US2625654A - Slotted cylindrical antenna - Google Patents

Description

Jan. 13, 1953 ALFQRD SLOTTED CYLINDRICAL ANTENNA Filed Jan. 12, 1946 2 SHEETS--SHEET 1 m a 6/ I, I4 jg 90 2 .56

Q on 9/? INVENTOR.

c ANDREW ALFORD. 90 62 57 BY Distqnce from Shortcuited end m free space Wave lengths ,qr mvgn Patented Jan. 13, 1953 UNITED. STATES PATENT, oFFI cE 2 Andrew Alford, Cambridge, Mass. Application January 12, 1946, Serial No. 640,690

This invention relates to antennas for ultrahigh frequencies, and more particularly'to antennas for radiating horizontally polarized waves at ultrahigh frequencies. I i "My invention makes use of a length of a waveguide which is provided with a longitudinal slot through which coupling is obtained between cur.-

rents' flowing inside and currents flowing out-' The antenna radiates nearly equal energy in different directions of the compass.

The antenna radiates stronger signals in direc-' tions near the horizon and relatively little en-'? ergy directly upward or downward, whereby 'the' power is distributed so that relatively little of the total is wasted by being sent directly to the sky or downward at large angles with respect to the horizon where the signal is normally stronger than it need be.

The antenna depends on metal parts ,for its mechanical strength and does not require the" use of insulating materials at points where there is substantial mechanical stress.

The antenna can be used as an element in a vertical array designed to' give further concentration, of energy near the horizon so that stronger signals may be sent out in diiferent directions of the compass towards distant points.

These and other features of the invention will more clearly appear from the following detailed description of'a few embodiments thereof and the appended claims. In the drawings:

Figs. 1, 1a and 1b are explanatory of the nature of the invention, Fig. 1a being a perspective view of a simple embodiment of the invention;

Figs. 2 and 2a are transverse cross sections of a simple embodiment of the invention illustrating the approximate distribution and electric field at a distance from the antenna when plotted in polar coordinates, Fig. 2 illustrating a cylinder whose'diameter is more than .15 of the operating wave length and Fig. one whose diameter is .138 of the operating wave length;

Figs. 3 and 3a illustrate, respectively, the distribution of voltage along the slotted part of the antenna and the distribution of the radiant field in a plane drawn through the longitudinal axis of the antenna;

9 Claims. (01. 250,-33.6.3)

Fig. 4 is a side elevationof a practical embodiment of my antenna;

- Figs. 5 and 6 illustrate diagrammatically modifications'of the'antenna of'Fig. 4; and

.Fig. 7 illustrates the voltage distribution characteristicsunder various frequency conditions.

Referring now to Fig. l, I represents a two-wire transmission line which is shunted by a number of loops of wire, such as 2, 3 and d. 5 is a source of ultrahigh frequency power which transmits an electromagnetic wave that propagates towards short-circuited terminal 6 of transmission line I. If the loops of wire 2, 3, 4, are sufficiently small in comparison with the wave length of the ultrahigh frequency source 5 they will have inductive input impedances as measured at terminals like I. These inductive impedances of the loops are in shunt with the distributed capacityof the transmission line and have the effect of reducing this capacity to a lower value when the spacing between the loops is sufiiciently small in comparison with the wave length; that is, when there area number of loops, for example, more than ten, per wave length. 3 One result of the reduction of the distributed capacity is that the phase velocity of the waves propagating along transmission line I is increased to a value which is greater than that of the velocityof light, and the standing waveswhich are formed because of the interference of the wave reflected at 6 are longer than th standing waves along the customary transmission line.

The dashed curve 8 in Fig. 1 indicates the voltagedistribution along transmission line I when a large number of loops 2, 3 and 4 are connected in shunt with it, while the dashed line 9 indicates the distribution of voltage which would be obtained along the same transmission line I if the loops were removed.

