US3453634A

US3453634A - Loopstick antennas

Info

Publication number: US3453634A
Application number: US550776A
Authority: US
Inventors: Roswell W Gilbert
Original assignee: Dictaphone Corp
Current assignee: Dictaphone Corp
Priority date: 1966-05-17
Filing date: 1966-05-17
Publication date: 1969-07-01
Anticipated expiration: 1986-07-01
Also published as: FR1522941A; BE698460A; DE1591099A1; NL6706758A; GB1166134A

Description

July 1, 1969 I R. w. GILBERT 3,453,634

LOOPSTI CK ANTENNAS Filed May 17. 1966 INVENTOR /Fa:/x zz 1 6715597 ATTORN United States Patent 3,453,634 LOOPSTICK ANTENNAS Roswell W. Gilbert, New York, N.Y., assignor to Dictaphone Corporation, Bridgeport, Conn. Filed May 17, 1966, Ser. No. 550,776 Int. Cl. H01q 7/08; H011 15/04 US. 'Cl. 343-788 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates, in general, to apparatus for radiating and receiving electrical signals and, in particular, to improved loopstick antennas.

A loopstick antenna, in its simplest form, includes a core of magnetic material, for example, a ferrite rod, around which is wound a coil of wire. The ends of the coil are connected to a transmitter or a receiver, dependent upon whether the antenna is being used to radiate or receive electrical signals.

A desirable feature of loopstick antennas is that they require relatively few inexpensive components so that they may be fabricated at relatively low cost. This renders loopstick antennas particularly attractive for use in portable radios, wireless intercoms, short-range transceivers and other low-cost, wireless radio-frequency units. Loopstick antennas, however, become less effective when constraints are imposed, for example, as to the size, shape and operating frequency. This has, to some extent, limited the use of such antennas in low-cost units such as those set forth above.

A number of techniques have been employed to improve the efliciency of a loopstick antenna, but none of these techniques has proven sufiiciently satisfactory when such an antenna is to be used in a compact, low-cost wireless unit. One approach has been to change the configuration of the antenna, either by way of an increase in its size" or a change in its shape. If the size of the antenna is increased to improve the etficiency to the desired extent, the antenna may become so large as to be unsuitable for the compact units under consideration. If the shape of the antenna is changed to a more complicated form, this may make it too expensive for a low-cost unit.

Another technique which has been employed for increasing the efiiciency of a loopstick antenna has been to wind the coil around the magnetic core in a special pattern and position. Such special coil position and pattern, however, are complex factors so that winding the coil to achieve the desired efiiciency may be a diflicult and costly operation, again, rendering the antenna unsuitable for compact, low-cost units.

Accordingly, it is an object of the present invention to provide new and improved loopstick antennas having relatively high efficiencies.

It is another object of the present invention to provide such highly eflicient loopstick'antennas of relatively small size.

It is a further object of the present invention to provide highly efiicient and compact loopstick antennas which 3,453,634 Patented July 1, 1969 are relatively simple in construction and inexpensive to fabricate.

Briefly stated, these objects are achieved by surrounding the magnetic core of a loopstick antenna with a conductive surface or shield which forces magnetic flux into a more effective pattern. In one embodiment of the invention, this shield is in the form of a cylindrical sheet of conductive material which is wrapped around the magnetic core. In another embodiment of the invention, the conductive surface or shield is in the form of an annular disc encircling the magnetic core intermediate its ends. In still another embodiment of the invention, the cylindrical shield and the annular disc shield are used in combination.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring to the drawing:

FIGURE 1 is a perspective view of one embodiment of a loopstick antenna constructed in accordance with the present invention;

FIGURE 2 is a horizontal 2-2 of FIGURE 1;

FIGURE 3 is a perspective view of a second embodiment of a loopstick antenna constructed in accordance with the present invention;

FIGURE 4 is a horizontal section taken along line 44 of FIGURE 3; and

FIGURE 5 is a perspective view of a third embodiment of a loopstick antenna constructed in accordance with the present invention.

The loopstick antenna illustrated in FIGURES 1 and 2 includes a core 10 of magnetic material and a coil 12 wound around the core 10. The core 10, in cylindrical form, may be a rod of the typical ferrite material which is commonly used in loopstick antennas. The ends 12a and 12b of the coil 12 are connected to a transmitter or a receiver (neither of which is shown) so that radio frequency signals may be conducted from the transmitter to the antenna when the antenna serves to radiate signals, or radio frequency signals may be conducted from the antenna to the receiver when the antenna serves to receive signals. For the sake of clarity, the coil 12 has been shown as having only three turns. It will be understood, however, that the coil 12 may have many more turns dependent upon the particular application of the antenna. Among the factors which determine the number of turns of the coil 12 are, typically, the operating frequency of the antenna and the parameters of the circuit to which it is connected.

