US2559693A - Antenna for broad frequency band operation - Google Patents

Description

Juiy 10, 1951 E. o. WILLOU GHBY 2,559,693

ANTENNA FOR BROAD FREQUENCY BAND OPERATION Filed Dec. 19, 1945 5 Sheets-Sheet l 'F/GZ Juiy 10, 1951 E. o. WILLOUGHBY 2,559,693

ANTENNA FOR BROAD FREQUENCY BAND OPERATION Filed Dec. 19, 1945 5 Sheets-Sheet 5 IOO- o -/ao -90 0 90 I80 g Z A/VGl/L/U? DIFECT/U/V/N DEGREES 8 M July 10, 1951 E. o. WILLOUGHBY 2,559,693

ANTENNA FOR BROAD FREQUENCY BAND OPERATION Filed Dec. 19, 1945 5 Sheets-Shet 4 o A vac -90 0 so lo A/VG'l/L/l/F DIRECT/fl/V/IV 066K565 F/ G. 3 g 200- Q, R 3

o 1 i. I80 90 0 90 I80 8; A N W ATTOR/VFV July 10, 1951 E. o. WILLOUGHBY 2,559,693

ANTENNA FOR BROAD FREQUENCY BAND OPERATION Filed Dec. 19, 1945 5 Sheets-Sheet 5 C A J I f s A I I A 03 Q /A/l 4 7'0,Q

N A Mm 0x441 Patented July 10, 1951 ANTENNA FOR BROAD FREQUENCY BAND OPERATION Eric Osborne Willoughby, London, England, assignor, by mesne assignments, to International Standard Electric Corporation, New York, N. 2.,- a corporation of Delaware Application December 19, 1945, Serial No. 636,021 In Great Britain January 1, 1945 7 Claims. 1

The present invention relates to antennae for operation over a broad band of frequencies and has for its object to provide an antenna which has directivity over a broad band of operational frequencies for modest dimensions.

An antenna according to this invention is of the end fire type and comprises in its broadest aspect two oppositely disposed edges of conduct ing areas with air or other dielectric therebetween; As will be made clear hereinafter the two oppositely disposed edges may be short circuited at one end or both ends and may hence be formed as a slot in a conducting sheet which may be plain or curved. Furthermore, the edges may be the edges of a groove in a conducting sheet, said groove having depth dimensions substantially one-quarter of the operating wave length. The most efficient antennae of this type from the directivity point of view are terminated at one or both ends in the characteristic impedance of the transmission line formed by the said opposing conductor edges, although such termination results in a loss of power of the order of 3 decibels (db).

The invention will be further explained in the following description with reference to the accompanying drawings in relation to reception antennae but it will be understood that the antennae may be used for receiving or transmitting equally well.

In the drawings:

Fig. 1 is a plan view of one embodiment of the invention;

Figs. 2A-2F show graphical curves used in the description and obtained practically;

Figs. 3A, 3B and 3C show graphical curves obtained theoretically;

Fig. 4 is an end view of another embodiment;

Fig. 5 shows a modification of the embodiment shown in Fig. 4;

Fig. 6 is a diagrammatic perspective view of another embodiment;

Fig. 7 is a plan view of a modification of the antenna shown in Fig. 1;

Fig. 8 is a plan view of a modification of the antenna shown in Fig. 1;

Figs. 9-11 are diagrams used in the description of the theory of the invention and Fig. 12 illustrates the electric field distribution in the neighbourhood of the slot.

Referring to Fig. l the antenna is formed by thin metal plates I, 2 whose adjacent edges 3, 4 oppositely disposed are separated by a narrow space 5. For high directivity over a wide frequency range the slot antenna thus formed is best terminated at each end in the characteristic impedance of the transmission line formed by the edges of the slot, although this results in a loss' of power of the order of 3 decibels (db).

Considering a wave polarised as shown by arrow 5 parallel to the plate and travelling in a direction parallel to the length of the slot, for example, in the direction of the arrow 1 a field is built up across the gap 34 which travels along the slot in opposite directions at the same velocity as the oncoming wave front; one portion in the same direction asthe received wave increases in the direction of propagation and gives a summation resultant at the end which is thus used as the receiving end or translating end of the slot while the other portion travelling in the opposite direction along the slot to the received wave is absorbed in the resistance 8 connected across the slot at the other or front end thereof. This resistance B is equal to the characteristic impedance aforementioned. At the receiving or translating end is a detector unit 9 connected across the slot.

