Retractable dual-band antenna system with parallel resonant trap

Dual-band retractable radiotelephone antennas have an elongated antenna element, a top load element, and a trap positioned between and electrically connected to the elongated antenna element and the top load element. The trap facilitates obtaining half-wave monopole performance at a first frequency band and half-wave monopole performance at a second higher frequency band.

FIELD OF THE INVENTION
 The present invention relates generally to radiotelephones, and, more
 particularly, to retractable antenna systems for use with radiotelephones.
 BACKGROUND OF THE INVENTION
 Radiotelephones generally refer to communications terminals which provide a
 wireless communications link to one or more other communications
 terminals. Radiotelephones may be used in a variety of different
 applications, including cellular telephone, land-mobile (e.g., police and
 fire departments), and satellite communications systems.
 Many radiotelephones, particularly handheld radiotelephones, employ
 retractable antennas which may be extended out of, and retracted back
 into, the radiotelephone housing. Typically, retractable antennas are
 electrically connected to a printed circuit board located within the
 radiotelephone housing that contains radio frequency circuitry. The
 antenna and the radio frequency circuitry are typically interconnected
 such that the impedance of the antenna and the impedance of the radio
 frequency circuitry are substantially matched. Because radiotelephones use
 50 ohm (.OMEGA.) impedance coaxial cable or microstrip transmission lines
 to connect the antenna to the radio frequency circuitry, such matching
 typically involves mechanically adjusting or electrically tuning the
 antenna so that it exhibits an impedance of approximately 50 ohms at its
 connection with the coaxial cable or microstrip transmission line.
 Unfortunately, matching the impedance of a retractable antenna may be
 difficult because the antenna impedance may be dependent on the position
 of the antenna with respect to both the housing of the radiotelephone and
 the printed circuit board which contains the radio frequency circuitry. As
 these respective positions change when the antenna is moved between the
 extended and retracted positions, an antenna typically exhibits at least
 two different impedance states, both of which should be matched to the 50
 ohm impedance of the feed from the printed circuit board. Accordingly,
 with retractable antennas, it is generally desirable to provide an
 impedance matching system that provides an acceptable impedance match
 between the antenna and the radio frequency circuitry, both when the
 antenna is retracted, and when the antenna is extended.
 "Dual-band" radiotelephones transmit and receive signals in two or more
 separated frequency bands. Exemplary dual-band radiotelephones are those
 used with various satellite communications systems that employ widely
 separated transmit and receive frequency bands (e.g., 800 MHz and 1900
 MHz). High performance 800 MHz radiotelephone antennas often take the form
 of a top loaded half-wave monopole. A helical top loading section may be
 used to mechanically shorten the antenna structure while maintaining the
 performance of a half-wave antenna. In the retracted position, the helical
 top loading section performs as a quarter-wave helical antenna. Dual-band
 performance may be achieved by either using a parasitic element in the
 helical top loading section that is resonant at 1900 MHz, or by inducing a
 secondary resonance in the helical top loading section at 1900 MHz.
 Unfortunately, it may be difficult to deliver sufficient power to resonate
 the parasitic element or the helical top loading section at 1900 MHz when
 the antenna is in an extended position. As a result, performance
 approaching a half-wave monopole at 1900 MHz may be difficult to achieve.
 Performance may often be better when the antenna is in a retracted
 position. Furthermore, severe constraints may be placed on a matching
 network to achieve the band width and power transfer necessary for
 satisfactory dual-band radiotelephone performance.
 SUMMARY OF THE INVENTION
 It is, therefore, an object of the present invention to provide retractable
 radiotelephone antennas that take the form of a top loaded half-wave
 monopole at one frequency band yet can still realize half-wave performance
 at a second, higher frequency band.
 It is another object of the present invention to provide dual-band
 retractable radiotelephone antennas without requiring complex impedance
 matching systems.
 These and other objects of the present invention are provided by a
 dual-band retractable radiotelephone antenna configured to radiate in
 separate frequency bands and having an elongated antenna element, a top
 load element, and a trap positioned between and electrically connected to
 the elongated antenna element and the top load element. The top load
 element includes a helical coil having a center axis generally parallel
 with a longitudinal direction of the elongated antenna element. A
 parasitic element is positioned adjacent to the coil to facilitate
 dual-band operation.
 The trap includes an inductor element and a capacitor element electrically
 connected in parallel. The trap is configured to have a predetermined
 first impedance at one frequency band and a second predetermined
 impedance, greater than the first impedance, at the second frequency band.
 The trap may be configured to be resonant at the second frequency band.
 The trap allows the elongated antenna element and the top load element to
 have a combined electrical length of approximately one-half a wavelength
 of a center frequency of the first frequency band. The trap also allows
 the elongated antenna element to have an electrical length of
 approximately one-half a wavelength of a center frequency of the second
 frequency band. Accordingly, the present invention provides a dual-band
 radiotelephone antenna with half-wave monopole performance at a first
 frequency band and half-wave monopole performance at a second, higher
 frequency band without requiring a complex impedance matching system.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention now will be described more fully hereinafter with
 reference to the accompanying drawings, in which preferred embodiments of
 the invention are shown. This invention may, however, be embodied in many
 different forms and should not be construed as limited to the embodiments
 set forth herein; rather, these embodiments are provided so that this
 disclosure will be thorough and complete, and will fully convey the scope
 of the invention to those skilled in the art. Like numbers refer to like
 elements throughout.
