US2511849A

US2511849A - Broad band antenna

Info

Publication number: US2511849A
Authority: US
Publication date: 1950-06-20
Anticipated expiration: 1967-06-20

Description

D. J. HILFERTY EI'AL June 20, 1950 BROAD BAND ANTENNA Filed April 29, 1943 W W W HA w ML mmf W7 W v Patented June 20, 1950 BROAD BAND ANTENNA Daniel J. Hilferty, Riverdale, and William E.

Withrow, Hyattsville, Md., and David L. Ringwalt, Washington, D. 0.; Ethel M. Hilferty executrix of said Daniel J. Hilferty, deceased Application April 29, 1943, Serial No. 485,016

15 Claims. (01. 25033) (Granted under the act of March 3, 1883, as amended April 36, 1928; 370 0. G. 757) This invention relates to antennas and is especially directed to broad band antennas particularly for high frequency operation.

In radio transmission it is necessary for many purposes to employ widely diiierent'frequencies under varying circumstances. The necessity for this arises from diurnal and seasonal variations of transmission characteristics and because of frequency allocation of various services. It has previously been required to provide a separate antenna for operation on each frequency, the antennas being precisely adjusted for operation on their selected frequencies. This is a very costly procedure and often, particularly in mobile installations, renders it impossible as a practical matter to obtain an efficient service.

Accordingly, it is an object of the present invention to provide an antenna capable of operation over a wider band of frequencies than has been possible with any previously known construction.

A further object of the invention is to provide an antenna presenting a practically constant input impedance over such an extended frequency range.

Another object of the invention i to provide an antenna system directly fed by a transmission line which may be employed over a wide frequency band without undue reflection back on the line.

Another object of the invention is to provide a wide band antenna which avoids the use of doubly curved surfaces, and which may therefore be easily constructed from electrically conductive sheet or screen material, although it may also be of reticular rod formation.

Whereas the antennas f the invention are especially useful for transmitting purposes, they are equally advantageous for reception and for duplex operation.

The present invention constitutes a development of the electromagnetic horn type of antenna, particularly the type which has been referred to in the literature as biconical horns. Such horns are characterized by two opposed members which flare outwardly away from each other to form an annular aperture. Normally the opposed members are of conical shape, but other formations have been employed. A11 horns of this type are characterized, however, by the fact that the separation between the opposed members increases as the aperture of the horn is approached. The specific shape of the members may be varied as required.

It has been recognized that biconical type horns may be employed in services where a sharply peaked antenna must be avoided, as in television service, for instance. It has been discovered that the electromagnetic horns of this type may be modified to cover a band width exceeding that of any previously known construction, the upper and lower frequencies of the range having a ratio in excess of two to one.

It will be understood that to obtain broad band operation, the antenna must present to its transmission line a reasonably flat matching impedance throughout the frequency range covered. Unless the antenna presents a pure resistance load to its associated transmission line and, in addition, this resistance is substantially equal to the characteristic impedance of the line, the input impedance to the line will vary with frequency,

Within the operating range the variations in impedance and phase angle of the antenna are so restricted that it may be directly fed by a transmission line without reflections exceeding ten per cent. With vertical mounting of the antenna, the radiation field characteristics of the antenna are so concentrated in a horizontal direction as to give a definitely improved signal over that of a vertical dipole tuned to the specific operating frequency.

The invention will be further described with reference to the embodiments shown in the drawings, wherein:

Fig. 1 is a diagrammatic representation of an antenna embodying the present invention, and

Fig. 2 is a sectional view of a portion of an antenna construction embodying the present invention.

In the embodiment illustrated in Fig. l, the antenna is shown as including conical opposed members I and 2, forming an electromagnetic horn with an annular aperture 3. The antenna is characterized by the fact that the lower cone 2 is larger than the upper cone I, and that a cylindrical member is positioned on the periphery of the upper cone l, extending axially of the cone and transversely away from the annula aperture 3. Cone 2 has a length a which exceeds the length I) of cone I and is also wider than cone l at the base.

The antenna is suitably excited by a concentric line 5, the outer conductor 6 connected to lower cone 2 and the inner conductor 1 connected to upper cone l Whereas the principles of operation of the antenna have not been fully ascertained, and consequently the discussion below is not to be considered limiting, it is believed that the function of the system is as follows. From the breadth of the band covered without undue reflection, it appears that, to an approximation, the total impedance presented to the transmission line is independent of frequency. This may be accounted for by the fact that the radiation resistance offered each branch of the system is equal to the square root of the inductance-capacity quotient, the larger cone being inductive and the smaller cone with the cylindrical member being capacitative.

The impedance of the antenna is matched to that of the transmission line at some intermediate frequency of the operating range, and the values over the range as a whole afford a sufficiently close match for entirely satisfactory operation.

The characteristic impedance of the antenna is primarily determined by the angles of the cones 01 and 02, as shown in Fig. 1. In this case Z =60 log, (cot cot 2 Usually the cones will be similar, so that 01:02, in which case Manifestly, however, the angles of the cones are not necessarily equal, and may differ as required. Furthermore, the biconical type horn includes the special case wherein one element is planar, as well as where the angle of one of the cones is obtuse, so that it envelopes at least in part the opposing cone.

The angles 01 and 02 are preferably selected to obtain an exact match with the transmission line.

Although various dimensions of the antenna elements may be employed, it has been determined that to cover the broadest range, the length a measured along the face of the larger cone should be approximately .36 of a wave length at the lower frequency of the range, and

that the lengths I) measured along the face of the smaller cone I and c of the cylindrical member 4 should each be approximately a quarter wave length at twice the lower frequency.