Several experimental curves are shown in Figs. 2A-2F. The curves are all taken using slots which had a Width of one sixteenth of an inchand a length of 1.3?\ at \=4.-5.2 cms. The total width of the plates 1 and 2 at right angles to the slot was also 1.3x at 45.2 cms., i. e. 58.8 cms.

The curve shown in Fig. 2A was taken with the slot open at the front end, that is, no matching resistance 8 was present.

The curves shown in Figs. 2B-2F were taken with the resistance .8 of approximate matching value connected across the front end and respectively using wavelengths of 71.4, 86, 9'7, 107 and cms.

It will be observed from a comparison of curves of Fig. 2A and the curves of Figs. 2B-2F that the presence of the matching resistance has some beneficial efiect on the directive properties of the antenna, but only at the expense of a loss of power amounting to about 3 db. Nevertheless all the curves show good directive properties with small side lobes and good front to back radiation ratio over a broad band of frequencies.

The plates l2 or aerial, may be placed over a parallel plate H3 at a distance less than \/2 from the plate, as shown in the end view diagram, Fig. 4 wherein dimension ll between the plates l2 and I9 is less than M2. In such a case the wave front between the plates i2 and i I is suppressed due to the fact that the spacing H is less than the critical spacing for a wave of polarisation parallel to the plates and only the upper surfaces of the plates l-2 are active, a fact that can readily be illustrated experimentally by altering the spacing ll within a range of values below the critical spacing at the operating wave length. It will be noted then that there is no appreciable change in the polar diagram. Second order changes may be expected from modification of the discontinuity at the end of the slot by the presence of the parallel plate 10.

It should be observed that if the plate 16 is of the order of a few wavelengths long in the direction of the slot, spacings ll greater than will not appreciably afiect the direction diagram because the propagation velocity between aerial l-Z and parallel plate i8 is not much greater than that in free space. Intermediate the two limits, however, with a spacing I I of dimension d such that considerable complication can arise because of the marked difierence in phase velocity above and below the plates i, 2, and this condition is not recommended as even approximate prediction of results is too difficult with plates of finite size.

The antenna system shown in Fig. l will operate equally well if the edges of the plates I2 parallel to the slot are connected to the earth plate IO as shown in Fig. 5, and thus forms an open ended pipe structure having a longitudinal slot in the wall thereof.

So far all considerations refer to coplanar parallel plates l2 but it is possible to use arrangements such as that shown in Fig. 6, in which I is an earth plate, 2 a strip of sheet metal angularly related preferably perpendicular to plate I to form a slot 5' between strip 2 and ground plate 1. Such an arrangement is really half the arrangement of Fig. 1, the ground plate I acting as an image sheet dividing the slot arrangement of Fig. 1 into two portions.

This arrangement is activated by polarisation across the slot as indicated by the arrow 6, otherwise the considerations afiecting its polar diagram are very much the same as those for Fig. 1.

Fig. '7 shows a modification of the arrangement shown in Fig. 1 in which the forward end as regards the direction of propagation of the received waves of the slot is flared out so as to increase the impedance to the backward portion of the energy induced in the plates l, 2. This also increases the radiation damping of the front termination when the antenna is to be used for transmitting, the flare, however, should be made but a small portion of the slot length so that the impedance of the flare portion matches the impedance of the slot portion to assist the phasing of the waves which gives the directivity. Provided wide operating band widths are not required, however, it is always possible to obtain good directivity by adjustment of slot length to a value which makes the best use of any reflections in aiding forward directivity, the slot length being determined experimentally.

So far only plane surfaces have been considered or coplanar surfaces forming the slot (if the image of plate 2 in the earth plate I is taken into account in Fig. 6). If distortion of thepolarisation of the wave front is of no moment, there is no reason why the plates l, 2 or i, 2" should not be inclined to each other at an obtuseangle or the plates themselves be curved provided the edges forming the slot are coplanar.

In regard to feeding energy to the slot antenna or taking away energy from the slot antenna, a.

transmission line is coupled to the edges of the slot at the translatin end. The two conductors of the line i 2 are connected directly to the respective edges of the slot as shown in Fig. 1.

In the case when the end of the slot is closed, the transmission line is then connected to the edges of the slot at a distance from the end of the slot equal to one quarter of the operating wavelength as shown in Fig. 8 where the slot is shown closed at the end i 3 and the transmission line is indicated at I2.

