GB2351849A

GB2351849A - Multi-band helical antenna with varying pitch

Info

Publication number: GB2351849A
Application number: GB0011790A
Authority: GB
Inventors: Guangping Zhou; Robert Kenoun
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1999-05-27
Filing date: 2000-05-16
Publication date: 2001-01-10
Also published as: CN1275824A; DE10025431A1; GB0011790D0; BR0002491A

Abstract

A helix antenna 129 for multi-band operation is achieved according to the present invention by varying the pitch angle, represented by s<SB>1</SB> and s<SB>2</SB>, on the helical coil in a suitable manner. In particular, by using an non-uniform pitch angle and keeping the total length of the wire constant, the second resonance of the antenna can be shifted high or low as a function of the pitch angle, while the first resonance is not affected. The diameter, d<SB>1</SB> and d<SB>2</SB> of the portions of the helical coil can be varied according to alternate embodiments of the invention. The antenna 129 is contained within an outer housing (Fig 11, 902) and connected to the wireless communication device(1130) via a monopole 802.

Description

2351849 HELICAL ANTENNA

Field of the Invention

This application is related to an antenna, and more particularly to a helical antenna adapted to operate in more than one frequency band.

Background of the Invention

With the increased use of wireless communication devices, spectrum has become scarce. In many cases, network operators providing services on one particular band have had to provide service on a separate band to accommodate its customers. For example, network operators providing service on a GSM system in a 900 MHz frequency band have had to rely on a DCS system at an 1800 MHz frequency band, Accordingly, wireless communication devices, such as cellular radio telephones, must be able to communicate at both frequencies, or even a third system, such as PCS 1900. Such a requirement to operate at two or more frequencies creates a number of problems. For example, the wireless communication device must have an antenna adapted to receive signals on more than one frequency band.

Also, as wireless communication devices decrease in size, there is a further need to reduce the size of an antenna associated with the device. Further, while an extendible antenna offers certain advantages, such an antenna poses problems to an end user. Because the antenna will typically perform better when in the extended position, the user is required to extend the antenna before operating the wireless communication device. As a result, many end users prefer a fixed or "stubby" antenna which do not need to be extended during operation. From a design standpoint, it is also considerably more difficult to match a retractable antenna in both the up and down positions when the antenna is designed to operate on two or more frequency bands.

A helix antenna may have several resonance frequencies as function of the wire length, pitch angle and the number of turns. Usually, these resonance frequencies can be adjusted by using proper techniques to obtain a singleband or a multi-band cellular operation. These techniques may include varying the helix wire dimensions and/or adding other components. A helix antenna having a uniform pitch angle can be tuned to operate at GSM band (900 MHz). However, the second resonant mode will not be at the DCS (1800 MHz) or PCS (1900 MHz) bands. Any changes in wire length, diameter and the pitch angle will shift both resonant frequencies high or low, thus making operation impossible for desired frequencies.

Accordingly, there is a need for a small helical antenna adapted to receive signals in multiple frequency bands.

Brief Description of the Drawings

FIG. I is a block diagram of a wireless communication device, such as a cellular radio telephone, according to the present invention; FIG. 2 is a helical coil antenna according to the present invention; FIG. 3 is a helical coil antenna according to an alternate embodiment of the present invention; FIG. 4 is a helical coil antenna according to an alternate embodiment of the present invention; FIG. 5 is a helical coil antenna according to an alternate embodiment of the present invention; FIG. 6 is a helical coil antenna according to an alternate embodiment of the present invention; FIG. 7 is a helical coil antenna coupled to a coaxial transmission line according to an alternate embodiment of the present invention; FIG. 8 is a helical coil antenna coupled to a monopole according to an alternate embodiment of the present invention; FIG. 9 is a cross-sectional view of the antenna of FIG. 8 according to the present invention; FIG. 10 is a plan view of an encapsulated antenna of FIG. 8 according to the present invention; and FIG. 11 is a partial perspective view of an antenna according to the present invention coupled to the wireless communication device of FIG. 1.

Description of the Preferred Embodiment

According to the method and apparatus set forth in the present disclosure, a fixed type antenna formed by a single helical wire has a non-uniform pitch angle. The dual-band operation is obtained according to one embodiment of the invention by adjusting the total length of the wire for the low band and the non-homogeneous pitch angle for the high band. In particular, with the use of a non-uniform pitch angle and keeping the total length of the wire constant according to the present disclosure, the second resonance can be shifted high or low as function of the pitch angle, while the first resonance is not affected. According to other embodiments of the invention, the diameter of the helical coil can be varied to achieve multi-band performance.

