Radiator for slowly varying electromagnetic waves

• US 5,307,081 A
• Filed: 07/31/1992
• Issued: 04/26/1994
• Est. Priority Date: 11/27/1990
• Status: Expired due to Fees

• Current Assignee: Geophysical Survey Systems, Inc.
• Original Assignee: Geophysical Survey Systems, Inc.
• Inventors: Harmuth, Henny F.
• Primary Examiner(s): Hille, Rolf
• Assistant Examiner(s): Brown, Peter T.
• Application Number:
  US07/924,368
• Time in Patent Office: 634 Days
• Field of Search: 343/741-744, 343/753, 343/787, 343/788, 343/834, 343/866, 343/867, 343/898, 343/912, 343/913, 343/756, 343/841, 343/842, 1/PC1/00, 1/1/48
• US Class Current: 343/842
• CPC Class Codes: H01Q 1/04 Adaptation for subterranean or subaqueous use
• CPC Class Codes: H01Q 7/00 Loop antennas with a substantially uniform current distribution around the loop and having a directional radiation pattern in a plane perpendicular to the plane of the loop

Abstract

A radiator useful for radiating pulses with a duration of about 10 ms is disclosed. Such pulses occupy the frequency band from zero to a few hundred Hertz. For a given time variation of an electromagnetic signal, the energy radiated in the far field is proportional to (Is)², where I is the current amplitude in the antenna and s is the length of the radiator. Typical antenna designs cannot be used at very low frequencies with large relative bandwidths. However, the large current radiator disclosed, herein, is small, has antenna currents in the order of 10⁸ A, and requires a drive voltage of about 1 volt and drive current of 10⁴ A. This large current radiator is designed with a small antenna length s by using a design wherein the antenna current is n times larger than the drive current. This is accomplished by winding electrically conductive means n times around a shield so that the n forward loop wires are all on one side of the shield, and cover a surface area sxW. The n return loop wires are on the opposite side of the shield and are confined so that they cover a surface area that is very small compared to the area of the forward loop. Furthermore, the shield is fabricated to reflect the electromagnetic energy produced by the forward loop and absorb the electromagnetic energy produced by the return loop. Hence, the antenna is highly efficient. And, since n can be 10,000 or more, the antenna current can be in the kilo Ampere range and beyond with a moderate drive current.

Claims

1. A non-resonant antenna for non-sinusoidal very low frequency (vlf) electromagnetic waves comprising relatively long pulses on the order of 10 ms in duration, comprising:

first electrically conductive means formed to cover a first surface area having a predetermined length;

second electrically conductive means formed to cover a second surface area which is substantially smaller than the first surface area, electrically connected in series with the first electrically conductive means having the predetermined length; and

shield means disposed substantially between the first electrically conductive means and the second electrically conductive means, including a pair of apertures through which said first conductive means and said second conductive means are serially connected so that the current in the first conductive means is equal to the current in the second conductive means, and wherein said shield means electromagnetically separates said first conductive means from said second conductive means, so that any electromagnetic energy radiated or received by the first conductive means is not radiated or received by the second conductive means, shield means having a first surface facing the first electrically conductive means and being made substantially of electromagnetically reflective material for reflecting, back towards the first conductive means, the electromagnetic energy incident on it from the first electrically conductive means.

2. An antenna according to claim 1 wherein said first surface of said shield means is an electromagnetically reflective material having a high permeability to said vlf pulses of relatively long duration in a first direction and substantially no conductivity to said pulses in a second direction that is perpendicular to said first direction.

3. An antenna according to claim 2 wherein:

said first direction is parallel to the magnetic field vector H of the electromagnetic wave produced by said first electrically conductive means; and

said second direction is parallel to the electric field vector E in the electromagnetic wave produced by said first electrically conductive means.

4. An antenna according to claim 2 wherein the first surface of the shield means comprises:

laminations of high permeability material;

at least one lamination of an electrically insulating material; and

the laminations of high permeability material being stacked in parallel to each other with a lamination of said electrically insulating material between them.

5. An antenna according to claim 1 further comprising:

electromagnetically absorbing means facing the second electrically conductive means for absorbing a substantial portion of the energy radiated by the second electrically conductive means such that substantially no electromagnetic energy is reflected from the absorbing means toward the second conductive means.

6. An antenna according to claim 1 wherein:

said first electrically conductive means includes an electrically conductive plate with a large surface area; and

said second electrically conductive means includes electrically conducting wire.

7. An antenna according to claim 1 wherein:

said first conductive means and said second conductive means are comprised of a set of N electrically conducting wires wound in series, wherein said first conductive means is and disposed over said first surface area on a first side of said shield means; and

said second electrically conductive means is disposed on a second side of said shield means, and disposed over said second surface area.

