WO1997026685A1

WO1997026685A1 - Shortened monopole antenna

Info

Publication number: WO1997026685A1
Application number: PCT/US1996/020713
Authority: WO
Inventors: Charles M. Day
Original assignee: Motorola Inc.
Priority date: 1996-01-16
Filing date: 1996-12-19
Publication date: 1997-07-24

Abstract

An antenna (200) for use with portable communication devices (300) includes a transmission line having a length substantially smaller than the quarter wavelength at an operating frequency. The transmission line includes a signal conductor (206) and a ground plane (204). A physical connection (210) made between the signal conductor (206) and the ground plane (204) provides for the transmission line to function as a quarter wavelength antenna without needing additional matching circuits.

Description

SHORTENED MONOPOLE ANTENNA

Field of the Invention

This invention if generally related to antennas and more particularly to antennas for portable radio communication devices.

Background of the Invention

As improved integrated circuit technology allows portable transceivers to be reduced in size, it is also desirable to reduce the overall length of the antenna structure used with such devices. The reduction of the size of the antenna is not only appealing from the point of view of aesthetics and marketability, it is a requirement in such applications as surveillance. Once style of antennas used with communication devices is the half wave dipole antenna which requires no extensive ground plane to operate. Half wave dipole antennas produce highly desirable and predictable electrical performance. These antennas are large and therefore undesirable for portable applications.

One of the smallest antenna structures frequently used with portable transceivers is a quarter wave length whip antenna. A quarter wave whip antenna radiates at acceptable levels below halfwave dipole, however with the benefit of reduced length. The much reduced size of portable communication devices has rendered even the quarter wave monopole too large and unpractical. In general, a standard quarter wave length monopole used on a

UHF (462 MHz) portable radio has a length of 6.4 inches in length. For a small portable of 3 to 4 inches in length, it is highly desirable to have an antenna that is also 3 to 4 inches in length to maintain a balanced look to the portable. One way to overcome this problem is to use a simple wire that is shorter in length than a quarter wave monopole and match its impedance to 50 ohms with a matching circuit. A monopole antenna that is this short has a very high impedance and requires a high Q matching circuit. This approach suffers high loss due to the realizable Q of the matching components. Because of the increased loss the effective radiated power is poor. This cutting of the coax in size works theoretically but in practice, the problem with matching has prevented designers from incorporating such an antenna in portable devices. Another approach to achieve acceptable radiation at reasonable length is , that is more expensive is to use a helical antenna design to shorten the quarter wave physical length by the helical's number of turns and pitch. This approach also suffers from poor effective radiated power and antenna bandwidth not to mention cost. An antenna is therefore desired that can be used with portable products with minimum size without degradation in performance.

Brief Description of the Drawings

FIG. 1 shows an antenna structure in accordance with the present invention.

FIG. 2 shows an alternative antenna structure in accordance with the present invention.

FIG. 3 shows a communication device in accordance with the present invention.

FIG. 4 shows a block diagram of a communication device in accordance with the present invention.

netailed nesπrintion of the Preferred Embodiment

To overcome the problems associated with the excessive length of presently available antennas in portable applications, the present invention provides an antenna that is only one eighth of the wavelength at the operating frequency. This antenna includes a center conductor that is shorted to the ground at the radiating end and left open at the radio interface end. The shorting of the radiating element to ground provides for the efficient operation of the antenna commensurate with quarter wave monopole antennas without the length disadvantages or complicated matching circuits of the prior art. The principles of the present invention will be better understood by referring to a series of drawings, where like numerals are carried forward. Specifically, referring to FIG. 1 a coaxial antenna 100 in accordance with the present invention is shown. A coax cable 112 is formed by an inner conductor 108 and an outer shell 106. These conductors are separated by the dielectric at the coax cable. The center conductor 108 is connected to the shell 106, at junction point 104 in one end of the antenna 100. At the other end of the antenna the center conductor 108 does not make contact with the ground shell 106 in order to provide the benefits of the present invention. A connector 110 is used to couple the antenna 100 to a radio communication device. The coupling of the center conductor 108 to the ground at the radiating end of the antenna provides a significant benefit of accomplishing radiation efficiency without the necessary length that is otherwise required or complicated matching circuits.