The loops 2, 3 and 4 are, in accordance with the present invention, replaced by a continuous sheet of metal It as shown in Fig. 1a In this figure, the edges] and I2 of the continuous sheet In formed into a longitudinally slotted cylinder perform the same function as transmission line I in Fig.1. Short-circuited end I 3 corresponds to the short-circuited end, 6, and generator I 4 corresponds to generator 5 in Fig. 1.

As in Fig. 1, the effect of the shunted inductance of the metal sheet Ill is to increase the phase velocity of the wave. propagating along transmission line ll, [2 so that the distribution of voltage is substantially as indicated by dotted line IS. The distance of the point such as [6 (which is the point of voltage maximum) from the shortcircuited end I3 is accordingly greater than the space quarter-wave length of the output of ultrahigh frequency generator l4. The exact distance between point I6 and short-circuited end l3 depends on the inductanceof the metal sheet. per unit length of transmission line H, I2. As

the diameter of the metal sheet cylinder is decreased, the shunted inductance per unit length is also decreased and the distance between points 13 and I6 is increased until a certain critical diameter of the cylinder is reached. Preferably this distance should be greater than two-fifths of the space wave length as long as the voltage distribution curve iii of Fig. 1a remains convex and does not become concave like curve IQ of Fig. 1b.

When the distributed inductance per unit length has a reactance lower than the reactance of the distributed capacitance between conductors H and I2, then the transmission line degenerates into one which is in a general way equivalent to the transmission line shown in Figblb.

The transmission line in Fig. 1b consists of distributed series inductances I1 and distributed shunt inductances 18. A transmissionline of this type does not transmit Waves without attenuation, and when the line is sufficiently long the voltage distribution will be approximately exponential. The transition between the voltage distribution of the type shown by dotted line -|5 in Fig. let to thetype Shown by dotted line I 9 in lb takes place when the diameter .of cylinder Iii in Fig. 1a is decreased beyond a certain critical value and, also, when the diameter of cylinder 10 is kept constant but the frequ ncy of generator i4 is decreased below a certain critical value.

On the outside of cylinder H! in Fig. 1a flow currents. The path of these currents is substantially circumferential so that the flow lines lie in planes which are at right angles to the axis of the cylinder. These currents flowing on the outside surface of the metal cylinder produce a radiation of elecromagnetic waves. The electrical field which is thus radiated by the cylinder has an electric vector which is directed at right angles to the axis of the cylinder. If the axis of the cylinder is vertical, the electric field is horizontal. Thedistribution of the electric field along a circle around the axis of the cylinder and in the plane at right angles to this axis depends on t e diameter of the cylinder. In general, asomewhat greater field occurs in the direction of the edges H, l2, as is shown in Fig. 2 where 2i indicates the approximate distribution and electric field at a distance from the cylinder It] When plotted in polar coordinates so that the radius vector from the center of the cylinder to any point along its surface is proportional to the magnitude of the electric field in the direction .of the radius vector. When the diameter of cylinder 1,0 is decreased, the shape of curve 2| changes slightly until the diameter reaches a value below .15 .of the operating wave length, when the shape of the radiation characteristic rapidly approaches a circle.

In Fig. 2a, 22 illustrates the approximate shape of the radiation characteristic obtained when the diameter of the cylinder I0 is .138 of the operating wave length. As the-diameter of the cylinder is decreased still further, the efiect of the lower shunted inductance per'unit length of transmission line ll, l2, together with the smaller effect of the radiation resistance, changes the distribution of standing waves along transmission line H, l2 in Fig. la into one similar to that shown-by l9 in Fig. lb. This effect takes place when thediameter of the cylinder is approximately .12). Thus, when the diameter of cylinder 1 ll is between .12 and preferably below .l4l\, the standing wave distributionalong the cylinderis of thetype indicated by dottedcurve l5 in Fig. .19; and the radiation pattern is nearly circular, as is shown in Fig. 2a.

Such antenna is particularly suitable for broadcasting ultrahigh frequency signals.

