US2248800A - Directional antenna array - Google Patents

Description

y 1941- A. ALFORD DIRECTIONAL ANTENNA ARRAY 4 Sheets-Sheet 1 Original Filed April 30, 1937 F IGJ.

.FIG.2.

ATTORNEY July 8, 1941. LFO D 2,248,800

DIRECTIONAL ANTENNA ARRAY Original Filed April 30, 1937 FIGA.

4 Sheets-Sheet 2 INVENTOR 444095 4L1 0RD ATTORNEY 2 July 8, 1941. ALFQRD 2,248,800

DIRECTIONAL ANTENNA ARRAY Original Filed April 50, 1937 FIG. 9.

4 Sheets-Sheet 3 BY 3 a ATTORNEY keen/Aer HIV/1RD A. ALFORD DIRECTIONAL ANTENNA ARRAY M K M .8

I I I July 8, 1941.

NN K

, II ATTORNEY so 22) M/DFGREES 40 20 44 64: orfz r Patented July 8, 194-1 U l'lE.

STATES PATENT OFFICE Mackay Radio and '1 York, N. Y., a corporat Original application April 140,039, now Patent No.

1940. Divided and this application Septem- Serial No. 297,063

ber 29, 1939,

2 Claims.

This invention is a division of my copending application 140,039 filed April 30, 1937, and relates to directional antenna arrays and pertains more particularly to antenna arrays utilizing a parasitic reflector and adapted for the transmission of a plurality of different frequencies at the same time.

It is an object of my invention to provide an antenna array which is simple in construction and still is adapted for the transmission of a plurality of different frequencies either simultaneously or separately.

Heretofore it has been customary to use the so-called dipole or half-wave antenna for the transmission of only a single frequency. In fact the very name of the antenna itself, expresses the thought that it is to be used for a single frequency only. But such antennae are, in truth, not extremely sharp in their transmitting qualities being, on the other hand, relatively aperiodic and adapted for the transmission of a fairly wide band of frequencies. The only difference between an antenna when it is operated as a true halfwave or dipole antenna and when it is energized with a frequency different from a half-wave value lies in the impedance which the antenna presents to the transmission line. In the case where the radiated frequency is such that half-wave operation results, the impedance presented by the antenna to the transmission as if a different frequency is transmitted the impedance will contain a capacitative or inductive component. But if proper provision is made for the matching of the antenna to the transmission line over the range of frequencies which it is desired to transmit, it will be found that the so-called half-wave type antenna, consisting of two equal halves fed at the center by means of a two-wir transmission line, may be operated in an efficient manner over at least a two to one range of frequencies.

Just as directional arrays may be constructed of a number of half-wave elements, so can they also be constructed of similar elements of other than a half-wave length. For the sake of simplicity such antennae as last mentioned will be called pseudo half-wave antennae or pseudo half-wave elements. A number of pseudo halfwave elements may be arranged in line and be fed in phase so as to produce a highly concentrated but a bi-directional radiation pattern. Unlike the ordinarily accepted half wave devices of the prior art, such an array is adapted for the transmission of two or three or more preselected frequencies. Generally speaking, an array of this line is resistive whereelegraph Company, New

ion of Delaware 30, 1937, Serial No. 2,195,880, dated April kind would have a maximum gain at that frequency at which the length of the individual pseudo half-wave element would be in the neighborhood of 1.2 wavelengths and would be somewhat less at all other frequencies. The decrease in gain with a change in frequency starting from the frequency of maximum gain is rather slight.

The main advantage of this multi-frequency array is relatively high gain per unit space, while a disadvantage is the fact that the array is bidirectional. This latter disadvantage is particularly serious when a directional array is to be used at the higher frequencies and for communication over relatively long distances. The bi-directional character of the radiation frequently results in the so-called echo difiiculties.

