US2566491A - Antenna construction - Google Patents

Description

p ,195.1 E. G. HILL v 2,566,491

ANTENNA CONSTRUCTION Filed March 15; 1946 {Shaw s-511551; 1

FIG. 1.

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' Elmr G. Hills Sept. 4, 1951 s 2,566,491

' ANTENNA CONSTRUCTION Filed March 15, 1946 2 Sheets-Sheet 2 IN VEN TOR.

I Attorneys Patented Sept. 4; 1951 ANTENNA CONSTRUCTION Elmer G. Hills, Chicago, Ill., assignor to Belmont Radio Corporation, Chicago, Ill., a corporation of Illinois Application March 15, 1946, Serial No. 654,619

The present invention relates to antenna constructions and more particularly to antennas for ultra high frequency applications, specifically for use in connection with mobile vehicles such as trains, trucks, busses and the like, where vertical space and durability are'at a premium.

Just recently 'the Federal Communications, Commission has promulgated rules governing railroad radio and they have allocated frequencies in the range of 158to 162 megacycles for train radio broadcasting. This is within the range generally referred .to as the ultra high frequency range. An antenna for use in train communication work in the 158 to 162 megacycle band should be essentially omni-directional in the horizontal plane or, in other'words should have a circular-horizontal radiation pattern. The reason'for this is that a train is likely to be orientated in practically all directions with respect to various fixed stations or other trains with which communication is to be carried on. It is accordingly undesirable to havedirectivity in any one horizontal direction since this would have to be at the expense of some other horizontal direction. Furthermore, since broadcasting in connection with railroad radio is either to or from a fixed station and a train or between trains orparts of trains on the same general'lev'el, it. is undesirable to have a high angle of radiation In fact, it is very desirable to have a lowangle of radiation to get maximum gain inthe horizontal direction close to the ground, or in other words vertical directivityis desired since radiation in a purely vertical direction would be wasted energy.

' It is absolutely essential that an antenna for a train which is usually mounted on the highest part of the train, be of'very rugged construction since workmen walking along the top of the train are very likely to grasp it for support or step on it Consequently, such an antenna should be sufficiently strong so that it will not be damaged by such handling or contact. It is also possible that material such as coal, for example, may acc identally be spilled on the antenna when the locomotive tender is loaded with coal and the construction of the antenna should be capable of withstanding such. treatment without dam-age thereto.

On the other hand, ithas been found that where an antenna is so mounted that portions of it are lower than adjacent parts of the train, reflections from such adjacent train parts. interfere with and tend to prevent the "formation of a circular horizontal radiationpattern and 13 Claims. (Cl. 250-33) in fact may cause radiation to be concentrated in the least desired direction. Therefore, a train antenna should be mounted on the highest part of the train. In view of this requirement, probably the most important consideration for a train antenna is that it uses only a very limited amount of space in the vertical direction above the highest part of the train. Modern railroad equipment has been so constructed as to avail itself of the maximum clearances under overhead structures such as bridges, tunnels and the like. Consequently, an antenna for train use should be limited to a vertical dimension of considerably less than a foot so as not to be damaged by striking existing overhead structures on the right-of-way. These same considerations of vertical space apply equally to trucks, taxicabs and tugboats. problem of windage is also important" It has also been found that train antennas should preferably be vertically polarized as contrasted with horizontal polarization since audiomodulation caused by the train moving past telephone poles or other structures along the rightof-way seems to be less troublesome for vertical polarization than for horizontal polarization. It is an object of' the present invention, therefore, to provide an improved high frequency antenna, which possesses the desirable physical and electrical characteristics enumerated above.

It is another object of the invention to provide an improved and simplified antenna, structure which is particularly adaptable for train and other mobile radio applications and which provides a substantially circular radiation pattern in the horizontal direction.

. Another object of the present invention is to provide a high frequency antenna of the vertical polarized type which has an overall height of less than foot and yet may be efficiently matched to a transmission line of practical impedance.

Still another object of the present invention is to provide a folded top loaded antenna for application to mobile vehicles in which the current distribution is a maximum at the top of the antenna and is substantially constant along the vertical length of the antenna.

A further object of the present invention is to provide a strong antenna construction for train radio use comprising a tubular pipe welded or otherwise rigidly fastened to a grounded portion of the train such, for example, as the top of the locomotive cab, thus becoming integral therewith.