When the length of the cylinder ii! is approximately 1.05 and its diameter is approximately 38x, the distribution of voltage along line H, I2 is as shown by dotted line 23 in Fig. 3. If the diameter of the cylinder is decreased, the effect on the voltage distribution is as shown by dotted line 24. If the diameter is increased, the voltage distribution changes to one indicated by dotted line 25. These changes in voltage distribution along transmission line H, l2 affect the concentration of radiated power in the planes through the axis of the cylinder. Most concentration is obtained when the voltage distribution is between conditions indicated by curves 23 and 24; that is, when the length .of the cylinder in terms of the virtual wave length as measured along conductors H, 12 is between .20 and .4)\. The distribution indicated by 25 in Fig. 3 gives definitely a lower concentration of energy than that shown by curves 24 and 23. A typical distribution of field in a plane through the axis of the cylinder is illustrated by 26 in Fig. 30;,

The changes in voltage distribution along transmission line H, i2 from a condition indicated by line 23 to the condition indicated by line it can be brought about by inserting a metal rod or a tube into the cylinder 10. The effect of such a metal rod or tube is to decrease the shunt inductance per unit length and is, in substance, similar to decreasing the diameter of the cylinder. Since a rod or tube of .02) in diameter has a n0- ticeable effect when inserted into the cylinder the details of the transmission line which is used to supply power to the antenna, in order to avoid the inconvenience of placing the generator itself at M as shown in Figs. 1a and 3, have some effect on the exact dimensions which are chosen so as to get the most eihcient voltage distribution along edges H, 12.

One convenient arrangement of the transmission line is shown in Fig. 4 in which concentric line 21 is brought in through the metal bottom 28 of the cylinder a short distance, of the order of .02 wave length, from edge H of the longitudinal gap up to the end of the cylinder where the inner conductor 29 of the concentric line 21 is connected to edge 12 of the cylinder. This arrangement provides means for applying the generator voltage to the open end of the transmission line H, 2. The opposite end is short-circulted by means of plate 28 which is conductively connected to cylinder l0. Since there is substantially no voltage across the short circuited end of transmission line ll 12, the outer conductor of line 21 is not energized and, therefore, does not act as a radiator and does not disturb the radiation patternof the antenna.

When the cylinder I 0 is energized, as shown in Fig. .4, .equal but opposite potentials exist at opposite'po-ints along edges H and 12. The potentials existing along the outer surfaces of the cylinder decrease to zero along a line on the surface of cylinder .IB and which is ODDO J 1 131 5 parallel with the center line of the gap H, [2. This fact makes it convenient to support cylinder H! by means of a metal mast, such as 30, to which the cylinder i0 is attached substantially along said line.

Experiments hav shown --that no serious changes'in radiation pattern result if the contact between the mast 30 and the cylinder lBr-ismade 5 along a strip whose center is said line provided that the width of this strip is only a fraction of the total circumference of the cylinder.

The transmission line 21 may be mounted also 6 lowest frequency and curve 51 is obtained at the highest frequency.

The distributions of potential shown by curves 55 and 56 are more desirable than 45 and 51, be-

