EP0458592B1 - Passive cellular telephone antenna system - Google Patents

Abstract

A mast radiator (12) is affixed to a glass or other non conductive exterior panel (16) of a vehicle and is provided with a dipole element (18) and one or more parasitic elements to exchange signals, with gain, between the mast (12) and the antenna of a portable cellular telephone unit within the vehicle. In the preferred embodiment, a separate base unit (14) with parasitic elements is mounted on the same glass panel (16) on the opposite surface and opposite the base of the mast radiator (12). <IMAGE>

Description

• The present invention relates to communications systems, and more particularly, to an improved combination for sending a radio transmission between a fixed antenna on the outside of a structure and a transceiver within the structure.
• Radio transmission and reception is difficult when a transceiver unit is located wholly within a structure that can act as a shield against radiation. For example, the interior of a motor vehicle may be isolated from radio signals that originate outside of the vehicle. Further, the transmission of radio signals from a transmitter which is located inside the vehicle may be blocked, as well.
BACKGROUND OF THE INVENTION
• Transceivers which are located inside vehicles are commonly connected to an antenna which is mounted on the exterior of the vehicle by means of a coaxial cable or other wire link. Frequently, the radiating and receiving element of the antenna, which is located on the exterior of the vehicle, is capacitively coupled to the coaxial cable termination through a glass window of the vehicle, thereby eliminating the necessity of drilling holes in the body of the vehicle.
• The increasingly common use of cellular telephones operating in the 800 to 1000 MHz frequency range in motor vehicles has promoted the use of such through the glass antenna units since the cellular telephone preferably utilizes an antenna whose mast extends above the roof line of the vehicle for optimum reception and transmission. Through the glass antennas are easily mounted near the top of the rear window and the antenna mast can extend vertically above the roof line.
• Several types of cellular telephones are common today. A permanently installed car telephone has a direct power connection to the vehicle electrical supply and has a coaxial link to the installed antenna. A so called "transportable" cellular telephone is a similar telephone unit which includes a self contained power supply and a movable antenna so that it can be carried in a brief case. The permanent and transportable telephones are permitted to have a maximum transmitted power of 3.0 watts, which generally mandates the use of a coaxial transmission line to an antenna.
• In recent years, a smaller, compact and lightweight cellular telephone has been developed which can be hand held. This hand held or "portable" telephone, which usually has an integral antenna as a part of the unit, is permitted a radiated power level of only .6 watts. Such devices can be quite small and can fit in one's pocket. When used in an open space, the portable can easily communicate with a "cell" of the cellular system. However, difficulties can be encountered if one wishes to use a portable when inside a vehicle since the metal body of the vehicle acts as a shield to both incoming and outgoing signals.
• These difficulties can be overcome if the portable unit can be connected to an exterior antenna or if the portable can be operated through an open window in the vehicle and the metallic mass of the vehicle doesn't affect the receiving or radiation patterns of the antenna. One approach has been marketed under the trademark LARSEN® ANTENNAS by Larsen Electronics, Inc., of Vancouver, Washington, Model KGB-825. This unit is described as a "passive repeater antenna" which passes signals to and from the externally mounted gain antenna elements. US-A-4794,319 and FR-A-2,081,211 discloses an antenna as described by the pre-characterising portion of Claim 1.
• Such an approach, however, fails to consider the low power available from the portable phone unit and the fact that the radiation pattern from the portable phone antenna is omnidirectional. This generally results in a very small fraction of the radiated power reaching the "repeater" and the external antenna unit. Similarly, the energy received by the external antenna is transferred to the "repeater" and omnidirectionally radiated within the vehicle. Only a small fraction of the received signal is acquired by the portable phone antenna. Further, the external antenna and the internal dipole repeater are coupled capacitively, through the glass window, thereby resulting in some signal loss.
• According to the present invention there is provided a directional passive repeater having the features described in the characterising portion of Claim 1. Using this arrangement, the radiated power reaching the repeater and external antenna unit is significantly improved. Preferably, the rear window is utilized to affix the antenna to the vehicle, since it does not create any significant visual obstruction to the driver's field of view. It is, of course, possible to mount the antenna to the front windshield or to any of the fixed glass side windows.
• In the simplest embodiment of the present invention, the dipole is made of a sheet material to increase the surface area that faces the interior of the vehicle. This increases the gain in a direction orthogonal to the surface by about 2 dB over conventional round wires. It is then possible for a portable unit on the interior of the vehicle to "see" the externally mounted dipole and communicate with it both in the sending and receiving modes.
• In order to create more "gain" in the direction of the portable phone unit and its antenna, one or more parasitic elements may be added. For example, a "reflector" radial, that is approximately .58 wavelength, is spaced at least 1/10 wavelength (or multiples thereof) away from the dipole in a direction that is substantially orthogonal to the axis of the mast and the axis of the dipole and which creates gain for signals being exchanged between the dipole and the antenna of the portable unit. This reflector dipole can also be mounted on the exterior of the vehicle and is in communication with the portable unit by radiation through the glass.
• In another embodiment, a second parasitic radial, or "director" that is at least .45 wavelength, is mounted on the interior of the vehicle, on the opposing surface of the glass and at least 1/10 wavelength from the dipole. In yet other embodiments, additional parasitic elements acting as directors and/or reflectors can be added to increase the directionality of the array and to increase the gain, as well, with respect to signals between the repeater and the antenna of the portable unit. In adding additional elements, it is important that each element be spaced at least 1/10 wavelength from the next adjacent element.