A still further object of thepresent invention is to'provide an improved high frequency antenna having an overall vertical dimension of the order of 5 inches or the like which can be efiiciently matched to a 50v ohm coaxial transmission line.

Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to 'and forming a part of this specification.

For a better understanding of the present in-.

vention reference may be had to the accompanying drawings in which:

Fig. 1 is an elevational view of an antenna embodying the present invention mounted on the cab of a train locomotive with the height of the antenna considerably exaggerated;

Fig. 2 is an enlarged perspective view partly. in section of the antenna shown in Fig. 1 having a coaxial transmission line schematically shown connected across the feed point thereof;

Fig. 3 is a diagram illustrating the current distribution in the vertical plane of the antenna along the length thereof;

Fig. 4 represents the equivalent circuit of the antenna;

Fig. 5 illustrates in polar coordinates the phase and amplitude of the current distribution in the antenna;

Fig. 6 is a curve diagram illustrating the voltage standing wave ratio of the antenna with respect to a 50 ohm line; and

Figs. 7, 8 and 9 illustrate vertical radiation patterns for the antenna withthe dimensions of the ground plane for each figure being different.

It should be understood that the antenna construction of the present invention may be utilized in many applications where high frequency antennas are required. However, since it is especialyl effective and desirable for use with mobile vehicles where the available vertical space is limited the invention has been illustrated in Fig. 1 of the drawings'in connection with arailway locomotive. Referring now to Fig. 1 the antenna of the present invention generally designated at Ii) is mounted on the cab ll of a steam locomo- L tive l2. Preferably the antenna should be mounted on the highest part of the train which in the illustrated embodiment is the cab ll. Other antennas, however, may be placed on the roof of any car wherein it is desiredto have a receiver.

' In Fig. 2 of the drawings the antenna I is shown in detail as comprising a folded loaded construction including two vertical conductors l3 and i4 positioned in parallel relationship and welded or otherwise suitably connected attheir upper ends to a top loading plate 15 in the form of a flat, oblong horizontally positioned disc. Preferably, conductors l3 and Mare in the form of short rods of pipes and in the 'illustratedembodiment they are specifically shown as pipes each having an outside diameter off2 inches with conductor [4 having a length of the orderof inches. The conductor or pipe. 'l i'. has its lowerv end welded to a ground plate'flfi which may be a separate conducting plate grounded td the 1000- motive cab. II or "which, may actually comprise the roof or top of the locomotive cab. If a separate ground plate such as I6 is to be'j'emplcyed, a thirty inch square of conducting material has been found. to be quite satisfactory: It is quite obvious that a short lehgth of pipe such as the conductor l ,which iswelded bothat thetb fi a d ott ntt he la es. Hi n e rativ -1a provides a very strong construction such that a trainman can readily grasp the antenna or stand thereon without damage thereto. In fact, actual tests on the antenna of the present invention have demonstrated that no damage is caused when a load of coal is dumped thereon.

In view of the fact that such an antenna when mounted on a train is subjected tosoot deposits, the lower end of the conductor or pipe 13 is separated from the ground plate l6 by a very wide base insulator ll of annular construction preferably formed of Bakelite or the like. This insures a long leakage path to minimize the effect of extensive. soot deposits on the antenna structure and insure against a short circuit of the capacitive eflect' As best shown'in Fig. 20f the drawings, the

antenna is fed from a coaxial transmission line generally indicated at [8. For railroad applications a 50 ohm coaxial transmission line has been chosen primarily because it has very desirable physical dimensions. Consequently, the antenna for railroad application should be capable of being efficiently matched to a 50 ohm coaxial transmission line. The outer conductor of the coaxial transmission line I8 is designated by the reference numeral IS in Fig. 2 of the draw-' ings and is grounded as'indicated at 20. The. ground plate l6, which may either comprise the cab roof or a plate being welded thereto, is obviously grounded. This connection is schematically illustrated at 2| in Fig. 2 of the drawings, The coaxial cable l8 also includes an inner conduc tor 22 which is connected to a brass cone 23 positioned within the lower end of the conducting member or leg [3 of the folded antenna and supported by an'annular insulator I! as clearly, disclosed in Fig. 2 of the drawings; In order that the coaxial transmission line 18 may readily be connected to and disconnected frornthe antenna duced by a steam locomotive, for example, it is preferable to. construct the exposed parts of the antenna from stainless steel.

By top loading the antenna ID with the oblon loading plate I5, it. is possible? to produce a substantially constant current'distribution from the top to the bottom of the vertical antenna;' Fig.

