BACKGROUND OF THE INVENTION

Field of the Invention

The invention pertains to antennas and more particularly to multiband antennas for use in the AM/FM/CB bands. Prior Art

Multiband antennas which simultaneously serve as receiving antennas for AM/FM broadcast radio and for Citizen Band transceivers are known. A problem in designing antennas of this type is to define an antenna which has near optimal receiving/transmission capabilities in the separate frequency bands in which the AM/FM and CB radios operate. The AM radio band falls in the comparatively low frequency range of 550 to 1600 KHz while FM radio operates in the 88 to 108 MHz range and CB operates in the relatively narrow range of 26.95 to 27.405 MHz. It is well known from antenna design principles that the optimum electrical length for a rod antenna is one-quarter of the wavelength of the transmitted signal. Thus, there is a design conflict when a single antenna is used for several frequency ranges. One option used in prior art antenna design is to tune the antenna to the separate frequencies when switching between bands. This has obvious disadvantages to the user of the radio.

Another option is to design an antenna which provides a compromise and is usable in several frequency bands. Such an antenna, by its nature, provides near optimal reception in at most one frequency range. For example, it is not uncommon in automobile antennas to use an antenna length equivalent to one-quarter wavelength to the midpoint of the FM range. As a consequence, AM reception suffers and such an antenna is unacceptable for use with a CB transceiver. Similarly, a CB antenna does not provide adequate FM reception.

In automobiles and trucks, it is common to use one antenna for CB and another for AM/FM. A problem has been encountered in trucks, where AM/FM antennas are commonly placed in the headliner of the cab, in that the modern cabs are often made of fiberglass or similar nonconductive materials which do not provide a ground plane for the antenna such as is present in metal cabs. As a result, the AM/FM reception suffers considerably, particularly in the AM band. Trucks typically use a pair of CB antennas connected in parallel and through a T-connection to the CB radio equipment. The antennas are often mounted on the side view mirrors on both sides of the cab which, because of their location outside of the cab and beyond the sides of the trailer or box behind the cab, provide a favorable signal reception position. It is not feasible, however, to put separate AM/FM and CB antennas on the mirrors because of space and interference considerations. Thus, what is needed is an improved combination AM/FM/CB antenna which can be mounted on the side view mirrors for best reception. Λ significant problem in multiple antenna systems of the prior art is the mismatch in electrical characteristics between the two separate antennas of a

dual antenna system and the mismatch between the antennas and the radio equipment. Such mismatches result in a loss of power and can cause damage to the radio equipment due to reflected energy. The loss of power is particularly noticeable in fiberglass cabs which lack the standard ground plane.

U.S. Patent No. 4,229,743 to Vo et al., issued October 21, 1980, discloses a multiband AM/FM/CB antenna having a plurality of resonant frequencies. This prior art antenna uses coil sections wound around portions of the antenna to form a network. The network is used to provide an impedance element having a resonant frequency at approximately 59 MHz. This is an approximate midpoint between the CB and FM band and does not provide optimal reception in the two separate bands.

U.S. Patent No. 5,057,849 to Dόrrie et al. issued October 15, 1991, discloses a rod antenna for multiband television reception. That antenna uses a support rod with two connected windings wound on the rod, one of the windings being spiraled with wide turns and the other being tightly wound. The two windings are capacitively coupled to the antenna connection element by a loop of a third winding. This antenna, when connected to a television receiver, allows the receiver to be switched between UHF and VHF without requiring specific tuning of the antenna. The antenna, however, does not provide optimal reception of two separate frequency bands. Frequency traps have been used by amateur radio operators to be able to use the same antenna for more than one frequency band. Such known frequency traps customarily consist of a coil in the antenna with a discrete capacitor connected across the coil and

external to the coil. Together, the coil and capacitor form an LC circuit which presents a high impedance at a selected frequency to effectively isolate a portion of the antenna at the selected frequency. Such an arrangement with discrete capacitors is not practical for automotive antennas and other applications.

U.S. Patent No. 4,404,564 to Wilson, _^ issued September 13, 1983, discloses an omni-directional antenna in which the electrically conductive antenna element is wound around a rod of insulating material and a tuning device comprising a hollow cylinder of non-conductive material mounted on the antenna rod and a metallic sleeve around a portion of the cylinder and an outer coil electrically isolated from the sleeve and the antenna conductor. Such an arrangement does not provide the desired frequency band separation.

