Linear Antenna Arrays

This invention relates to a linear antenna array wherein three or more element antennae are arranged along a straight line, and more particularly an antenna array suitable for receiving television waves and frequency modulated signals.

As an television receiving antenna so called unidirectional antenna which is sensitive to electric waves coming from a given direction but not to electric waves in other directions has been used for the purpose of eliminating ghost signals caused by electric waves reflected by buildings. One example of such antenna is disclosed, for example, in Japanese laid open patent specification No. 4146 of 1977 dated Jan. 13, 1977. The most simple unidirectional antenna disclosed therein is a two element dipole array antenna of the phase difference feeding system in which two dipole antenna elements are arranged along a straight line with their antenna conductors disposed in parallel with a spacing d, and two feeder lines l.sub.1 and l.sub.2 having different lengths are connected between respective element antennae and the split ports of a splitting circuit which are divided into two ports from a common feed ports. The phase of either one of these two feeder lines is reversed by, for example, crossing two parallel conductors. When the lengths l.sub.1 and l.sub.2 of the two feeder lines and the spacing d between the antenna elements are selected to satisfy an equation: ##EQU1## where K is a wavelength compression coefficient, the antenna would receive electric waves in the direction of a straight line along which the antenna elements are arrayed, but not electric waves coming from other directions. The unidirectional antenna of the phase difference feeding type is characterized in that the unidirectional performance of the directivity does not vary irrespective of the variation in the frequency of the received waves. In other words, it is possible to obtain a directive antenna over a wide frequency band.

However, in such two element antenna, the direction in which the directivity is the maximum or minimum (null point direction) is only one for each direction. However, the direction of arriving the ghost signals in the reception of television waves is not generally limited to one, and in most cases the ghost signals come from a number of directions. For this reason, in order to eliminate ghost signals, it has been strongly desired to provide an antenna which have a plurality of null directions and which can install the null points in an arbitrary direction. However, until today there was no antenna that can meet such requirement over a wide frequency band.

To satisfy these requirements it is necessary to increase the number of the antenna elements and to use a special method of feeding that assures a desired directivity. According to said Japanese laid open patent specification No. 4146 of 1977, 2.sup.n dipole antenna elements are arrayed along a straight line and the phase difference feed system is used. However, the object of this antenna array is to make more sharper the directivity of the two elements antenna thereby improving the ratio between the maximum and the minimum values of the directivity, that is the front to back ratio characteristic, and this laid open patent specification does not teach the installation of null points in a plurality of difference directions as contemplated by the instant invention.

Accordingly, it is an object of this invention to provide an improved linear antenna array having more than two null point directions and can not only install these null point directions in desired directions but also always maintain in specified directions these null points regardless of the frequency.

Another object of this invention is to provide a novel antenna array capable of providing a maximum number of the null points with a minimum number of the element antennae.

A further object of this invention is to provide a linear antenna array having a definite directivity over a wide frequency band.

According to this invention there is provided a linear antenna array comprising three or more than three element antennae which are arranged on a straight line; a plurality of power dividers respectively corresponding to the element antennae, each power divider being provided with split ports of a number corresponding to a binomial coefficient, which is expressed by an equation p.sub.i =.sub.n-1 C.sub.i-1 where p.sub.i represents the number of ports, n the number of antenna elements, and i the number of a corresponding antenna counted from one end of the array and a common port connected to a feed port of an element antenna; a plurality of phase shifters having one ends connected to the split ports of the power dividers; each of the phase shifters producing a phase shift which varies in proportion to frequency; and a common power divider having split ports connected to the other ends of the phase shifters and common ports acting as feed ports, whereby to alternately reverse the phase between the feed port of the common power divider and the feed ports of respective element antennae.

These and further objects, the principle of operation and the specific advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIGS. 1A, 1B and 1C are a block diagram and vector diagrams respectively showing the principle of operation of one embodiment of this invention utilizing a three-element antenna;

FIGS. 2A, 2B and 2C are a block diagram and its detailed diagrams respectively useful to explain the principle of this invention utilizing n element antennae;

FIG. 3 is a block diagram showing one embodiment of this invention wherein the number of the element antennae is 3;

FIG. 4 is a block diagram showing another embodiment of this invention wherein the number of the element antennae is 4;

FIG. 5 is a block diagram showing still further embodiments of this invention wherein the number of the element antennae is n;

FIGS. 6A, 6B and 6C are polar diagrams showing the directivity of the linear antenna array according to this invention; and

FIGS. 7A, 7B and 8A, 8B and 8C are polar diagrams respectively showing the E-plane directivity and the H-plane directivity of this invention utilizing 3 element antennae.

