US2531476A - Ultra high frequency antenna - Google Patents

Description

Nov. 28, 1950 R. G. scHRlEFER ULTRA HIGH FREQUENCY ANTENNA Filed April 28, 1947 R OR TE NF EE V m NH IC S G. T R E B O R ATTORNEY Patented Nov. 28, 1950 UNITED STATES PATENTv OFFICE ULTRA HIGH FREQUENCY ANTENNA Application April 28, 1947, Serial No. 744,435

(Cl. Z50-33) 6 Claims.

This invention relates to antennae and particularly to an ultra-high frequency vertical antenna adapted for use in mobile communications.

For railroad radio communications an antenna is required which has a low clearance. The space limitations imposed on a railroad antenna require that the antenna be mounted in close proximity to the metallic vehicle body which may cause distortion of the radiation pattern, and may reduce the eld strength due to the absorption of a portion of the radiant energy by adjacent metallic surfaces.

Furthermore it has usually been required in Y antenna installations to provide an impedance matching device for the purpose of obtaining an ecient energy transfer between the radiating element and the antenna feeder line. Alternatively, the length of the antenna may be varied until the desired resistive component is obtained, whereupon the reactive component may be tuned out. These conventional arrangements necessarily complicate the installation and adjustment of the antenna and often cause ineicient operation due to the instability, or mechanical slippage of the tuning or impedance matching components. Further deficiencies of the prior art antennae reside in the lack of adequate weatherproofing for the protection of the antenna and its feeder line and the absence of suiicient mechanical ruggedness required to withstand the severe vibration and shocks to which a mobile antenna is subjected.

it is, therefore, an object of the present invention to provide an ultra-high frequency antenna especially adapted for use in mobile radio communication services in View of its low vertical dimensions.

Another object of the invention is to provide a self-supporting antenna of great mechanical strength having structural simplicity and Wherein the elements are dimensioned so as to provide a resonant structure having a predetermined input impedance for matching the antenna impedance to that of the antenna feeder line.

A further object of the invention is to provide an antenna structure devoid of insulation and substantially impervious to accumulations of snow, ice or sleet, and which furthermore provides a weatherproof housing for the connecting elements contained within the antenna structure.

In accordance with the present invention there is provided an antenna comprising a coaxial transmission line having an inner conductor and an outer conductor. A cylindrical member is provided which has a diameter different from that of the outer conductor, that is, it may be larger than the outer conductor. This cylindrical member extends beyond the transmission line. Means are provided for connecting the inner conductor to an extremity of the member and further means are provided for mechanically supporting the cylindrical member.

For a better understanding of the invention, together 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 drawing, the single figure is a cross sectional view of a vertical antenna embodying the present invention.

Referring now to the drawing, there is illustrated a coaxial vertical antenna comprising outer cylinder I and inner cylinder I I. The upper rim of outer cylinder I is aiiixed to a metallic plug i2 which is, for example, screwed upon the upper rim of the inner cylinder Il. The outer cylinder iii extends downwardly, in skirt fashion, concentrically With a substantial portion of the upper length of inner cylinder II. The inner cylinder i E serves as a support for the antenna structure and is mounted on base plate I5 and may be fixed thereto by set screw I 8. The base plate I5 is then fastened to a ground plane or metallic surface I6 by the mounting screws II. Metallic surface I5 is preferably grounded, as shown, thereby to maintain the cylinders II and IQ at direct current ground potential.

By this arrangement, a rigid and self-supporting metallic antenna structure is provided, wherein no insulation is required, and wherein the entire antenna structure may be maintained at a direct current ground potential.

There is provided a coaxial transmission line comprising the lower section of inner cylinder I I and the center conductor 20 which serve as an antenna feeder line for supplying alternating excitation current to the antenna or for feeding alternating current from the antenna to a receiver. Aperture 2i is arranged in the sidewall of inner cylinder II, and the free end of the center conductor 2B extends through this aperture and is electrically connected to the lowermost portion of outer cylinder I0, that is, to its lower eX- tremity.