3 of thedrawings shows this current distribution for an aritennajconstructed in accordance with the presentinventionhaving a height of 5 inches or. an angular height. of approximately 24. It will be understood that the current distribution in an antenna which is shortened in length by top loading is substantially the same in'the portion remaining as it would havebeen if the antenna were not shortened by top loading. From an examination of Fig. 3 it will be observed that the current distribution represented by the curve I is a maximum at the top of the antenna lll whereby the angular height from the top to'the, point at which the, antennacurrent would reach:

aminimumiif it were. not suppressed by thetop loading plate I5,is 101. One} quarter wave" length. Consequently the valu'of G' for the 5 antenna I0, where G is the angular height above the base of the antenna to the point at which the antenna current would reach a minimum if it were not suppressed by the top loading plate is, i 114. It is known that the radiation along the horizontal is increased at the expense of the vertical radiation if the current is a maximum at the top of the antenna and for mobile radio applications the desirable feature of getting the current maximum as high above the vehicle as possible is obtained with the antenna I0, while still maintaining the antenna height at a fraction of a foot. With this arrangement, the antenna has the desirable characteristic of vertical directivity with maximum gain in the horizontal direction, very little energy being wasted in the vertical directions. Also with the antenna fed at the bottom from coaxial transmission line [8, it is fed below the current maximum or current loop which as is obvious from Fig. 3 is at the top of the antenna, and this has the effect of making the antenna slightly inductive. This is desirable to compensate for the inherent capacitance which shunts the feed point of antenna III by virtue of the large base insulator II, it being essential that the antenna I be resonant at the desired frequency.

In Fig. 4 of the drawings the equivalent circuit for antenna I0 which is a parallel resonant circuit, is illustrated, the capacitance I1 representing the capacitance across insulator H, the inductance 25 representing the radiation reactance of the antenna, and the resistance 26 representing the radiation resistance of the antenna.

Since the antenna should be tuned to resonance at the center of the desired frequency band,.

angular length G of 90 is approximately 36 ohms. Where the antenna has a physical dimension of less than a quarter wave length it is known that this radiation resistance decreases as the length of the antenna decreases because not as much power is radiated, power P in watts being equal to I Rr, where I is the antenna current and R1 is the radiation resistance. If P is decreased without changing I obviously R1 is also decreased. For a pipe such as the pipe I4 in Fig. 2 of the drawings the radiation resistance is only about 6 ohms. However, by feeding the antenna at a point below the current maximum or current loop the radiation resistance is increased by a factor l/sin G. From the curve of Fig. 3 of the drawings where G is equal to 114, the factor l/sin G. is approximately equal to 1.2.

'Fig. 5 shows the phase and amplitude of, the measured current distribution in the two conducting members I3 and I4 of the antenna. II]. By changing the shape of the loading plate I5 at the top of the antenna III it is possible to control this current distribution within limits; In order to make the current approximately umform around the two members in the interest of efliciency it was found necessary to extend the end I 5a of the loading plate I5 of antenna I0 slightly farther on the side of the bottom fed member I3 than the end I5b extended from top red member I4. By using the folded construction of antenna I0 comprising the two members I3 and It not only is a. mechanically strong con struction provided but the radiation resistance can be increased to the desired 50. ohms in a simple manner while maintaining high efflciency. If the two members I3 and I4 carried equal in-- phase currents the resultant effect would be the same as having twice the current flowing in a single conducting member or in other words radiating 4 times the power.

it is clear that increasing the power 4 times increases the radiation resistance RT 4 times without increasing the current I. Thus by keeping the current low the losses are considerably less and the efiiciency of the antenna is greatly increased. It will also be understood that if the same power radiation were desired without the folded construction a much higher voltage would be necessary with considerably higher dielectric as well as heat losses. With the current distribution illustrated in Fig. 5 of the drawings for the construction described above the radiation resistance is increased by a factor of 2.62 when employing a folded construction. The radiation resistance which is increased by factors of 1.2 and.

2.62 as described above is of course dependent upon the fact that a parallel resonant circuit is employed in which the inductive effect thereof is cancelled out by capacitance connection with the base fed member so as to leave an impedance which is purely resistive. Although in the antenna I0 described above the members I3 and I4 have the same diameter it is possible to increase the antenna impedance still more by vary ing the diameters of members I3 and I4 as will be brought out by the following description.