Turning first to FIG. 1, a block diagram of a wireless communication device such as a dual band cellular radiotelephone incorporating the present invention is shown. In the preferred embodiment, a frame generator ASIC 10 1, such as a CMOS ASIC available from Motorola, Inc. and a microprocessor 103, such as a 68HC I I microprocessor also available from Motorola, Inc., combine to generate the necessary communication protocol for operating in a cellular system. Microprocessor 103 uses memory 104 comprising RAM 105, EEPROM 107, and ROM 109, preferably consolidated in one package I 11, to execute the steps necessary to generate the protocol and to perform other functions for the communication unit, such as writing to a display 113, accepting information from a keypad 115, controlling a frequency synthesizer 125, or performing steps necessary to amplify a signal according to the method of the present invention. ASIC 10 1 processes audio transformed by audio circuitry 119 ftom a microphone 117 and to a speaker 12 1.

A transceiver processes the radio frequency signals. In particular, a transmitters 123 and 124 transmit through an antenna 129 using carrier frequencies produced by a frequency synthesizer 125. Information received by the communication device's antenna 129 enters receivers 127 and 128 through a matching network and transmit/receive switch 130. A preferred matching network and transmit/receive switch 130 will be shown in more detail in FIG. 10. Receivers 127 and 128 demodulate the symbols comprising the message frame using the carrier frequencies from frequency synthesizer 125. The transmitters and receivers are collectively called a transceiver. The communication device may optionally include a message receiver and storage device 131 including digital signal processing means. The message receiver and storage device could be, for example, a digital answering machine or a paging receiver.

A helix antenna is suitable for cellular phone communications due to size and polarization considerations. A simple helix antenna has N turns, each turn has a diameter of d and the spacing between the turns is s. The total height of the antenna is h=Ns and the total length of the wire is I=Nlo. Here lo is the length of the wire between each turn. The pitch angle is defined as the inverse tangent of the s/d ratio.

Usually, when the antenna dimensions are small compared to the wavelength, the antenna is in normal mode of operation and the radiation characteristics are similar to that of a quarter-wave monopole, but the profile (height) of the helical antenna is much smaller (shorter) than the monopole.

The helical antenna provides an elliptical polarization that is a useful feature especially when the antenna is rotated at an angle with respect to a linearly polarized fixed transmitter. A good example is that when the user holds the cellular phone at various angles, the antenna will still be able to receive signal from the fixed and linearly polarized base station, since an elliptical polarization can be decomposed to the two orthogonal linear polarizations.

A helix antenna may have several resonance frequencies as function of the wire length, pitch angle and the number of turns. Usually, these resonance frequencies can be adjusted by using proper techniques to obtain a single-band or a multi-band cellular operation. These techniques may include varying the helix wire dimensions and/or adding other components. A helix antenna having a uniform pitch angle can be tuned to operate at GSM band (900 Nffiz). However, the second resonant mode will not be at the DCS (1800 MHz) or PCS (1900 Nmz) bands. Any changes in wire length, diameter and the pitch angle will shift both resonant frequencies high or low, and thus making multi-band operation impossible. A multi- band operation can be achieved only if the pitch angle is modified in a suitable manner. In particular, with the use of a non-uniform pitch angle and keeping the total length of the wire constant according to the present disclosure, the second resonance can be shifted high or low as function of the pitch angle, while the first resonance is not affected.

Turning now to FIG. 2, a helix with decreasing pitch angle will move the second resonance closer to the first one. In particular, a helix has a feedpoint 203 coupled to a first portion 202 extending a height pi from a proximal end 204 to a distal end 206 and having a pitch represented by sl. Although pitch s, could be fixed throughout first portion 202, s, could represent an avarage of a pitch which would vary throughout first portion 202. The diameter of the helical coil throughout first portion 202 is represented by dl. The helix also has a second portion 208 extending a height P2 from a proximal end 210 to a distal end 212 and having a pitch represented byS2. Although pitchS2could be fixed throughout second portion 208, S2could represent an avarage of a pitch which would vary throughout second portion 208, whereS2is different from sl. The diameter of the helical coil for second portion 208 is represented by d2. In the embodiment of FIG. 2, d, is equal to d2- Turning now to FIG. 3, a helix with increasing pitch angle will move the second resonance away from the first one. In particular, a helix has a feedpoint 301 coupled to a first portion 302 extending a height p, from a proximal end 304 to a distal end 306 and having a pitch represented by si. Although pitch si could be fixed throughout first portion 302, s, could represent an avarage of a pitch which would vary throughout first portion 302. The diameter of the helical coil throughtout first portion 302 is represented by dl. The helix also has a second portion 308 extending a height P2 from a proximal end 3 10 to a distal end 3 12 and having a pitch represented by S2. Although pitch S2 could be fixed throughout second portion 308, S2 could represent an avarage of a pitch which would vary throughout second portion 208, where S2 is different from si. The diameter of the helical coil throughout second portion 308 is represented by d2- In the embodiment of FIG. 3, d, is equal to d2 In the design process, one can choose the total length of the wire as the varying parameter to align the AMPS/GSM band first and then adjust the pitch angle until the second resonance is aligned with the DCS band. The total length of the antenna can be chosen by the number of turns and the diameters of the two portions of the coil. The bandwidth and the return loss can be improved further by using a proper matching circuit, which is well known in the art. The efficiency of the nonuniform pitch antenna is measured to be better than 80% at the center frequency of each band.