on the outside of cylinder l but it should come cause they approach more closely to the ideal out through the base disc 28 or near a neutral distribution in which the difference of potential point, i. e. where the cylinder is attached to the and, therefore, also the amplitude of the circumpole 30. ferential currents are uniform throughout the The width of the slot between edges 1 I and 12 length of the cylinder. It will be noticed that the of the cylinder is not critical but is preferably potential at the minima 58,59 of curves 55, 56 are of the order of one-sixth of the diameter of the not less than of the maxima 60, 6|. The cylinder and is closed by means of a thin strip minimum 62 of curve 51 is very low. By operat- 3! of insulating material bound along its edges ing near the cut-off frequency, that is, with the by brass strips 32. The open upper end of cylinpotential distributions such as 55, 56, it is possible der I0 is closed by a dome 33 of insulating mateto achieve a greater gain because the currents rial. The cylinder is fastened to the steel tube are more nearly uniform along the length of the 30 constituting the mast by means of a plurality antenna and because the antenna may be made of brass rings like 34. longer without them being a substantial change It will be obvious to those skilled in the art in the phase of the currents. that other embodiments than the cylindrical an- 50 and 5! show the distribution of relative tenna it may be practiced without departing phase along the cylinder. The phase is nearly from the spirit of the invention. It should be constant between 52, 53 where the phase begins understood that the word cylinder as used in to rise rapidly. The phase .of the currents near the specification and claims'is meant to cover the potential is approximately in quadrature with bodies whose transverse cross sections have other the large currents at other points closer to the shapes than circular. No matter of what shape, short-circuited end. If such currents are allowed the effective area of the cross section should be to flow they radiate power which is advanced in of the same order of magnitude as of cylinder phase with respect to the power radiated by our- !ll, i. e. between .0178 1 and .0113 )3, correspondrents along other portions of the antenna, proing to a diameter of .15 A to .12 A for circular 0371- ducing an undesirable tilt of the maximum radiinders. ation from the plane perpendicular to the cylin- In Fig. 5, for instance, indicates an antenna der and passing through it. This undesirable of metal bent into a cylinder which has a trieifect is avoided by cutting the cylinder so that angular cross section. The cylinder has a longiits open end is closer than the first minimum in tudinal slot, the edges of which are 36, 3! and 35 the potential distribution. The preferred length a metal bottom 38. The transmission line 21 is of the cylinder is of the distance between the brought into contact with edge 36, and the inner short-circuited end and the minimum. conductor 29 of said transmission line is 0011- These principles apply not only to the antenna nected to edge 31. Antenna 35 can be properof Fig. 4 but also to cylinders of other cross-sectioned to operate in accordance with the princitional shapes. pies described in connection with antenna 10. The following datagiven in Tables I and II will In order to allow for the effect of the details serve as examples of the effects which may be such as the thickness of the edges of the gap, the expected. Table I gives the effect of the gap width of the gap, the reinforcement, the diameter width in the case of circular cylinder with thin of the transmission line installed in the cylinder edges. and others, it is desirable to proceed as follows: Table I Construct the cylinder too long by making it about twelve times the square root of the cross section area which is chosen to be about .014A at ll i i n t l b f of ogliitifi tltat the design frequency. Connect a variable fre- Cylinder age Distribumn quency oscillator and measure the voltage distribution along edges 36, 31 at frequencies below gig and above the design frequency. At a frequency .224 .1501 somewhat below the design frequency the voltage distribution is Substantially p n i l, as is The effect of increased thickness of edges is Shown y 9 in i B a y 45 i AS similar to the effect of decrease in the gap width. the frequency is increased, the voltage distribu- Table II compares cylinders having elliptical tion changes progressively, as shown in Fig. '7 by cross sections with an antenna having a circular curves 55, 55 and 51. Curve 45 is obtained at the 0 cross section.

Table II For Optimum Distribution 52:53? w wish air as g Mn In.

' Inches Inches Mc. Circular 0ylinder-... 1.0 Y .s 14.9 -l8.0 .0158 350 33.8 Elli I 1.7 .s. 14.0 15.7 .0141 355 33.3 Ellipse II- 3.15 .s 14;0 10.9 .0107 310 32.0 Ellipse 1.7 .s. 14.9 15.7 0153 380 31.1 Ellipse Iv- 2.5 .s 114.9 12.2 0155 v420 28.1

accuse-4.

In ellipses I and II the gapshould beat the end of the major axis, and in the two other ellipses at the end of the minor axis.

Antenna in Fig. 6 has a rectangular cross section. In this figure,4l,'42iare the edges of the gap. Metal sheet 43 is the short-circuiting' bottom and 21 is the concentric transmission line used to energize the transmitter.

The proportioning of cylinder "49 can be carried out in accordance 'with the procedure described in connection with cylinder 35.