In Fig. 6 the standing wave ratio of the antenna is shown with reference to a 50 ohm transmission line for the frequency range encountered in railway radio, and with a ground plane comprising a 30" x 30" steel plate. This standing wave ratio is very low, falling in the range of 1.1 to 1 thus indicating a very close match with the 50 ohm transmission line. Actually the size of the ground plane has an effect on the impicted by a comparison of Figs. 7, 8 and 9 of the drawings. Fig. 7 shows the vertical radiation pattern of antenna II] when the ground plane comprises. a 30" x 30 steel plate I6.

Fig. 8 shows the vertical radiation pattern with a 6' x 6 screen as the ground plane and Fig. 9 shows both the measured and theoretical vertical radiation patterns with an infinite ground plane. The computed curve of Fig. 9 is calculated on the assumption that there exists no radiation from the top loading plate I5. This is not a valid assumption because the loading plate I5 is an appreciable portion of a wave length long and even if the current distribution in the plate I5 were symmetrical about the two axes of the plate. radiation from the plate would be zero only in directions normal to the plate. The currents in the vertical members I3 and I4 of the antenna, as shown by Fig. 5 of the drawings, indicate that the distribution in the loading plate I5 cannot be symmetrical so that radiation should be expected even in directions normal to the loading plate. This is shown by the measured curve of Fig. 9. The horizontal radiation pattern with From the equation antenna I ll issubstantially circular as desirledfor omniedirectional antennas with the radiation, in

a direction atright angles to the plane formed by the axes of members l3 and M slightly less than in the other directions.

It should be understood that although the illustrated embodiment only a single antenna H1 is. employed on the train, the possibility exists,

of'placing several such top. loaded antennas on the train to form an array to get greater directivity, particularly when it is desired to carry on communications from a station or stations which have a constant directionwith respect. to the axis of the train. An array of'extremely high directivity could be obtained by placing many such antennas along the topof the train in the'form 01 an end fire array and switching the transmitting and receiving equipment to this array when the train is going to be on a stretch of straight track for many miles. I Y

It should also be understood that there are other ways by which the. antenna! can be matched to a transmissionline. For example, the antenna may be top loaded by means. of a plate such'as [5, in such a-manner that the value of G would be 9.0". 'Then' the diameter of the top fed member [3 may be varied. until the radiation resistance of the antenna equals the characteristic impedance of the transmission line.

From the above description it will be noted that there .has been provided a top loaded folded'antenna for use with mobile vehicles wherein a very shortantenna. of the order of 5 inches or the like is providedby employing the top loading feature and an antenna of high efiiciency is provided by employing the folded feature. Since the top fed member I4 is a steel pipe welded at the top and bottom to steel platesv the antenna is very strong mechanically and can be readily grasped by atrainman or subjected to other severe strain without damage'thereto. This differs materially from'prior art constructions which are weak mechanically because of being mounted on an insulator. Furthermore the antenna is constructed so that the current loop occurs at the highest point or top thereof which is of the order of 5 inches above the highest point on the train' so that it can readily be employed on any raliroad regard-- less of the overhead structures thereon. In ad dition the antenna 16 has a radiation resistance of 50 ohms so that it canraccurately be matched with a 50- ohm coaxial transmission line. Its

' radiation pattern in the horizontal direction is substantially circular and consequently it meets the omnidirectional requirement of s ch an antenna. Furthermore there isdirectivity in the vertical direction in that there is gain along the horizontal at the expense of radiation in the vertical direction.

It will be understood by those skilled in the art that while there has been illustrated and described a particular embodiment, of the invention, modifications thereof will, occur to those skilled in the art. Itshould be understood, therefore, that the present invention is not limited to the particular arrangement disclosed and ii? is intended in the appended claims to cover all modifieations and changes which fall, within thfiqtrue. spirit and scope of the invention.

I claim:

1. An antenna comprising two. closely spaced vertically positioned parallel conductors,:.each

.beingonly a fraction of afoot. in length, a flat lower. endolf one of said conductors to ground and an insulator separating thelower endof theother of said conductors from ground. 1 A V 1 2. An antenna comprising two closely spaced vertically positioned parallel tubular conductors each being only a fraction of afoot in length, a flat oblong loading plate formed of conducting. material welded to the upper ends of each of said conductors, means for connectingthe lower end of one of said conductors to ground, and an annu lar insulator separating the lower end of the other of said conductors from ground.

