US3514780A - Circularly polarized loop v antenna - Google Patents

Description

May 26, 1970 GNP. PETRIcK T AL 3,514,780

CIRCULARLY POLARIZED LOOP V ANTENNA Filed March 51, 1967 FIG. I

INVENTORS Ont-I. I. from Mt: r. NLS

BY HOBI'RT G. FIIRELL -IR W x .4 TTOR/VE/F United States Patent 3,514,780 CIRCULARLY POLARIZED LOOP V ANTENNA George P. Petrick, St. Petersburg, James P. Jones, Senunole, and Robert G. Ferrell, Jr., St. Petersburg, Fla., assignors to Electronic Communications, Inc. Filed Mar. 3-1, 1967, Ser. No. 627,433 Int. Cl. H01q 7/38 US. Cl. 343-830 7 Claims ABSTRACT OF THE DISCLOSURE An antenna for launching and receiving circularly and eliptically polarized electromagnetic waves consisting of a plurality of elements lying on the surface of an imaginary cone, the apex being removed from the ground plane and being shielded therefrom by a coupling transformer and electrically grounded spaced radials resonant at the highest operating frequency of the antenna.

Background of the invention A conventional loop V antenna consists of a plurality of radiating elements each having vertical and horizontal portions lying on an imaginary cone, the apex of which forms a common feed junction. While this type of antenna provides a convenient expedient for launching and receiving circularly polarized waves, it has been found that circular polarization exists for a particular frequency only and that over a range of frequencies, the contribution of each of the elements varies, thereby affecting the axial ratio (a measure of the circular polarization). Further, it has been found that the radiation pattern of the conventional loop V configuration degrades at low angles. This is of particular significance since receivers located at the horizon would not be positioned for optimum reception.

Objects of the invention Accordingly, it is the object of this invention to provide an antenna exhibiting circular polarization char acteristics over a broad frequency range at a low look or vertical angle. In other words, an antenna which optimizes both circularity and pattern gain for angles near horizontal.

It is a further object of this invention to achieve the foregoing object with an antenna of minimum complexity.

Summary of the invention Briefly, the invention is predicated upon the concept of eliminating negative vertical vector components by a feed choke which doubles as an impedance matching transformer and providing an image ground resonant at the highest frequency in the desired band.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawing.

Brief description of the drawing FIGS. 1 and 2 show partial plan and elevation views, respectively, of a modified loop V antenna according to the invention.

3,514,780 Patented May 26, 1970 Description of the preferred embodiment The basic loop V antenna, which may be seen by reference to the upper portion (above the apex) of FIG. 1, consists of a plurality of quarter wave loop elements 10 supported and excited by an equal plurality of quarter wave stubs 12 tied together by common feed junction 20.

The radiation coverage for horizontal polarization is obtained from the loop potrion of the loop V and is closely approximated by an analogy to an ungrounded circular loop over a ground plane. For approximating the radiation coverage for vertical polarization, the vertical elements may be treated as an ungrounded plural element array.

In order to obtain circular polarization, space and time quadrature vectors are necessary. That is, the basic criteria for circular polarization are two elements oriented with respect to one another, both in space and time. The space quadrature is apparent from the drawing; the horizontal vector component being available from the horizontal 1001;) member 10 and the vertical component being derived from angled element 12. With elements 10 and 12 each being one quarter of a wavelength long at the design frequency, every point on the horizontal loop 10 is 90 out of time phase with a corresponding point on the vertical element 12. Hence, phase quadrature is also achieved.

While the horizontal component element is shown to be arcuate, it may also be straight. However, looping is the more convenient configuration and permits a proximity between the end of one element and the beginning of the other, thereby improving the omnidirectional characteristic in the azimuth plane. The use of four elements, rather than three or five, is also a matter of convenience, and it is to be understood that a single element would also launch and receive circularly polarized waves. The omnidirectional characteristics, however, would obviously be degraded.

In the conventional loop V antenna the common feed apex junction 20 is disposed -a predetermined height above ground. It has been theorized that the vertical feed stem of the conventional loop V antenna (not shown) contributes to the vertically polarized field vector. An analysis of current distribution indicates that the stem produces a component out of time phase with the vertical antenna element field component, resulting in a degradation of the axial ratio. The degradation becomes even more pronounced as the frequency is increased within the band.

An attempt to improvethe antenna circularity by shorting the feed stem and lowering the antenna height results in a decreased axial ratio; however, the horizontal gain is significantly reduced more than offsetting the improvement. Further, as the frequency is increased over the bandwidth, the polarization reduces to linear at an angle of 45. At this juncture, it bears mentioning that in the strict technical sense circular polarization requires an axial ratio of zero db. In practice, however, an axial ratio of 3 db or less is considered circular polarization.

In the following, the inventive antenna wil be described with reference to an embodiment having a bandwidth of from 225 to 300 MHZ. It wil be assumed that refers to the wavelength at the lowest operating frequency (in this case 4.36 feet) and A refers to the wavelength at the highest operating frequency (in this case 3.28 feet) within the band.

As may be seen from the drawing, a peripheral grounded choke sleeve 18 is provided about the antenna feed stem (elements 24 and 22) to eliminate the out of phase vertical field vector. The use of a choke is not accompanied by a reduction in low look angle gain. Further, the choke sleeve simultaneously supports an important secondary function without the addition of extraneous elements. This function, as will be explained, is a reduction in the VSWR (voltage standing wave ratio) which inherently accompanies a mis-match between the antenna input impedance and the characteristic impedance of the feeding coaxial cable.

