US 2611865 A
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Sept. 23, 1952 A. ALFORD 2,511,865
TRANSVERSELY GAPPED CYLINDRICAL ANTENNA Filed June 19, 1946 2 SHEETS-SHEET 1 lNVENTOR ANDREW ALFORD fPM/MMA ATTORNEY 2'SHEETSSHEET 2 Filed June 19, 1946 Fig. 3.
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W III III. a I- I: 7 2M H .9 5 .8 T HI Patented Sept. 23, 1952 UNITED STATES PATENT TRANSVERSELY GAPPED CYLINDRICAL ANTENNA This invention relates to new and useful improvements in antennas and particularly in mobile transmitting and receiving antennas for use on automobiles, buses, trains, aircraft and the like.
' In the art of radio transmission where at least one station .is mobile it is the general practice to employ vertically polarized radio frequency waves, since they have good field strength near to the ground.
'A simple and widely used antenna for such waves is a vertical flexible whip antenna. This antenna has several serious shortcomings. In order to function etficiently it must be near to a quarter wave length long, a length at which it is resonant and has a narrow pass band. The pass band can be broadened by increasing the thickness of the radiating element, but it then may cease to be a whip antenna and may present too rigid and hazardous a projection from a moving vehicle. The band width of an ordinary quarter wave Whip antenna is of the order of ten percent (.10) and, even if compensating circuits are used, this can only be increased to approximately twenty-five percent (.25).
If the length of a whip antenna is reduced below a quarter wave length, then its radiation resistance will become materially lowered and its input impedance will include capacitive reactance. The expedient of tuning out this reaotance with a coil will further decreasethe efficiency of the antenna and its already small band width.
I Tilting a vertical whip antenna away from its perpendicular position, so as to lower the height of its tip, will diminish both its band width and its radiation resistance.
At the relatively high frequency of 156 mc., a quarter wave length whip would be all of sixteen inches long. A sixteen-inch protrusion from the top of a vehicle as high as a bus, or a truck, or a railroad car can be very troublesome. Such vehicles frequently are built nearly as high as the minimum head-room tolerance legally permitted for bridges, tunnels, viaducts and terminals. Moreover, in certain areas such obtrusions would most likely encounter low hanging branches of trees.
Foldable mounting of a whip antenna to avoid overhead obstacles will make continued operation of associated radio equipment impossible. Furthermore, such a mounting, as well as one permitting the whip antenna to tilt when striking an object, will necessitate complicated and expensive flexible feed line means and will add to servicing requirements.
- A whip antenna attached tov some part of a 15 Claims. (Cl. 250-33) the presence of radio equipment in a vehicle and to the type of vehicle it is, for example, that it is a special police car.
The protrusion of whip antennas is objection able on aircraft for the additional reasons that at ordinary high speeds they produce substan: tial drag, and at very high speeds their drag attains not only inordinately large magnitudes, but the-vibration will either break off thean tenna or-damage the aircraft.
Among the objects of this invention areto provide a vertically polarized transmitting or.'receiving antenna having a band width .of 1.2 a. substantially circular radiation pattern in horizontal planes, and one which is very compact and therefore will, when installed on a vehicle, present a minimum protrusion. p
With this object in view. I provide a hollow conducting body having a transmission-line formed therein intermediate its ends. Preferably, the transmission line is constituted by a'transverse gap in the side wall of a cylinder, which returns on itself and with which the feederiS: connected. The hollow conducting body isconnected with, and preferably mounted on, a fiat conducting surface, e. g. the surface of a vehicle,
According to one of the features of the present invention, if a cross sectional area of the space within said cylinder, taken in a planeth'ro i'gh' its axis, is of'the order of one-hundredth of a; wave length squared, then the structure will satisfy the above-mentioned objects.
