WO1994026000A1

WO1994026000A1 - Anti-electromagnetic field antenna for cellular telephone

Info

Publication number: WO1994026000A1
Application number: PCT/US1994/004685
Authority: WO
Inventors: Christopher E. Combest
Original assignee: Multi Service Corporation
Priority date: 1993-04-28
Filing date: 1994-04-28
Publication date: 1994-11-10

Abstract

An anti-electromagnetic field antenna for cellular telephones (12). The antenna comprises a folded monopole driver antenna (14) adapted to be attached to a coaxial cable and means for shielding the user's head from radio-frequency electromagnetic fields. The shielding means consists of a ground-plane circuit (16) including at least two radials electrically connected to the shielding of the transceiver coaxial cable and means for providing inductance and capacitance to the ground-plane circuit. The ground-plane circuit is contained on an electrical circuit board and the radials are of a metallic foil composition and are printed on the circuit board. This specially designed ground-plane acts as a shield by absorbing and reflecting radio-frequency electromagnetic fields away from the user's head and eliminating stray currents and radiation from the transceiver and coaxial cable.

Description

ANTI-ELECTROMAGNETIC FIELD ANTENNA FOR CELLULAR TELEPHONE

Field Of The Invention; The present invention is concerned with the field of antennas and specifically with antennas used for portable transceiver devices such as hand-held cellular telephones and radios. The invention is an antenna which shields the user's head from radio-frequency electromagnetic fields emitted from cellular phones and radios while transmitting and receiving.

Background Of The Invention;

Cellular phones and radios emit low-level radio- frequency electromagnetic fields when in use, primarily when transmitting. The electromagnetic fields produced by these phones and radios are in part inherently directed toward the head of the user. As a result, users of these hand-held radios and cellular phones are subjected to low levels of radio-frequency electromagnetic fields, especially when the radio phone is in the transmit mode.

Cellular phones and hand-held radios generally use antennas which are nominally one-fourth wave length in length. However, the minimum electrical length of a "balanced" antenna is one-half wave length. A shorter antenna of only one-fourth wave length has good transmission characteristics; however, the shorter antenna is unbalanced. When a one-fourth wave length whip antenna is employed, radiation is produced not only from the whip, but also from the ground. Thus, the chassis and the shield of the coaxial cable leading from the chassis to the whip, both become radiators. This results from the unbalanced condition.

In recent years, there have been many studies of the health effects of electromagnetic radiation, including a number that indicate that health hazards may exist even at low power levels such as those found in hand-held radios and cellular phones. Although none of the research to date conclusively proves that low-level radio-frequency electromagnetic fields cause adverse health effects, the public is concerned with the safety of cellular phones and radios and further studies may discover additional risks.

Thus, the prior art points out the need for an improved antenna that shields the user's head from radio- frequency electromagnetic fields.

«iiinn»irv Of The Invention;

The present invention solves the prior art problems discussed above and provides a distinct advance in the state of the hand-held transceiver art. More particularly, the antenna of the present invention provides a means for shielding the radio and cellular phone user's head from radio-frequency electromagnetic fields emitted during use of the transceiver device. The improved antenna for hand-held transceivers of the present invention provides a ground plane effect which serves as the missing one-fourth wave length when the driver or whip element is only one-fourth wave length. The ground plane accomplishes this by functioning like an electrical mirror without itself becoming a significant radiator. The energy that would otherwise be radiated from the chassis or coax shield is focused into the ground plane antenna, where it is radiated in a clean, symmetrical pattern. To the extent that the ground plane portion of the antenna is a radiator, the radiation is in the same symmetrical pattern as the monopole or whip antenna element, because of the symmetry of the ground plane. The resultant radiation pattern has a natural null area at each end of the monopole. This is of significance in the present antenna construction because the bottom or inner extremity of the monopole antenna is where the user's head is located during use of the transceiver. .