The input impedance of an antenna is dictated by, among other things, its configuration. The characteristic impedance of the cable feeding the antenna depend upon ancillary equipment considerations. In order to avoid reflections due to the diverse considerations and resultant mis-match, a transitional element must be introduced. The characteristic impedance of the cable may, for practical purposes, be assumed to be 50 ohms. The impedance of a single branch of the antenna shown in the figures is approximately 500 ohms; consequently, four elements in parallel yield an antenna input impedance of approximately 125 ohms.

A quarter wavelength transformer having an impedance value equal to the square root of 50 125, or approximately 80 ohms, will serve as the transition to match the antenna input impedance to the coaxial line.

Utilizing the formula for characteristic impedance of a coaxial line as:

138 D Z 10 VER g d the conductor diameters may be determined where:

E =relative dielectric constant D=the outer conductor diameter d=the inner conductor diameter Since the choke sleeve length (which will house and form a part of the transformer) is dictated by the look angle requirements and the axial ratio and not the desired quarter wavelength, the dielectric material must be carefully chosen to compensate. It was found that Teflon, having a dielectric constant of 2.1 and yielding a D/d ratio of 4.68, gives a one quarter wavelength choketransformer at the center frequency (which, in this case, would be 262 MHZ).

Due to the unique choke-transformer arrangement and the necessity for coupling to and mechanically supporting the antenna, the transformer is formed in two stages having inner conductors 24 and 22 and outer conductors 24' and 22 respectively. Tapering between the two allows a smooth transition in the transformer, obviating a discontinuity. Both transformer stages retain the same 4.68 ratio of outer to inner conductor diameters and hence have the same Z The upper inner conductor supports the antenna proper while the smaller conductor 22 is available to serve as the center conductor of the coupler 26. Flange 16 is provided as a convenient means for mounting the antenna upon the platform which will serve as ground.

The antenna thus far described is substantially improved at the lower operating frequency of 225 MHZ. At the upper operating frequency, however, both the axial ratio and the gain at the low look angle start to degrade. This situation is completely ameliorated by radials 14.

Radials 14 are of a length to be resonant at the higher operating frequency All/4 and produce an effective ground for the antenna at this frequency. At the lower frequency, the radials are not seen by the antenna and the ground is that coupled to flange 16. This arrangement not only significantly improves the gain and axial ratio at the higher frequency but also at the lower frequency. Altering the height of the radials with respect to ground flange 16, as well as the radial angle qb, produces variations in the axial ratio and the horizon gain; these may be empirically determined to give the best results for the frequency range desired.

While the foregoing manifested a substantially improved axial ratio and low look gain vis-a-vis a conventional loop V antenna, an analysis of the radiation patterns indicated the ratio of horizontally to vertically polarized vectors had not been completely optimized. Since a lowering of the antenna height affects the matching transformer, the location of the radial with respect to ground, and the low look gain, a shortening of the feed stem to compensate is precluded.

It was found, however, that if the vertical to horizontal element length ratio is varied while the approximate same overall length is maintained, the ratio is optimized. That is, the sum of antenna element arms 10 and 12 is maintained at a constant approximately equal to /2 the Wavelength at the lower operating frequency, while the vertical element is increased at the expense of the horizontal element 10.

For the frequency range of 225 to 300 mHz., the following parameters were found to yield the antenna attributes desired and described herein.

(1) Vertical antenna element 12-.268)\ 14". (2) Horizontal antenna element 10.232 12". (3) Cone angle 045.

(4) Radial angle 622.

(5) Ground radials 14.25)\ 13".

(6) Transformer height.14)\ -7.3".

(7) Radial height.12 ,5.2".

(8) Outer to inner conductor ratio4.68.

(9) Choke outer diameter3 /z.

While the principles of the invention have been described in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. A loop V antenna of the type having a plurality of vertical and horizontal elements lying on the surface of an imaginary cone and fed via a stem coupled to the cone apex, wherein the improved comprises a grounded choke sleeve surrounding said stem, the dimensions of said choke sleeve and said stem being diameter-related to form a coaxial line having substantially the characteristic impedance of a matching transformer.

2. A loop V antenna of the type having a plurality of vertical and horizonal elements lying on the surface of an imaginary cone and fed via a stem coupled to the cone apex, wherein the improvement comprises a grounded choke sleeve surrounding said stem, and a plurality of radial elements spaced about said choke sleeve and resonant at a predetermined frequency within the frequency band of the antenna.

3. The antenna claimed in claim 2 wherein the dimensions of said choke sleeve and stem are diameter-related to form a coaxial line having substantially the characteristic impedance of a matching transformer.

4. The antenna claimed in claim 3 wherein the sum of the lengths of the serial vertical and horizontal elements is approximately equal to one-half the wavelength of the lowest inband frequency of the antenna, and wherein said vertical and horizontal elements are unequal in length.

5. The antenna claimed in claim 1 wherein the space between said stem and said choke sleeve is filled with dielectric material, and wherein the diameters of said stem and choke sleeve vary, respectively, with substantially the same ratio therebetween.

6. The antenna claimed in claim 2 wherein said plurality of radial elements intersects said choke sleeve at an angle less than perpendicular.

7. The antenna claimed in claim 2 wherein said radials are of a length which is substantially an integral multiple of one-quarter wavelength at the highest inband frequency of the antenna.

References Cited UNITED STATES PATENTS 2,284,434 5/1942 Lindenblad 343830 X 2,489,287 11/1949 Guarino et al. 343830 X 6 2,508,438 5/ 1950 Wilson et a1. 343830 2,532,920 12/ 1950 Johnson 343743 3,247,515 4/1966 Boyer 343-743 X HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner US. Cl. X.R.

Images