US2419480A - Loop antenna shield - Google Patents

Description

April 22, 1947. R. H.'BRYAN ET'AI. 2,419,480

LOOP ANTENNA SHIELD Filed May 18, 1944 ll wc( 1,6" f7 221411.@@11/ if :fa/1 145 /mz/ Y y Wvw 5MM Q Patented Apr. 22, 1947 LOOP ANTENNA SHIELD Robert H. Bryan, Oak Park, and Truman L.

Johnston, Wilmette, Ill., assgnors to Stewart- Warner Corporation, Chicago, Ill., a corporation of Virginia Application May 18, 1944, Serial No. 536,116

(Cl. Z50-33) 7 Claims.

The present invention relates in general to Aa loop antenna shield, and more particularly to an electrostatic shield for such antennas.

One of the objects of the present invention is to provide an improved antenna shield of tear 'drop or streamlined design to reduce the aerodynamic drag when used on airships or airplanes. Another object is to provide an electrostatic shield that will shield the loop antenna electrostatically in any position without producing unde` magnetic eld which is used to determine the direction of the incoming signal is able to pass through to the loop antenna very readily.

Another object is to provide improved means for shielding the loop antenna so as to reduce so called precipitation static.

A further object is to provide a simple and eiective method of constructing an electrostatic shielding shell.

Still another object is to provide a simple method of providing shielding conductors on the streamlined supporting shell.

A stil1 further object is to provide an electrostatic shield on a loop antenna in which there is no appreciable loss of sensitivity as a result of the shielding.

There are other objects of the invention which, together with the foregoing, will be described more in detail in the specification that is to follow, with reference to the accompanying drawings illustrating certain specific embodiments thereof.

It is well known that when a loop antenna is employed in aerial-navigation for direction finding, it is desirable for the loop to be shielded in order to eliminate the pickup of non-directional electrostatic fields. Suchv shielding will insure that the null in the directive electromagnetic eld Y being employed is definite and certain. However, if such shielding is placed about the loop in position to rotate with it, undesirable aerodynamic drag occurs.

The suggestion has therefore been made to employ a streamlined shell or casing of fixed position about the loop antenna. However, in such instances almost invariably the shielding is not uniform with respect to varying angular positions of the loop and the nulls in the directive signal being received vary and are uncertain.

Precipitation static is the name given to extraneous electrical charges which have caused considerable difficulty in the radio receiving sets of airplanes, and became much more troublesome with the advent of high speed ying. It was thought to be caused by the accumulation of electrical charges from rain, mist, and snow, and more recently has been assumed to be a charge caused by the friction of the wind on the surfaces of the airplane. It is, however, very evident that a static charge of considerable magnitude is built up and sporadically discharged, causing great difficulties of radio reception in the operation of the radio direction nders.

Shielding the loop antenna reduces the elect of precipitation static very materially and in conjunction with precipitation static discharge devices eliminates it to the point where a radio compass can function. The present inventionv combines a loop shield and a means for discharging precipitation static. The invention employs two sets of parallel conductors each open at one end with their other ends connected to ground, providing a so called Faraday method of shielding that permits the electromagnetic Waves to penetrate therethrough but substantially stops electrostatic penetration. By the employment of two sets of open ended parallel conductors the electromagnetic ei'ect from the electrostatic currents discharging to ground cancel one another. The tear drop design of the casing or shell supporting the conductors permits the free rotation of the loop antenna Within, as well as locating the grounding bars for the parallel conductors as far away from the loop as possible. These conductors, being located on the outside of the plastic shell, discharge the precipitation static before it can accumulate.

In practicing the invention, it will thus be seen that a plastic shell of streamlined or tear drop design is molded of sufficient size so as to enclose the loop antenna and permit its free rotation. Parallel grooves are molded in the outer surface of the plastic shell which is constructed from a dielectric plastic such as laminated Bakelite. A longitudinally extending groove is formed in each end of the plastic shell. Separate grounding conductors are mounted in each longitudinal groove and connect to an annular metallic ring molded in the base of the shell. Each longitudinal conductor is connected to conductors lying parallel to one another in the grooves about the shell, one half being connected to one longitudinal conductor and the other half to the other. The parallel conductors are open ended. The shielding shell is secured to the metal surface of the airplane in such position that its axis parallels the line of ilight of the plane, thereby effectively shielding and streamlining the loop antenna.

The drawings, comprising Figs. 1 to 4, inclusive, show two specific embodiments of the invention.

