US2541107A - Low-clearance antenna - Google Patents

Description

ZJMLW Feb. 13, 1951 P. N. sELGxN Low-CLEARANCE ANTENNA Filed April l2, 1947 INVENTOR PAW. J. SELGIN ATTORNEY llnwl Patented Feb. 13, 1951 UNITED STATES PATENT OFFICE LOW-CLEARANCE ANTENNA n Paul J. Selgin, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application April 12, 1947, Serial No. 741,109

claims. (c1. 25o- 33) This invention relates to high frequency antennae and particularly to an antenna for radiating a vertically polarized wave substantially uniformly in all horizontal directions.

It is conventional practice to establish radio communications between a moving vehicle such as a train and another train or a xed station. For a radio communications system of this type an antenna is required which is usually mounted on the roof of a vehicle such as a railroad car. Since the train must be able to pass under bridges or through tunnels of xed clearance, it is desirable to provide an antenna which has low overall vertical dimensions and a high efficiency comparable to that of a conventional half-wave dipole antenna. The horizontal radiation pattern developed by the antenna should preferably be substantially circular.

It has been suggested to provide an antenna comprising a lower cone and an upper disc. This so-called discone antenna has a very wide frequency range. However, the height of the conventional` discone antenna is approximately a quarter wave length at the cuto" frequency which is too high for a low clearance antenna for vehiL cles. It has also been proposed to provide a horn or radiating wave guide consisting of two inverted `cones having their apices adjacent to each other. Another conventional antenna of this general type' has two radiating cones coupled to the two conductors of a coaxial transmission line. The 'two cones of the biconical antenna are of equal diameter, and the overall vertical height of the antenna is not low enough for the purposes referred to. Furthermore, certain diiculties have been experienced with a biconical antenna.

should be adjustable While the antenna is in D operation so that waves of different frequencies ,may be either radiated from or received by the antenna.

It is an object of the present invention, theretore. t0 provide ahish frequency antenna which combines low vertical overall dimensions with an efficiency comparable to that of a conventional half-wave dipole. i

Another object of the invention is to provide a novel high frequency antenna which will radiate a vertically polarized wave having a substantially circular radiation pattern ina horizontal plane. n

A further object of the invention is to provide an antenna suitable for intercepting and radiating high frequency energy and having a low re-A active impedance with an impedance matching network which may be adjusted during operation of the antenna for properly matching the impedance of the transmission line feeding the antenna to that of the antenna at a desired frequency.

In accordance with the present invention there is provided an antenna ccmprising a first an.. nular antenna element and a second annular an-u tenna element. There are further provided la transmission line and impedance means for coupling the transmission line to the two antenna elements. The rst antenna element has a diameter which is smaller than that of the second an'- tenna element. Preferably the second or lower antenna element has a diameter of the order of one half the operating Wave length of the antenna, while the total height of the antenna may be of the order of one-sixteenth of its operating wave length. Further in accordance with the present invention the antenna is provided with an adjustable impedance matching network, such as a reactive network, for coupling the transmission line to the two antenna elements.

For a better understanding of the invention, to? gether with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing:

Fig. 1 is a cross sectional View of an antenna embodying the present invention;

Fig. 2 is a top plan view of the antenna of Fig. 1; and

Fig. 3 is a schematic electric diagram of the antenna of the invention illustrating the adjustable impedance matching network for couplingV a transmission line to the antenna. y

Referring now to the drawing and particularly to Figs. l and 2, there is illustrated a high frequency antenna including lower antenna element I and upper antenna element 2. As shown in Fig. 1, the lower antenna element l is of substanfI tially conical shape. The upper antenna element 2 has also been illustrated as being of substantially conical shape, the apices of the two cones formed by antenna elements i and 2 are arranged adjacent to each other. However, the shape of the upper antenna element 2 is not critical, and accordingly, antenna element 2 may, for example, consist of `a flat disc instead of a cone..