THEORY OF END FIRE SLOT AERIAL All that is attempted herein is an approximate elementary theory sufficiently accurate to indicate the main properties of the slot antenna as a radiator. Secondary efiects associated with the discontinuities at the ends of the slot have been neglected although it is admitted that with short slots they may have considerable eiiect on the radiation pattern.

Case I 7 21rl kE cos 9 exp-z cut-I- cos 0 in which w=21r frequency and 7c is a constant and exp.=exponential, L is the length of the slot.

Hence integrating the total current at A becomes i. e. taking the square root of the sum of the squares of the real and imaginary parts, the field i. e. the resultant vector has the form of the chord of a circular are.

This is a maximum at cos 6:1, where Case II Referring to Fig. 10, neither the direction of propagation P nor the electrical vector E lie in the plane of the slot.

Let (7) be the angle between the line of the slot and the direction of propagation and let 0 be the angle between the electrical vector E and the perpendicular on: to the slot in the plane of the plates forming the slot. Then due to slot element aZ and therefore the field for direction of propagation in the direction (p corresponds to (1cos from similar considerations to those of Case I.

Case III The purpose of this case is to consider the effect on the radiation pattern when the forward end of the slot is terminated in an impedance ZR, the receiving end being of impedance Z matching the slot as before.

To simplify subsequent expressions let Referring to Fig. 11 and using the most general case with an electrostatic field of the type of Case II, the E. M. F, 651 at distance I from the left hand side of theplates is composed of two E. M. F.s, one acting to the left and one to the right. The E. M. F. acting to the left is Z+Z tanh 71 in which v is the propagation constant in the slot and this E. M. F. is applied to Zia-through a length of line Z. The E. M. F. to the right is (Z+Z tanh 71) (Z-l-Z) (1+tanh v1) and this looks into a matching impedance Z through a length of line Z.

Now finally to determine the current at Z it is necessary to find the E. M. F. sending the reflected wave at ZR from B back to A. Y

The current in ZR is given by Z +Z tanh 5 1 51 1 -(Z-l-Z) (1+tanh 7t) 6 Z cosh l-l-Z sinh l i. e. by

This is obviously correct since it is the same current as if 1:0, multiplied by exp. ('yl), and therefore the E. M. F. available to send current back to the receiving end at A is Therefore the current at A is hence the total current at A due to element 6Z becomes 6 Hence integrating with respect to Z the total current at A becomes hence the magnitude of the current at A is to equal the absolute amplitude of I Y 0E cos0 Now it is a good approximation for a narrow slot to assume damping being small compared to the stored voltamps. in the slot transmission line, hence the magnitude of the current at A is the absolute magnitude of If ZR=, i. e. the front end is open circuited, this becomes 2 cos (1cos 1 3 A 2 1 sin %(l-I-cos (1)) sin %(lcos T 21r 21r fl-l-cos o) A (1 cos 5) It is fully appreciated that none of these terminations are physically possible, but they are approached if the slot width is very narrow compared with the operating wavelength and in the case of the matched slot if the optimum termination is determined experimentally.

The curves shown in Figs. 3A, 3B and 30 have been deduced from the above theory.

The curve in Fig. 3A is for the case in which a matching resistance 8 is connected across the front end of the slot. The curve in Fig. 3B is for the case in which the front end of the slot is short circuited and the curve in Fig. 3C is for the case in which the front end of the slot is open. In the particular examples shown the length of the slot is 95 cms. and the operating wave length is 45.2 cms. It will be observed that the curves obtained practically and shown in Figs. 2AF approximate very near to the curves in Figs. 3A-B obtained theoretically.

The curves in Fig. 2 relate to an approximately matched slot line and if the slot is relatively long and the plates wide, the field plot in the vicinity of the slot is as shown in Fig. 12 which indicates the concentration of the field near the slot, and shows that if such a field is formed on an electrostatic basis, the characteristic impedance of the slot for the end fire mode of operation is substantially independent of wavelength for wavelengths small compared with the slot length but large compared to slot width.

Two nodal points NN On the field plot indicate the boundaries of the zones associated with the field across the slot.

For wide frequency bands with high front to back radiation ratio, the 3 db sacrifice is necessary and excellent results can be obtained over a very wide frequency range. That is a relatively narrow forward beam, side lobes Well down and good front to back radiation can be obtained.