In order to allow for inaccuracies in proportionin'g of the cy'linder,a 'short-circuiting bar, as shown in Fig. 6. 'may be provided. This will enable the adjustmentof the voltage distribution along the edges of' the slot 'to the optimum, the short-circuiting bar having the effect of providing essentially thesame reflecting action as the bottom 43 of the cylinder,-but can be more readily displaced.

Refer-ring again to Fig. 7 which assumes 2, cyl inder having a circular cross section and-asap equal to A; of its diameter, it will beseen-that as the frequency'is decreased the attenuation increases. The standing waves are superposed on an exponential attenuation curve so that as the frequency is reduced the voltage minimum rises higher andhigher until there is no'longe'r a minimum. Finally the attenuation becomes so rapid that the wave from the feeder-does not reach the short-circuited end, and there are no standing waves.

The distribution of phase of the voltageacross the gap and the voltage amplitude are also shown in Fig. 7. The phase'curves 50, 5l=also show that f" the phases of the circumferential currents at particular sections diifer only by a -constant amount fromthe'phase of the potential-across the gap at this section.

The phase curve 50 has a shelf portion 52, 53

along which the phase remains approximately constant, around 90. At the point of minimum of potential the phase is advanced by about '75 with respectto the phase along the shelf. Still farther from the short-circuited end the phase advances rapidly. When the attenuation is reduced as in the case of curve 51 similar phenomena take place except thatthe phase error at the voltage minimum approaches closer'to v90.

It will be clear from these curves that the cylinder should not be longer than about .9 of the distance from the short-circuited end to the minimum because, Otherwise, currents near the open end of the cylinder, being advanced in phase, will tend to tilt the maximum of radiation away from the plane perpendicular to the axis of the antenna. The length'of the cylinder may beincreased near the cut-off frequency. Since at a given frequency the power gain increases even somewhat faster than the length oft'he cylinder, it is very desirable to operate with a voltage distribution such as is shown by curve 55 or 56. The voltage distribution 56, while still usable, is not as eiiicient as 55. The voltage distribution shown by curve 51 is not very efiicient. I

The-emcie'nt voltage distributionls one in which voltage-minimum is not less than .3 o'f the maximum and preferably not less than .5 of the maximum.

It should be noted that the antenna is operated under conditions where the attenuation is very nential curve because there would be practically no amplitude left to be reflected.

What I claim is:

1. An antenna for radiating horizontally polarized high frequency radio waves of a given band, comprising a conducting cylinder having a longitudinal slot and having ashort circuit across the slot at one end of the cylinder and means for feeding the cylinder at its other end across the slot, said cylinderhaving an effective diameter between 15 and .12). where A is the free space wave length corresponding-to a frequency of the given band, said cylinder having a length equivalent to substantially .9 of thehalf standing wave length established'along said cylinder by said frequency of the given band.

2. An antenna for radiating horizontally polarized high frequency radio waves of a given band, comprising a conducting cylinder havinga longitudinal slot and having 'a shortcircuit across the slot at one end of the cylinder and means for feeding the cylinder at its other end across the slot, said cylinder having an efiective diameter between .15 and 12% where A is the free space wave length corresponding to a frequency of the given band, said cylinder having a length equivalent to .4-of the virtual wave length, said virtual wave length being the length of the standing wave on the cylinder, coresponding to said frequency of the given band as increased by the phase velocity of propagation corresponding to the diameter of the cylinder.

3. An antenna for radiating horizontally polarized high frequency radio waves of a given band, comprising a conducting cylinder having a longitudinal slot and having ashort circuit across the slot at one end of the cylinder and means for feeding the cylinderat its other end across the slot, said cylinder having an efiective cross sectional area between .OI'IBA and .0112). where A is the free space wave length corresponding to a frequency of the given band, said cylinder having a length equivalent to substantially .9 of the half standing wave lengthestablished along said cylinder by said frequency of the given band.

