Near Field Spiral Antenna

Abstract

A near field antenna is formed of a coil printed on an insulating board mounted in parallel fixed spaced relationship to its own ground plane. The coil is capacitively shunt tuned and capacitively coupled serially to a coaxial line. A planar ferrite sheet may be incorporated to enhance the sensitivity of the antenna. The printed coil is provided with means for adjustably fixing its length.

Description

SUMMARY OF THE INVENTION

This invention serves to establish a transmission link using a VHF near field. The particular utility for this invention is in the link between a transmitting antenna mounted within the tire of a wheel of a vehicle and a receiving antenna mounted on the vehicle adjacent the wheel within the near field of the transmitter. The purpose of the near field antenna is the reception of a VHF signal from the transmitting antenna upon the occurrence of an abnormal condition such as a low pressure or high temperature within the tire. The receiving antenna comprises a planar coil printed on an insulating circuit board which is mounted in parallel fixed spaced relationship to a conducting ground plane for the antenna. The coil is capacitively shunt tuned and capacitively coupled serially to a coaxial line. A planar ferrite sheet may be added to enhance the sensitivity of the antenna. The coil is provided with fine adjustment means to permit relatively fine tuning to compensate for the relatively coarse capacity tuning mandatory in a mass production environment.

BACKGROUND OF THE INVENTION

In communications systems which establish a transmission link from within or adjacent to an automobile or truck tire to a receiver within the automobile or truck, a small transmitter located within or close to the tire radiates a VHF signal indicating an abnormal condition such as low pressure or high temperature. The transmitted signal is coupled to a receiving antenna located adjacent the tire in a convenient place on the body or fender of the vehicle in the near field of the transmitter. The receiving antenna is connected to a signal processor and receiver through a coaxial cable. Such a system is shown in U.S. Pat. application Ser. No. 289,702, filed Sept. 15, 1972, assigned to the same assignee as this invention.

The relatively short distance between the transmitter and receiver antennas places the receiver antenna within the near field of the transmitting antenna and the problem of optimum transmission is therefore simplified to the optimization of the coupling between the antennas, taking into consideration the available space. As disclosed in the aforementioned application, the transmitter may comprise an oscillator which generates an oscillatory electrical signal in a tuned circuit. The magnetic A. C. field generated by the inductive component of the tuned circuit couples into the receiving antenna and thereby effects the desired transmission. The receiving antenna comprises a resonating coil and shunt capacitor optimally capacitively coupled to an R. F. transmission line.

This invention provides a receiving antenna which includes a metallic groundplace spaced a fixed distance in parallel relationship to a planar printed circuit coil to eliminate or minimize the effect of variations in the ground plane on which the antenna is mounted. The printed circuit coil is inductively tunable after manufacture to the precise frequency of the transmission. A planar ferrite sheet may be added to enhance the sensitivity of the antenna.

THE DRAWINGS

FIG. 1 is a schematic representation of an antenna in accordance with this invention;

FIG. 2 is a cross-sectional view showing a preferred antenna construction made in accordance with this invention;

FIG. 3 is a plan view of a preferred form printed circuit board with an adjustable length antenna coil mounted thereon;

FIG. 4 is a modified printed circuit board; and

FIGS. 5 and 6 represent another form of this invention.

Referring to FIG. 1, the near field receiving antenna comprises a coil 10 tuned to the frequency of the transmitter by means of capacitors 12 and 14 additionally coupled to a processor-amplifier (not shown) by means of a capacitor 14 through a coaxial cable 16.

The mechanical configuration for the antenna shown in FIG. 1 is illustrated in FIGS. 2 and 3 which includes an insulating printed circuit board 18 on which all of the elements are mounted. As best seen in FIG. 3, the antenna coil is planar and the windings are spirally printed on the surface of the printed circuit board 18. The coil 10 has a predetermined length to provide a given inductance within relatively close tolerances. The coil 10 is tuned by the lumped capacitors 12 and 14 which are the subject of much wider tolerance variations than the coil 10. To overcome those capacity tolerances, the inductance of the antenna coil 10 is made adjustable. The antenna coil 10 includes a plurality of turns 20 beginning at an outer terminal 22 and ending at an inner terminal 24. The precise length of the coil is determined by the location of a connection consisting of a shunt 26 made to any one of a plurality of spaced parallel taps 28 all of equal length and all connected to lead 29 which completes the coil. If the shunt 26, which may consist of a short length of bare wire, is located adjacent the point A, the length and therefore the inductance of the coil 10 is at its maximum. If the shunt 26 is connected to an intermediate point, the length of the inductor is shortened, and its length and inductance becomes a minimum when the shunt 26 is moved adjacent point B.

As previously noted, the coil 10 is tuned to the transmitter frequency by means of capacitors 12 and 14 and is also coupled to the receiver by means of capacitor 14 which in turn is connected to the inner terminal 30 of a coaxial connector 32. The values of the capacitor 12 and 14 are chosen to properly resonate the antenna coil 10. However, because of the fact that the length of the coil 10 is adjustable by selecting the position of the shunt 26, the capacitors 12 and 14 may have commercial tolerances which are subsequently compensated for by proper selection of a tap 28 by the shunt 26.