In order to reduce the back radiation of a multi-frequency array of pseudo half-wave elements it is necessary to provide a suitable multifrequency reflector. While it is possible to install another multi-frequency array directly in back or directly in front of the first array and identical to it, and feed it in such a way that the radiation in the backward direction will be completely cancelled, I prefer, in accordance with my invention, to utilize a parasitic reflector. By the use of a parasitic reflector the difficulty of supplying a feeding system for the second or refleeting array, is avoided.

The usual type of parasitic reflector, consisting simply of a length of wire slightly longer than an electrical half-wavelength, is not suitable for use in a multi-frequency array since this type of reflector will function at one frequency only.

In accordance with my invention I provide a pseudo half-wave antenna which is associated with a novel type of reflector, the latter being adapted to operate at a plurality of difierent frequencies. This reflector instead of being a single length of wire slightly longer than an electrical half-wavelength, is similar to the pseudo half -wave antenna, consisting of two equal halves connected to a sharp balancing or phase adjusting section of transmission line at the center, the two halves having an over-all length which is about the same as that of the pseudo half-wave antenna itself. The balancing or phase adjust ing transmission line is provided with auxiliary wires which may be either of the short circuit or open circuit type to insure the correct phase relationship of the reflected waves at the several frequencies which it is desired to transmit.

The above mentioned and further objects and advantages of my invention and the manner of attaining them, will be more fully explained in the following description taken in conjunction with the accompanying drawings; in which Fig. 1 shows a pseudo half-wave antenna; Fig. 2 is a field strength curve for such an antenna; Fig. 3 is a diagram used in explaining the operation of an antenna according to my invention; Figs. 4, 5 and 6 are forms of pseudo half-wave dipoles; Figs. 7 and 8 are end-on arrays of such dipoles; Figs. 9, 10 and 11 are reflector structures according to my invention, suitable for operation at several frequencies and Figs. 12 and 13 are curves showing the relation of impedanceto the square of the field strength for the reflectors.

Fig. 1 shows a pseudo half-waveantenna i fed with a source of unit powenZ. i.The antenna impedance is assumed to beimatchedata given frequency by the matching device 3 to the surge impedance of the transmission line and the impedance of the source of unit power 2 is assumed to be the same at all frequencies. When the arrangement in Fig. 1 is operated at a number ofdifferent frequencies itwill be assumed unless otherwise stated, that the matching device 3 is such that the antenna impedance remains matched to the transmission line at each frequency. Under the assumed conditions the pseudo half-wave will radiate, if the resistance of the wires isneglected, a unit of power at each frequency.

Then,- if an antenna of length L were in space so that theeifects of ground reflection could be neglectedthe field'producedby the antenna at a distant point located in the planebisecting the antenna at right angles would vary with frequency in the manner shown in Fig. 2. In this overall length of figure the abscissa LA is the the antenna in terms of wavelength and is thus proportional to frequency. The ordinate is proportional to the field strength at the distant point.

Fig.2 clearly shows that the pseudo half-wave antenna is per se capable of producing comparable fieldstrengthat the distant point over a wide range of frequencies.

When the ohmic resistance of the conductors is considered this range of frequencies over which the antenna can function successfully is somewhat reduced. Indeed when L/% is less than .5 the radiation resistance on a current loopbasis falls rather rapidly as LA is decreased, with the resultthat the currents produced by the unit power inthe section of transmission line between the antenna and the matching device 3 is increased. The current in the matching device.3 also increases. Thus for a given power delivered to the antenna the PR losses increase as L/x decreases so that for very small values of L/X the field produced at the distant point falls off and approaches zero. With the usual type of construction this decrease in radiation efiiciency does not take place immediately upon a decrease of L/A below .5, but takes place gradually and may be considered to be relatively small when L/A is more than .25.

.Thus in actual practice a pseudo half-wave antenna may be considered to be an efficient radiator fromL/A of say .3 up to L/A=1.45 or almost a.5:l frequency range. Over a two to one frequency range from .7 to 1.4 it is always more efficient than a. half-wave.