Located between the core 10 and the coil of wire 12 is a conductive surface in the form of a sheet or shield of conductive material 14 Wrapped cylindrically around the core 10 for one and a fraction turns so that the fractional turn overlaps a portion of the first turn. In the embodiment illustrated, the overlapped region extends over approximately one-quarter of a turn. The cylindrical shield 14 is seen to extend over a substantial portion of the length of the core 10. The shield 14 may have such thickness as to be self-supporting, or it may be made of thin foil material.

It is important that the overlapped portions of the cylindrical shield 14 do not make electrical contact So as to form a short-circuited turn around the core 10. In order to prevent this, a sheet 16 of insulating material, for example, waxed paper, is wrapped with the sheet of conductive material 14 around the core 10 so as to be disposed between the two layers of this shield 14 throughout the overlapped region. In the drawing, the spaces between the core 10, the shield 14 and insulating sheet 16 are exaggerated section taken along line simply for purposes of illustration. It will be appreciated, however, that, in practice, the conductive sheet 14 and insulating sheet 16 preferably are Wrapped tightly around the core in contact with each other.

It is believed that the cylindrical shield 14 forces magnetic flux in the core 10 into a more effective pattern than would be the case in the absence of the conductive sheet 14. Without the cylindrical shield 14, the magnetic flux pattern along the core would include appreciable lines of flux which would emerge from and return to the core throughout the intermediate portion of its length. These intermediate flux lines contribute little to a radiated signal and do not contribute significantly to the current induced in the coil 12 when the antenna serves to receive signals since only a few of the turns of the coil would be cut by these flux lines.

By providing the conductive cylindrical shield 14, magnetic flux is impeded in emerging from and returning to the core 10 throughout the intermediate portion thereof within the confines of the cylindrical shield 14. Instead, the magnetic flux lines are forced to follow longitudinal paths in the core 10 so that they emerge from and return to the core near its ends where the conductive sheet 14 terminates. This causes a concentration of flux in that portion of the flux pattern which is more effective in the radiation or reception of signals. As a result, the radiation resistance of the antenna is increased, causing a lowering of the Q-factor which, in turn, improves the figure-ofmerit of the antenna. Another way of viewing this result is that the effective polar spacing of the antenna is increased, which is comparable to lengthening the antenna. Any closed circular circuit around the core 10 is broken along edges 14a and 14b of the cylindrical conductive sheet 14 by the insulation 16 in the overlapped region so that the net current around the core is zero.

As previously mentioned, the cylindrical conductive sheet 14 may be of material having such physical thickness as to be self-supporting, or it may be of foil material. When foil is employed, the cylindrical sheet is likely to require a plurality of turns so that the conductivity thickness of the shield is sufficient to impede flux penetration. The conductivity thickness of the cylindrical wrap is detenmined by the physical thickness of the conductive surface and the particular material selected.

A second embodiment of a loopstick antenna constructed in accordance with the present invention is illustrated in FIGURES 3 and 4. This antenna includes the usual magnetic core 20 and coil 22 wound around the core. Surrounding the core 20 and the coil 22 is a conductive disc or shield 24 which is spaced slightly from the coil. The disc shield 24 is in the form of an annular plate, with its ends overlapping, which is positioned near the center of the core 20 and disposed perpendicular to the longitudinal axis of the core. The overlapping portions of the disc shield 24 are spaced apart slightly to prevent electrical contact therebetween, similar to the insulation between the overlapped layers of the cylindrical shield 14 of FIGURES l and 2. The disc shield 24 may be supported in place with respect to the core 20 by any suitable means (not shown) which does not interfere with the radiation or reception of signals by the antenna.

The disc shield 24, like the cylindrical shield .14 of FIG- URES 1 and 2, forces magnetic flux in the core 10 into a more effective pattern than would be the case in the absence of the shield. Again, those flux lines which might otherwise contribute little to the effective operation of the antenna are forced to follow almost complete longitudinal paths through the core with the result that there is a maximum concentration of flux lines in the core. The effect is similar to increasing appreciably the effective crosssectional area of the antenna. Any circular circuit around the core 20 is broken along the

edges

24a and 24b of the disc shield 24 by the spacing between its overlapping end portions.

It should be noted that foil material also may be used in the loopstick antenna in FIGURES 3 and 4 instead. of the thicker annular disc 24. When using foil material, a plastic ring, for example, may be employed to support the foil.

The spacing between the core 20 and the disc shield.24 is exaggerated in the drawing simply to illustrate better that the coil 22 passes between the core and the shield and extends to either side of the shield. In practice, the shield 24 preferably is snug against the coil 22.

Another embodiment of a loopstick antenna constructed in accordance with the present invention is illustrated in FIGURE 5. This embodiment combines the cylindrical shield 14 of the embodiment of FIGURES l and 2, and the annular disc shield 24 of the embodiment of FIGURES 3 and 4. Specifically, the loopstick antenna in FIGURE 5 includes the customary core 30 and coil 32. Interposed therebetween and surrounding the core is a cylindrical shield 34a of conductive material. Positioned at the center of this cylindrical shield 34a and surrounding the coil 32 is an overlapped annular disc shield 34b.