3. An antenna comprising two losely' spaced vertically positioned parallel tubular conductors each being only a fraction of a foot in length, a flat loading plate electrically connected to the upper ends of each of said conductors, aground.

plate welded to the lower end or oneof said conductors, and an insulator separating thejlower vertically positioned parallel conductors each b eing only a fraction of a foot in length, a fiat load-, ing plate welded to the upper ends of each of said conductors, a ground plate welded to the lower end of one of said conductors, an annular insulator separating the lower end of theother of said conductors from ground, and a-coaxial transmission line connected to the lower end or said other of said conductors. p 1

5. A train antenna comprising two hollow closely spaced vertically positioned parallel conductors having an electrical length of the order of 24, a loading plate weldedto the upper ends Of said conductors, a ground plate comprisingthe roof of a train locomotive cab welded to the lower end of One of said conductors, and an insulator separating the other of said conductors from said electrical length of the order 015242 a loading plate of conducting material. electrically connected to the upper ends of saidconductolfsi. a ground plate electrically connected to thenlower end of one. of said conductors, an annular insulator separating the other of said conductors from said ground plate, and means for feeding one;of said conductors with radio frequency energyfrom one end while feeding the other conductor from the other end. r j

7. An antenna for mobile vehicles comprising two hollow closely spaced Vertically positioned parallel conductors having a length of the order of 5 inches, a loading plate electrically connected to the upper ends ofsaid conductors; a ground plate of conducting material adapted to be fa stened to the highest part of said vehicle and elec= trically connected to the lower end of one of said conductors, and an insulator separating the other of said. conductors from said ground plate.

8.A high frequency folded top loaded antenna for mobile vehicles comprising two hollow closely spaced verticallypositioned parallel conductors having a length of the order of 5 inches, a load a ing plate electrically connected to the upper ends of said conductors, aground plate electrically. connected to the lower end of one of em oriductors, said electrical connections being such as to provide a rigidmechanical structure adapt;

d t w h ds b ant m forces applied t ere,

to, and an insulator separating the other of said conductors from said ground plate.

9. An antenna for railroad radio having a substantially circular horizontal radiation pattern comprising two closely spaced vertically positioned parallel tubular conductors formed of stainless steel having a length of the order of 5 inches, a loading plate of stainless steel welded to the upper ends of said conductors, a ground plate electricallyconnected to the lower end of one of said conductors, a Wide base insulator separating the other of said conductors from said ground plate, and means for feeding one of said conductors with radio frequency from one end while feeding the other conductor from the other end.

10. An antenna for mobile vehicles having a substantially circular horizontal radiation pattern comprising two closely spaced vertically positioned parallel tubular conductors having an electrical length of considerably less than a quarter wave and a physical length of less than a foot, a loading plate of conducting material electrically connected to the upper ends of said conductors, a ground plate electrically connected to the lower end of one of said conductors, an insulator separating the other of said conductors from said ground plate, and a radio frequency energy connection for said antenna near the end of said conductor adjacent said insulator.

11. An antenna for mobile Vehicles comprising two closely spaced vertically positioned tubular parallel positioned conductors having a length of the order of 5 inches and an outside diameter of between 2 and 3 inches, a loading plate electrically connected to the upper ends of said conductors, a ground plate of conducting material adapted to be fastened to the highest part of said vehicle and electrically connected to the lower 10 end of one of said conductors, and a wide base insulator separating thelower end of the other of said conductors from said ground plate.

12. A high frequency antenna of the loaded folded type comprising two vertically extending conductors each having a length of less than a quarter wave and being disposed in spaced-apart parallel relation, with one of said conductors being grounded at its lower end and the other of said conductors being insulated from ground at its lower end, and a flat loading plate united to the upper ends of said conductors.

13. A high frequency antenna of loaded folded construction for operating in the 158 to 162 megacycle frequency band, such antenna comprising two vertically extending conductors each having a length of less than one foot and being disposed in spaced-apart parallel relation, with the lower end of one of said conductors being grounded and the lower end of the other of said conductors being insulated from ground, and a flat loading plate united to the upper ends of said conductors.

ELMER. G. HILLS.

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

UNITED STATES PATENTS Number Name Date 2,175,252 Carter Oct. 10, 1939 2,283,914 Carter May 26, 1942 2,285,669 Lehmann June 9, 1942 2,287,220 Alford June 23, 1942 2,293,112 Carlson et a1. Aug. 18, 1942 2,359,620 Carter Oct. 3, 1944 2,368,663 Kandoian Feb. 6, 1945 2,417,793 Wehner Mar. 18, 1947

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