Due to the modifications of the antenna from the simple biconical horn, the calculated value of impedance will vary from the actual value, and in order to obtain correct adjustment in specific installations the spacing d of the apices of the cones should be adjusted. The optimum distance may be determined by adjustment for minimum line reflections.

The impedance of annular aperture electromagnetic horns formed by opposed members of other than conical shape may be determined experimentally and precisely adjusted on installation as described.

In Fig. 2 is shown a fragmentary sectional view of the constructional details of an antennasystem of the present invention, wherein the opposed members are supported by conducting elements 6 and I constituting the coaxial transmission line. Cone 2 is mounted on the outer conductor 6 by ring II integrally afiixed thereto. Central conductor I is positioned in the outer conductor 6 by insulating members 8 and 9. Member 9 is a capping element for the coaxial system and is held in place by screws I0 passing through cone 2 and threaded into ring II. The central conductor is reduced at I3 to form a shoulder Ill engaging the lower face of the capping element. The reduced portion I3 is threaded and is maintained in position by engagement of threaded ring I2 with the upper face of the capping element.

The upper conical element I includes a block I5 positioned in its apex by screws I8, and provided with a threaded axial bore I9 receiving the reduced extension I3 of central conductor 1. The upper conical assembly I, carrying cylinder 4 (not shown in Fig. 2) is therefore adjustably mounted relative to cone 2.

After adjustment of the spacing for the purposes above described, the upper cone I may be fixed in position by engagement of set screw I'I received in threaded bore I6 in block I5.

It should be understood that the practice of the invention is not limited to the embodiments illustrated and described but is circumscribed only by the scope and limitations of the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

'We claim:

1. An antenna comprising a biconical electrically conductive electromagnetic horn having two members, one member of the horn extending radially beyond the other member, and a cylindrical electrically conductive member positioned on the outer periphery of one of the members.

2, An antenna comprising a biconical electrically conductive electromagnetic horn having two members, one member of the horn extending radially beyond the other member, and a cylindrical electrically conductive member positioned on the outer periphery of the other member.

'3. An antenna including a biconical electrically conductive electromagnetic horn formed by two opposed conical members, one of said members being shorter than the other, and a cylindrical electrically conductive member of a length equal to that of the shorter conical member positioned-on its periphery.

4. In a wide band antenna covering a frequency range, a biconical electrically conductive electromagnetic horn formed by two opposed conical members, one of said conical members having a length substantially a quarter wave length at twice a lower frequency of said range, and the other conical member having a length substantially a third wave length at said lower frequency of said range.

5. A wide band antenna covering a frequency range extending above a lower frequency including two opposed electrically conductive members "flaring outwardly away from each other to form an electromagnetic horn with an annular aperture, one of said members being of conical shape, and having a length approximating a quarter wave at twice said lower frequency, and a cylindrical electrically conductive member having a length equal to that of said conical member mounted on its periphery.

6. A wide band antenna covering a frequency range extending above a lower frequency including an electromagnetic horn formed by two opposed electrically conductive members flaring outwardly to form an annular aperture, said members being of conical shape, one conical member having a length approximating a quarter wave length at twice said lower frequency and a cylindrical member of equal length positioned on its periphery, the other conical member having a length approximating a third wave length at the said lower frequency.

'7. An antenna including two opposed electrically conductive sections flaring outwardly away from each other to form an electromagnetic horn with an annular aperture, the first of said sections expanding continuously to its end, and the second of said sections terminating in portion extending transversely away from the aperture the outer diameter of one of said sections being not more than half the operating wave length of the antenna.

8. An antenna including two opposed electrically conductive sections flaring outwardly away from each other to form an electromagnetic horn with an annular aperture, the first of said sections expanding continuously to its end, and the second of said sections terminating in a cylindrical portion the outer diameter of one of said sections being not more than half the operating wave length of the antenna.

9. An antenna including two opposed electrically conductive sections flaring outwardly away from each other to form an electromagnetic horn with an annular aperture, one of said sections expanding continuously to its end, the other of said sections terminating in a portion extending transversely away from the aperture, and said one section being longer along its surface than the other section.

10. An antenna including two opposed electrically conductive sections flaring outwardly away from each other to form an electromagnetic horn with an aperture, the first of said sections terminating at said aperture, and the second of said sections having a portion extending transversely away from said aperture the outer diameter of one of said sections being not more than half the operating wave length of the antenna.

11. An antenna including two opposed electrically conductive members flaring outwardly away from each other to form an electromagnetic horn with an annular aperture, the first of said members having a circular periphery, the second of said members expanding continuously to its end, and a cylindrical electrically conductive member positioned on the periphery of said one member the outer diameter of one of said members being not more than half the operating wave length of the antenna.

12. An antenna including two opposed electrically conductive members flaring outwardly away from each other to form an electromagnetic horn with an annular aperture, the first of said members expanding continuously to its end, the second of said members being of conical shape, and a cylindrical electrically conductive member positioned on the periphery of the conical member the outer diameter of one of said members being not more half the operating wave length of the antenna.

13. An antenna comprising two opposed electrically conductive sections flaring outwardly away from each other to form an electromagnetic horn, one of said sections terminating in a portion extending longitudinally of the antenna, and the other of said sections being longer than said one section.

14. An antenna comprising two opposed electrically conductive members flaring outwardly away from each other to form an electromagnetic horn, one member of the horn extending outwardly beyond the other, and a cylindrical electrically conductive member positioned on the outer periphery of one of the members.

15. An antenna comprising two opposed members flaring outwardly away from each other to form an electromagnetic horn, one member of the horn extending outwardly beyond the other, and a cylindrical electrically conductive member positioned on the outer periphery of the smaller member.

DANIEL J. HILFERI'Y. WILLIAM E. WITHROW. DAVID L. RINGWALT.

REFERENGES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS