US3183510A

US3183510A - Underground loop antenna

Info

Publication number: US3183510A
Application number: US77778A
Authority: US
Inventors: Lucien E Rawls
Original assignee: Deco Electronics Inc
Current assignee: Deco Electronics Inc
Priority date: 1960-12-22
Filing date: 1960-12-22
Publication date: 1965-05-11
Anticipated expiration: 1982-05-11

Description

May 11,1965 L. E. RAwLs UNDERGROUND LOOP ANTENNA .2 Sheets-Sheet 1 Filed Dec. 22, 1960 FIG. |A

\\\\\w 1 Sig l f WAV@ FIC-5.2

FIG. 4

INVENTOR LUCIEN E. RAwLS BY Wo 072e/ W0@ ATTORNEYS May 11, 1965 E. RAwLs 3,183,510

` UNDERGROUND LOOP ANTENNA Filed Dec.y 22. 1960 2 Sheets-Sheet 2 INVENTOR. LUCIEN E. RAWLS BY 7X0@ @fw/Wo@ .i ATTORNEYS United States Patent O 3,183,510 UNDERGROUND LGOP ANTENNA Lucien E. Rawls, Leesburg, Va., assigner, by mesne assignments, to Deco Electronics, Inc., Washington, D.C., a corporation of Virginia Filed Dec. 22, 1960, Ser. No. 77,778 3 Claims. (Cl. 343-719) The present invention relates to hardened antennas and particularly to a buried antenna capable of withstanding nuclear blasts or theY like.

It is an object of the present invention to provide an antenna which presents a small target cross section and which has an optimum launching efficiency relative to its cost.

Another object of the invention is to provide a buried antenna which radiates a vertical polarized field above ground.

It is another object of the invention to provide an efficient buried antenna having a subs-tantially omnidirectional radiation pattern.

It is another object of the invention to provide a buried antenna whose efficiency is not impaired by subfsurface and surface water.

According to the invention, radio waves, preferably below 500 kc., are transmitted from a buried loop antenna or loop antenna array. Radio waves in the frequencies ranging downward from 500 kc. are transmitted more and more efficiently by a buried loop antenna or loop antenna array than by other antenna types of cornparable dimensions in terms of wavelengths. Each loop antenna is located in a vertical plane and has a length of the order of a quarter wavelength of the radiated waves in air. The height or width of the antenna is small relative to its length. The antenna radiates vertically polarized waves in a fgure-of-eight pattern having its axis along the length of the loop and nulls at right angles to the loop. To achieve an omnidirectional pattern, a pair of loops may be arranged at right angles or four loops may be arranged in the form of a turnstile. The antenna is located in low loss material and provided with means for shedding surface water and draining sub-surface water to maintain a high launching efficiency.

Other objects and advantages of the invention will become apparent and the invention itself will be fully understood from the following description and the drawing wherein:

FIG. l is a schematic sectional view showing a loop antenna according to the invention;

FIG. 2 is a cross sectional view taken along the line 2--2 of FIG. l;

FIG. 3 is an end View of a wire forming the loop antenna;

FIG. 4 is a sectional view showing another embodiment of the invention;

FIG. 5 is a schematic view of a two element orthogonal loop antenna array; and

FIG. 6 is a schematic view of a quadrangular array of four loop antennas.

Referring to the drawing, a two-wire coaxial cable 1t) connects loop antenna 12 to a transmitter. The radio `Waves transmitted may be anywhere in the range below 500 kc., but preferably below 100 kc. For the sake of definitencss, the present invention will be described particularly with reference to an antenna operating at a frequency near 500 kc. Wires 13, 14 of cable 10 and

connections

15 and 16 feed arms 17 to 2t) of the loop. The loop has a length of approximately a quarter wavelength in air of the Waves to be transmitted. The length of a quarter Wavelength is a rough upper limit to the overall length. The length may be more with some resultant ICC distortion of the figure 8 horizontal radiation pattern of a single loop. If less, the efficiency will be decreased. For a frequency of 500 kc. a suitable length of the loop is approximately 150 meters. Loop 12 is located in a vertical plane with

arms

17 and 20 immediately above

arms

18 and 19, respectively, and spaced therefrom a distance of about 3 feet. Loop 12 is tuned by a capacitor 21 connected between conductors 13 and 14.