Lower antenna element I is provided with horizontal supporting plate 3 to which it may be secured by suitable rivets or screws indicated at 4. Thus, lower antenna element I and supporting plate 3 form an enclosed space. Upper antenna element 2 is provided at its outer rim with reinforcing ring-shaped tube 5 to provide mechanical is connected through coaxial transmission line S When high `frequency'translator consists of a transmitter, the antenna will radiate averticall-y polarized wave having a substantially circular radiation pattern in a horizontal plane. It

. will be noted. that antenna elements IV and 2 as illustrated consist of A'two very `flat cones while cone-2 vmay also consist of adiscso that the overall vertical height of the antenna may be rnade extremely small. Thus, the total height of the antenna need be no more than one-sixteenth of the operating wave length. The diameter of lower antenna element l should preferably be slightly less than one-half the operating ywave length, lwhile the diameter of upper antenna element 2 should be approximately Vone-.half that of the lower antenna element I.

rEhe antenna as described has a low impedance whichis reactive. Accordingly, the radiation impedance of the antenna depends upon the frequency of the wave intercepted or radiated. VIt is desirable, therefore, to provide a reactive impedancematching network between coaxial Ytransmission lline E andantennae I and 2 which should 'be adjustable .for dierent frequencies.

Y "The impedance matching network is 4preferably disposed in lower antenna element i and further comprises means .for supporting the up-` per antenna element 42 from the 'lower element i This .construction makes it vpossible to support 'the two antenna elements in such a manner that a radiated wave does not forman interferenceV pattern which would be the case if any .obstructions .such as insulating Vsupports were arranged in the space between elements Yl and .2.

For the `purpose oi electrically connecting outer conductor ID of transmission line 6 to the lower Y .antennaelement l, there is provided cup-shaped Inner conductor 8 of L tion of outer sleeve "I5 is secured to upper antenna j element 2 through bushing plate YII'T- which con- Ajust the capacitance Vof condenser ,30.

sists of a conducting material. Upper antenna element 2 is accordingly supported by housing l2, cover I4 and outer sleeve I5.

Inner conductor 8 of transmission line 6 is connected to lower condenser plate `2 I'I as illustrated. Condenser plate is secured to housing I2 through collar 2l which consists of an insulating material. .Condenser plate ..20 is thusfiixed to housing I2. Upper condenser plate 22 is secured to inner sleeve 23 which is slidably arranged in outer sleeve I5 and consists of insulating material.

Upper Vcondenser plate 22 is electrically coupled to upper antenna element 2 through inductance element 24 provided in the space formed by inner sleeve 23. Stud 25 which consists of a conducting material is electrically connected to inductance element. "24 and may be screwed into inner sleeve 23 or otherwise secured thereto. Stud 25 extends through outer sleeve I5 and is screwed into bushing 265v which in turn is threaded into bushing yplate I1. Set screw '.21 is provided in bushing -26 for vlocking Vstud 25. Another set screwv 28 `is provided `in housing vI2 for `locking it to cover I4.

An impedance matching network is vthus provided for coupling coaxial transmission line -6 to antennae I and 2. As show-n schematicallylin Fig. 3, inner conductor 8 is'connected to ladjustable condenser 30 which is represented in 1 by condenser plates 20 and v 22. Condenser -30 `-is coupled to Vupper antenna element 2 through in"- ductance element 24 yand stud 25 as shown rin Fig. 3. Another condenser 31 is provided between the junction point of condenser 3B and inductance element 24 on 'the one hand `and housingY l2 on the other hand as `shown schema-tically in Fig. 3. Thus, condenser 3l -is effectively connected between inner conductor 8 and lower antenna element I. Condenser 3| represents-the capacitance between condenser plate 22 and coverv I4.

It will now be seen that vcondensers L30 and 35| are individually adjustable. Thus, Yin order to adjust the capacitance of condenser 3|, set screw 2 is loosened and stud y25 is rotated thereby to move inner sleeve 23 with respect 'to outer sleeve l5. Consequently rthe distance between condenser plate 22 and cover I4 maybe varied 'or adjusted. It will be obvious that by this adjustment the distance between condenser Aplates '20 and 22 will be varied simultaneously. However, lthe distance between condenser plates 20 and 22 may be adjusted .by-loosening set screw 23 and Y rotating upper antenna elementl which ,may tbe effected, 'for example. by .gripping tube 5- .Accordnsly, upperantenna element .2.,outers1eeve l5, cover I4, inner `sleeve 123 and upper condenser plate y22 will rotate in .unison with respect to housing I2 thereby to adjust the Ydistance vbetween condenser plates .20 and .22. 'This in turn will .ad-

It will accordingly be evident that the .im-

pedance matching network including .condensers 3U, V3l and inductance element '24 .may beV adjusted while the antenna ,is in operation and tween condenser plates'20 and 22 onthe-one hand' `and between condenser plate 22 and cover I4-on, the other handiifthe antenna is Vintended tooperate at va `predetermined frequency. In that case the adjustment may be effected while 'fthe 'antenna is assembled.

