This invention pertains to antennas and, more particularly, to antennas for car phones.
Cellular mobile telephone service utilizes radio transmission to provide direct-dial telephone service to car phones in automobiles, vans, trucks, buses, and other vehicles. Since there are no overhead telephone lines which connect the vehicles, both speech and signaling are transmitted by radio waves. This is accomplished through the use of special tones rather than applying a voltage level or detecting a current as is done in overhead telephone lines. The tones will ring the mobile telephone to indicate an incoming call or will indicate a busy signal.
Until recent years, the growth of mobile telephones had been restricted due to the limited number of radio channels available and the limited range, i.e. 25-30 miles, of single base transmitting units. Subsequently, the cellular concept was developed to provide high quality mobile service for more customers at an affordable cost. The basic concept of a cellular system is to reduce the area covered by the transmitter by reducing the power of transmissions. In this manner, concentrated areas of population can have more transmitting stations and therefore, more channels because each transmitter handles numerous telephone conversations. Moreover, because the lower power transmitters cover less area, the same frequency can be reused in a common geographical area.
In a cellular system, the service area is divided into regions called cells. Each cell has equipment to switch, transmit, and receive calls to and from the mobile telephone units located in the cell. A typical cell can have a radius of one to 12 miles. Each cell transmitter and receiver operates on a specified channel. Desirably, each channel is used for many simultaneous conversations at locations which are far enough apart to avoid interference. As the number of mobile users grow, the overloaded cells can be split or divided into smaller cells by adding more transmitters.
Cell sites form the radio link between individual cellular telephones and the telephone system. Each cell station is equipped with a transmitter and receiver coupled to an array of antennas. The cell sites also have a telephone switching network and other support equipment. The cells are located where they will operate most effectively in the radio environment. In urban area, cell sites are usually found on top of the tall buildings. In rural areas, cell sites are located at the highest levels, such as on mountains or on radio towers. To minimize interference, adjacent cells use different frequencies.
The cell sites are interconnected and controlled by radio transmission by switching circuitry through a central mobile telephone switching office (MTSO). The MTSO is linked to the cell sites by a group of voice circuits for conversations, together with one or more data links for signaling and control. A call initiated by a mobile telephone unit can be interconnected by the MTSO to a stationary telephone just as any other telephone call.
The mobile telephone units have a telephone control unit, a handset, a mobile telephone antenna, and interconnecting cables. Each mobile telephone unit is assigned a ten digit telephone number. The control unit performs all the functions associated with normal telephone use. The receiver and transmitter provide a transceiver and they are the means by which signals are sent between the cellular telephone and the cell site. The receiver can tune to frequency modulated (FM) channels assigned to the cellular system, such as in the 800-900 MHz range. The band from 824-849 MHz is often used to receive signals from the mobile units. The transmitter can tune to the appropriate FM band. The band from 869-894 MHz is often used to transmit signals to the mobile units. The mobile units transmits at a low power. The base station transmits at a higher power.
Generally, each cellular telephone has a small integrated circuit or chip providing a numeric assignment module (NAM). The NAM chip is programmed usually by the cellular telephone dealer or installer to contain the information that uniquely identifies the cellular telephone with a cell site. The information programmed in the NAM chip includes the telephone number and serial number of the cellular telephone. The computers at the MTSO and the cell site use this information to identify the particular cellular telephone when transmitting and receiving calls.
Most cellular telephone manufacturers and supplies offer hands-free accessories that make it unnecessary to hold the handset while talking on the cellular telephone. A typical hands-free installation, includes a small microphone, which clips onto the windshield's sun visor to pick up the driver's end of the conversation and a speaker in the control head of the cellular telephone. Removing the handset from its cradle restores the cellular telephone to its normal operation and deactivates the microphone external speaker (speaker phone) to end the hands-free mode.
When the handset is lifted or a button is pressed to place a call, the MTSO automatically selects an available channel. The user hears the normal dial tone and can continue dialing, i.e. pressing the alpha/numeric buttons, in the same manner as a stationary telephone with overhead telephone lines. An incoming call to the mobile unit is signaled by a ringing tone and is answered simply by lifting the handset or pressing the receive button. The mobile telephone transmitter is linked by a voice channel to another telephone, such as another mobile telephone or a stationary telephone.
Each cell site has at least one setup channel dedicated to signal between the cell and its mobile units. The remaining channels are for conversations. Two channels are required for duplex operation. A mobile telephone unit is called by transmitting its number over the setup channel. This occurs when the cellular telephone is taken off-hook. The cellular transmitter links to an available set-up channel and send a request tone to the serving cell site. The cell site assigns a voice channel set over which dialing and voice signals will take place. The cellular control circuit automatically switches to the voice channel. The cell site interprets incoming dial tones and send the digits along to the MTSO which will connect the cellular telephone to the recipient telephone. When the recipient telephone is taken off-hook, the telephone are linked and a conversation can take place. As vehicles move (roam) out of the cells and the cellular signal becomes weaker, the MTSO switches to another cell to establish a new voice channel with a stronger signal, by a switching procedure known as a handoff. Handoffs take place so rapidly by a computerized switching system, that cellular users do not even know that it has occurred.