An ordinary ground plane antenna having two or more radials produces a slight asymmetric effect between the two null areas because it slightly simulates real ground, absorbing and reflecting radiation at the bottom end of the monopole. Thus, the bottom null area has less radiation than the top null area remote from the transceiver housing. In the present antenna, which is unique in this field, the pattern of the conductors making up the ground plane defining portion of the antenna not only serves to provide the primary electrical function of an efficient ground plane, but also provides significant enhancement of the reflection and absorption characteristics of the antenna, while at the same time presenting a very compact design. The result is less radiation in the bottom null area, and minimum radiation directly under the ground plane itself, which is the very area of the user's head. The preferred anti-electromagnetic field antenna hereof includes a folded monopole driver antenna which is adapted to be attached to the coaxial cable, along with ground plane means for shielding the user's head from radio-frequency electromagnetic fields. The ground plane defining device includes a circuit having at least two radials electrically connected to the shielding of the transceiver coaxial cable, and means for providing inductance and capacitance in the ground-plane circuit. In a particularly preferred form, the ground plane circuit is supported by an electrical circuit board and the radials are of a metallic foil composition which are applied to the circuit board. This configuration provides an inexpensive, yet compact and rugged anti- electromagnetic field antenna which is easy to manufacture using conventional materials. Brief Description Of The Drawings;

Figure 1 is a perspective view of the antenna structure shown mounted on a cellular telephone.

Figure 2 is a plan view of the ground-plane circuit. Figure 3 is a perspective view of the antenna structure showing the internal wiring of the driver antenna.

Figure 4 is an elevational view of the antenna structure shown mounted on a cellular telephone. Figure 5 is a sectional view of the antenna structure showing the internal wiring of the driver antenna.

Figure 6 is a perspective view of the simulated radiation pattern emitted from a transmitting cellular phone using a prior art antenna. Figure 7 is a perspective view of the simulated radiation pattern emitted from a transmitting cellular phone using the present anti-electromagnetic field antenna.

Detailed Description Of The Preferred Embodiment

Referring to the drawing figures, the improved anti- electromagnetic field antenna 10 of this invention is shown in its normal operative position on a cellular phone 12. Antenna 10 broadly includes a driver antenna section 14 ground plane circuit 16 adapted to be connected to a coaxial cable 50 of a cellular phone or other transceiver device. Driver antenna 14 is a folded monopole antenna having an approximate electrical length of 1/4 λ and an approximate actual length of 1/5-1/4 λ contained within a conventional whip antenna housing 48. The whip antenna housing includes a longitudinal shaft portion 28 and a spherical end portion 30.

The folded monopole driver antenna 14 includes a conductor 18 folded over on itself made up of a main wire 20 and a drop wire 22. In a preferred form, the conductors 20 and 22 are flexible, stranded copper cables having diameters in the range of .01-.1 inch. The main wire 20 is connected to the shielding of the cellular phone coaxial cable near the ground-plane circuit 16. The main wire 20 is also provided with a plurality of outwardly extending capacitance hats 24 at the spherical end portion of the whip antenna 30, each of which extends at a positive angle of approximately 45° with respect to the longitudinal axis of the driver antenna 14. The capacitance hats 24 serve to provide a capacitive effect at the end of the main wire 20, drawing current and the associated electromagnetic field along the main wire 20 and thus away from the base of the antenna and the user's head. This also allows the folded monopole to be shortened. In a preferred form, the capacitance hats are rigid copper conductors approximately .15-.25 inch in length and .01-.1 inch in diameter. The drop wire 22 is connected to the feed wire of the cellular phone coaxial cable. The drop wire 22 includes a plurality of outwardly extending capacitance hats 26 at the spherical end portion of the whip antenna 30 extending at a negative angle of approximately 45° with respect to the longitudinal axis of the driver antenna 14. A second set of capacitance hats 27 is located approximately .075-.1 λ above the base on drop wire 22, and extend at a negative angle of approximately 45° with respect to the longitudinal axis of the driver antenna 14. The capacitance hats 26 and 27 function to provide a capacitive effect to the drop wire 22, drawing current and the associated electromagnetic field through the drop wire and thereby away from the base of the antenna and the user's head and allowing the folded monopole to be shortened. In a preferred form, the capacitance hats 26 are rigid copper conductors approximately .15-.25 inch in length and .01-.1 inch in diameter, and capacitance hats 27 are .2-.4 inch in length and .Ol-.l inch in diameter. The placement of the capacitance hats and the positive and negative angles referred to above enhance performance and increase bandwidth while minimizing radiation near the base of the antenna.