` Experiments carried out :with "the antennalof the invention have revealed that the antenna has an eiciency comparable to that of a conventional vertical half-wave dipole antenna. Since the overall height of the antenna may be as low as one-sixteenth of the operating wave length, the antenna is particularly suitable for use on moving vehicles where a low clearance antenna is required. In spite of its small height the efliciency of the antenna is not impaired.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An antenna comprising a first antenna element, a second antenna element, a line having first and second conductors, a housing for electrically connecting said second conductor to said second element and fixed to said second element, a first condenser member connected to said iirst conductor and xedly disposed in said housing, a second condenser member, means for electrically coupling said second condenser member to said first element, and movable means mounted on said housing and connected to said second condenser member for adjusting the distance and the capacitance between said condenser members, thereby to provide an adjustable impedance matching network between said antenna and said line.

2. An antenna comprising a rst antenna element, a second antenna element, a line having rst and second conductors, a housing for electrically connecting said rst conductor to said second element and xed to said second element, a rst condenser plate connected to said second conductor and iixedly disposed in said housing, a second condenser plate, a cover for said housing electrically connected to said second element and insulated from said second condenser plate, said second condenser plate being arranged to have its distance from said cover adjusted to vary the capacitance between said second condenser member and said cover, said cover being arranged to be moved in unison with said second condenser plate for adjusting the distance and capacitance between said condenser plates, thereby to provide an adjustable impedance matching network between said antenna and said line.

3. An antenna comprising an upper antenna element, a lower antenna element, a coaxial transmission line having an inner and an outer conductor, a housing for electrically connecting said outer conductor to one of said elements and disposed within said one element, a cover for said housing electrically connected thereto and mechanically connected to and insulated from the other of said elements, a first condenser plate connected to said inner conductor and fixedly disposed in said housing, a second condenser plate electrically coupled to said first condenser plate and to said cover, means for electrically connecting said second condenser plate to said other element and for adjusting its distance from said cover to adjust the capacitance therebetween, and means for simultaneously moving said cover and said second condenser plate to adjust the distance and the capacitance between said condenser plates,

6 thereby to provide an adjustable impedance matching network between said antenna and said transmission line.

4. An antenna comprising an upper antenna element, a lower antenna element of substantially conical shape, the diameter of said lower element being larger than that of said upper element, a coaxial transmission line having an inner and an outer conductor, a cupshaped housing for electrically connecting said outer conductor to said lower element and disposed within said lower element, a cover threaded into said housing and electrically connected thereto, an insulating support iiXed to said cover and to said upper element, a first condenser plate connected to said inner conductor and iiXedly disposed in said housing, a second condenser plate electrically coupled to said rst condenser plate and to said cover, means for electrically connecting said second condenser plate to said upper element and for adjusting its distance from said cover to adjust the capacitance therebetween, thereby to adjust the distance and the capacitance between said condenser plates by rotating said upper element and said cover simultaneously with said second condenser plate.

5. An antenna comprising an upper antennal element, a lower antenna element of substantially conical shape, the diameter of said lower element being larger than that of said upper element, a coaxial transmission line having an inner and an outer conductor, a cup-shaped housing for electrically connecting said outer conductor to said lower element and disposed within said lower element, a cover threaded into said housing and electrically connected thereto. an insulating support fixed to said cover and to said upper element, a first condenser plate connected to said inner conductor and xed'ly disposed in said housing, a second condenser plate electrically coupled to said first condenser plate and to said cover, a stud threaded into said upper element, an inductance element for electrically connecting said second condenser plate to said stud and to said upper element, an insulating member for securing said stud to said second condenser plate, thereby to adjust the distance and the capacitance between said cover and said second condenser plate by rotating said stud and to adjust the distance and the capacitance between said condenser plates by rotating said upper element to rotate said cover simultaneously with said second condenser plate.

PAUL J. SELGIN.

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

UNITED STATES PATENTS Number Name Date 2,031,103 George s- Feb. 1i8, 1936 2,175,252 Carter Oct. 10, 1939 2,272,608 Hoiman Feb. 10, 1942 2,344,171 Rote Mar. 14, 1944 2,368,663 Kandoian Feb. 6, 1945 2,401,601 Atwood June 4, 1946 FOREIGN PATENTS Number Country Date 114,368 Australia Dec. 24, 1941 OTHER REFERENCES Three New Antenna Types and Their Applications, by A. G. Kandoian, Proceedings of the IRE, February 1946, pp. 70W-75W.

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