Like all other end fire aerials the slot aerial is useful for moderate directivity but has a law of decreasing returns for large increases of slot length.

Case IV Parallel conducting plate underneath slot as described in connection with Fig. 4. There are two arrangements for consideration.

(a) Of these cases, the most important is that in which the parallel plate If! is less than M2 from the plate l-2, as then on the basis of the spacing being less than the critical one for propagation between the plates and polarised parallel to them, the upper plate I2 is only effective on its upper side plus a small zone below the slot confined to the immediate neighbourhood of the slot. The treatment is otherwise precisely as indicated above.

(b) For wide spacing H between the plates l--2 and ill of the order of or more provided the slot is not very long compared to Wavelength the theory given in the Cases I, II and 111 is substantially correct when the cosine of the angle of the direction of propagation of the guided wave between the plates and the plane of the plates approaches unity.

For intermediate spacing a more elaborate theory is necessary corresponding to a wave front above the plates with normal phase velocity and another wave front below the plates with a guided wave travelling at some higher phase velocity. This is not considered likely to give results of more engineering value than the simpler arrangements (a) and (b) and is of no further interest.

Experiment, however, verifies the Case IVa since change of spacing of the plates l-2 and I I well below critical spacing has negligible effect on the polar or directional diagram.

Case V A general theory could be evolved very similar to that given hereinbefore for plates inclined at an obtuse angle to form a slot or curved plate sections of a cylinder arranged to form a slot therebetween. Both these arrangements are similar in general principles to that of the plane slot, but have the disadvantage of causing greater distortion to the polarisation of the wave front.

CHARACTERISTIC IMPEDANCE OF THE SLOT AERIAL The active portion of the plate on either side of the slot 3-4, Fig. 1, is determined from electrostatic considerations as shown by the field plot in Fig. 12, and this enables the capacity per unit length of slot to be obtained, and thereby the effective characteristic impedance of the active portion between the nodal lines NN.

While only slot antennae have been referred to, an antenna according to this invention may comprise a groove in a conducting sheet providing the groove has a depth substantially equal to an odd number of quarter wave lengths at the operating frequency and said groove may be closed or open at one or both ends so as to short circuit the sides of the groove and as with the slot antennae, the groove may be terminated at one or both ends by its characteristic impedance.

What is claimed is:

1. An end fire type of antenna system for broad frequency band operation comprising a of conducting sheets arranged with oppositely disposed edges coplanar and spaced to form a slot, an impedance substantially equal to the characteristic impedance of said slot connected across each end of the edges of said sheets formim said slot. a source of high frequency energy, and means for feeding said antenna system with said high frequency energy at given points on the edges of said sheets.

2. An end f re type of antenna system according to claim 1 in which said conducting sheets are angularly related with respect to each other.

3. An end fire type of antenna system according to claim 1 further comprising a conducting sheet mounted on one side of said first mentioned sheets and at a distance therefrom less than half a wave length at the operating frequency.

a. An end fire type of antenna system accord ing to claim 3 wherein said further conducting sheet and said first mentioned sheets are connected together to form an open ended tubular conductor.

5. An end fire type of antenna system according to claim 1 wherein the edges of said sheets forming said slot are flared whereby the slot is flared out at one end.

6. An end fire type of antenna system according to claim 1 comprising a further conducting sheet mounted on one side of the first mentioned sheets and at a distance therefrom greater than one wave length at the operating frequency.

7. An end fire type of antenna system for broad frequency band operation comprising a pair of conducting sheets arranged with oppositely disposed edges coplanar and spaced to form a slot, an impedance substantially equal 1 to the characteristic impedance of said slot connected across one end of the edges of said sheets forming said slot, at source of high frequency energy, and means for feeding said antenna system with said high frequency energy at given points on the edges of said sheets.

ERIC OSBORNE WILLOUGHBY.

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

UNITED STATES PATENTS Number Name Date 2,206,923 Southworth July 9, 1940 2,238,770 Blumlein Apr. 15, 1941 2,234,293 Usselman Mar. 11, 1941 2,400,867 Lindenblad May 21, 1946 2,402,622 Hansen June 25, 1946 2,407,068 Fiske et a1. Sept. 3, 1946 2,414,266 Lindenblad Jan. 14, 1947 2,433,368 Johnson et a1 Dec. 30, 1947 FOREIGN PATENTS Number Country Date 818,696 France Oct. 1, 1937

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