A and 5B show the extendable antenna of the present invention in the extended position and in the retracted position, respectively. FIGS. 6A and 6B are simplified equivalent circuits of the respective antenna arrangements of FIGS. 5A and 5B. Now referring to FIG. 5A, when the whip antenna 31 is fully extended from the telephone housing, the stopper 24 makes contact with the sleeve 46 and the junction therebetween becomes the feed point of the antenna. As a result, the whip antenna 31 in the extended position becomes operative for transmitting and receiving radio frequency signals. At the same time, since the metallic base 44 is capacitively coupled to the sleeve 46 by means of the second insulator 45, the helical antenna 21 is capacitively coupled to the whip antenna 31 in parallel with respect to the feed point of the antenna. Thus, in terms of the equivalent circuit as shown in FIG. 6A, the whip antenna 52 and the helical antenna 53 are coupled in parallel to the feed point 51 of the antenna.

On the other hand, when the whip antenna 31 is retracted into the telephone housing as shown in FIG. 5b, the whip antenna 31 is capacitively coupled to the sleeve 46 by means of the insulator tube 23, and the helical antenna 21 becomes operative through capacitive coupling between the sleeve 46 and the metallic base 44. With such an arrangement, the capacitive coupling between the sleeve 46 and the metallic base 44 operates to compensate for the deficient capacity component of the helical antenna 21 so that, the operational bandwidth of the present antenna in the retracted position becomes wider than that of the prior art extendable antenna. In addition, such capacitive coupling arrangement improves the radiation efficiency of the antenna in the retracted mode of operation. Furthermore, the operating characteristics of the present antenna in the retracted position can be made more stable than that of the prior art extendable antenna by making the capacitive coupled whip antenna 31 short-circuited at a point of 90.degree. in electrical phase so as to form a .lambda./4 balun. The .lambda./4 balun can be formed by providing a .lambda./4 stripline within the telephone housing. The .lambda./4 stripline is connected at one end to the sleeve 46 and is adapted to make contact with the stopper 24 at the other end when the whip antenna 31 is fully retracted into the housing. Thus, in terms of the equivalent circuit as shown in FIG. 6B, the present antenna in the retracted position operates only with the helical antenna 53 via the feed point 51 while the whip antenna 52 forms a .lambda./4 balun through the capacitive coupling with the sleeve 46.

From the foregoing it should be evident that there has been described a new and advantageous extendable antenna utilizing capacitive coupling method. In particular, the present extendable antenna operates over a wider operating bandwidth with improved radiation efficiency in the retracted mode of operation. The present extendable antenna is simple in structure and mechanically reliable, and exhibits more stabilized operating characteristics.

While this invention has been described with reference to its presently preferred embodiments, its scope is not limited thereto. The present invention can be implemented in various additional configurations and by utilizing other materials, mediums, devices, or structures exhibiting similarly desirable characteristics or traits. In particular, the electrical lengths of the helical and whip antennas in the present antenna are not limited to a quarter-wavelength, but may also be an integral multiple of that. The elements typically constructed of electrically conductive material may be fabricated with an insulator material coated with an electrically conductive material. The size, shape or location of the antenna elements, coupling means, and support structures may be varied, depending upon the particular operating frequency or the amount of coupling desired for a particular application.

It will now be apparent to one skilled in the art that many and other various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended, therefore, that all those changes and modifications as fairly fall within the scope of the appended claims be considered as part of the present invention. The scope of the invention is only limited insofar as defined by the following set of claims and all equivalents thereof.