Reference is made to my copending application U. s. Serial No. 669,758, filed May '15, 1946; now Patent No. 2,508,084, granted May 16,1950, although part of its disclosure will be repeated for the convenience of the reader and to' facili tate the understanding of the present invention. Other objects, features and advantagesof this invention will become apparent to those familiar with the art from the following descriptiongand the appended claims and drawings, in whichr Fig. 1 is an isometric view, partly cut away. of the radiating element of one embodimentpf the antenna;
Fig. 2 shows the antenna of Fig. 1 in combina-' tion with a conductive ground plane;
Fig. 3 is a transverse cross sectional view 0 the antenna shown in Fig. 2, the cross section being taken along line 3-3 of Fig. 2; and" OFFICE the roof, such as on a- Fig. 4 shows a modification of the form shown in Fig. 1.
Referring now more particularly to Fig. 1, the radiating element shown therein consists of two principal circular body elements I, 2 which may be metallic castings or spinning-s shaped like pie plates. These elements include as respective integral parts. thereof bottom discs 3, 4- and upstanding rims attached thereto, which, in the figure, bear the above-mentioned numerals I and 2. Body elements i, 2 are joined by a tube which is mechanically attached to inside surfaces of bottoms 3 and 4 at substantially theircenters;
In practice tube 5 may be formed as an integral part either of element I or of element 2 andthen the main assembly will consist of two parts. Tube 5 may be of metal and as a result will be conductive. However, the inside portion of tube 5 joins areas of the outer sides of discs 3 and 4 where R. F. voltages are of zero (or extremely ljowipotential and, therefore, the inside maybe nonconductive without preventing this device fronr' functicning. On the other hand, it willdo'no harmito its functioning if the inside" of thetube is a'good-conductor. Theouter surface of tube 5-whose surface ispart oftheinner surface of the hollow cylinder (composed of elementsl and 2), should be conductive as there must be a low impedance axialconnection bet'weenthe inner surfaces ofelementsl and 2. Where -a'metallic-supporting mast is used the portion ofit' which passes-through the hollow cylindermay take the-place of tube 5. Its outer skin--willperfo'rm' this conductive function and there'willbe no need fortube- 5', except that 5 ordinarily serves as'a convenient'gauge for properly spacinggl "and 2. I fTuberi i's' of such length and is so arranged thattbodyelements I and 2 do not touch each other? it holdsthem with their bottom discs 3 and 4" parallel and: their' rims evenly separated bya gap'fil Gap 6 maybe an air. gap and is so shownin the drawing. However, it may be physicallysealed by a dielectric substance so long as' thearea of the-gap. still remains non-conmauve, in the ordinary sense, and constitutes, a "hiatus" in..the. conductive surfaces of the.cylin der. l-, .2 wherever it intercepts them. Gap filis shont-lcircuitedby metal band- I.
, Bottoms 3. and. 4. have centrally. located holes whichregister with the-ends of the annular opening.. through tube 5. The entire structure I -I may be described as a: hollow body or cylinder whose ends. are closed-by discs 3-, 4-and whose side walls havea gap fi wh-ich runs round. the cylinder ina plane'parallel to and midway between discs 3 and 4, andreturnson. itself, the cylinder halves being: held together by an internally locatedaxial tuber fastened to the inside surfaces of discs- 3 and 4.
- The'hollow cylinder maybe made of solid sheet metal (or an equivalent, such as solid sheet plastic including: av conductive layer) and, accordingly, the rinner. and outerareas, will comprise wall surfacesphysically unbroken except for gap 6. How ever...this:physical continuity'is not necessary and the: device will operate satisfactorily ifmade' of wire'screen, or of perforated sheet metal ifzthe openings are not too large. with respect torone wave length corresponding to the highest-operatingjfrequency, e. g. not larger. than i ofia wave length.
Theantennais energized over a: coaxial transmissiondinehaving an; outer conductor; 8* and innenconductor 9: Inner; con'ductorrB isconnected to the edge of the rim of body element I, and the outer conductor 8 is similarly connected to element 2. These connections are made at points opposite each other across gap 6, and this part of the gap is at a place on the side of the hollow cylinder which is diametrically opposite the position of short-circuiting band 1'.