When the antenna has been installed at a certain height, above. groundthe effectiverange of frequencies which may be used for communicating with a givenpoint may be still further re- .duced under certain circumstances because the producedfor smaller values of correctv results may usually sidering. each pseudo half-wave antenna as two separate half-wave antennae spaced a distance maximum of radiation may take place at the desired angle to the horizon at one frequency and at some other undesired and ineffective angle at a second frequency. When the two frequencies are used for communication with two different points which are approximately on the same great circle this may or may not be true. This situation is common to all kinds of antennae operated at more than one frequency and is not a peculiarity of the pseudo half-wave antennae or pseudo half-wave arrays, and it need not be discussed here in detail.

The pseudo half-wave antennae which have lengths between .8)\ and IAOA are particularly useful as elements in arrays because they result in greater signal strength at the distant point per feeder.

Inorder that the action of these pseudo halfwaveantennae in arrays may be more clearly understood the following elementary explanation of theaction cf-such anantcnna with L/ l.0 is

' offered.

It .is well known that the field produced at the distant point byan array consisting of two half-wave antennae which are fed individually and placed end-on as shown in Fig. v3 depends on the distance S between the half-wave elements. :For small values of S the mutual impedance between. the elements tends to reduce the currents produced in the half-wave elements by a unit of power and thus decreases the field at the distant point. For small values of S the field at a distant point increases as S is increased.

:A pseudo half-wave such as shown in Fig. 4 hasa current distribution which is very similar to that of two separate half-wave antennae shown in Fig. 3. The only difference between the .two current distributions is the additional current in opposite phase indicated by the shaded area in Fig. v4. When. distance S is relatively small the current in the shaded areas is also small and the field produced by this current at the distant pointis likewise quite small. This field, howeven'tends to oppose the field produced bythe main radiating portion of the structure. Thus asLzis increased beyond 1.0x, at first the effect isrto decrease the mutual impedance be tween the two halves of the antenna and thus to increasethe field produced at the distant point-bya transmitter of .unit power. When L isstill furtherincreased the radiation from the shadedarea in Fi .4 .becomes appreciable and begins to cancel. out themain radiation thereby decreasing :thefield'H at the distant point. is this phenomenon which causes the decrease of thefieldH when L l.30 This effect may be-made to take place for values of.L l.3 by cancelling out some of the radiation from the shaded portion in:Fig. thy providing an auxiliary radiator i-4 energized 180 out of phase with the main radiator, as shown inFig. 5.

On the other hand the peak radiation may be L by loading the ends of theantenna with some form of capacity .5-5, for example as shown intFig. 6.

When a numberof .psuedo half-wave antennae are operated together in an array it becomes necessary to insurethat they cooperate with each other in such a way as to produce maximum field at the distant point. The exact calculation of rmutual interaction between pseudo half-wave elements is rather complicated but approximately be obtained by con- S=L- This assumption becomes more nearly correct when an auxiliary radiator as shown in Fig. is employed. When such an auxiliary radiator is not used then the error in the calculation is due to the field produced by the auxiliary radiator. As this field is usually not very large it may be neglected in the calculation of interaction between the elements.

Thus it is found that when two pseudo halfwave antennae are connected in broadside and parallel to each other they should be about .6 to .75) from each other for best results. Separations between ends of .2A to .67\ are found to be satisfactory when two pseudo half-wave antennae are in broadside and in line with each other.

From the above discussion it is also clear that a broadside antenna such as shown in Fig. '7, consisting of a number of sections 6, I, 8, 9, each approximately 154% long, which radiates maximum energy in the plane at right angles to the antenna produces greatest radiation when the phase changers iii, H] are so adjusted that the phase delay is about and not x as is generally assumed. This type of an antenna becomes less and less aperiodic per se as the number of half-wave radiating portions is increased. Moreover, when a large number of radiating portions are employed the phase changers located further away from the ends also begin to radiate. For this reason it is probably best to use coils instead of loops as phase changers in the elements next to the feeder.

