US 3550136 A
Description (OCR text may contain errors)
Dec. 22,1970 c. H. WALTER ET L 3,550,136
SEMI-HELICAL ANTENNA Filed March 14, 1968 5 Sheets-Sheet 1 FIGJAv CABLE v M 2o 40 Q WIRE LOOP GROUND PLANE,
. INVENTOR CARLTON H. WALTER ANTHONY G. JENNETTI ATTORNEY Sheets-Sheet 2 MZZMM c. H. WALTER ET AL SEMI-HELICAL ANTENNA Dec. 22, 1970 Filed Marqh 14. 1968 BY ANTHONY e. JENNETTI ATTORNEY R m m 2 W M m m 1 H a \A \A N L H. u 0 U 532 m c N |Z |Z .Z L WT L... w a v. m H c N n h A h B l 5 N H R 3 Q 3. M a a A N .N N N m G A G U 9 m N 7 U F F m m M n. 7 W G o m R I L E F. z P N N, A m m o z N w 6 w w W. m GmmmuuExEi fim 52 975? m w N H N N N United States Patent O 3,550,136 SEMI-HELICAL ANTENNA Carlton H. Walter, Columbus, Ohio, and Anthony G. Jennetti, Lebanon, Pa., assignors to The Ohio State University Research Foundation, Columbus, Ohio Filed Mar. 14, 1968, Ser. No. 713,161 Int. Cl. H01q 11/12, 1/48 U.S. Cl. 343-742 4 Claims ABSTRACT OF THE DISCLOSURE The invention is for super-directive semi-helical antennas, with increased directivity accomplished by excitation of a three-lobed cosine current mode.
BACKGROUND The invention relates to a super-directive antenna consisting of a semi-helix mounted on a ground plane. The combination of the ground plane and the semi-helix produce an antenna which acts as a one and one-half wave-. length dipole antenna but has a physical length of one half wavelength. The antenna is a narrow band device which has an increase in directivity of approximately 1.5 over that of a conventional dipole. The increased directivity is accomplished through excitation of a single 3rd mode in the semi-helix.
The invention has applications in communications, homing, and direction finding. The invention may be used on surface vehicles as well as supersonic aircraft. The antenna would be inexpensive to construct and the narrow beam characteristic would reduce interference. The an tenna has a coaxial input and is therefore easier to feed than the helical dipole.
OBJECTS Accordingly it is a principal object of the invention to provide an improved helical antenna.
Another object of the invention is to provide a helical antenna with substantially increased directivity when compared to conventional helical dipole antennas.
Another object of the invention is to provide a helical antenna that is super-directive over a narrow bandwidth to reduce interference.
Another object of the invention is to provide a superdirective antenna which permits in a single half-wavelength helical antenna many of the characteristics of a two-element array with sin element factor, thereby resulting in a space reduction.
Another object of the invention is to provide a superdirective antenna that is inexpensive to construct.
A further object of the invention is to provide a superdirective antenna which is low profile.
Still a further object of the invention is to provide a super-directive antenna which has applications in communications, homing, and direction finding on both supersonic aircraft and surface vehicles.
For a complete understanding of the invention, together with other objects and advantages thereof, reference may be made to the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of the side (FIG. la) and top (FIG. 1b)'views of the preferred embodiment of the invention;
FIG. 2 is a graphical representation of typical patterns (FIGS, 2a, 2b, and 2c) obtained from the antenna illustrated in FIG. 6;
FIG. 3 is an illustration of mode distributions and farfield patterns of a line source of length L for the 1st (FIG. 3a), 3rd (FIG. 3b), and 5th (FIG. 30) modes;
FIG. 4 is a graphical representation of the theoretical directivity as a function of aperture length for the oddorder modes of a line source of length L FIG. 5 is a graphical representation of the theoretical side-lobe levels as a function of aperture length for the odd-order modes of a line source of length L 5 FIG. 6 is a graphical representation of the theoretical super-gain ratio (SGR) as a function of aperture length for the odd-order modes of a line source of length L,; and,
FIG. 7 is a graphical representation of the calculated half-power beam widths as a function of aperture length for the odd-order modes of a line source of length L DETAILED DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention is illustrated in FIG. 1. The desired three lobed (3rd order) aperture distribution is characteristic of a one and one-half wavelength dipole, whereas a one-half wavelength dipole antenna has only a one-lobe configuration. To overcome this difiiculty the one and one-half wavelength dipole is coiled to a physical length of one-half wavelength forming a helix. The antenna of this embodiment consists of a semi-helix 10 mounted on a ground plane 20. The semihelix 10 is constructed so that it has a physical length of one-half wavelength and an electrical length of one and one-half wavelength.