4. An antenna for radiating horizontally polarized high frequency radio waves of a given band, comprising a conducting cylinderhaving a longitudinal slot and having a short circuit across the slot at one end of the cylinderandmeans for feeding the cylinder at its other end across the slot, said cylinder having an effective cross sectional area between .01"78 and .0112 where i is the free space wave length corresponding to a frequency of the given band, said cylinder having a length not substantially greater than .9 of the half standing Wave length established along said cylinder by said frequency of the given band.

5. An antenna for radiating horizontally polarized high frequency radio wavesof a given band comprising a conducting cylinder having a longitudinal slot, means closing the cylinder at one end having conductive means connected across space wavelengths'the"voltagedistribution would be represented by a substantially smooth expothe slot, a concentric cable extending through the closed end of the cylinder to the opposite end having one conductor connected to one side of the slot and the other conductor to the other side of the slot, said cylinder having an effective cross sectional area between .0178). and .0112k where k is the free space wave length corresponding to a frequency of the given band, said cylinder having a length equivalent to substantially .9 of the half standing wave length established along said cylinder by said frequency of the given band.

6. An antenna for radiating horizontally polarized high frequency radio waves of -a given band comprising a conducting cylinder having a longitudinal slot, means closing the cylinder at one end only by conductive means connected across the slot, a concentric cable extending through the closed end of the cylinder to th -Iopposite end having one conductor connected to one side of the slot and the other conductor to the other side of the slot at the other end, said cylinder having an effective cross sectional "area between .0178A and .0112A where A is the free space wave length corresponding to a frequency of the given band, said cylinder having a length not substantially greater than 1.05A.

7. An antenna for radiating horizontally polarized high frequency radio waves of a given band comprising a conducting cylinder having a longitudinal slot, means closing the cylinder at one end having conductive means connected across the slot, a concentric cable extending ;through the closed end of the cylinder to the opposite end having one conductor connected to one side of the slot and the other conductor to the other side of the slot, said cylinder having an effective cross sectional area between .0178)? and .0112A where A is the free space wave length corresponding to a frequency of the given band, said cylinder having a length equivalent to substantially19 of the half standing wave length established along said cylinder by said frequency of the given band, and means supporting said antenna in external contact therewith along a strip of the cylinder parallel to and diametrically opposite the center line of the longitudinal gap. f

8. An antenna for radiating horizontally polarized high frequency radio waves of a given band comprising a conducting cylinder having a longitudinal slot, means closing the cylinder at one end having conductive means connected across the slot, a concentric cable extending through the closed end of the cylinder to the opposite end having one conductor connected to one side of the slot and the other conductor to the other side of the slot, said cylinder having an effective cross sectional area between .0178A and .0112A where A is the free space wave length corresponding to a frequency of the given band, said cylinder having a length equivalent to substantially .9 of the half standing wave length established along said cylinder by said frequency of the given band, and means supporting said antenna in external contact therewith along a strip of the cylinder parallel to and diametrically opposite the center line of the longitudinal gap, and strapping means extending around the metallic part of said cylinder and said supporting means.

9. An antenna for radiating horizontally polarized high frequency radio Waves of a given band, comprising a conducting cylinder having a longitudinal slot and having a short circuit across the slot at one end of the cylinder and means for feeding the cylinder at its other end across the slot, said cylinder having an efiective diameter between .15A and .12A where A is the free space wave length corresponding to a frequency of the given band, said cylinder having a length equivalent to substantially .9 of the half standing wave length established along said cylinder by said frequency of the given band, said longitudinal slot having a width substantially one sixth of the diameter of the cylinder or less.

ANDREW ALFORD.

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

UNITED STATES PATENTS Number Name Date 2,234,293 Usselman Mar. 11, 1941 2,238,770 Blumlein Apr. 15, 1941 2,321,454 Brown June 8, 1943 2,400,867 Lindenblad May 21, 1946 2,414,266 Lindenblad Jan. 14, 1947 2,415,094 Hansen et a1. Feb. 4, 1947 2,513,007 Darling v June 27, 1950

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