US2518933A - Antenna for radiating circularly polarized waves - Google Patents

Description

ug- 1950 R. M.REDHEFFER 2,518,933

ANTENNA FOR RADIATING CIRCULARLY POLARIZED WAVES Filed Aug. 1, 1945 INVENTOR RAYMOND M. REDHEFFER ATTORNEY dipoles.

Patented Aug. 15, 1950 UNITED STATES PATENT OFFICE ANTENNA FOR RADIATING CIRCULARLY POLARIZED WAVES Raymond M. Redheifer, Cambridge, Mass., as-

signor, by mesne assignments, to the United States of America as represented by the Secretary of War Application August 1, 1945, Serial No. 608,296

1 Claim.

This invention relates in general to antennas and more particularly to antennas in which the resultant radiation is circularly polarized.

Normally antennas have the electromagnetic radiation emanating from them polarized in one plane, usually a plane either perpendicular to the 7 ground or horizontal to the ground, giving rise to either vertical polarization or horizontal polarization, respectively. Horizontal polarization may be caused by the elements of the antenna, say a number of dipoles, being positioned horizontally with respect to the ground. The electric field radiated from these dipoles is oriented in a hori- The maximum voltage will be induced in this receiving antenna when it is parallel in space to the The same analysis may be applied to vertical polarization, which may be represented by an electric field, or E, vector vertical in space.

For some purposes it is desirable to have the radiation pattern from an antenna system circularly polarized. In this case the angle of polarization is constantly changing. It may be represented by an electric field, or E, vector rotating in space at a constant angular velocity w radians per second, where w=21rf, being the frequency of oscillations of the radio-frequency generator causing the field in cycles per second. It may furthermor be seen that a receiving antenna need not be oriented in a direction substantially 2. To provide such a system in which is incor-, porated a means to cause the polarization of the radiation to be circular.

In accordance with the present invention there are provided two embodiments of a means for changing linear polarized waves to circularly po-. The first incorporation hereof inlarized Waves. volves a dipole antenna array with dielectric strips wound about the antenna and at a short distance from it. These dielectric strips are chosen of a fibrous material in which the dielectric constant along the fibers is different from that normal to the fibers. The orientation of the dielectric fibers must be substantially 45 from the polarization of the dipoles forming the array.

Taking into account the difference in the dielec-.

tric constants for waves having their electric vectors parallel to and those having their electric vectors perpendicular to the fibers, a thickness of the dielectric is chosen so that the difference in time of passage of the two abovementioned electric vectors will be a quarter of a cycle. This will produce the circular polarization desired.

The second embodiment provides a dielectric housing for an antenna in which the outer surface of the housing makes a high angle of incidence with respect to the polarization of the wave from the antenna. The dipoles are oriented at substantially 45 with respect to the vertical. The properties of the housing are such that reflection oil the interior walls causes different efiective path lengths for the horizontal component of the electric vector and for the vertical component. The result is a circularly polarized wave.

This invention will best be understood by reference to the drawings in which Figs. 1 and 2 show two embodiments of the present invention.

Referring now to the description of one embodiment of the apparatus and to Fig. 1, there is shown a functional diagram of an antenna array which is essentially non-directional in the azimuthal horizontal plane. To lead-in member IE! there are coupled four groups of curved dipoles, such as the group containing dipoles l I, I2, and

7 [3. These dipoles are of such a curvature that all those of a group lie along the circumference of the same circle. The antenna array is usually oriented so that the azimuthal plane is the plane of the aforementioned circle. Any number of groups of dipoles may be used, the greater the number of groups, the narrower will be the beam of the radiation pattern of the array in a plane perpendicular to the azimuthal plane. It is understood that the system is not limited to this type of non-directional array. Also, if a uniform azimuthal radiation pattern is not desired, the system should not be limited to a non-directional type of array.

A housing [4 is placed about the antenna array which consists of strips of dielectric material wound around the array at an angle of substantially 45 with the plane of the dipoles, the azimuthal plane. This dielectric material must be such that it have a different effective dielectric constant along the direction of winding than normal to the direction of winding. One substance having this property is so-called Polyfiber. Another method of obtaining this property is to use a number of closely-spaced thin conducting wires wound in a direction substantially 45 to the plane of the dipoles. The size and spacing of the wires must be carefully chosen.

. The latter construction is therefore equivalent in efiect to a dielectric material such as housing M of Fig. 1 or housing 24 of Fig. 2.

A functional diagram of a second embodiment

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