• At the radio frequencies of operation, the glass has no shielding effect and a capacitative coupling through the glass is unnecessary. The resulting combination according to the present invention is a passive array which can be highly directional and can effectively impart "gain" to signals which are passed between the portable phone antenna and the external mast.
• In alternative embodiments, additional parasitic elements can be provided inside or outside of the vehicle to improve the gain and directionality of the array. Because there is no capacitative coupling through the glass, there are no "coupling" losses.
• The novel features which are characteristic of the invention, both as to structure and method of operation thereof, together with further objects and advantages thereof, will be understood from the following description, considered in connection with the accompanying drawings, in which the preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and they are not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a front view of a repeater according to the present invention;
• FIG. 2 is a front view of an alternative repeater;
• FIG. 3 is a side view of the repeater of FIG. 2;
• FIG. 4 is a diagram of a passive repeater array according to the present invention;
• FIG. 5 is a side view of an alternative embodiment including a parasitic element on each side of the vehicle glass;
• FIG. 6 is a side view of a preferred embodiment with two parasitic elements on one side of the glass and one on the other; and
• FIG. 7 is an electrical diagram of the repeater of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
• Turning first to FIG. 1, there is shown acellular repeater 10 according to a primitive embodiment of the present invention. As shown, therepeater 10 includes amast radiator 12 and abase 14 which is adhesively affixed to the exterior of aglass window element 16 of a vehicle. Theglass window element 16 is preferably the rear window of the vehicle, but can be any of the non conductive panels of the vehicle. While the rear window or back light is the mounting place of choice, the side windows or the front windshield of the vehicle can serve, as well.
• Embedded in thebase 14 and extending substantially (but not necessarily) at right angles to the axis of themast radiator 12 are a pair ofquarter wave radials 18 that are connected to serve as a 1/2 wave dipole. Theradials 18 are coupled to themast radiator 12 and aid in the passive repeater function with respect to signals received by themast radiator 12 and with respect to signals that are received from a portable transmitter (not shown) in the near vicinity.
• To improve the efficiency of theradials 18, a modifiedantenna 20 is shown in FIGS. 2 and 3. Rather than using a small diameter round wire as the radial element, flatelongated plates 22 in the base 14' extend approximately one quarter wave from themast radiator 12 and serve as the dipole. Using theplates 22 permits slightly higher gain in the horizontal plane and thus better communication between theportable telephone 24 antenna on the interior of the vehicle and thedipole plates 22 which are mounted on the exterior of the vehicle.
• To further improve the gain, an additional parasitic element can be added to the base to create another alternative repeater 30 as is shown in FIG. 4. A base 14" is modified to include two pairs of elements. A first dipole made up of onequarter wave segments 22 is positioned adjacent theglass 16 while a second pair ofelements 26 are spaced at least 1/10 wavelength from the dipole pair and function as a reflector with a sensitive gain axis in the direction toward the front of the vehicle where the portable unit is most likely to be found.
• In the embodiment of FIG. 5, a similar result is achieved in repeater 40 by utilizing a parasitic element on the interior of the vehicle. As shown, the base 14' containing thequarter wave segments 22 is affixed to the exterior of aglass plate 16 and aninterior base 42 has a similar pair ofparasitic elements 44 which function as a director. The combination is then more sensitive to radiation in the horizontal plane along a line generally parallel to the vehicular axis when the base 14' is affixed to the rear window.
• Generally, thedirector element 44 should be less than one quarter wave length and should be spaced apart from thedipole segments 22 by at least 1/10 wavelength, which includes the thickness of theglass 16.
• A preferred embodiment of the invention has been shown in FIG. 6. Here, the alternative antenna 30 of FIG. 4 is combined with theinterior base 42 of FIG. 5 to form arepeater combination 50 that includesdipole segments 22,reflector elements 26 and, on theinterior base 42,director elements 44. This arrangement more nearly approximates a passive dipole array that is highly directional and which exhibits considerable gain along the sensitive axis which is orthogonal to the parasitic elements.
• As in the other embodiments, the spacing between adjacent elements is at least 1/10 wavelength and the effective length of the director is less than 1/2 wavelength while the reflector is more than 1/2 wavelength. In one experimental model, the director was set at .45 wave length while the radiator was set at .58 wave length
• FIG. 7 is an electrical diagram of thedipole 18 of FIG. 1 connected to theradiating mast 12. As shown, the connection to one arm of thedipole 18 is through a capacitive element 46 and through aninductive element 48 to the other arm of thedipole 18. The impedance values are selected for optimum electrical coupling between theradiator mast 12 and thedipole 18 at the frequencies of interest. Since the dipole elements of the other embodiments are electrically equivalent to the dipole of FIG. 1, the electrical interconnection would be similar.
• Thus there has been shown a passive antenna repeater for a portable cellular telephone which is to be used in the interior of a vehicle. The repeater unit in its simplest embodiment includes a passive dipole which is coupled to a mast radiator that has unobstructed communication with a "cell". The portable telephone is operated within the vehicle which would otherwise effectively shield the telephone antenna from the "cell".
• As a result, the telephone antenna and the dipole are now in direct, line of sight communication for transmission and reception of electrical signals. The signals received by the passive dipole are radiated from the radiator mast and the signals received by the radiator mast are radiated from the dipole to the interior telephone antenna at power levels which are sufficiently low to pose no human health hazard.
• In alternative embodiments, additional parasitic elements are added, either to the externally mounted device or to an internally mounted device which is placed on the inner surface of the vehicle glass opposite the externally mounted device.