According to alternate embodiments of the present invention, the diameter of the coil can also be varied. As shown in FIG. 4, the diameter of a first portion 402 of the coil extending from a proximal end 404 to a distal end 406 is generally larger, while the diameter of a second portion 408 of the coil extending from a proximal end 410 to a distal end 412 is generally smaller. In particular, a helix having a feedpoint 401 is coupled to a first portion 402 has a pitch represented by sl. The diameter of the helical coil for p, is represented by di. The helix also has a second portion 408 has a pitch represented by S2. The diameter of the helical coil for pi is represented by d2- In the embodiment of FIG. 4, di is greater than d2 In contrast, the diameter of the lower portion of the coil in the embodiment of FIG. 5 is generally smaller, while the diameter of the upper portion of the coil is generally larger. In particular, a helix having a feedpoint 501 coupled to a first portion 502 extending from a proximal end 504 to a distal end 506 has a pitch represented by s). The diameter of the helical coil throughout the first portion 502 is represented by d 1. The helix also has a second portion 508 extending from a proximal end 5 10 to a distal end 512 and has a pitch represented by S2. The diameter of the helical coil throughout the second portion 508 is represented by d2, In the embodiment of FIG. 5, d, is less than d2. Although pitch s, and S2 could be fixed throughout first and second portions 302 and 308 in the embodiments of FIGs. 4 and 5, they could represent an avarage of a pitch which would vary throughout respective portions. Similarly, although the diameters d, and d2 are preferably fixed, they could also vary according to the present invention. Also, although diameters d, and d2 could be fixed in FIGs. 4 and 5, they could also represent the average of the variable diameters throughout portions 402 and 408.

Turning now to FIG. 6, an alternate embodiment of the present invention includes both variable pitch and variable diameter. In particular, both the pitch and the diameter decrease from a proximal end 604 of the antenna (near a feedpoint 605) to a distal end 606 of the antenna. While one combination of variations is shown, any combinations of varying pitch and diameter, including a fixed pitch or diameter, and the relative locations of the varying pitch or diameter could be employed according to the present invention. Also, more than two segments of varying pitch or diameter could be employed according to the present invention.

Turning now to FIG. 7, a coaxial transmission line is coupled to the feed point of the helical coil. In particular, a helical coil such as that shown in FIG. 2 is attached to a coaxial transmission line 712 having an outer conductor 714 and an inner conductor 716 coupled at feedpoint 203. Although the helical coil of FIG. 2 is shown, any helical coil of FIGs. 3-7 could be coupled to the coaxial transmission line according to the present invention. The advantage of the coaxial transmission line feeding the variable pitch antenna is that the length of the coaxial transmission line can be used to change the current distribution on the PC board, so the optimum radiating efficiency can be obtained, particularly when the phone is being used. The reason for the optimization is that the coaxial transmission line changes the input impedance of the variable pitch, so does the current distribution on the PC board.

Because the current on the PC board has significant radiation for the far field, then varying the current distribution can optimize the antenna radiation efficiency. When the phone is in use, the current on the antenna and PC board is significantly changed by the proximity of the user's body. With the coaxial transmission line feeding, the changed current can be compensated.

Turning now to FIGs. 8-10, the perspective view shows a helical coil antenna 801 according to the present invention coupled to a monopole 802 having a threaded portion 804 and a contact portion 806. Although the helical coil of FIG. 2 is shown coupled to the monopole, any helical coil of FIGs. 3-7 could be coupled to the monopole. A cross-sectional view in FIG, 9 shows the cross-section of one embodiment of the antenna of FIG. 10. In particular, a dielectric core 904 within the overmold 902 preferably comprises a dielectric material. For example, the core could be a dielectric material comprising santaprene and polypropylene. For example, the dielectric core could be composed of 75% santoprene and 25% polypropylene to create dielectric material having a dielectric constant of 2.0. A plan view in FIG. 10 shows antenna 129 detached from the wireless communication device. As will be described in FIG. 11, any antenna described in this Application could be coupled to a whip portion to be incorporated in a retractable antenna.

