Dual band antenna for radio terminal

Details Dual band antenna for radio terminal Dual band antenna for radio terminal

1999-02-19

2001-03-06

Wong, Don (Department: 2821)

Communications: radio wave antennas

Antennas

With radio cabinet

C343S895000, C343S725000, C343S729000

Reexamination Certificate

active

06198440

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dual band antenna for a radio terminal capable of efficient operation at two different frequency bands.
2. Description of the Related Art
In general, to implement a dual band antenna using a single antenna, an additional element such as a choke is required for enabling respective parts of the antenna to independently operate at different frequencies. U.S. Pat. Nos. 3,139,620 and 4,509,056 disclose an antenna employing a choke, to permit operation at multiple frequencies.
U.S. Pat. No. 4,509,056 (the '056 patent) discloses a multi-frequency antenna employing tuned sleeve chokes. FIG. 1 of the '056 patent illustrates a cross sectional view of a monopole antenna operating at dual frequencies. This antenna is suitable for a radio terminal in which the frequency is not isolated by harmonics and the frequency ratio is greater than 1.25. As illustrated, the antenna is composed of a common monopole antenna, a coaxial transmission line having an open end, a shorted end, and a ground plane.
In
FIG. 1
, a coaxial transmission line choke
12
i
is formed at the middle of the antenna and has an electrical length &lgr;/4 at the higher frequency band of the dual frequency band. At the higher frequency band, the &lgr;/4 sleeve choke
12
i
forms a very high impedance between the open end and an extension element
100
of the coaxial feed line, thereby preventing coupling therebetween. Accordingly, at the higher frequency band, only the portion represented by l functions as the antenna as illustrated in FIG.
1
. However, at the lower frequency band, the sleeve choke
12
i
does not serve as an isolation element so that the entire portion represented by P functions as a monopole antenna.
A drawback associated with the conventional dual band antenna employing a choke is that it is both complicated and large, as compared with a single band antenna. Further, the large antenna may be easily damaged by a trivial impact. In addition, the conventional fixed (i.e., irretractable) antenna may inconvenience a user in carrying the radio terminal.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a dual band antenna for a radio terminal, consisting of a retractable whip antenna and a helical antenna with irregular pitches, wherein the whip antenna is independent of the helical antenna.
To achieve the above object, there is provided a dual band antenna for a radio terminal, including a helical antenna having first and second helical portions having first and second pitches. The first and second helical portions being independently operable at different frequency bands. The dual band antenna further includes a whip antenna including a conductive core line, a conductive substance covering a first portion of the conductive core line to serve as a choke and an isolation element extending from an upper end of the conductive core line, for filling a gap between the conductive core line and the conductive substance, Wherein only the first portion of the conductive core line is operable at a first frequency band and the entire conductive core line is operable at a second frequency; and a fixing element for fixing the helical antenna and the whip antenna to the radio terminal, wherein the fixing element has an upper end connected to a lower end of the helical antenna and a through hole via which the whip antenna is inserted into an interior of the radio terminal. Here, the first pitch of the first helical portion is narrower than the second pitch of the second helical portion.
A feature of the present invention is that when the whip antenna is retracted into the radio terminal, only the helical antenna is operable and the isolation element of the whip antenna is located in the through hole of the fixing element, so as to decouple the whip antenna from the helical antenna.
A ratio of the first frequency band to the second frequency band is controlled by adjusting the number of turns of a coil constituting the helical antenna, while the first and second pitches of the first and second helical portions are fixed to specified values.
Further, the fixing element has screwed teeth formed at a lower, outer wall for fixing the fixing element to the radio terminal.


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