It has already been explained that pseudo halfwave antennae per se are efficient radiators over a fairly wide band of frequencies. cordance with the teachings of my copending applications, Serial Nos. 12,451 filed March 22, 1935, 18,995 filed April 3, 1935, and 118,886 filed January 2, 1937, the impedances of these antennae may be matched to the impedance of the transmission line at a plurality of frequencies under substantially the conditions herein above assumed. Moreover, by using the procedure explained in my copending application, Serial No. 118,886, it is possible to operate a pseudo halfwave antenna or a number of'them on several frequencies simultaneously.

In order that echo phenomena on the longer radio circuits may be reduced it is usually necessary to provide an antenna which is substantially unidirectional. To achieve this end an array of pseudo half-waves may be provided with a reflector. This reflector may be either of the fed or of the parasitic type. When the reflector is fed it is usually made similar to the radiator in every respect and is placed at some fraction of the operating wavelength behind or ahead of the radiator and is fed in such phase that the back radiation is cancelled.

When an array of pseudo half-waves is operated at more than one frequency reflectors of the parasitic type are usually easier to tune, require less material and as a rule provide sufficiently high front to back ratio for practical purposes. For these reasons this type of reflector will be described in some detail.

The parasitic reflector which is well known consists simply of a length of wire placed in back of the radiator anywhere from a 2x to .SA and adjusted to such length that the back radiation from it in the back direction is about 180 out of phase with the radiation from the antenna. Parasitic reflectors of this type are suitable atone frequency only.

Now, in acmagnitude and of such A type of parasitic reflector which is suitable for use at several frequencies is shown in Figs, 9 and 10. This reflector consists of a pseudo halfwave antenna H, a length of transmission line I2 and a number of auxiliary sections l3, 14 of transmission line shunted across the main transmission line. When such a reflector is located a fraction of a wavelength behind the radiator then because of the mutual impedance between them there is produced in the reflector a certain voltage E, referred to some convenient point such as a current loop. This electromotive force in turn produces a current which depends on the total impedance of the reflector. This total impedance consists of two parts: the impedance of reflector itself Z1 and the impedance of the transmission line Z2. At a given frequency the impedance Z1 remains fixed as long as the pseudo half-wave antenna itself is unchanged. The impedance of the transmission line Z2, however, may be varied at will, for example, by varying the position of the short circuiting member [5 in Fig. 10. As Z2 is varied the current in the reflec tor is also varied in magnitude as well as in phase, the current following the formula Thus by adjusting Z2 it is possible to produce a current which is of approximately correct phase and amplitude in the reflector provided that voltage E has suiiicient magnitude and approximately correct phase. This condition exists when the reflector is located between .1 and .3)\ from the antenna. However, as will be mentioned later certain spacings are preferable to others, moreover, in some cases, depending on the length of the antenna and of the reflector a spacing of somewhat less than .1) may be utilized.

Assuming the induced voltage E is of sufficient phase that the range of phase adjustment provided by Z2 is sufficient for producing the current of the correct phase, then it follows that by moving the short I5 in Fig. 11 into various positions any value of Z2 between -7'. and +a'. may be obtained. Thus for a given frequency h the reflector is in adjustment when the short i5 is in some position A. At another frequency f2, the reflector is in proper adjustment when the short is in some other position B. If it is desired to have the reflector in adjustment at both frequencies at the same time, the simple short would no longer do as it would have to be in both positions simultaneously. In order that this result may be achieved in effect the system shown in Fig. 9 may be employed. In this figure the short in position A has been replacedby a building out section 13 and the length of the open-ended line it is made AA at frequency f1.