In Fig. l the radiating element is shown mounted on a mast 10. For reasons already explained, the mast may be conductive and, therefore, may be of metal such as steel. It passes through tube 5 and through the centrally located holes in bottoms 3 and 4. The antenna may be prevented from sliding up or down the mast by suitable collars or clamps similar to the one which is shown in the drawing, in a displaced position at Mia. The coaxial transmission lines 8, 9 may be. clamped to mast l0 and to element 2 by any suitable means.
When the antenna is energized the adjacent edges 'ofthexcylind'er halves;i.e. the edges of gap 6, constitute-a: balanced transmission line: Thehollow body portion 'ofthis antenna: (which-maybe further described as surrounding and 9110105:
ing, a toroidal space about the outer perimeter of"- tweentheinner surfacesof bottoms. 3 and. 4las: H, then .this cross. sectional-area may be. calcu-.
latedby. using the formula A: (Dd.) XH Certain...cylinders, whichhave proper magnitudesof distributed reactance. and perform. in. a preferred. manner as radiators. in accordance with this in-. vention; have such dimensions that, when converted into-Wave lengths, the area derived. by solving the equation is-A .0105
Short-circuiting band 1 causes nearly complete.'
reflection of incident waves traveling. along; gap 6 around both sides of. the hollow cylinder toward it. The interference between incident wavesand. reflected waves results in a standing-wave distribution of voltage acrossgap 6.
One minimum. of voltage across-the gapoccurs at shorting band 1; two maxima of voltage occur at points on oppositesides of the:hol1o-w cylinder each of which is about midway between. band. I and the feed point. Under certain conditions, two-additional voltageminima, which ordinarily are undesirable,- mayoccur on gap 6. They will be between band 1 and the feed point and are likely to benear to, and on each side of, thefeed point. A condition which may cause this isthat the gapis too long inproportion to the cross sectional area A and that, therefore, the velocity of propagation along the gap is. not. great enough for the-gaplength.
When the only voltage minima are-at'the shorting bandand the feed point, thevoltage, diiferences across gap 6, though they vary in magni tude, are. in; the: same phase, and currentsrmov-ingbetween the edges of gap 6 and the discbcttoms move-in the-same direction.
7 It has been; empirically ascertained inactual tests,- and it can be furtherdemonstrated inother ways, that this in-phase conditionof theiR. F. voltagesacross gap 6 and the geometriczshapeof 5. the hollow cylinder will cause this element to act as a horizontally omnidirectional radiator of vertically polarized energy, assuming, of course, that the axis of tube is perpendicular to the ground.
When undesired voltage minima occur between the feed point and band I (along the gap 6 on each side of the hollowcylinder), the phases of the R. F. voltages near those minima shift rapidly. This alters the current distribution pattern and'results in asymmetry of the horizontal radiation pattern. The hollow cylinder should pref erably be so proportioned that the distances from hand I to the first voltage minima (or to imaginary positions of the first minima constructively existing beyond the feed point) in each direction around the hollow cylinder, are equal to (or greater than) one-half of the circumference of the cylinder.
The'desired standing wave condition can be obtained even when gap 6 is greater than M2 in length by sufiiciently increasing the velocity of propagation along the gap, even substantially beyond the speed of light under certain conditions. This is controlled in part and within limits by the cross sectional area described above and by the capacity per unit length between the edges of the gap.
A cross sectional area A:.0l05 together with a gap which is of the order of .0048x Wide and is formed by thin metal edges, results in a velocity of propagation along the gap about twenty percent greater than the velocity in free space. This permits the use of a cylinder whose circumference is proportionally increased so that each half circumference is in the neighborhood of twenty percent more than a half wave length. The physical enlargement of the hollow cylinder with respect to the operating wave length, which is effected at the same time that undesired voltage minima are averted, will make its input impedance low enough to match a 50-ohm coaxial line. Thus, in addition to being compact and vertically polarized (when properly positioned with respect to the earth), the device requires no input transformer, making it an ideal coupling between a 50- ohm cable and the radiation resistance of space.
