WO1996002075A1

WO1996002075A1 - Combined am/fm/cellular telephone antenna system

Info

Publication number: WO1996002075A1
Application number: PCT/US1995/008528
Authority: WO
Inventors: Glen J. Seward; Robert M. Lynas; Paul E. Miller; Robert B. Ennenga
Original assignee: R.A. Miller Industries, Inc.
Priority date: 1994-07-08
Filing date: 1995-07-07
Publication date: 1996-01-25

Abstract

A combined AM/FM/cellular telephone antenna system (100) includes a standard length AM/FM antenna rod (101) provided with a coaxial trap (105) mounted on the antenna rod at position removed from the ground plane (114) by a distance (103) equal to one and one-quarter wavelengths in the cellular frequency range. The coaxial trap (105), which comprises a cylindrically shaped sleeve section, is mounted on and electrically connected to the antenna rod at the lower end of the sleeve section. The lower portion of the antenna rod, below the sleeve section, is defined as a cellular phone antenna and the coaxial trap functions to prevent the flow of currents in the cellular frequency range from the upper portion of the antenna to the lower portion. A coaxial sleeve (120) is mounted on the lower part of the antenna. The coaxial sleeve functions to minimize the effects of phase reversal in the cellular telephone frequency antenna.

Description

COMBINED AM/FM/CELLULAR TELEPHONE ANTENNA SYSTEM BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to motor vehicle antennae and more particularly to a multiband AM/FM/cellular telephone antenna system suitable for use on automotive vehicles. Description of the Related Art

In recent years with the introduction of cellular telephone systems, the use of telephones in motor vehicles has increased dramatically. The cellular telephone system typically requires a separate antenna. On automobiles, the cellular antenna may be factory-installed original equipment or subsequently installed after-market equipment. Earlier cellular telephones required substantial amount of electrical equipment stored in the trunk of the car and wiring was required from the front of the passenger compartment to the rear of the car and to the antenna. The antenna is typically a rear window mounted antenna or a trunk lid antenna. Particularly for after-market equipment, the wiring is a time-consuming job which adds substantially to the cost of installation of the cellular telephone in the vehicle. More modern cellular telephone systems use more compact circuitry which is often installed underneath the front seat. Thus, much of the wiring between the passenger compartment and the trunk is avoided. However, the wiring between the cellular telephone, typically mounted near the driver, and the antenna, typically mounted near the rear window, still requires a substantial amount of wiring. The wiring is preferably hidden from view behind the roof headliner, or the like, which adds substantially to the cost of installation, particularly for after-market installation.

Since practically all cars, trucks and the like are equipped with an AM/FM radio, more and more such vehicles now have two antennas, one for the AM/FM radio and another for the cellular telephone. The AM/FM antenna is almost always a rod antenna. The cellular telephone antenna typically includes a helically wound coil near the middle of the antenna. The coil improves the effectiveness of the antenna by impeding phase reversal, but detracts from the overall appearance. Furthermore, a well-known problem with the cellular telephone antenna of that type is the wind noise or whistle caused by the coil when the vehicle travels at highway speeds.

Portable vehicular cellular telephones offer flexibility in that they may be taken from vehicle to vehicle. A serious drawback of these portable cellular phones is that their range is limited. This is due in large part to the limitations of the antenna of the portable units. The portable vehicular cellular telephones are typically provided with an electrically shortened inductively loaded whip antenna referred to as a "rubber ducky." When they are used with such an antenna in cars or trucks and the like, the range of these units, and their usefulness, is limited.

SUMMARY OF INVENTION

These and other problems associated with the use of cellular telephones in automotive vehicles are solved in accordance with the principles of this invention by a combined AM/FM/cellular telephone antenna system which employs a single modified rod antenna. In accordance with one aspect of the invention, the modified rod antenna is a standard length AM/FM antenna provided with a coaxial trap positioned at a point removed from the lower end of the antenna by a distance equivalent to one and one- quarter wavelength of a signal in the cellular telephone frequency range, i.e., 821 to 896 megahertz (MHz). The coaxial trap serves to isolate the upper portion of the antenna from the lower portion to form a cellular telephone antenna in the lower portion. Advantageously, the trap does not significantly alter the effectiveness of the full-length antenna in the AM frequency range, i.e., 550 to 1,600 KHz or the FM frequency range, i.e., 88 to 108 MHz range. In one embodiment of the invention, the coaxial trap is formed from a length of conductive tubing having one end electrically connected to the rod at a specific location and spaced apart from the rod antenna by a dielectric layer.