The operation of this structure is as follows: at frequency ii the quarter wave line it produces a nearly zero impedance at A and thus acts substantially as though there were a physical short at A. This action of the quarter wave line is in no way disturbed by the presence of the building outsection it since the total impedance obtained by paralleling a finite impedance across a zero impedance is, of course, always zero. For this reason the length of section [3 may be varied at will without in any way disturbing the operation of the tuning of the reflector at frequency f1. On the other hand, the impedance Z2 at the second frequency it may be varied by changing the length-of section l2. In fact, the length of this section may be so adjusted as to produce the same value of Z2 which would result if there were a physical short in position A. Assuming that the distance between position A and position B in degrees at frequency f2 is and assuming further that n is the wavelength at frequency f1 and that A2 is the wavelength of frequency f2 and that the length of section l3 in degrees of frequency f2 is 0.

Then in order that the impedance at B be equal to zero at frequency f2 it is necessary that the impedance at A at the same frequency be equal to cot (90).

The impedance which is actually obtained at this point is Z which is given by the following equation:

1 1 1 (1) 2 3 tan j cot (90 j cot 0+j tan (90%) when section l3 has been properly adjusted Z=7' cot (90) so that This equation enables the calculation of 0 for any given f1 and Thus 0 may be found from 0 S-m-AZ where 0 is in degrees and S in same units as )2.

When the reflector is to be tuned to three frequencies there will be three positions of the short which have to be considered. Suppose that these positions for frequencies 71, f2, is, are respectively A, B and C-Figs. 9 and 11. This tuning may be accomplished as follows: The first step is to determine the points corresponding to A, B and C, at which shorts would properly tune the reflector at the respective frequencies f1, f2 and fa. This is done by moving up or down the transmission line of the reflector a short circuiting member, while energizing with the desired frequency the antenna with which the reflector is associated, and at the same time measuring the backward radiation to determine the minimum value. This value shows the proper location for the short circuit. Or conversely the maximum forward radiation may be used to determine this location, if the forward radiation is the prime consideration. The three points A, B and C are then marked on the transmission line.

The next step is to install a quarter wave section at frequency 11 at A and to calculate the length S of a shorted building out section which connected at point A would tune the reflector to In as has already been explained above. Then instead of installing this shorted building out section of length S an open section of length S +1 is installed. After this is done the reflector will be tuned to two frequencies f1 and f2. Now in order to tune the reflector to the third frequency a section of length D is connected across the section of length A2 8 +11 at the point which is A2 I from the open end of that section. This new section of length D will not disturb the operation of the reflector at frequencies f1 and f2 but will permit the tuning at the third frequency.

The length D of the new section may be calculated by assuming for the moment, that only frequencies f1 and is are concerned. Then the length S of the building out section which would tune the reflector to frequency is may be calculated from Equation 2.

When this phantom length S is known, the desired length D may be calculated at once. In fact the impedance presented by the shorted section of length S at frequency is is equal to 21r J tan 5 In order that the complex section consisting y 4 D and 0 present the same impedance at frequency is as a simple shorted section of length S, the following equation must be satisfied j tan and hence from which D may be calculated. When the length D calculated from Equations 2 and 3 comes out longer than a quarter wavelength it may be shortened by a quarter wavelength and left open, instead of shorted at its far end.

So far it has been shown how the reflector may be tuned to a number of different frequencies so that it would produce a maximum amount of forward radiation or a minimum amount of backward radiation for a given position of the reflector with respect to the antenna. As already explained above the voltage which induces a current in the reflector depends in magnitude and phase on the spacing between the reflector and the radiator. While for any given length of the radiator and the reflector there exists the best value of distance between them which results in maximum radiation forward and still another distance between them which results in a maximum amount of back radiation, when the complete aerial consisting of a radiator and a reflector is to be operated at a numbe of frequencies a compromise must be had.