' One example of satisfactory proportioning of the several parts of the antenna is the following: D=.32 d=.32 H=.072 and g=.0048 These dimensions are expressed in terms of a wave length corresponding to the middle frequency of the operating band of frequencies. Where that middle frequency is 378 me. and these dimensions are converted into inches, they are D=10", d=l", H=21%;" and 9:96. When mounted so that l or 2 is in contact with ground (Figs. 2 and 3), H should be reduced to one-half of this amount, i. e. 1%. Obviously, if the operating frequency is changed, all of the physical proportions of the antenna should be proportionally changed.
It should be borne in mind that the controlling geometric characteristic for a gap which has a fixed amount of distributed capacitance per unit length, is the cross sectional area described above. For example, d may be made larger with respect to D, reducing the value of D-d, but by increasing H by an easily determined amount, A will remain substantially the same and the standing wave condition will be substantially unchanged.
Moreover, the hollow cylinder may be shaped so that its toroidal cross section on each side of the central tube is of other shapes than rectangular. This cross section may be circular,
elliptical or irregularly shaped. In the same manner. it is not essential that cross sections of" the hollow cylinder, taken in planes perpendicww lar-totlie axis of tube 5, should be circular. The important factors are that A be generally ofthe order of .0105 and that the exact value of A can vary depending upon the capacitive charac teri'stics of the gap. In-this application, and in particular in. the claims, the word cylinder is used in a broad sense to indicate all hollow struc-' tures which electrically conform with the require perpendicular to the axis of tube 5, and figures-- of-8 vertical patterns in planes passing through that axis. The band width of the antenna is of the order of 1.2. I Obviously the antenna described above may be employed at frequencies and for purposes other than those given above by way ofexemplification.
Referring now to Fig. 2, a radiating element similar to that of Fig. 1 is attached to a conductive ground plane I l, which may be a metal sheet, with disc 4 of the radiating element juxtaposed to it. In practical use, ground plane ll may be attached to the roof of a vehicle or form an integral portion thereof, and does not have to be perfectly flat. As well known, a ground plane of finite size, above a certain limit, will behave like an infinite reflective plane and afiectthe radiation patterns of any radiator associated with it; a physically infinite plane is unnecessary. Similarly, a substantially flat ground plane will, at
ordinary wave lengths, behave like a perfectly flat ground plane. Therefore, the curved ,metal roof of a vehicle may serve as ground plane II.
It is obvious that ground plane II will affect the vertical distribution pattern of the radiator. However, this is not a disadvantage. If the antenna is close to the earth, e. g. on thereof ofa bus, the areas masked by the ground plane will be unimportant. Even if the antenna is not close to the earth, i. e. if it is in an airplane, the vertical masking is not a disadvantage. For in an aircraft installation the radiatingelement could.
be installed on the underside of the surface acting as a ground plane. The important-patterns, those in horizontal planes, are not appreciably affected. I v
The presence of ground plane H affects both the velocity of propagation and the standing wave along the gap. Therefore, if a ground plane used, the proportioning of the antenna body should be appropriate to compensate for its presence. Its cross sectional area A should be about one-half as great as that of a correspondingantenna of the type shown inFig. 1.
The non-conductive space inside the hollow cylinder need not necessarilybe filled with air. Instead, other dielectrics may be used including dielectrics which are gases, liquids and solids. However, substances whose dielectric constants differ from that of air will increase the distributed capacity and reduce the velocity of propagation. The proportions of the hollow cylindershould, therefore, be somewhat altered. Other changes and adaptations will readily suggest themselves to those skilled in the art.
; The shorting element which has been describe in the present application as located on the side of the slot diametrically opposite to the feed point need not in practice be of exactly zero impedance.
sired 'standing wave-condition'on the gap. or-slot v can be produced with this two feedpointarranga ment-iust=,as1.wellas by one in which there is. a singlefeed ,point;and" a'reflection point or low, impedance pointdiametrically opposite. to-it. It
has been found that usually a. double feed affects; the operating characteristics of the antennabybroadening the operating frequency band.