In accordance with another aspect of the invention, a conductive coaxial sleeve is placed on the rod antenna in the approximate middle of the lower portion of the modified antenna rod, which is defined as the cellular telephone portion of the antenna. This coaxial sleeve impedes the phase reversal encountered in cellular telephone antennas and, in that sense, perform the function of the helical coil used in prior art antennas. Advantageously, the coaxial sleeve does not have the noise disadvantage associated with the helical coil and is of considerably smaller diameter than the helical coil of the prior art cellular telephone antennas. The entire antenna, including the coaxial trap and the coaxial sleeve may advantageously be encased in a sleeve-like cover to give the antenna a pleasing appearance.

The combined AM/FM/cellular telephone antenna is preferably mounted on a fender or cowl of the automobile in the same manner an AM/FM antenna is typically mounted. In accordance with one aspect of the invention, a coaxial connector is incorporated in the dashboard of the automobile which allows a portable telephone system to be connected to the more efficient combined AM/FM/cellular telephone antenna, thereby significantly enhancing the usefulness of the portable units in automobiles. The portable telephone may be connected to the vehicle battery via a connector in the dashboard.

Advantageously, Applicants' combined AM/FM/cellular telephone antenna provides a better match to vehicular cellular phone systems than many prior art antennas and provides substantial improvements in gain over standard magnetic mount and glass mount cellular telephone antennas. Furthermore, with the antenna of the present invention, operation of the cellular phone does not create any discernable interference in AM/FM reception.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below with reference to the drawing in which:

FIG. 1 is a representation of a combined AM/FM/cellular telephone antenna incorporating a coaxial trap and a helical coil;

FIG. 2 is an enlarged cross-sectional view of the coaxial trap of FIG. 1; FIG. 3 is a AM/FM/cellular telephone antenna including an upper coaxial trap and a lower coaxial sleeve; FIG. 4 is an enlarged cross-sectional view of the coaxial sleeve of

FIG. 3;

FIG. 5 is a representation of the antenna of FIG. 3 encased in a sleeve-type cover; and FIG. 6 is a circuit diagram representation of a diplexer circuit connected to the antenna of FIG. 1 or FIG. 3.

DETAILED DESCRIPTION

FIG. 1 is a representation of an automotive antenna incorporating principals of the invention. The antenna 100 includes an antenna rod 101 consisting of an upper portion 102, shown in a partial breakaway representation, and a lower portion 103. The full-length of the antenna rod 101 is on the order of 30 to 36 inches which is generally the preferred length for an AM/FM antenna. The upper portion 102 and the lower portion 103 are divided by a high-frequency, coaxial trap 105. The trap 105 presents a high impedance, essentially open circuit, to signals in the cellular telephone frequency range and serves to impede the flow of current in the cellular frequency range from the upper portion 102 of the antenna rod 101 to the lower portion 103. The coaxial trap 105 does not substantially effect the current flow in the rod 101 in the AM/FM frequency range. In this manner, the coaxial trap 105 serves to define a cellular telephone antenna, in the lower portion 103, as an integral part of an AM/FM antenna.