A problem with prior art antennas is that, for proper reception, the antennas have to be individually tuned to the vehicle in which they are installed. This is commonly done by adjusting the electrical length of the antenna which may be accomplished through adjustment of the physical length of the antenna or adjustment of a loading coil section typically provided at the top part of the antenna. Such installation tuning tends to be time consuming and difficult to control, resulting in a poor performance characteristics. This is particularly true in vehicles using the cabs made of material other than metal and therefore do not have a well defined ground plane which typically enhances signal reception.

SUMMARY OF THE INVENTION These and other problems of the prior art are overcome in accordance with this invention by means of

a multiband antenna having a resonant circuit section referred to as a "frequency trap" at a selected location in a rod antenna. The trap is designed to present a high impedance for one frequency band and a very low impedance in other frequency bands. By isolating the upper portion , the trap controls the vertical pattern of the antenna. This prevents deep nulls in the pattern and better receptio is maintained while the vehicle is moving. Advantageously the trap provides an antenna section of the desired length at one frequency, for example at a point in the FM range, and provides a different length rod antenna for another frequency or frequencies.

A frequency trap, in accordance with the present invention, comprises of a plurality of turns of antenna wire forming a coiled section, a conductive material disposed internal to the coiled section and a dielectric material disposed between the conductive material and the coiled antenna wire. The coiled section with the conductive material and the dielectric material form the equivalent of a parallel LC circuit in which only parasitic currents flow.

In one particular embodiment of the invention, the antenna is formed by an antenna wire loosely wound around a nonconductive cylindrically- shaped core. The antenna wire is wound more tightly around the core in the coiled section. The conductive material, in the form of a cylindrical tube, extends over a section of the core and the dielectric material extends over the tube such that the tightly wound coiled section is wound around the section of the core occupied by the tube and is separated from the tube by the dielectric material.

Advantageously, the frequency trap in accordance with this invention is easy to manufacture. Since many antennas are loosely wound around a nonconductive core of fiberglass or other like material, the incorporation of the FM trap requires only the addition of a thin walled tube externally to the core and a thin layer of dielectric over the tube and the formation of a coiled section by increasing the number of turns per unit length of the core in the area of the tube.

One embodiment of the invention, a multiband radio antenna system comprises a pair of spaced apart rod antennas each comprising a conductive antenna wire including frequency trap comprising a coiled section having a layer of conductive material disposed internal to the coiled section and a layer of dielectric material disposed between the layer of conductive material and the antenna wire thereby insulating the conductive layer from the antenna wire. The arrangement forms a resonant circuit section having a resonant frequency falling in the FM frequency range. A multiplexer circuit is provided to couple the pair of antennas to an AM/FM radio and to a CB radio. In one specific embodiment of the invention, the antennas have an overall electrical length equivalent to a quarter wavelength within the CB range and the resonant section is positioned at an electrical distance from one end of the antenna equivalent to a quarter wavelength for a frequency falling in the FM frequency range. Advantageously, the entire length of the antenna is available for use as a CB antenna or AM antenna and FM frequency currents in the antenna are limited to the section of the antenna delineated by the resonant section.

The two spaced apart antennas preferably each have a relatively tightly wound coiled section forming a trap and a loosely wound sections above and below the trap and the windings of the corresponding sections of the two antennas are preferably substantially identical in angular dimension and in spacing. Advantageously, such substantially identically wound sections provide substantially identical matching electrical characteristics for the two antennas, thereby significantly increasing the gain of the two-antenna system over mismatched antennas.

In a particular embodiment of the invention, a pair of antennas, each incorporating an FM trap, are electrically connected to a CB transceiver and an AM/FM radio through a multiplexer circuit. The multiplexer circuit includes an LC circuit having an inductor and a capacitor connected in series between the connection to the antenna and the CB transceiver, forming a resonant circuit designed to pass signals in the CB frequency range. A second capacitor is connected between the CB transceiver input and system ground and the component values of the inductor and first and second capacitor are selected such that the sum of the impedances of the inductor and the capacitors and the antennas equals the output impedance of the CB transceiver.

Advantageously, using the resonant circuit and an additional capacitor to match the impedance of the CB transceiver improves significantly the strength of the CB signal. In accordance with one aspect of the invention, an antenna connected to a radio apparatus via a multiplexer is tuned to a selected frequency, such as the CB mid-band frequency, by adjusting the electrical length of the antenna for the desired

frequency on the vehicle and adjusting the impedance values of the multiplexer circuit such that the sum of the impedances of the multiplexer circuit and the antenna equals the output impedance of the radio apparatus. Advantageously, by this method, the antenna is tuned for proper reception and transmission of the desired frequency and the antenna and multiplexer circuit are properly matched to the impedance of the radio apparatus.