The electrical length of the antenna extending above metallic surface I6 corresponds to one quarter of the operating wave length as indicated in the drawing. L1 indicates the length of outer cylinder I0, while La is the length of the exposed portion of inner cylinder II, that is, the portion of inner cylinder II which is not surrounded by outer cylinder I and which extends beyond metallic surface I6. D1 and D2 indicate the diameter of the outer cylinder I0 and of the inner cylinder; I I; respectively.

The inductive and capacitive reactive components and the radiation resistance of the coaxial antenna of the invention are mainly dependent upon the ratios of the lengths LinL'z' and ofthe,-

diameters D1, D2.

The antenna of the invention-isxsubstantially equivalent to a vertical quarter wave dipole and accordingly will have an omnidirectional 'horizontal radiation pattern and will radiate a vertically polarized wave. A high frequency excitation current applied to the antenna through the coaxial feeder line II, 2i] will excite the outer surfaces of inner cylinder II and o'outer cylinder--Ill -which together form a resonant str-ucture thereby to establish an omnidirectional horizontal radiation -eld Yabout the antenna.

When the antenna is excited; high frequency currents will be present onfthey outer surface of outer'vcylinder I0 `as Well asl upon the outerfsuriace of theexposedportion-of rinner cylinder II. These currents must be in phasefand are indicate'd-for a giveninstant by arrows 22 and 23. At thevsarnetime the'high frequency currents flow through coaxial transmission line vII, 2S. Thus, acurrent indicated loyal-row 2li flows along thefouter-surfa'ce of inner conductor 2Q in the same direction as the current llovvin;r on the z..

outer surfaces of cylinders Iii and II. Since the exposed portion ci inner cylinder II forms part of the coaxial transmission line, a current indicated by arrow will ow on the inner surface of cylinder I I inthe opposite direction from that flowing-'on its outer surface and indicated by arrow 23. The currents distributed over the comn bined outer conducting -surfaces'of cylinders II andfl -as indicated by arrows 22 vand 23 for a certain instant-causethe structure to resonate and to function as an antenna. It is to be understood that the currents represented by arrows 22 `to-25-willfilow -in the reverse direction during the opposite portion of a cycle of operation.

The portion-of cylinder iI varranged within outercylinder IB is not impertantfor the operation or the antenna but'mainly serves as asupport for the outerl cylinder IG.- However, the electricalconnection established from the metallic surface I8 through innercylinder II, metallic plug I2 to outer cylinder` I@ keeps the entire antenna structure at a direct current ground potential.`

Experiments yhave revealed that when the entire physical. length of theantenna extending.'

beyond metallic surfaced@ is kept constant, while the length L1 of outer cylinder Ill is reduced, whichsi-multaneously,will increase the length L2 of the exposed portion of inner cylinder II, the antenna resonates at a slightly higher frequency \vhileth'e antenna input impedanceis simultaneously increased. Inversely,v if the length L1 is increased while the `length L2 -is decreased and the total length of the antenna maintained con-- stant, the antenna will* assume a slightly lower resonant frequency and the antenna input impedance will be decreased.

When-the dimensions L1, L2 and Dz'are maintained constantlwhile thediameter D1 of outer cylihderelll is reduced,` the antenna resonates at a slightly higher frequency While the antenna input impedance is increased. A decrease of diameter D2, that is, the diameter of inner cylin der II causes a reduction of the resonant frequency of the antenna as well as of the antenna input impedance provided that L1, L2 and D1 are kept constant.

B3n-suitably choosing ,the ratios .of the lengths Li, Lz' and of the diameters D1, D2', the antenna input impedance may be adjusted to match the impedance of the coaxial transmission line which feedsj-the antenna.y Accordingly, an impedance matching network is not required between the feeder line and the antenna. The dimensions of the antenna ofthe invention suitable for operation on a frequency of approximately megacycles .and having an antenna input impedance of 50 ohms are as follows:

Inches Inches L2 3%; D2v 1 Forf-fprovidingfan antenna which will resonate at a frequency different from 160 megacycles and which will` have the `same antennainput 'impedance 4of 50. ohms, theabove dimensions may be Vmultiplied byrthe ratio ofith'e desired fref quency to the given frequency of 160 megacycles.