The lower portion 103 of the antenna 100 includes a helical coil 107 such as is typically used in cellular telephone antennas to minimize the effects of phase reversal. FIG 1 further shows a conventional antenna mount 115 which threadably engages a lower housing 110 for mounting the antenna rod 101 to the housing 110. The housing 110 is mounted to fender 114 in a conventional manner by means of a threaded mounting ring 108. The feed point 106 of the antenna rod 101 at the lower end thereof is electrically connected to a diplexer network 109 contained within the housing 110 and from which emanate a pair of conductors 116, 117 for connection to a cellular telephone and a AM-FM radio, respectively. FIG. 2 is an enlarged, cross-sectional view of the coaxial trap 105. The trap 105 has an electrical length of 1/4 wavelength in the cellular frequency range, e.g., 850 MHz, and includes a thin-walled, cylindrically shaped sleeve section 125. The sleeve section 125 includes a lower wall 123, having a central opening 124, and is mounted to the antenna rod 101 at its lower end by means of a collar 126 and a conventional screw (not shown in the drawing). The collar and screw provide electrical contact between the sleeve section 125 and the rod 101. The space between the wall of the sleeve section 125 and the rod 101 is preferably filled with a dielectric 127 such as Teflon or other suitable dielectric. The sleeve section 125 may be constructed of copper or other suitable conductive material such as steel. In one embodiment, the walls of the sleeve section 125 are 0.012 inches thick, the outer diameter is approximately 5/16 inches, and the space between the sleeve section and the rod is filled with a dielectric having a dielectric constant of approximately 2. The coaxial trap, at its upper end, presents essentially an open circuit to signals in the cellular telephone frequency range. Referring again to FIG. 1, the trap 105 is preferably mounted on the antenna rod 101 such that its upper edge is at a distance equivalent to an electrical length of one and one-quarter wavelengths in the cellular telephone frequency range from the ground plane, e.g., the fender 114. Shown in FIG. 1 is a dashed line 119 representing current distribution in the cellular telephone range with current essentially at 0 at the upper edge of the trap 105 and a standard current distribution between that point and the ground plane of fender 114.

A preferred embodiment of the invention is shown in FIG. 3. In additional to the coaxial trap 105, the antenna shown in FIG. 3 includes a coaxial sleeve 120 which replaces the helical coil 107 of FIG. 1. The typical cellular telephone antenna has an electrical length of one and one-quarter wavelengths in the corresponding frequency range for improved gain. The prior art helical coil 107 serves to reduce the effects of phase reversal at the approximate midpoint of the antenna, which would otherwise occur tending to cancel the groundwave signal and negatively affecting antenna performance. Even with the addition of the coil, a relatively small phase reversal of the current in the cellular frequency range does tend to occur, as indicated by line 119 in FIG. 1. However, this does not significantly affect signal strength in the antenna of FIG. 3. The coaxial sleeve 120 is used instead of a helical coil and performs the function of reducing the effect of phase reversal and the accompanying signal degradation. The coaxial sleeve produces a phase shift by electrically adding one-half wavelength to the antenna.

The coaxial sleeve 120, which is constructed in a manner analogous to trap 105, is shown in greater detail in FIG. 4. The coaxial sleeve

120 comprises a thin-walled tubular sleeve section 121 which is open at its lower edge and comprises an upper wall 128 having an opening 131 extending over antenna rod 101. The sleeve 120 is mounted to the antenna rod 101 by means of a collar 122 and a conventional screw (not shown in the drawing). The collar and screw provide electrical contact with the antenna rod 101 at the upper end of the coaxial sleeve 120. The space between the antenna rod 101 and the wall of the sleeve section 121 is preferably filled with a dielectric 130. The sleeve section 121 may be made of copper or other suitable conductor. In one embodiment, the sleeve section 121 is made of copper and has a thickness on the order of 0.012 inches and an outer diameter of approximately 5/16 inches. In that embodiment, the dielectric material has a dielectric constant of approximately 2. Referring again to FIG. 3, the coaxial sleeve 120 has a physical length of approximately one-quarter wavelength and an electrical length equal to one-half wavelength in the cellular frequency range and the lower edge of coaxial sleeve 120 is removed from the antenna feed point 106, at the upper end of the housing 110 by a distance equivalent to an electrical length equal to one- quarter wavelength in the cellular telephone frequency range. FIG. 3 shows the antenna feed point 106 positioned above the fender 114 by a distance, e.g., on the order of 3 inches. This position of the feed point above fender 114 was empirically determined to provide improved antenna operation. The position of the feed point and the diplexer network relative to the ground plane provided by the vehicle may be adjusted for optimum operation on different vehicle types. The antenna may readily be adjusted in the vertical direction by adjusting the position of the housing 110 in the threaded mounting ring 108. Line 129 in FIG. 3 shows a current distribution similar to that shown by line 119 in FIG. 1. It will be apparent that the coaxial sleeve 120 provides the same function of minimizing the effects of phase reversal as the prior art helical coil 107. However, the coaxial sleeve 120 conveniently avoids the wind noise which accompanies such helical coil. The coaxial trap 105 and the coaxial sleeve 120, together with the remainder of the antenna rod 101 may be conveniently encased in a sleeve-like cover or the like as depicted for example in FIG. 5. The antenna rod 101 may be a standard stainless steel whip antenna and the cover 130 may be made of fiberglass or the like. The cover serves for aesthetic purposes and is not required for proper operation of the antenna. The cover is preferably tapered in the area above the coaxial trap 105.