Referring now to the drawings:

Fig. 1 is a perspective elevation of the invention with a certain portion of the plastic shell cut away to show the loop antenna;

Fig, 2 is an end elevation of the same, illustrating how the parallel conductors are secured to the grounding longitudinal conductor on one side and open ended on the other;

Fig. 3 is a sectional view of Fig, 1, taken along the line 3 3 thereof, looking in the direction of the arrows; and

Fig. 4 is a side elevation of a modified form of the invention illustrating a little different type of streamlining wherein the shell is reduced in diameter at the bottom to provide a pedestal.

In the drawings, like reference characters apply to similar parts throughout.

Referring now to the drawings, the reference character 6 indicates a shell of a dielectric plastic of general streamline shape of sufficient size to enclose a loop antenna l rotatably mounted therein. The shell or casing S may be formed from laminated Bakelite and pressed into the shape shown, or may be molded. Into each side of the shell ll, parallel grooves 8 are formed and extend into a channel 9 at one end of lthe shell and terminate short of a channel iii, while on the other side of the shell 6 similar grooves join the channel l and terminate short of the channel 9.

Metallic conductors ll and E2 are secured in the channels 9 and I0. respectively, at each end of the shell 6 and are connected to charge collecting conductors ifi and i5, respectively, secured in parallel grooves 8 lying in each hemisphere of the shell. The longitudinal conductors l I and I2 are suitably connected with the metallic ring Il' extending about the base of the shell 8, and suitably secured thereto by molding or in any other convenient manner.

A number of openings IB are provided about the bottom periphery of the shell 6 and permit the fastening of the same to the surface i9 of .the airplane by means of rivets or screws 28. This brings the annular ring Il into direct electrical connection with the metal surface of the plane, thus grounding it,

rIhe parallel conductors lil and l may be made from any good conducting wire, such as copper or silver, and may be soldered to the longitudinal connecting wires H and i2 respectively. The longitudinal connecting wires ll and i2 extend to the openings 2i and Z2, and are soldered to the annular conducting ring il.

A very convenient method of constructing this parallel conductor arrangement is to employ a metallic spray such as copper or silver, whereby the metal will lie in both the horizontal and longitudinal grooves formed in the plastic shell and the excess metal is wiped or scraped away.

The loop antenna 7 may be of any usual and well known construction and is freely rotatable within the shell S by any convenient means. The shell 5 is secured to the surface of the plane in such direction that its blunt nose faces the direction of travel of the plane, thus reducing aerodynamic drag to its minimum.

It will be seen that the parallel conductors are so placed about the enclosing plastic shell that the conductors in each hemisphere are grounded at one end and open at the other. As before pointed out, this type of construction not only forms a very eifective electrostatic shield of the Faraday type for the loop antenna l, but neutralizes any currents in the conductors. As a result, the conductors do not form a closed circuit and the electromagnetic iield produced as the electrostatic charges iiow to ground is therefore negligible. The electromagnetic field from the directive wave can. however, freely penetrate the structure.

Since precipitation static is mainly electrostatic in character, the improved construction is very effective in eliminating it, the electromagnetic fields around the parallel conductors substantially canceling each other. The conductors are spaced sufficiently from the loop antenna that any uncanceled fields will have no appreciable effect upon the loop.

Use of parallel conductors in this manner about the two sides of the insulating shell 6 also prevents any appreciable efect on the nulls in the directive signal for any position of the loop antenna since free passage of the electromagnetic wave is provided,

The construction of the modified form of the invention shown in Fig. 4 is essentially the same as that of l, except that the lower portion of the streamlined shell 2E has a reduced diameter of streamline cross section shown at 25, furnishing a base or pedestal. In this case the horizontal grooves containing the parallel conductors it and i5 do not need to extend below the level of the loop antenna 'i shown in the dotted lines.

The longitudinal conductors Il and IZ may be connected to an annular metallic ring at the bottom of the pedestal the same as il, Fig. 3, which furnishes `the method of grounding the shield to the airplane structure. The shielding conductors may be grounded in any other suitable manner,

The construction of Fig. l furnishes a more convenient mounting at times, though the operation of the two forms or shapes of shields are essentially the same.