Turning now to FIG. 11, a partial cross-sectional view shows an antenna according to the present invention coupled to a wireless communication device, such as that shown in FIG. 1. Antenna 129 comprises an outer housing or overmold 902 having a sleeve 1004. Monopole 802 comprises a threaded portion 804 which extends to a coupling portion 806. The length of the monopole generally effects vertical polarization, where a longer monopole generally provides greater vertical polarization.

The monopole will be described in more detail in reference to the remaining figures.

The antenna is engaged with a clip I 110 having a contact element 1112 at the end of a flexible arm 1114 which is attached to a base portion 1116. Base portion 1116 is preferably attached to a circuit board having circuitry of FIG. I or some other suitable circuit. Bracket 1110 further includes a second contact I 118 coupled to flexible arm 1120 which also extends to base portion 1116. Coupling portion 1108 is retained by flexible anns 1114 and 1120 which also provide an electrical contact. The dimensions of the flexible arms are preferably selected to optimize the efficiency of the antenna. That is, the length and width of the flexible arms are selected to provide the proper inductance or capacitance for the antenna, where a narrower an-n provides greater inductance and wider arm provides greater capacitance.

FIG. I I also shows a housing 1130 of the wireless communication device of FIG. 1. The housing includes a receiving sleeve 113 2, shown in partial cross-section, which retains a threaded nut 234 for receiving threaded portion 1106 of the antenna. Although the feed point of the antenna is preferably made at contact elements 1112 and 1118 near the base of coupling portion 1105, the feed point could be made at the threaded nut 1134 according to the present invention. Although the present description focuses on a stubby antenna, any antenna disclosed in this application could be incorporated in a retracted antenna, such as a retractable antenna disclosed in Application Serial No. 09/219,561 for AN ANTENNA ADAPTED TO OPERATE IN A PLURALITY OF FREQUENCY BAND filed on December 23, 1998 and assigned to the assignee of the present invention. Further, matching circuit arrangements according to Application Serial No. 09/219,561, which Application is incorporated by reference, could also be employed.

In summary, a multi-band antenna can be achieved according to the present invention by modifing the pitch angle of a helical coil in a suitable manner. In particular, by using a non-uniform pitch angle and keeping the total length of the wire constant, the second resonance can be shifted high or low as a function of the pitch angle, while the first resonance is not affected.

Although the invention has been described and illustrated in the above description and drawings, it is understood that this description is by way of example only and that numerous changes and modifications can be made by those skilled in the art without departing from the true spirit and scope of the invention. Although the present invention finds particular application in portable cellular radiotelephones, the invention could be applied to any wireless communication device, including pagers, electronic organizers, or computers. Applicants' invention should be limited only by the following claims.

Claims

We claim:

Claims 1 An antenna adapted to operate in at least two frequency bands, said antenna comprising:

a helical coil extending from a proximal end to a distal end, said helical coil having a pitch which varies from said distal end to said proximal end.
2. The antenna of claim 1 wherein the diameter of said helical coil varies from said proximal end to said distal end.
3. The antenna of claim 1, further comprising:

a first helical portion extending from a first distal end to a first proximal end, said first helical portion having a first pitch; and a second helical portion extending from a second proximal end, coupled to said first distal end of said first helical portion, to a second distal end, said second helical portion having a second pitch which is different ftom said first pitch.
4. The antenna of claim 3 wherein said first pitch is larger than said second pitch.
5. The antenna of claim 3 wherein said first pitch is substantially fixed.
6. The antenna of claim 3 wherein said first pitch varies.
7. The antenna of claim 3 wherein said second pitch varies.
8. The antenna of claim 3 wherein said first pitch is substantially fixed and said second pitch is varying.
9. The antenna of claim 3 wherein said first helical portion has a first diameter, and said second helical portion has a second diameter which is less than said first diameter.
10. The antenna of claim 3 further comprising a monopole, wherein a first end of said first helical portion is coupled to said monopole.
11. A method for adapting an antenna to operate in at least two frequency bands, said method comprising the steps of:

winding a first portion of a helical coil at a first pitch; and winding a second portion of said helical coil at a second pitch.
12. The method for adapting an antenna according to claim I I wherein said step of winding a first portion of said helical coil comprises winding said first portion at a substantially fixed pitch.
13. The method for adapting an antenna according to claim I I wherein said step of winding a first portion of said helical coil comprises winding said first portion at a variable pitch.
14. The method for adapting an antenna according to claim 11 wherein said step of winding a first portion of said helical coil comprises winding said first portion at a substantially fixed pitch and wherein said step of winding a second portion of said helical coil coniprises winding said second portion at a variable pitch.
15. The method for adapting an antenna according to claim I I further comprising a step of coupling said first portion to a monopole.