FIG. 6 is a circuit diagram representation of the diplexer network 109 contained within the housing 110. The diplexer circuit is connected to the antenna 100 by means of a conductor 150, which may, for example, be a short length of #18 or #20 conductor. A self-resonant inductor 157 is connected between the antenna and the connection to an AM/FM radio. Because the frequency separation between cellular and AM/FM is relatively great, a self- resonant inductor, resonant at 850 MHz, will present a relatively low impedance in the AM/FM range (approximately 100 MHz) and present a high impedance at the cellular frequency (approximately 850 MHz). In one particular embodiment, the inductor 157 has an inductance of approximately 25 nanohenrys. An open circuit stub 159 is connected to the conductor 117 between the coil 157 and the connection to the AM/FM radio. The open circuit stub 159 has an electrical length equivalent to one-quarter of the wavelength at 850 MHz, e.g., a length on the order of 2.4 inches, using a teflon dielectric. The open circuit stub provides a reflecting termination at the selected frequency, having a very low impedance at the one-quarter wavelength. Other standard circuit arrangements may be provided to isolate the AM/FM radio from the cellular frequency signal. A capacitor 155, presenting essentially an open circuit in the AM/FM frequency range, is connected in series between the conductor 150 and the connection to a cellular telephone. The capacitor 155 may, for example, have a value of 3.3 picofarads. The capacitor 155 is connected to conductor 150 via a relatively short conductor section 154 having an inductance of 10 nanohenrys will tend to resonate at cellular frequencies, thereby presenting a very low impedance at those frequencies while offering a high impedance to AM/FM frequencies. A coaxial connector 152 is provided to allow a portable cellular phone to readily connected to and disconnected from the antenna.

It will be understood that the above-described arrangement is illustrative of the principles of the invention and that other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. An antenna comprising: an antenna rod having an upper end and a lower end and a longitudinally centerline extending between the upper and lower ends and a frequency band separating coaxial trap mounted on the antenna rod and providing a high impedance to signals in a selected frequency range to impede transmission of signals in the selected frequency range between an upper portion of the antenna rod above the coaxial trap and a lower portion of the antenna rod below the coaxial trap; the coaxial trap comprising a cylindrically shaped sleeve of conductive material having a longitudinal center line extending coextensively with the centerline of the antenna rod and having an upper end and a lower end; the coaxial trap having an electrical length of one-quarter wavelength at a frequency in the selected frequency range and mounted to the antenna rod and electrically connected to the antenna rod at the lower end of the cylindrically shaped sleeve section; the upper end of the cylindrically shaped sleeve section removed from the lower end of the antenna rod by a distance equivalent to an electrical length equal to an integer multiple of one-quarter wavelengths of a signal in the selected frequency range.

2. The antenna in accordance with claim 1 wherein the cylindrically shaped sleeve section comprises a vertical side wall and a lower wall having a central opening and wherein the frequency band separating coaxial trap is mounted to the antenna rod and electrically connected to the antenna rod at the lower wall.

3. The antenna in accordance with claim 2 wherein the selected frequency range is the cellular telephone frequency range and the antenna rod has an electrical length greater than one and one-quarter wavelength of a signal in the cellular telephone frequency range and the upper end of cylindrically shaped sleeve section is removed from the lower end of the by a distance equivalent to an electrical length of one and one-quarter wavelengths of a signal in the cellular telephone frequency range.

4. The antenna in accordance with claim 3 and further comprising a phase reversal impeding coiled section formed in the lower portion of the antenna rod and removed from the lower end of the antenna rod by a distance equivalent to an electrical length equal to one-quarter wavelength of a signal in the cellular telephone frequency range, the coiled section minimizing the effects of phase reversal of signals in the cellular frequency range.