=What: I' claim is;
1. In an antenna, a hollow conducting body havingcontmuoussidewalls, and end walls PO51! tioned opposed to one another at opposite ends oil-thehollow: body, said side walls having a. linear discontinuity extending around the side walls and spaced from.- the end walls, a conductingrelement within the hollow body, extending between theend' walls and electrically connected thereto, means havinga fiat conducting surface onwhich the hollow conducting body at one of its. ends is mounted, said linear discontinuity forming an a i r' ga1;v with'the conductive material adjacent both sides of the-gap forming a'transmission line about the hollow body, and a feeder connected to the body withone terminal thereof connected to one side of the transmission line and the other terminal to the other side.
2. The-antenna according to claim I, and in. which the transmission line. forms a closedloop in the side wallsoi the hollow conducting body.
3. The antenna according to claim 1, and in which-the transmission line forms. a circular loop intheside walls of the hollow conducting body.
4 4; The antenna according to claim 1, and in whichthe transmission line forms a closed loop inthe sidewalls of the hollow conducting body, and short circuit means connected across the line opposite the connection of the feeder.
5. Thea-ntenna according to claim 1, and in which the transmission line forms a circular loop with both sides of the line in a common vertical cylindrical surface, and short circuit means connected across the line near-a point halfthe way around the loopfrom where the terminals of said feeder are connected.
G J'Ihe antenna according to claim 1, in which the transmission line forms a closed loop having alow impedance radio frequency reflecting means connected across the transmission line at a point half the way around the loop from where the ter'minalsof saidfeeder are connected.
"7; The antenna according to claim 1 having a sccondfeeder conn'ected'across the transmission lineat a point half the way around the loop from: where the terminals of said feeder are connected.
8*. An antenna comprising a. conducting cylinderclosed'at both ends and having a gap formed in'thematerial around the body of the, cylinder intermediate its ends providing two portions, a conducting surface attached to one end of the cylinder; a: conductor-extending axially within the, cylinder and holding. the two portions spaced to .iorm said'gapebetween opposing edges of the.
t ortions, alreeding line having one conductor 75 connected with one and :the other conductor with:
the other portion ofthecylinder neansaid gap,
and; short circuit means across thegap at-a point. diametrically opposite the points. ofv connection,
of the feeding line.
9. The antenna according to claim18, and in,
which the gap divides the cylinder into halves. 10-. The antenna, according to claim, 8, and in which the cross-sectional area of the spacezwithin.
the cylinder taken in a plane containing its axis. and on one side of, the conductor being substan-- tially one-hundredth multiplied; by the wave length squared wherein said-wave length is-the= wave. length corresponding substantially to the.
center frequency of the transmission band.
11. The. antenna according to claim, 8 andinv which. they diameter of thecylinder is; substantially 32A where ll is the wave length corresponding to the center frequency; of( the operating transmission band.
12. The antenna according to claim: 8,- andin.
which the height of the cylinderis substantially .0367\ where A is the wave: length correspondingto the center frequency of the operating transmission band.
13. The: antenna according to claim 8-, and; in
which the width of the gap is a small fractionof-a wave length corresponding-to the. center frequency of the operating transmission band.
- 4. An antenna comprising a metal, cylinder substantially .05-7l high where A is the wavelength corresponding to the center, frequency of the operating transmission band. closed at both .ends
' and having a circular gap around it parallelwith and midwaybetween the ends, said cylinderhaving a circular cross v section substantially 44w diameter, a metal plate on whichv one-endof the cylinder is mounted, a metal tube havingits ends fastened to the ends of thecylinder and holding the edges of the cylinder halves .to form said gap substantially .0048x wide, the. diameter of said tube being substantially .2737\, a; concentric feed line having an innerconductor. connected withv one and an outer'conductor with the other half of the cylinder nearsaid gap, and short circuit.
. means across the gap at a point diametrically opposite the points. of connection of the feed line to'the cylinder.
15. In an antenna, a hollow conducting cylinder, having between the ends, thereof a gap in its. side wall providing thereby a closed; loop, a means providing a conductive closure for one end of the v cylinder wholly within the side wall thereof means providing a conductive closure for the other end of the! cylinder extending beyondthe side wall in a plane perpendicular thereto, a; conductive connection. between the closures, connected therewith near the axis of the cylinder. and means for feeding. the cylinder near the gap;
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES. PATENTS