The structure of the antenna 100 solves the problems of the prior art by shorting the signal conductor to ground at the radiating end. The shorted transmission line that is produced acts as a high Q matching element and feeds the transmitted power to transmission line's shield or ground. The center conductor 108 of the coaxial cable 112 feeds the transmitted power up to the shorted end 104 and radiates off the outer shield 106. The shortened length of the coax 112 is a shorted transmission line which becomes a high Q inductance as the center conductor 108 exits the coax 112 at the end that attaches to the portable radio. Thus, the antenna length can be greatly shortened, and the high radio impedance of the shortened monopole can be matched to 50 ohms without any discrete matching components.

Referring to FIG. 2, a transmission line antenna 200 in accordance with the present invention is shown. The transmission line 200 may be a microstrip or a stripline. The length of the transmission line antenna 200 is substantially shorter than a quarter wave length at the operating frequency of the transmit signal. In the preferred embodiment, the length of the antenna 200 is substantially one eighth of the wavelength at the operating frequency. The antenna 200 includes a center conductor 206, a ground plane 204 and a dielectric 210. The dielectric 210 may be any well known dielectric used in transmission line applications such as Teflon® and any printed circuit board material. The center conductor 206 is coupled to the radio communication device via a connector 208 on one end of the antenna. On a second end, the radiating end, the center conductor 206 is coupled to a ground plane 204 via junction point 210. This junction point 210 is the only contact between the center conductor 206 and the ground plane 204. Conversely, a gap in the opposite side of the antenna close to connector 208 prevents coupling between the center conductor 206 and the ground plane 202. Similarly, this antenna structure provides for the efficient radiation of the transmit radio frequency signal without the historical lengths required of such efficient antennas.

The length that the antenna 200 can be shortened to is determined by the transmission line's impedance (50 ohm, 100 ohm, etc.) and the dielectric constant of the dielectric material between the center conductor and ground. In general, The radiated power using this antenna 200 is maintained as high as that of a quarter wave length antenna without using any matching circuits. Referring to FIG. 3, a communication device in accordance with the present invention is shown. The communication device 300 includes a housing 302 on which knobs, such as, volume control knob 304 are located. The antenna 100 or 200 may be attached to the radio communication device via connectors 110, 208, respectively. By connecting the center conductor to the outer sleeve of a coax or the ground plane of a transmission line, a one eighth wave is forced to function as a quarter wave resonant antenna while simultaneously removing the need for a matching element to connect the antenna to the amplifier. The shorted center conductor acts like a large inductance at the frequency of operation and hence moves the eighth wave antenna to the center of the chart (50 ohm point). The key is the shortening of the center conductor to the ground plane in order to get an antenna that is a fraction of a quarter wave antenna in length to function as a quarter wave antenna without the detriment of matching circuits. The matching is automatically provided by this function of shorting the two ends.

Referring to FIG. 4, a block diagram of a communication device is shown. The transmitter 404 is used for the transmission of signals received through a microphone 402. A receiver 410 couples received signals to a speaker 412. The antenna 100 or 200 is coupled to the transmitter 404 and the receiver 410 through a coupler 408. This coupler 408 provides for switching between the transmitter 404 and the receiver 410 when directed by controller 412.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

What is claimed is:

Claims

1. An antenna, comprising: a transmission line having a length substantially smaller than the quarter wavelength at an operating frequency, the transmission line including a signal conductor and a ground plane; and a physical connection made between the signal conductor and the ground plane to allow the transmission line to function as a quarter wavelength antenna without needing an additional matching circuits.

2. The antenna of claim 1, wherein the transmission line includes a coaxial cable.

3. The antenna of claim 1, wherein the transmission line includes a micro-strip structure.

4. The antenna of claim 1, wherein the transmission line includes a stripline structure.

5. An antenna, comprising: a transmission line cut in size to be substantially smaller than a quarter wavelength structure, the transmission line comprising: a center conductor; a dielectric; and a ground plane; the center conductor being physically connected to a signal generator at one end and to the ground plane on the other end in order to provide for the effective matching of the transmission line to the signal generator.

6. The antenna of claim 5, wherein the dielectric includes Teflon®.

7. The antenna of claim 5, wherein the dielectric includes PC board material.

8. The antenna of claim 5, further including a connector.

9. A communication device, comprising: a transmitter for producing a transmit signal having an operating frequency; an antenna for radiating the transmit signal, the antenna comprising: a transmission line having a length substantially smaller than the quarter wavelength at the operating frequency, the transmission line including a signal conductor and a ground plane; and a physical connection made between the signal conductor and the ground plane to allow the transmission line to function as a quarter wavelength antenna without needing an additional matching circuits.

10. The communication device of claim 9, further including a connector for coupling the antenna to the transmitter.