In particularly preferred forms the dropwire 22 has a smaller diameter than the main wire 20, resulting in a decrease of radiation near the base of the antenna near the user's head. The drop wire 22 may also be slightly shorter or longer than the main wire 20 to aid in overall tuning while simultaneously minimizing radiation near the base of the antenna.

The ground-plane circuit broadly designated 16 includes a plurality of radials 32, linear loads 34, capacitance hats 36 and a conductive ring 40 mounted on a circular circuit board 38. More particularly, the circuit board 38 is of the conventional 2 oz. copper type used in electrical and electronic applications and is made of fiberglass/epoxy, or similar dielectric material and covered with epoxy solder mask or similar material. The preferred circuit board is circular in shape, is of a diameter of approximately 2 1/2 inches, and has a small circular cut-out 42 at its center. The center cut-out 42 is surrounded by a conductive ring 40 of essentially square-shaped or circular configuration which is printed on the circular circuit board 38. The driver antenna 16 extends through the circuit board cut-out 42, centering and securing the circuit board 38 on the longitudinal axis of the driver antenna 16. The radials 32 are formed of a copper foil material applied to the circular circuit board 38, each having an approximate electrical length of 1/4 λ and an approximate actual length of .05-.1 λ. The radials 32 extend outwardly from the center of the circular circuit board and present distal and proximate portions in relation to the center of the circuit board. In a preferred form, the ground plane circuit contains four radials 32 which are equally spaced on the circuit board 38. To enhance reflectivity and absorbance, one pair of the radials is tuned to resonate in the radio receive frequency band, and the other pair is tuned to resonate in the radio transmit frequency band.

The linear loading means 34 consists of a copper foil material applied to the circuit board 38. The linear loads 34 provide bottom loading to the radials 32, adding inductive load and allowing shorter radials to be used. The linear loads 34 also help absorb and reflect radiation. In the preferred embodiment, the linear loads extend outwardly from the center of the circular circuit board 38 and are essentially configured in the shape of a "Y". One linear load 34 is provided for each radial 32. As illustrated, one end of each of the linear loads is connected to the proximate end of its respective radial, and the other end of each of the linear loads is connected to the conductive ring 40 at the center of the circuit board 38. The conductive ring 40 is electrically connected to the conventional shielding of the coaxial antenna cable of the cellular phone or other transceiver device. With this configuration, each of the linear loads 34 is electrically connected to the shielding of the transceiver coaxial cable.

The capacitance hats 36 and 37 consist of a copper foil material applied to the circuit board 38. The capacitance hats 36 provide a capacitive effect to the radials 32, drawing current to the distal end of each of the radials, thus shortening the radials and improving performance and shielding of the ground-plane circuit. In a preferred form, the ground-plane circuit 16 contains four capacitance hats 36, one for each radial 32. As illustrated, the capacitance hats 36 include a longitudinal portion 44 and two perpendicular depending leg portions 46. The longitudinal portions 44 are centered on and connected to the distal ends of the radials 32. The longitudinal portions 44 are perpendicular to their respective radials, and the depending leg portions 46 run parallel to the distal ends of their respective radials. As illustrated in Fig. 2, each capacitance hat 36 is connected to two adjacent capacitance hats at the depending leg portions 46 and at the ends of the longitudinal portions 44, forming a continuous capacitive circuit. The capacitance hats 37 connected near the distal end of the radials and the interconnected depending leg portions 47 provide additional capacitance to the radials for a more compact ground-plane and additional reflection and absorption. Interconnection of the capacitance hats further enhances the capacitance effect, providing for a more compact ground-plane circuit.