Loop 12 is formed of a conductor having a plastic core 24 surrounded by a cylindrical stranded conductor 25, such as a heavy duty Copperweld wire. Conductor 25 is embedded in a plastic cover 26 formed of material such as polystyrene having a low effective dielectric constant, say less than 3. The plastic cover 26 should have a relatively larger diameter, for example, 4V inches, when the diameter of conductor 2S is about 2 inches. The larger part of the latter dimension is that of plastic core 24. The plastic cover 26, having a low dielectric constant and low loss and relatively large diameter, reduces losses which would otherwise occur in the material surrounding the loop wires.

The construction of the antenna will now be described particularly with reference to FIG. t2. The ground is graded so as to provide a run-off for surface water. For

this purpose a

suitable incline

27, 28 is provided on both sides of the position of the loop antenna, and this grad ing, of course, extends the entire length of the antenna. Three

trenches

3d, 31 and 32, are dug to a suitable depth, for example, 10 feet. The trenches can have any convenient shape; for example, they can be triangular or V-shaped, having sloping sides, or straight sides, as shown in FIG. 2. These trenches run the entire length of the loop antenna. At the bottom of the outer trenches drainage means are provided, such as

clay tile

33 and 34. The clay tile drains extend the entire length of the loop antenna and slope downwardly from the center of the loop toward the two ends thereof. The tile may be in the form of pipe having openings therealong for the admission of water. After the clay :

tile

33 and 34 are laid at the bottom of

trenches

31 and 32, these trenches are re-filled with a suitable material which will impede the movement of water toward the center trench. Such a material is stabilized soil which will remain dry. Trench 3i) is filled to a suitable depth of, say, two feet, with silica sand or vermiculite. The loop antenna is then placed in the trench and connected to the twin coaxial line 1@ as shown in FlG. l. The ltrench 30 is further filled with coarse silica sand or vermiculite to a height slightly above the loop antenna. The remainder of trench 3f) is then filled with stabilized soil to a depth of about l foot within the surface. A plastic Pliofilm 40 is -then laid over trenches 30-32 for the entire length of the antenna to form a Waterproof shed over the trenches. To increase th? effectiveness of lm 40, it is preferably extended a few feet beyond the

outer trenches

31, 32. Film 4t) is then covered with earth 41 to hold down and hide the film and to provide a suitable slope of the earth to facilitate run-off of surface water.

In the embodiment of the invention shown in FIG. 4 a triangular trench 47 has a low loss fill 42 such as concrete or processed, dried earth. Upper loop wire 43 is buried at about one meter, or deeper, and lower loop wire 44 at one meter or more below the upper wire. A weatherproof cover or Pliofilm 45 is placed over the trench, slightly below the surface of the earth, to shed surface water. lf the fill 42 is sufficiently resistant to water, the

side trenches

31 and 32 of FIG. 2 may be dispensed with. The length of the loop depends on the frequency, a length of about 250 to 300 feet being suitable at 1 megacycle and a length of about 500v feet at 500 kc. Generally, the loop length and the number of Crossovers of the loop 3 cable wire are such that the phase shift along the loop is negligible, the length being uncritical.

In order to provide an omnidirectional radiation pattern a pair of

loop antennas

46, 48 are placed across each other as shown in FIG. 5.

Antennas

46 and 48 are each connected by cables, as shown in FIG. 1, to a transmitter which energizes these loop antennas in phase quadrature. Another method of obtaining a substantially omnidirectional radiation pattern is to use four loop antennas of the type disclosed in FIG. 1, but arranged as in FIG. 6.

Loops

51 and 53, FIG. 6, are preferably energized in opposite phase, as are

loops

52 and 54, and

loops

51 and 53 are energized in phase quadrature with

loops

52 and 54. All four antennas 51 to 54 are connected by suitable cables to the same transmitter. It is to be understood that the antennas in FIGS. and 6 are of the same construction and are buried in the same manner as the antenna of FIGS. 1 and 2.