The cylindrical shield 34a is arranged in a similar manner to the cylindrical shield 14 in FIGURES 1 and 2 in that the cylindrical shield 34a also includes one and a fraction turns of conductive material wrapped around the core 30 with a sheet 36 of insulating material disposed between layers of the cylindrical shield 34a throughout the overlapped region.

The disc shield 34b is arranged in a generally similar manner to the disc shield 24 in FIGURES 3 and 4, and preferably is positioned snugly against the coil 32. These two annular shields differ, however, in that the annular shield 34b is provided with a strip 38 of insulating material along its inner edge at the gap between its overlapping portions to prevent it from conductively bridging the insulated overlap gap of the cylindrical shield 34a, and to prevent the conductive cylindrical shield 34a from conductively bridging the spaced overlap gap of the disc shield 34b. Thus, the broken circular circuits of the cylindrical shield 34a and the disc shield 34b are not short-circuited by each other. Hence, there are no closed turns around the core 30 so that the net current around the core is zero.

The two conductive surfaces, namely, cylindrical shield 34a and disc shield 34b, together force the magnetic flux lines into a more effective pattern in the same manner that they do separately. However, they provide a marked improvement over the improvement accomplished by either separately.

In a typical antenna embodying the present invention, in the form illustrated in FIGURES 1 and 2, the length of the core 10 was 4 /2, and its diameter was /2". The material of cylindrical shield 14 was copper, which was wrapped around the core one and a quarter turns. The length of cylindrical shield 14 was 3" and the thickness was 0.010". The insulation 16 was waxed paper and the coil 12 had turns.

While there have been described what are at present considered to be the preferred embodiments of this invention it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A loopstick antenna comprising: a cylindrical ferrite rod; a sheet of conductive material and a sheet of insulating material wrapped around said rod into a cylindrical wrap of more than one turn whereby the edges of said conductive sheet overlap one another, said sheet of insulating material insulating portions of said conductive material from each other over the region of overlap; and a coil of wire wound around said cylindrical wrap for conducting radio frequency current.

2. A loopstick antenna comprising: an elongated magnetic core, a conductive disc encircling said core and having a non-conductive gap, said disc having a thickness which is small relative to the length of said core, and a width which is of the same order of magnitude as the thickness of said core, said disc extending in a direction which is transverse to the longitudinal axis of said core, and a coil of wire wound around said core for conducting radio frequency current.

3. A loopstick antenna according to claim 2 wherein the conductive disc is perpendicular to the longitudinal axis of the core and is positioned at the center of said core.

4. A loopstick antenna comprising:

a cylindrical ferrite rod;

an overlapped annular conductive plate encircling said rod at the center of said rod, said plate being perpendicular to the longitudinal axis of said rod and overlapping portions of said plate being spaced apart;

and a coil of wire wound around said rod for conducting radio frequency current.

5. A loopstick antenna according to claim 4 wherein the coil of wire on the rod extends on both sides of the annular plate.

6. A loopstick antenna comprising:

a magnetic core;

a conductive sheet wrapped around said core into a cylindrical shield of one and a fraction turns whereby said fractional turn overlaps a portion of the first turn;

an insulating material disposed between layers of said conductive sheet over the region of overlap;

a coil of wire wound around said cylindrical shield for conducting radio frequency current;

and an overlapped annular conductive plate encircling said coil, overlapping portions of said plate being spaced apart.

7. A loopstick antenna according to claim 6 wherein the annular plate is disposed perpendicular to the longitudinal axis of the core and is positioned at the center of said core.

8. A loopstick antenna comprising:

a magnetic core;

a conductive sheet wrapped around said core into a cylindrical wrap of one and a fraction turns whereby said fractional turn overlaps a portion of the first turn;

an insulating material disposed in the gap bet-ween layers of said conductive sheet over the region of overlap;

a coil of wire wound around said cylindrical wrap for conducting radio frequency current;

an overlapped annular conductive plate encircling said core, overlapping portions of said plate being spaced apart;

and insulating means interposed between said cylindrical wrap and said overlapped annular plate for preventing said plate from conductively bridging said insulated overlap gap between layers of said cylindrical wrap and for preventing said cylindrical wrap from conductively bridging the gap between said spaced overlapping portions of said plate.

References Cited UNITED STATES PATENTS 2,981,945 4/1961 Fyler et al. 343787 3,267,478 8/1966 Schiefer 343-788 3,378,626 4/1968 Tucker 336-84 FOREIGN PATENTS 468,973 3/1924 Germany.

ELI LIEBERMAN, Primary Examiner.

US. Cl. X.R. 336-84