BRIEF DESCRIPTION OF THE DRAWING

An illustrative embodiment of the invention is described below with reference to the drawing in which:

FIG. 1 is a diagrammatic representation of a dual antenna system incorporating the principles of the invention;

FIG. 2 is a partially cutaway view of an FM trap in accordance with the invention; and FIG. 3 is an equivalent circuit representation of the FM trap of FIG. 2.

DETAILED DESCRIPTION FIG. 1 is a diagrammatic representation of an antenna system 10 comprising a pair of substantially identical antennas 11. The antennas each comprises a well-known, enamel-coated conductive antenna wire 12 wound around an essentially cylindrically-shaped core 13 and extends continuously from the top of the core 13 to the bottom thereof where it is connected to a coaxial cable 18. Each antenna has a lower section 14 and an upper section 15. The upper section 15 is preferably relatively loosely wound and the lower section 14 is wound somewhat tighter. Disposed between

the upper and lower sections is a resonant section referred as an FM trap 16, which will be described in greater detail later herein with respect to FIG. 2. The trap section 16 is tightly wound. Each antenna 11 is further provided with a loading coil 17 at its upper end consisting of several tightly wound turns of the antenna wire 12 with approximately the same number of turns per unit length as the trap section 16. The two antennas are connected via a pair of coaxial cables 18, with matching impedance characteristics, to a circuit board 30 comprising a multiplexer circuit for coupling the two antennas to a CB transceiver and an AM/FM receiver. The antenna of this invention may be used advantageously as an AM/FM antenna, without being used as a CB antenna. The lower section 14 of the antenna provides a tuned FM antenna and the overall length of the antenna will be available for AM reception and provide a significant improvement over standard automotive antennas. Each of the antennas 11 of FIG. 1 has an electrical length equivalent to one-quarter wavelength for a frequency in the CB frequency band, i.e. approximately 27 MHz. The FM trap 16 in each antenna is located at a position above the lower end of the antenna such that the electrical length of the lower section 14 is equivalent to one-quarter wavelength for a frequency in the FM frequency band, i.e. approximately 100 MHz. The FM trap 16 presents a high impedance in the FM frequency band and prevents the flow of FM frequency signals from the upper antenna section 15 to the lower section 14. Accordingly, the lower section 14 provides an FM antenna which is resonant at the FM mid-band frequency. The FM trap has a very low impedance at the CB and AM frequencies and

therefore does not affect signals at those frequencies. Hence, the entire length of the antenna is available for both CB or AM or both.

The coiled antenna advantageously has a considerably shorter physical length than its electrical length and may be on the order of 50 to 60 inches in length as a CB antenna having an electrical length of one-quarter wavelength in the CB frequency range or approximately 112 inches. The core 13 may be a solid fiberglass or the like material core having a diameter, for example, on the order of one-quarter inch and serves as a support for the coiled wire antenna. The two antennas 11 may each be provided with a mounting clamp (not shown in drawing) for mounting the antenna on side view mirrors or the like preferable separated by a distance equivalent to approximately one-quarter wavelength in the CB frequency range.

The loading coil 17 of each antenna may be used for impedance matching of the antenna for a particular application or vehicle. For example, in the production of truck antennas, an antenna is produced with a specified number of turns in the loading coil to obtain a specific desired electrical characteristic of the antenna for a particular model of vehicle. All antennas for vehicles of that model are then produced to the same specifications with the same number of turns per unit length in the lower and upper sections 14 and 15, in the FM trap 16 and the loading coil 17 of the two antennas. By maintaining close tolerances in the manufacturing process, antennas of substantially identical electrical characteristics are produced, providing properly matched antennas when used in pairs, and providing antennas which are properly tuned to a

particular model of motor vehicle prior to installation of the antenna on the vehicle.