In preferred use, the above-described antenna is tuned to broadly cover the transmit frequency band of approximately 825-845 MHZ and receive frequency band of 870-890 MHZ and to minimize radiation in the bottom null area. As those skilled in the art will appreciate, the tuning process for the main driver antenna 14 is primarily performed by varying the length and diameter of the main and drop wires 20 and 22 and the spacing between them. The tuning process for the ground-plane circuit, is primarily performed by varying the length of the radials and linear loads, and the width of the copper foil for the radials, linear loads, and capacitance hats.

In operation, the ground-plane circuit 16 operates as an artificial radio-frequency ground and adds the electrical equivalent of another 1/4 λ to the driver element by electrically balancing that element. As illustrated in Figures 6 and 7, this specially designed ground-plane circuit acts as a shield by absorbing and reflecting radio-frequency electromagnetic fields away from the user's head and eliminating stray currents and radiation from the transceiver and coaxial cable. Any metal elements added to the antenna whether attached or not, become active and carry current. Therefore the "screen" is electrically optimized for the sum of all currents, including those induced by the electromagnetic field. In this unique design the ground plane in the circuit board is not only a set of radials with inductive loads and capacitance hats, but also a screen.

The entire ground-plane circuit is mounted on a compact and inexpensive circuit board. This configuration provides an inexpensive, yet compact and rugged anti- electromagnetic field antenna which is easy to manufacture using conventional materials.

Having thus described the preferred embodiment of the present invention, the following is claimed as new and desired to be secured by Letters Patent:

Claims

Claims ;

1. An antenna system adapted to be attached to the coaxial cable of portable transceiver devices, comprising: a monopole driver antenna fed by said coaxial cable; and means for shielding the user's head from radio- frequency electromagnetic fields emitted from said antenna, said shielding means including: a ground-plane circuit including at least two radials grounded to the shield of said coaxial cable and extending outwardly from said driver element, and support structure for said ground-plane circuit.

2. The antenna as set forth in claim 1, said ground-plane circuit further including means for providing capacitance to said radials.

3. The antenna as set forth in claim 1, said ground-plane circuit further including linear loading means for providing inductive load to said radials.

^ 4. The antenna as set forth in claim 1, said ground-plane circuit support structure being a circuit board.

5. The antenna as set forth in claim 4, said ground-plane circuit including four radials, two of said radials being tuned to resonate in the radio receive frequency band, and two of said radials being tuned to resonate in the radio transmit frequency band.

6. The antenna as set forth in claim 5, said radials being formed of a copper foil material applied to said circuit board, said radials extending outwardly from the center of said circular circuit board and presenting proximate and distal portions relative to the center of said circuit board.

7. The antenna as set forth in claim 6, said ground-plane circuit further including means for providing capacitance to said radials, said capacitive means comprising capacitance hats connected to said distal portions of said radials.

8. The antenna as set forth in claim 6, said ground-plane circuit further including linear loading means for providing inductive load to said radials, said linear loading means being formed of a copper foil material printed on said circuit board and electrically connected to said proximate portions of said radials.

9. The antenna as set forth in claim l, said monopole driver element consisting of a conductor folded over itself and including a main wire and drop wire, said main wire being connected to said shield of said coaxial cable, said drop wire being connected to said feed wire of said coaxial cable, said driver element extending through and centered on said circular circuit board.

10. The antenna as set forth in claim 9, said main wire and said drop wire including a plurality of outwardly extending capacitance hats.

11. The antenna as set forth in claim 9, said drop wire being a smaller diameter than said main wire.

12. An antenna system for portable phones and radios, comprising: a coaxial cable including a feed wire and shielding for electrically connecting said antenna to said phone and radio; a folded monopole driver element consisting of a conductor folded over itself and including a main wire and drop wire, said main wire being connected to said shield of said coaxial cable, said drop wire being connected to said feed wire of said coaxial cable; means for shielding the user's head from radio- frequency electromagnetic fields emitted from said antenna, said shielding means including: a ground-plane circuit including at least two radials extending outward from said driver element at an angle with respect to said driver element, at least two of said radials grounded to the shield of the radio coaxial cable, and support structure for said ground-plane, said support structure being a circuit board.