8. The antenna of claim 1 wherein the shield means substantially encircles the second electrically conductive means.

9. The antenna of claim 8 wherein the second electrically conductive means is a set of wires and the shield means is a three-dimensional solid shape with the set of wires passing through the apertures therein.

10. The antenna of claim 9 wherein the three-dimensional solid shape is cylindrical.

11. An antenna according to claim 1 wherein the first and second electrically conductive means are comprised of k conductors electrically connected in series, each carrying the same current, so that the current carrying capacity of the antenna is k times the current in each conductor.

12. An array of M antennas, each constructed according to claim 1 wherein the M antennas are electrically connected in series, and physically arranged to form one long antenna.

13. An antenna according to claim 1 further including a step down transformer, a first current being supplied to an input of the step down transformer, a second current, produced at the output of the step down transformer, being connected to drive the first conductive means, whereby the first current is smaller than the second current.

14. A non-resonant antenna for non-sinusoidal very low frequency (vlf) electromagnetic waves comprising relatively long pulses on the other of 10 ms in duration, comprising:

first electrically conductive means formed to cover a first surface area having a predetermined length;

second electrically conductive means formed to cover a second surface area which is substantially smaller than the first surface area, electrically connected in series with the first electrically conductive means having the predetermined length; and

shield means disposed substantially between the first electrically conductive means and the second electrically conductive means, including a pair of apertures through which said first conductive means and said second conductive means are serially connected so that the current in the first conductive means is equal to the current in the second conductive means, and wherein said shield means electromagnetically separates said first conductive means from said second conductive means so that any electromagnetic energy radiated or received by the first conductive means is not also radiated or received by the second conductive means, said shield means having a first surface facing the second electrically conductive means and an opposite second surface facing the first electrically conductive means, wherein the first shield surface is made at least partly of electromagnetically absorptive material for absorbing a substantial portion of the electromagnetic energy incident on it from the second electrically conductive means, such that substantially no electromagnetic energy is reflected from the first shield surface toward the second conductive means.

15. An antenna according to claim 14 wherein said first surface of said shield means is an electromagnetically absorptive material having a high permeability to said vlf pulses of relatively long duration in a first direction and a non-zero conductivity to said pulses in a second direction that is perpendicular to said first direction.

16. An antenna according to claim 15 wherein:

said first direction is parallel to the magnetic field vector H of the electromagnetic wave produced by said second electrically conductive means; and

said second direction is parallel to the electric field vector E of the electromagnetic wave produced by said second electrically conductive means.

17. An antenna according to claim 15 wherein the first surface of the shield means comprises:

first laminations of high permeability material that are electrically conductive; and

second laminations of known electrical conductivity;

wherein said first laminations are stacked parallel to each other with at least one second lamination between each pair of first laminations.

18. An antenna according to claim 14 wherein:

said first electrically conductive means includes an electrically conductive plate with a large surface area; and

said second electrically conductive means includes an electrically conducting wire.

19. An antenna according to claim 14 wherein:

said first conductive means and said second conductive means are comprised of a set of N electrically conducting wires wound in series, wherein said first conductive means is disposed over said first surface area on a first side of said shield means; and

said second electrically conductive means is disposed on a second side of said shield means, and disposed over said second surface area.

20. The antenna of claim 14 wherein the shield means substantially encircles the second electrically conductive means.

21. The antenna of claim 20 wherein the second electrically conductive means is a set of wires and the shield means is a three-dimensionally solid shape with the set of wires passing through the apertures therein.

22. The antenna of claim 21 wherein the three-dimensional solid shape is cylindrical.

23. A method which includes the steps of:

a) providing an antenna with, connected in series through a pair of apertures in a shield,

1) a forward loop that covers a large area surrounding at least a section of the shield that electromagnetically reflects signals incident upon it back towards the forward loop, and

2) a return loop that is surrounded by the shield wherein a second section of the shield electromagnetically absorbs signals, radiated by the return loop;

b) driving a high current through the forward and return loops for radiating large relative bandwidth electromagnetic energy; and

c) sensing current in both the forward and return loops when receiving electromagnetic energy in the forward loop.

24. A non-resonant antenna for radiating and receiving non-sinusoidal, very low frequency electromagnetic waves comprising:

an electromagnetic shield including a pair of apertures therein;

a large number of electrically conductive means for carrying large current, connected in series through said apertures, and disposed so as to form;

a) a forward loop that covers a first surface area on a first side of said electromagnetic shield, and

b) a return loop that covers a second surface area that is substantially smaller than the first surface, and on a second side of said electromagnetic shield opposite the first side of said electromagnetic shield;

wherein the first side of said shield reflects the electromagnetic energy radiated by or received by the forward loop.