While there has been.- described what is at present considered the preferred embodimentl of theA invention, it will be obvious tothose skilled in the .art that 'various changes andmodications may be made therein without` departing from the invention,` and it is, therefore,aimed in the appended vclaims to cover allllsuch changes and modificationsk as fail withinl the true spirit and scope of-the invention.

What is claimed is z l. An antennacomprising a coaxial transmission 'line having an inner conductor and an outer conductor, acylindrical member surrounding a portion ofsaid'outer conductor, a connecting element for electrically and mechanically connecting one extremity of said member to said outer conductor, means for connecting the other end of'said member to said'inner conductonand a metallic surface electricallyvconnected to said outer conductor, the length of said antenna extendingI beyond said surface being electrically equivalent to one-quarter of the operating waveA length or"y said antenna.-

2. An antenna comprising al coaxial transmission line having an inner conductor and an outer f conductor, a Acylindrical member coaxial with and surrounding the Youter portion of said outerv conductor, a connecting element for electrically and mechanically interconnecting the upper extremities of said member and of said outer conductor, meansffor-connecting the free end of said inner conductor to the lowermost portion of said member, and a metallic surface electrically connected' to saidouter conductor, the length of said antenna extending beyond said surface being electrically equivalent to one quarter of the operating'wave length of said antenna.

3. A vertical antenna comprising a coaxial transmission line having an inner conductor and an outer conductor, a-cylindrical member coaxial with and surrounding the upper portion of said outer conductor, meansior connecting the free end-of said inner conductor to the lowermost portion of said member, anda connecting element for electrically and mechanically interconnecting-,the upperV extremities .of said 'member and of said outer conductor, the relative length of said member and of the radiating portion of said outer conductor not surrounded by said member being proportioned in such a manner as to provide a predetermined input impedance of said antenna and a predetermined electrical length.

4. A vertical antenna comprising a coaxial transmission line having an inner conductor and an outer conductor, a cylindrical member coaxial with and surrounding the upper portion of said outer conductor, a connecting element for electrically and mechanically connecting the upper extremities of said member and of said outer conductor, means for connecting the lowermost portion of said member to the free end of said inner conductor, and a surface electrically connected to said outer conductor, the length of said antenna beyond said surface being electrically equivalent to one quarter of the operating Wave length of said antenna, the relative length of said member and of the radiating portion of said outer conductor arranged between said surface and said member being pre portioned in such a manner as to provide a predetermined input impedance of said antenna and an electrical length equivalent to one quarter of said wave length.

5. A Vertical quarter-wave antenna comprising a coaxial transmission line having an inner conductor and an outer conductor, a cylindrical member coaxial with and surrounding the upper portion of said outer conductor, means for connecting the free end of said inner conductor to the lowermost portion of said member, and a connecting element for electrically and mechanically interconnecting the upper extremities of said member and of said outer conductor, the diameter of said member and of said outer conductor being such as to provide a predetermined input impedance of said antenna and a predetermined electrical length.

6. A vertical antenna comprising a coaxial transmission line having an inner conductor and an outer conductor, a cylindrical member coaxial with and surrounding the upper portion of said outer conductor, a connecting element for electrically and mechanically connecting the upper extremities of said member and of said outer conductor, means for connecting the lowermost portion of said member to the free end of said inner conductor, and a surface electrically connected to said outer conductor, the length of said antenna beyond said surface being electrically equivalent to one quarter of the operating wave length of said antenna, the diameter of said member and of said outer conductor being such as to provide a predetermined input impedance of said antenna and an electrical length equivalent to one quarter of said wave length.

ROBERT G. SCHRIEFER.

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

UNITED STATES PATENTS Number Name Date 2,113,136 Hansell et al. Apr. 5, 1938 2,234,234 Cork et al. Mar. 11, 1941 2,284,434 Lindenblad May 26, 1942 2,321,454 Brown June 8, 1943 2,323,641 Bailey July 6, 1943 FOREIGN PATENTS Number Country Date 260,005 Great Britain Mar. 24, 1927

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