5. The antenna in accordance with claim 3 and further comprising a coaxial sleeve comprising a cylindrically shaped sleeve section of conductive material having a longitudinal center line extending coextensively with the centerline of the antenna rod and having an upper end and a lower end, the cylindrically shaped sleeve section of the coaxial sleeve mounted to the antenna rod and electrically connected to the antenna rod at the upper end of the cylindrically shaped sleeve section of the coaxial sleeve.

6. The antenna in accordance with claim 5 wherein the cylindrically shaped sleeve section of the coaxial sleeve comprises a side wall and an upper wall having a central opening and wherein the cylindrically shaped sleeve section of the coaxial sleeve is mounted to the antenna rod and electrically connected to the antenna rod at the upper wall.

7. The antenna in accordance with claim 6 wherein spatial areas are formed between the side wall of each of the cylindrically shaped sleeve sections and the antenna rod and wherein the spatial areas between the side walls and the antenna rod contain a dielectric material.

8. The antenna in accordance with claim 7 wherein the dielectric material has a dielectric constant substantially equal to 2.

9. The antenna in accordance with claim 8 and further comprising an outer cover encasing the antenna rod together with the band separating coaxial trap and the coaxial sleeve.

10. An antenna comprising: an antenna rod having all overall length greater than the electrical length of one-quarter wavelength of a signal in a selected frequency range and having an upper end and a lower end and a longitudinal centerline extending between the upper and lower ends; a coaxial sleeve mounted on the antenna rod; the coaxial sleeve comprising a cylindrically shaped sleeve section of conductive material having a longitudinal centerline extending coextensively with the centerline of the antenna rod and having an upper end and a lower end; the coaxial sleeve electrically connected to the antenna rod at the upper end of the cylindrically shaped sleeve section and at a position on the antenna rod removed from the lower end of the antenna rod by a distance equivalent to an electrical length approximately equal to three-quarter wavelength of a signal in the selected frequency range.

11. The antenna in accordance with claim 10 wherein the selected frequency range is the cellular radio frequency range.

12. The antenna in accordance with claim 11 wherein the cylindrically shaped sleeve section comprises a side wall and an upper wall having a central opening and defining a spatial area between the upper wall and the side wall and the antenna rod and wherein the spatial area contains a dielectric material.

13. A combined AM/FM/Cellular antenna system comprising: a longitudinally extending conductive antenna rod having an upper end and having a lower end electrically connected to a diplexer circuit providing connection to an AM/FM radio receiver and to a cellular telephone transmitter- receiver; and a coaxial trap mounted on the antenna rod and having a lower end electrically connected to the antenna rod and having an upper end removed from the lower end of the antenna rod by a distance equivalent to an electrical length equal to one and one-quarter wavelength of a signal in the cellular telephone frequency range and providing a high impedance for signals in the cellular telephone frequency range to impede the coupling of energy at the cellular telephone frequency from a portion of the antenna rod above the coaxial trap to a portion of the antenna below the coaxial trap; a coaxial sleeve mounted on the antenna rod and having an upper end electrically connected to the antenna rod and removed from the lower end of the antenna rod by a distance equivalent to an electrical length equal to three-quarters wavelength of a signal in the cellular telephone frequency range and impeding phase reversal of signals in the cellular frequency range; the coaxial trap and the coaxial sleeve each comprising a cylindrically shaped sleeve section of conductive material having an outer wall and an end wall electrically connected to the antenna rod.

14. The antenna system in accordance with claim 13 wherein the cylindrically shaped sleeves each define a spatial area between the antenna rod and the outer wall and a dielectric material is disposed in the spatial areas.

15. The antenna system in accordance with claim 14 and wherein the diplexer circuit comprises a conductive cable connected to the antenna and a pluggable connector connected to the conductive cable for pluggably connecting the antenna to a portable cellular telephone.

16. The antenna system in accordance with claim 15 wherein the pluggable connector is connected to the conductive cable via a capacitor connected in series with a length of conductor forming a resonant circuit providing low impedance in the cellular frequency range and high impedance in the AM/FM frequency ranges.

17. The antenna system in accordance with claim 16 wherein the diplexer circuit further comprises an inductor connected in series between the conductive cable and a conductor connected to an AM/FM receiver and an open, one-quarter wave transmission line stub connected to the conductor connected to the AM/FM receiver, the inductor and stub together providing a high impedance to signals in the cellular telephone frequency range and a low impedance to signals in the AM/FM frequency ranges.