From the foregoing, it will appear that the shield is suiiiciently streamlined to add but little drag, and this slight drag is constant, irrespective of the position of the loop antenna. Since the grounded collector conductors are at opposite ends of the shield, the arrangement of the shielding wires is symmetrical, and fields induced by current ilow through the wires of one group are substantially canceled by the elds due to current ilow through conductors of the other group. Because all of the circuits are open, that is, none of the conductors completely encircles the shield, these conductors cannot produce fields of substantial strength and as a result the loop antenna will be eective to detect the direction of the incoming directional signal with a high degree of precision. The charges due to precipitation static are rapidly dissipated to the skin and frame of the airplane since the conductors are located on the streamlined shell and preferably on the outside of the shell, and thus the harmful interfering eifects of precipitation static are substantially completely eliminated,

While we have shown and described particular embodiments of our invention, it will be apparent that numerous variations and modifications thereof may be made without departing from the underlying principles of the invention. We therefore desire, by the following claims, to include within the scope of our invention all such variations and modications by which substantially the results of our invention may be obtained through the use of substantially the same or equivalent means.

We claim:

1. An electrostatic shield for a loop antenna comprising, a shell of dielectric material, a plurality of spaced parallel conductors mounted around said shell, each extending slightly less than one-half the distance around the shell, and a pair of collecting conductors connected` to ground and located at opposite ends of the shell, one half of said parallel conductors being connected to one collecting conductor and th'e other half of said parallel conductors being connected to the other collecting conductor.

2. An electrostatic shield for a loop antenna comprising, a shell of dielectric material, a plurality of spaced parallel conductors mounted around said shell and substantially enclosing it, each of said conductors extending slightly less than one-half the distance around the shell, a pair of collecting conductors connected to ground and located at opposite ends of the shell, one half of said parallel conductors each having one end connected to one of said collecting conductors, the opposite end being free, and the other half of said parallel conductors each having one end connected to the other of said collecting conductors with their opposite ends free.

3. An electrostatic shield for a loop antenna comprising a shell of dielectric material having a streamline shape, a plurality of parallel conductors on said shell, a grounded metal ring secured to said shell, a pair of longitudinal conductors each connected t0 said ring and located at diametrically opposed ends of said shell, one half of said parallel conductors being connected at one end to one of the longitudinal conductors and the other half of said parallel conductors being connected at one end of the other longitudinal conductor.

4. An electrostatic shield for a loop antenna comprising, a, shell of dielectric material having a streamline shape, a plurality of spaced parallel conductors on said shell, a grounded metal part secured to said shell, a pair of longitudinal conductors at the opposite ends of said shell and each connected to said part, one half of said parallel conductors being connected at one end to one of the longitudinal conductors and the other half of said parallel conductors being connected at one end to the other longitudinal conductor, the other ends of both sets of parallel conductors being open.

5. An electrostatic shield for a loop antenna comprising a shell of dielectric material having a streamline shape,` a grounded metal part secured to the base of said shell, a pair of longitudinal conductors each connected to said metal part and located 4on the shell substantially 180 apart, a plurality of parallel'oonductors mounted around said shell, one half of said parallel con ductors being connected at one end to one of the longitudinal conductors and the other half of said parallel conductors being connected at one end to the other longitudinal conductor, the other ends of both sets of parallel conductors being open, and means for clamping said shell to the frame of an airplane.

6. An electrostatic shield for a loop antenna, comprising a shell of dielectric material having a streamline shape and having a base formed for attachment for the skin of an airplane, a plurality of groups of parallel conductors extending partly around said shell, a metal part secured to said shell at the base thereof so as to make grounding contact with the skin of an airplane to which the shell is secured, a plurality of collecting conductors each connected to said part, said collecting conductors being spaced substantially 180 apart, the parallel conductors of each group being connected at one end only to one of said collecting conductors.

7. A stationary electrostatic shield for a loop antenna, comprising a streamlined dielectric housing in which the loop antenna may rotate,

n said housing having a plurality of sets of parallel grooves therein, the grooves of each set having their adjacent ends joined by a groove extending transversely thereof, the transverse grooves being spaced apart substantially 180, metallic conductors substantially iilling all of said grooves, and means for grounding the conductors in the transverse grooves.

ROBERT H. BRYAN.

TRUMAN L. JOHNSTON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 644,530 Mercer Feb. 27, 1900 2,297,488 Luderitz Sept. 29, 1942 1,824,579 Threlkeld et al. Sept. 22, 1931 2,234,998 Worrall Mar. 18, 1941 2,343,306 Lear Mar. 7, 1944 D. 122,616 Lear Sept. 17, 1940 D. 132,380 Page Sept, 1'?, 1940

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