In most mobile cellular phone installations, the phone receives its power from a vehicle battery. If it is part of a permanent installation, the phone may be permanently connected, or hard wired, to the vehicle's electrical system. If the phone is intended to be removed occasionally, a quick disconnect plug or an adapter that lets the telephone take its power from the vehicle's cigarette lighter can be used.
For permanent installations, the transceiver/logic unit can be mounted in the trunk of the automobile and connected to the control head and car battery by cables. This location also places a trunk-mounted transceiver close to a rear trunk-mounted antenna. This arrangement shortens the length of feedline cable connecting the rear antenna to the transceiver. Generally, the shorter the feedline cable in cellular telephones, the stronger the transmission and the better the reception.
An antenna is a length of wire that radiates or captures radio signals. Without an antenna, the cellular telephone would be virtually useless since the telephone would have no means to transmit and receive signals to and from a cell site. Because of the microwave frequencies utilized in cellular telephones, it is possible to make the cellular telephone antennas quite small. Cellular telephone antennas come in many shapes and sizes. Some antennas are intended for permanent mounting and some for easy removal. Some are intended for vehicle use and others are to be attached to a portable phone. Some antennas are mounted on metal. Other antennas are mounted on glass.
An antenna typically needs a ground plane to provide a surface to launch radio waves, even though there is no electrical connection between it and the antenna. A ground plane should have a radius equal to at least one quarter of the wavelength of the radio wave. Since the frequencies used by cellular telephone wavelengths are quite short, very little in the way of a ground plane is needed. Generally for an antenna to radiate an omnidirectional signal, i.e. one that radiates outwardly in all directions from the antenna, the antenna should be mounted as close to the center of the ground plane as possible.
Roof-mounted antennas are secured to the roof of the vehicle. Metal roofs provide a convenient ground plane. The roof is also the highest point on the vehicle so that signals emitted from and transmitted to the antenna are less likely to be obstructed by nearby objects. The roof-mounted antenna can be connected to the internal dome light on the ceiling. The cable can then be placed in the ceiling liner for connection to the telephone. Roof-mounted antennas require a hole in the roof. Roof-mounted antennas usually extend at least a foot above the roof of the car. Roof-mounted antennas are not very popular with automobile drivers because they can be easily damaged by low garage doors and car washes. Roof-mounted antennas are not very popular with truck drivers because they can be readily damaged by low overpasses. Furthermore, roof-mounted antennas are not easily accessible for removal or repair.
Trunk-mounted antennas can be mounted on the trunk of an automobile through a hole in the trunk or with a clip that attaches to the edge of the trunk. The cable can be positioned in the space between the trunk and the body of the car. The metal trunk provides the ground plane. Elevated feed antennas can also be used in which the signal is received into the antenna above its base.
Magnetic-mount antennas are intended for easy installation and removal on the roof or trunk lid of a car. The magnetic-mount antenna has a base with a powerful permanent magnet that holds base of the antenna firmly to the metal surface of the vehicle.
External glass-mounted antennas are mounted on the exterior surface of the vehicles's rear window. The external glass-mounted antenna have an antenna rod, base, and a mounting plate. The mounting plate is cemented to the rear window with a weatherproof adhesive or can be placed on the rear window with a suction cup. The base, which can also serve as a mounting plate, contains a circuit that substitutes for the metal ground plane that would otherwise be provided by the roof or trunk of the car. The other parts of the antenna are glued to the inside of the window directly opposite the outside portion. The signal transmitted to and from the external glass-mounted antenna are coupled through the glass by capacitive action, which allows a current to develop between the two section of the antenna system without any physical connection between them. The coaxial cable is connected to the inner portion of the external glass-mounted antenna and extends into the vehicle's rear deck to the telephone. The efficiency of external glass-mounted antennas can be adversely effected by the rear window defogger and rear window heating elements. External glass-mounted antennas are useful but do not perform as well as roof-mounted antennas.
External cellular antennas typically have a pigtail or squiggle providing a phasing coil partway up its length. The phasing coil serves to divide the antenna into two antennas: one above the phasing coil and one below the phasing coil. The phasing coil is designed to maintain the proper phase relationship of the radio waves received by the antenna so that each half of the antenna receives the proper portion of the signal. A 5 dB antenna usually has two phasing coils or pigtails to properly distribute the signal.
The efficiency of external antennas can be adversely effected by external weather conditions, e.g. ice, snow, sleet, rain, winds, smog. Furthermore, external antennas can be readily bent, broken, or otherwise damaged in car washes. External antennas are also attractive to thieves since external antennas are readily accessible and can be easily removed with proper tools. Sometimes, external antennas are broken in parking garages and parking lots by vandals.
In-vehicle antennas mount inside the vehicle. In-vehicle rear window antennas can be secured to the inside surface of the rear windows of vehicles but peel-back double sided tape or by Velcro tabs. The rear window antennas are mounted vertically in the center of the rear window, but can also be attached instead to the front or side windows. In-vehicle antennas perform almost as well as external glass-mounted antennas.
It is, therefore, desirable to develop an improved cellular telephone system and antenna for use in vehicle which overcomes most, if not all, of the preceding problems.