As is indicated in patent application Serial No. 77,5 S7, tiled on December 22, 1960, in behalf of George F. Leydorf and Lucien E. Rawls, launching losses are significantly reduced if power losses occassioned by skin effect and proximity efr'ect are minimized. Advantageously, such losses are minimized if the subject antenna, or antenna array, is buried below the ground surface at a depth of the order of one skin depth; i.e., about one skin depth from the surface of the ground to the longitudinal central axis of the loop. Further, proximity effect losses are reduced if the upper and lower loop conductors are spaced-apart by about the order of a skin depth. Skin effect and proximity effect are well known phenomena, which are explained in great detail in the following books: Electronics and Radio Engineering 4th edition, authored by Frederick E. Terman and published by McGraw-Hill Company, at pages 2l 'to 24 and pages 803 to 808; and, American Institute of Physics Handbook, published in 1957 by the McGraw-Hill Company, at pages 5-85 to 5-95. Brieily, the phenomenon called skin effect is the tendency of an alternating, or varying, current to redistribute over the conducting mediums cross section in such a pattern that the current density is greater where it is encircled by fewer lines of magnetic llux; proximity eiect being the term given to the skin eiect phenomenon in connection with interacting conducting media such as looped conductors and coils. A radiated electromagnetic eld near a conducting medium induces a current flow in the conducting medium: The current density in the conducting medium being greatest near the inducing iield; the current density decreasing exponentially in the conducting medium at distances more and more remote from plate where the eld is being applied. At a remote distance, or depth of penetration, from the place where the eld is being applied, the current density will be about 36.8% of the maximum current density.

`sions have been given.

The remote distance at which this reduction in current density occurs is called one skin depth.

For the sake of deniteness, a particular embodiment of the invention has been described and particular dimen- It will be understood, however, that many variations and modications of the arrangements herein disclosed will be apparent to those skilled in the art. For example, other meanswhich might be used for maintaining a low conductivity and low loss in the vicinity of the antenna include lling the loop-containing trench with oil or applying heat to the trench. Therefore, it is not desired to limit the invention except vas deiined in the following claims.

What is claimed is:

l. A loop antenna for transmitting radio waves, comprising a single turn loop having an upper conductor and a lower conductor located in a vertical plane, a transmission line connected to said loop for feeding the same, said loop, having a length approximately equal to a quarter wavelength in air ofthe radio waves transmitted thereby, the distance between the upper and lower wires of theloop being a very` small fraction of the length of the loop, said loop being buried in a trench within the earth with the center of said loop at a depth below the earths surface of no more thanone skin depth, said trench being filled to above the loop with a material having a lower loss than the surrounding earth, separate drainage trenches on each side of said loop antenna extending the entire length thereof, and a liquid impermeable cover located above said trench and extending the entire length of the loop for shielding said loop antenna from surface water.

2. A loop antenna according to claim 1, wherein said drainage trenches include pipe means at the bottoms thereof covered by a stabilized soil, and wherein said impermeable cover also covers said drainage trenches.

3. A loop antenna according to claim 2, wherein said material is coarse silica sand.

References Cited by the Examiner UNITED STATES PATENTS 1,220,005! 3/ 17 Lyon et al 343-719 1,365,579 1/21 Appleby et al. 343-719 X 1,373,612 4/21 Hanson 343-719 1,530,129 3/25 Loftin et al. 343-719 2,690,509 9/54 Toth 343-855 2,910,695 10/59 Troost et al. 343-855 X 2,980,793 4/61 Daniel 343-719 X FOREIGN PATENTS 149,532 8/20 GreatfBritain. 224,661 11/ 24 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

BENNETT G. MILLER, Examiner.

Claims

1. A LOOP ANTENNA FOR TRANSMITTING RADIO WAVES, COMPRISING A SINGLE TURN LOOP HAVING AN UPPER CONDUCTOR AND A LOWER CONDUCTOR LOCATED IN A VERTICAL PLANE, A TRANSMISSION LINE CONNECTED TO SAID LOOP FOR FEEDING THE SAME, SAID LOOP HAVING A LENGTH APPROXIMATELY EQUAL TO A QUARTER WAVELENGTH IN AIR OF THE RADIO WAVES TRANSMITTED THEREBY, THE DISTANCE BETWEEN THE UPPER AND LOWER WIRES OF THE LOOP BEING A VERY SMALL FRACTION OF THE LENGTH OF THE LOOP, SAID LOOP BEING BURIED IN A TRENCH WITHIN THE EARTH WITH THE CENTER OF SAID LOOP AT A DEPTH BELOW THE EARTH''S SURFACE OF NO MORE THAN ONE SKIN DEPTH, SAID TRENCH BEING FILLED TO ABOVE THE LOOP WITH A MATERIAL HAVING A LOWER LOSS THAN THE SURROUNDING EARTH, SEPARATE DRAINAGE TRENCHES ON EACH SIDE OF SAID LOOP ANTENNA EXTENDING THE ENTIRE LENGTH THEREOF, AND A LIQUID IMPERMEABLE COVER LOCATED ABOVE SAID TRENCH AND EXTENDING THE ENTIRE LENGTH OF THE LOOP FOR SHIELDING SAID LOOP ANTENNA FROM SURFACE WATER.