This is accomplished by securely mounting on the top surface 30 of the ground plane 20 a plurality of rigid semi-loops, such as at 40, of wire which are electrically insulated, by conventional means, from the ground plane 20. These semi-loops 40 are then connected together electrically in series on the bottom surface 50 of the ground plane 20 by sections of coaxial shielded cable such as at 60. The interconductor of the coaxial cable 60 being .electrically connected to the wire loops and the outer shielding of the cables 60 electrically connected to the ground plane 20. The combinationof the wire semi-loops 40 and coaxial cables 60 is arranged on the ground plane 20 in such a manner that the wire loops 40 are evenly spaced over a length of one-half-wavelength in the form of a semi-helix and the combination has a total electrical length of one and one-half wavelength. A coaxial feed 70 is used to couple electromagnetic energy to the antenna. The antenna is fed at oneend and the other end is left open.
The ground plane 20 will act as a mirror and produce an image that will replace the portion of the helix which has been removed. This phenomenon, which is termed image theory, is useful since an aircraft is almost an ideal ground plane. This semi-helix construction also provides an antenna with a very low profile. The patterns shown in FIG. 2 illustrate typical patterns obtained from the semi-helical antenna.
The following analysis provides a complete understanding of the theory underlying the operation of the antenna disclosed herein.
Patented Dec. 22, 1970 For the sake of simplicity we will consider the antenna to be a line source of length L located along the z axis centered at z'=0 and a resonant mode A(z) exists along the structure and is given by I where n is the mode number and is considered odd. The far-field F (k cos is related to the aperture distribution by Thus, it approaches the pattern of a two-element linear array with sin 0 element factor. However, as will be seen later, n=3 gives a good approximation to this. Three examples of the mode distribution and far-field patterns are given in FIG. 3. As can be seen in FIG. 3, n=3, the resulting patterns from a half-wavelength aperture is 52, which is a substantial improvement over a conventional dipole of the same length -(78).
A theoretical plot of directivity vs. aperture length (L,) is given in FIG. 4. As can be seen, 3rd order mode excitation results in better than a 33 percent increase in directivity compared to the conventional dipole antenna mode (n=1) over a 2:1 bandwidth and also that little is to be gained in directivity by exciting the 5th or 7th mode. A 2-element array of isotropic point sources is shown also for comparison.
A theoretical plot of side-lobe level is given in FIG. 5, where it can be seen that for the 11:3 case the side-lobe level is below db for L,g0.75. This figure is better than a conventional 2-element isotropic array. This improvement is due to the sin 0 factor in Eq. 3.
An indication of the bandwidth properties of an antenna is the super-gain ratio (SGR), since SGR is related to the aperture Q by SGR=l +Q. The definition used here is F(k cos 0)E sin 0 cos (ii-L, cos 0) 1 2 +L). L (q)I p where F(q) is related to A(p) by q)=ff A p p SGR= 4 and A(z') is transformed to A(p). A plot of SGR for the various modes is given in FIG. 6. A working approximation is to say that SGR increases as n FIG. 7 is a graphical representation of the calculated half-power beam widths obtained from individual excitation of the various modes. The calculations in FIG. 7 are correct within 02. In FIG. 7 it can be seen that for L,=O.50, the half-power beam widths from the 3rd order mode is better than the 1st order mode of twice the aperture (L,=l.0). Thus resulting in better than 2:1 size reduction. Comparable results also hold for directivity as shown in FIG. 4. Consequently, our interest is in the 3rd order mode.
As has been shown in the theory, the aperture distribution along a dipole is a cosine function of the Wavelength and FIGS. 3 through 7 apply exactly, with the n=3 case, for the patterns and results of the helical antenna disclosed herein.
Although a certain and specific embodiment is shown, departures may be made without departing from the true spirit and scope of the invention.
What is claimed is:
1. A super-directive narrow-band semi-helical antenna comprising a plurality of semi-loop of electrical conductors, a ground plane, means for securely attaching and electrically insulating said semi-loops from a first surface of said ground plane, connecting means for connecting said semi-loops together electrically in series on the opposite surface of said ground plane, and means for coupling electromagnetic energy to said semi-helical antenna.
2. A super-directive narrow-band semi-helical antenna as set forth in claim 1 wherein said connecting means is a coaxial cable having its inner conductor connected to said semi-loops and its outer conductor connected to said ground plane.
3. A super-directive narrow-band semi-helical antenna as set forth in claim 1 wherein said semi-loops are arranged on said first surface of said ground plane in an even spaced relationship over a length of one-half wavegength, thereby having the physical appearance of a semielix.
4. A super-directive narrow-band semi-helical antenna as set forth in claim 1 wherein said semi-loops are arranged on said first surface of said ground plane in an even spaced relationship over a length of one-half wavelength, thereby having the physical appearance of a semihelix, the combination of said loops and said connecting means having a total electrical length of one and onehalf wavelength.
References Cited UNITED STATES PATENTS 3/1957 Carter 343741 5/1965 Chatelain 343742 US. Cl. X.R.