Claims (8)

1. A repeater assembly (10,20,30,40,50) for use with a transceiver (24) located within a shielding environment having areas (16) that are transparent to electromagnetic radiation, comprising in combination:

   an antenna, including a radiator (12) having an axis, said antenna being mounted to the exterior of the shielding environment in a radiation transparent area (16) thereof; and tuning means (46,48) electrically coupling a dipole element (18,22) to said antenna radiator (12); characterised by the dipole element (18,22) being exterior of the shielding environment and attached to said antenna and extending in a direction to maximise transmission and reception in a plane that intersects said antenna axis;

   whereby the transceiver (24) within the shielding environment is in radiant energy communication with said dipole element (18,22) and, by means of said radiator (12), communicates with remote transmitters and receivers that are in radiant energy communication with said radiator (12).
2. A repeater assembly as claimed in Claim 1, wherein the antenna is coupled to a first base member (14, 14', 14"), adapted to be mounted to the exterior of the shielding environment in a radiation-transparent area (16) thereof.
3. A repeater assembly as in Claim 2, further including a parasitic element (26) in the first base member substantially parallel to said dipole (22) for enhancing transmission and reception in a selected direction whereby gain is imparted to signals transmitted in the selected direction between the repeater assembly (10,20,30,40,50) and a transceiver (24) within the shielded environment.
4. A repeater assembly as in Claim 1 or Claim 2, further including a parasitic element (44) mounted on the interior of the shielding environment in the transparent area (16) thereof adjacent said antenna, said parasitic element (44) being spaced at least 1/10 wavelength from said dipole (22) and arranged to be substantially parallel thereto for enhancing transmission and reception in a selected direction, whereby gain is imparted to signals transmitted in the selected direction between the repeater assembly and a transceiver (24) within the shielded environment.
5. A repeater assembly as claimed in Claim 4 wherein the parasitic element (44) mounted on the interior of the shielding environment is contained within a second base member (42).
6. A repeater assembly as claimed in Claim 5 when dependent upon Claim 2, wherein the second base member (42) is adjacent the first base member (14).
7. A repeater assembly as in Claim 3, further including a parasitic element (44) mounted on the interior of the shielding environment in the transparent area (16) thereof adjacent said antenna, said parasitic element (44) being spaced at least 1/10 wavelength from said dipole (22) and arranged to be substantially parallel thereto for enhancing transmission and reception in a selected direction, whereby gain is imparted to signals transmitted in the selected direction between the repeater assembly and a transceiver (24) within the shielded environment.
8. A repeater assembly as claimed in Claim 7 wherein the parasitic element (44) mounted on the interior of the shielding environment is contained within a second base member (42) that is adapted to be installed on the interior of the shielding environment in the transparent area (16) thereof, and adjacent to the first base member (14).

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