From experiments with such aerials I have found that in order to obtain a structure which is efficient over a range of frequencies of 2 to 1 it is best to choose such spacing between the reflector and radiator that it is equal to approximately .2A 0 .22A at the highest frequency at which the aerial is to be operated. Under such conditions it is found that good reflector action is obtained at twice the said lowest frequency.

The aerial consisting of a radiator and a reflector is not necessarily most efiicient when the length of the radiator alone would be most emcient, for example it is found that when the spacing between the reflector and radiator is .2 the aerial is more efflcient when both the radiator and the reflector are 1A long rather than 128A long. On the other hand the latter condition results in greater front to back ratio.

Figs. 12 and 13 in which the abscissa represent the phase angle of Z1+Z2 which is the total impedance of the reflector while the ordinates show the field squared which is produced in the forward direction and also in the backward direction. It is seen from Fig. 12 that when the phase angle Z1+Z2=0 the radiation in a forward direction is a maximum and when the phase angle of Z1+Zz=40 the radiation in a backward direction is a minimum. The maximum front to back ratio is obtained with the phase angle of Z1+Z2 between these two values.

The maximum radiation forward obtained with the aerial corresponding to Fig. 13 is also obtained when the phase angle of Z1+Z2=0 but the minimum back radiation occurs when this angle is about 23. Thus the minimum back condition is more nearly equal to the maximum front condition and a much higher front to back ratio is obtainable with this aerial. The maximum forward radiation obtainable is somewhat less than that obtainable with the antenna corresponding to Fig, 12.

It may be noted tion of maximum nearly twice the power that in both cases the condiforward radiation results in being radiated in a forward direction clearly showing that with parasitic reflectors much longer than one-half wave length when excited by radiators of the same length it is possible to obtain approximately the same or in fact a better reflector action than is obtainable with a parasitic reflector a half wave length long energized by a half Wave antenna well known in the art. It may be seen that the antenna of Fig. 12 when operated at half the frequency becomes a half wave with a reflector located .1 from the radiator and which as is well known can be adjusted to deliver a considerable forward radiation with a high front to back ratio. I have found from experiments that also with radiators and reflectors of lengths intermediate between 1.3x and .5) it is possible to secure forward gain with a ratio when the reflector radiator spacing is 22x at the highest frequency.

high front to back about At times it may be desired to retune an existing aerial consisting of a pseudo half-wave antenna with a pseudo half-wave reflector of the type herein disclosed at some frequency at which the spacing is greater than 22k. Under these conditions it will be found that the reflector can still be tuned so as to increase the forward radiation at the expense of side and back radiation. This action will not, however, be quite as pronounced because of the comparatively low current induced in the reflector.

A number of aerials, each consisting of a pseudo half-wave radiator and a pseudo halfwave reflector may be arranged in broadside or in line and operated as an array. When arranged in broadside it is best to keep the spacing between the ends of the adjacent antennae 2x or greater at the lowest frequency so as to avoid the interaction between the various radiators and reflectors which makes the tuning of the various reflectors dependent on each other and for that reason a somewhat laborious procedure.

While I have described particular embodiments of my invention for the purposes of illustration, it will be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

For example while in the above description antenna arrangements embodying my invention have been described in connection with transmission the same arrangements may be used for reception of signals. Furthermore while the tuning of the reflector to three different frequencies has been described it should be understood that tuning to more than three frequencies may be carried out by a repetition of the procedure already described.

What I claim is:

1. An antenna array comprising a radiator and a reflector, and means for tuning said reflector simultaneously to a plurality of different frequencies, the spacing between said radiator and reflector being not over .25 at the highest frequency and greater than .08)\ at the lowest frequency.

2. An antenna array comprising a radiator and a reflector, each having two radiating elements of approximately equal lengths, said reflector having a transmission line connected thereto intermediate said lengths, said line terminating in a plurality of auxiliary lines of different lengths, the number of said auxiliary lines being at least equal to the number of frequencies at which it is desired to use the array.

ANDREW ALFORD.

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