 Referring now to FIG. 1, a conventional radiotelephone 5 includes a handset
 unit 6 enclosed within a housing 7. The housing 7 encloses a transceiver
 that enables the radiotelephone 5 to transmit and receive
 telecommunications signals. A keypad 8, display window 9, and retractable
 antenna 10 for receiving telecommunications signals, facilitate
 radiotelephone operation. Other elements of radiotelephones are
 conventional and need not be described herein.
 Referring now to FIGS. 2A and 2B, a conventional dual-band retractable
 radiotelephone antenna 10 is schematically illustrated. The antenna 10
 includes a linear rod 12 slidably mounted within a radiotelephone housing
 14, and movable between a retracted position (FIG. 2A) and an extended
 position (FIG. 2B) via an aperture (not shown) in the housing. Mounted at
 an upper end 12a of the linear rod 12 is a top load element 16. The top
 load element 16 is configured as a helical coil 17 and has a center axis
 that coincides essentially with the longitudinal direction of the linear
 rod 12. One end of the helical coil is free-standing and the other end of
 the helical coil is electrically connected to the linear rod 12. As is
 known to those skilled in the art, dual-band operation may be achieved
 through the use of a parasitic element 18 positioned adjacent to the
 helical coil 17 and generally parallel with the center axis thereof.
 A matching network 20 is provided to match the impedance of the antenna 10
 to the 50 ohm impedance of the radio frequency (RF) circuitry (not shown)
 of the radiotelephone. The matching network 20 employs dual impedance
 matching circuits, one of which is associated with the retracted antenna
 position (FIG. 2A), and the other which is associated with the extended
 antenna position (FIG. 2B). In the retracted position (FIG. 2A), the base
 17a of the helical coil 17 presents a 50 ohm match to the terminal at 800
 MHz. Operation at 1900 MHz is supported by the parasitic element 18
 adjacent to the helical coil 17. Electrically, the linear rod 12 of the
 antenna 10 is connected at 1900 MHz and generates some level of
 performance degradation due to energy radiating away from it. In the
 extended position (FIG. 2B), the antenna 10 represents a half-wave
 monopole at 800 MHz which is matched to 50 ohms through the matching
 network 20.
 Referring now to FIG. 3, a parallel resonant trap 22 is positioned between
 the linear rod 12 of the antenna 10 and the top load element 16. The
 illustrated parallel resonant trap 22 includes an inductor element 24 and
 a capacitor element 26 connected in parallel with each other. However, as
 known to those skilled in the art, the parallel resonant trap 22 may be
 implemented as lumped elements, in printed wire board patterns, or as
 coaxial components, and the like. The parallel resonant trap 22 is
 configured to be resonant at 1900 MHz, thereby having a high impedance,
 yet have a relatively small impedance at 800 MHz. Accordingly, at 800 MHz
 the antenna 10 still performs as a half-wave monopole in the extended
 position. It is to be understood that the ratio of the inductor element 24
 and capacitor element 26 can be configured to allow the parallel resonant
 trap 22 to have low and high impedance at various selected frequencies.
 The relative dimensions of the linear rod 12 and the helical coil 17 are
 adjusted by the parallel resonant trap 22 so that the linear rod 12 is
 near a half-wave length at 1900 MHz and the helical coil 17 is near a
 quarter-wave at 800 MHz. At 1900 MHz, when the antenna 10 is in the
 extended position, the parallel resonant trap 22 prevents energy from
 entering the helical coil, due to the high impedance of the parallel
 resonant trap.
 Referring to FIGS. 4A-4B, the equivalent circuits at 800 MHz and 1900 MHz
 caused by the parallel resonant trap 22 are illustrated. At 800 MHz, the
 parallel resonant trap 22 has small reactance (low impedance) thereby
 allowing energy to reach the helical coil 17 (FIG. 4A). At 1900 MHz, the
 parallel resonant trap 22 effectively opens the circuit because of high
 impedance, thereby preventing energy from passing from the helical coil 17
 through the linear rod 12 (FIG. 4B).
 Accordingly, in the extended position, the antenna 10 performs as a
 half-wave monopole with a small series reactance at 800 MHz and as a
 half-wave monopole at 1900 MHz. In the retracted position, the helical
 coil 17 of the antenna 10 performs as a quarter-wave monopole at 800 MHz
 and as a quarter-wave monopole at 1900 MHz with the parasitic element 18.
 In the retracted position as illustrated in FIG. 5, the linear rod 12 is
 effectively electrically disconnected from the helical coil so that energy
 is not permitted to leak down the linear rod and be absorbed by the
 radiotelephone user's hand. Accordingly, the present invention can provide
 a radiotelephone antenna with half-wave monopole performance at 800 MHz
 and half-wave monopole performance at 1900 MHz without requiring a complex
 mechanical structure.
 The foregoing is illustrative of the present invention and is not to be
 construed as limiting thereof. Although a few exemplary embodiments of
 this invention have been described, those skilled in the art will readily
 appreciate that many modifications are possible in the exemplary
 embodiments without materially departing from the novel teachings and
 advantages of this invention. Accordingly, all such modifications are
 intended to be included within the scope of this invention as defined in
 the claims. In the claims, means-plus-function clauses are intended to
 cover the structures described herein as performing the recited function
 and not only structural equivalents but also equivalent structures.
 Therefore, it is to be understood that the foregoing is illustrative of
 the present invention and is not to be construed as limited to the
 specific embodiments disclosed, and that modifications to the disclosed
 embodiments, as well as other embodiments, are intended to be included
 within the scope of the appended claims. The invention is defined by the
 following claims, with equivalents of the claims to be included therein.