The circuit board 30, connected via the coaxial cables 18 to the two antennas, comprises multiplexer circuitry which couples the two antennas to a CB transceiver 48 on the one hand and an AM/FM receiver 49 on the other hand. An input conductor 31 connects the two coaxial cables 18 to an output conductor 34 and the CB transceiver 48 via a series connected L-C circuit 21 consisting of capacitor 32 and inductor 33. Capacitor 32 may be a variable capacitor. The component values of capacitor 32 and inductor 33 are selected to form a resonant LC circuit tuned to the 27 MHz CB mid-band frequency. The L-C circuit 21 serves to isolate the CB receiver from AM/FM frequency signals, thereby preventing signal degradation, while presenting essentially a short circuit for CB frequency signals transmitted between the CB transceiver 48 and the antennas. As mentioned earlier, the two antennas 11 are produced with matching electrical characteristics. Furthermore, in addition to being selected to form a resonant circuit, the component values of capacitor 32 and inductor 33 are preferably selected, such that the sum of the impedances of the L-C circuit 21 and the antennas is substantially equal to and matches the output impedance of the CB transceiver. In this manner, reflections are minimized and power transfer between the transceiver and the antennas is enhanced. A variable capacitor 23 is connected between the output of transceiver 48 and system ground to aid in impedance matching.

The input conductor 31 is connected to an output conductor 35 and the AM/FM receiver 49 via a

parallel L-C circuit 25 consisting of the coil 36 and variable capacitor 37. The parallel L-C circuit 25 serves to block the CB signal from the AM/FM receiver, which is particularly important during CB transmission, while passing signals in the AM/FM frequency ranges. A series L-C circuit 27 comprising coil 38 and variable capacitor 39, is connected between output conductor 35 and system ground. L-C circuit 27 is tuned to the 27 MHz CB mid-band frequency to shunt any CB signal to ground. The series LC circuit 27 may be used for impedance matching of the antenna to the FM receiver. The values of the coils 36, 38 and capacitors 37, 39 of circuits 25 and 27 may be readily selected in a known fashion for the 27 MHz frequency and the capacitors may be variable capacitors to assist in fine tuning the circuit.

As stated earlier, the antennas 11 are preferably manufactured to precise specifications in order to have matching electrical characteristics. Since the operation of an antenna is influenced by its environment, the antennas 11 are preferably designed for specific model or type of vehicles. Some trimming of the antennas may be required when they are installed on the vehicle. This may be done in a standard fashion by trimming the loading coil 17. Additionally, the antennas may be matched to the CB transceiver by adjustment of the variable capacitor 23.

FIG. 2 shows the FM trap 16 in partial cut away. Shown in FIG. 2 is a section of the fiberglass core 13 around which the antenna conductor 12 is loosely wound above and below the FM trap 16. In the area of the FM trap the antenna wire is tightly wound to form a coiled section 47 with the successive turns of the coil essentially immediately adjacent one

another. A thin walled brass tube 45 is extended over the core 13 with its horizontal centerline at the electrical length from the lower end of the antenna equivalent to one-quarter wavelength in the FM frequency range, at approximately 100 MHz. A thin dielectric film 46 is applied over the exterior surface of the tube 45 and the antenna wire 12 is tightly wound over the dielectric film. In this manner a capacitor is formed with the brass tube 45 acting as one plate of the capacitor and the antenna wire in the coiled section 47 as the other plate, while the film 46 acts as a dielectric between the two plates.

FIG. 3 shows an equivalent circuit of the FM trap which includes an inductance L introduced by the tightly wound coiled section 47 and a capacitance C resulting from the tube 45 disposed within the coiled section and separated from the coiled section 47 by the dielectric 46. There is no direct electrical connection between the antenna wire 12 and the tube 45 and the capacitance between the antenna wire 12 and the tube 45 is essentially only stray capacitance. For this reason, the connections between the coil L and capacitor C, in FIG. 3, are shown in the form of dotted lines. An antenna incorporating an FM trap in accordance with the invention may be readily constructed by sliding the metallic tube, having an inner diameter slightly larger than the core, over the core and taping a thin layer of dielectric material over the core prior to coiling the antenna wire on the core. In one particular embodiment of the invention, the brass tube 45 is approximately 2 inches long and has walls which are 0.012 inches thick. The dielectric film in this particular embodiment is a single-layer

Kapton ^® film with a thickness in the range of 0.002 to 0.004 inches. The antenna conductor 12 may be a 20- gauge, enamel-coated wire or the like which is tightly wound to form the coiled section 47 with on the order of 35 to 40 turns over the 2 inch length of the tube 45. This arrangement has been found to be self resonating at approximately 100 MHz. The dimensions of the tube and dielectric and the antenna wire as well as the number of turns in the coiled section 47 clearly can be varied and adjusted by one skilled in the art to obtain the resonance at the desired frequency and the above-noted dimensions are provided only as an exemplary embodiment.