The connector 32 is clamped to a metal sheet 34 by means of nuts 36 which provide a ground connection for the outer connector terminal 38. The support for the printed circuit board 18 includes a rectangular-shaped insulating housing 40, the periphery of which is recessed at 41 so as to provide a shoulder to seat the printed circuit board 18. The sheet 34 is positioned on the outside of the housing 40 opposite the printed circuit board 18 thereby establishing a ground plane in spaced relationship to the coil 10 and minimizing the effect of variations in the ground plane. It is maintained fixed on the bottom of the housing 40 by tensioning the lead 42 from the capacitor 14 to the inner terminal 30 of the connector 32 as well as by means of the ground connection 44 from the capacitor 12.

The entire assembly is provided with a cover 46 which is provided with holes 48 and 50 through which attaching screws may be passed to provide for convenient attachment to a vehicle.

A modification of the printed circuit board of FIG. 3 is shown in FIG. 4. In FIG. 4 the length of the coil 10 is determined by the position of a pivotable shunt 52 to any one of a plurality of taps 54 arcuately positioned with respect to the connector 52. The position of shunt 52 is fixed by soldering.

The embodiment of FIGS. 5 and 6 differs from that of FIG. 2 in the means for housing and spacing the components. As seen in FIG. 5, the sheet 34 is positioned in fixed spaced parallel relationship to the printed circuit board 18 by means of spacer pins 56, thereby establishing a ground plane for coil 10. Instead of using the housing 46, the coil 10 is coated with a layer of insulating plastic 58 and the entire volume between the sheet 34 and the printed circuit board 18 is filled with a potting material 60 to provide a rigid and moistureproof enclosing means for the assembly.

In addition, the embodiment of FIGS. 5 and 6 is provided with an inductance adjustment by making the last turn 62 of the coil 10 a solid wire. The inductance can be reduced before coating by flipping the turn 180.degree. on the imaginary axis 64.

The sensitivity of the antenna may be increased by addition of a planar ferrite sheet 66 inserted between the printed circuit board 18 and the ground plane 34. The ferrite sheet serves to increase the coupling to a transmitting coil located on the other side of the printed circuit board.

Various modifications and adaptations of this invention will be apparent to persons skilled in the art. For example, while there are shown ordinary circuit capacitors 12 and 14, it is within the scope of this invention to provide such capacitors by means of printed circuit techniques. Furthermore, while only an antenna circuit is mounted on the circuit board, it is apparent that for other applications it may be appropriate to mount all or at least some of the signal processing circuitry on the antenna circuit board along with the antenna.

Claims

We claim:

1. A near field antenna comprising:

an insulated printed circut board having at least first and second terminals thereon;

a planar spiral conductor on said board, one end of said spiral conductor being connected to said first terminal;

a length of additional conductor;

means for connecting selected portions of said additional conductor between the other end of said spiral conductor and said second terminal;

capacity shunt tuning means connected across said spiral conductor and said additional conductor;

an output circuit;

capacity coupling means between said spiral conductor and said output circuit; and

a metal sheet mounted under said printed circuit in parallel fixed spaced relationship to the plane of said spiral conductor.

2. The invention as defined in claim 1 wherein a plurality of taps each are connected to a respective point along the length of said additional conductor, one end of said additional conductor being connected to said second terminal; and

means for connecting the other end of said spiral conductor to a selected tap.

3. The invention as defined in claim 2 wherein said means for connecting said other end of said spiral conductor to a selected tap comprises a second additional conductor.

4. The invention as defined in claim 3 wherein said second additional conductor is pivotally connected to said other end of said spiral conductor, the other end of said additional conductor being connectable to a selected tap.

5. The invention as defined in claim 3 wherein said means for connecting the other end of said spiral conductor to a selected tap comprises a bare wire electrically connected between said other end and a selected one of said taps.

6. The invention as defined in claim 1, and an insulated rectangular housing, said printed circuit board being seated on one end of said housing, said metal sheet being seated on the opposing end of said housing, whereby said planar sheet and said printed circuit board are maintained in parallel fixed spaced relationship.

7. The invention as defined in claim 1 wherein said metal sheet is maintained in parallel fixed spaced relationship to the plane of said spiral conductor by means of a plurality of spacer pins, the space between said printed circuit board and said metal sheet being filled with an insulating potting material.

8. The invention as defined in claim 1 wherein a ferrite sheet is positioned between said insulated printed circuit board and said metal sheet.

9. A near field antenna comprising:

an insulated printed circuit board having at least first and second terminals thereon;

a planar spiral conductor on said board, one end of said spiral conductor being connected to said first terminal;

a length of additional wire conductor connected between the other end of said spiral conductor and said second terminal;

said wire conductor being movably arranged to change the area enclosed by the inner turn of said spiral conductor;

capacity shunt tuning means connected across said spiral conductor and said additional conductor;

an output circuit;

capacity coupling means between said spiral conductor and said output circuit; and

a metal sheet mounted under said printed circuit in parallel fixed spaced relationship to the plane of said spiral conductor.