With reference to FIGS. 1A, 1B and 1C, the principle of this invention utilizing 3 element antennae will firstly be described.

The embodiment shown in FIG. 1 comprises three element antennae #1, #2 and #3 which are arranged along a straight line with a spacing d therebetween. Each element antenna comprises a dipole antenna, for example.

The feed lines of the second antenna cross each other as its feed port so as to invert 180.degree. the phase of the current fed to this element antenna #2. A feed network bounded by dotted lines and connected to three element antennae #1, #2 and #3 is constructed as follows. More particularly, the feed network comprises a common power divider D.sub.s constructed to apply four signals having substantially the same phase to respective split ports, phase shifters L.sub.11, L.sub.21, L.sub.22 and L.sub.31 respectively connected to the split ports of the common power divider D.sub.s, a one-way divider D.sub.1 connected between the phase shifter L.sub.11 and the first element antenna #1, a two-way divider D.sub.2 with its split port connected to the outputs of phase shifters L.sub.21 and L.sub.22 and its common port connected to the second element antenna #2, and a one-way divider D.sub.3 connected between the phase shifter L.sub.3 and the third element antenna #3.

Each power divider functions to divide a signal among the split ports when the signal is supplied thereto from the common port. Conversely, when the signal is applied to the divider through the split port, it produces a superposed signal output on the common port, thus acting as a combiner. For the sake of brevity, the term "divider" is used not only for a case where a signal is applied through the common port but also for a case where the signal is applied through the split port.

The one-way dividers D.sub.1 and D.sub.3 are not required to have a dividing performance. But for the purpose of clarifying the relationship between the explanation for FIG. 1A and the description of the general principle it is designated as a one-way divider.

Let us denote the exciting currents of the first, second and third element antennae #1, #2 and #3 by I.sub.1, I.sub.2 and I.sub.3 respectively. Neglecting the loss of the circuit elements in the circuit, the exciting current I.sub.1 of the first element antenna #1 accompanies a delay thereof which is supplied from the power divider D.sub.s to the phase shifter L.sub.11 and then supplied to the first element antenna #1 via the phase shifter L.sub.11, and the one-way divider D.sub.1. The exciting current I.sub.3 supplied to the third element antenna #3 accompanies a delay thereof which is supplied from power divider D.sub.s to the third element antenna #3 through the phase shifter L.sub.31 and the one-way divider D.sub.3.

Accordingly, if the amplitude of the current produced at the split port connected to the phase shifter L.sub.11 of the power divider D.sub.s is made to be equal to that of the current produced at the split port connected to the phase shifter L.sub.31 of the power divider D.sub.s, current I.sub.1 and I.sub.3 would have the same amplitude but different phases.

Current I.sub.2 is a vector sum of currents I.sub.21 and I.sub.22. I.sub.21 represents current which is supplied from the common divider D.sub.s to the second element antenna #2 and delayed by the phase shifter L.sub.21 and the two-way divider D.sub.2. I.sub.22 represents which is supplied from the common divider D.sub.s to the second element antenna #2 and delayed by the phase shifter L.sub.22 and the two-way divider D.sub.2.

Each of the phase shifters L.sub.11, L.sub.21, L.sub.22 and L.sub.31 is a line type phase shifter utilizing a transmission line whose amount of phase shift is varied by varying the length thereof, and for convenience the lengths of the transmission lines of respective phase shifters are designated by the same reference characters L.sub.11, L.sub.21 L.sub.22 and L.sub.31.

In the linear antenna array shown in FIG. 1A, since the number of the element antennae is 3 it is possible to form null points having no radiation in any two directions as will be described hereinunder.

Suppose now that a first null point is to be formed in a direction .gamma..sub.1 as shown in FIG. 1A. This can be accomplished by making zero the vector sum of electric fields E.sub.1 (I.sub.1), E.sub.2 (I.sub.2) and E.sub.3 (I.sub.3) created in a wave front O.sub.1 -Q.sub.1 corresponding to the direction .gamma..sub.1 by the exciting currents I.sub.1, I.sub.2 and I.sub.3 supplied to the first, second and third antennae. Taking the electric field E.sub.1 (I.sub.1) created by the first antenna #1 in the wave front O.sub.1 -Q.sub.1 as a reference, field the electric field E.sub.3 (I.sub.3) created by the third element antenna #3 is delayed by an amount corresponding to the distance between the third element antenna #3 and the O.sub.1 -Q.sub.1 wave front so that it can be shown by a vector E(I.sub.3) shown in FIG. 1B.

Since the amount of the phase lag at point q.sub.1 can be varied as desired by varying the amount of phase shift effected by the phase shifter L.sub.31 the direction of vector E(I.sub.3) shown in FIG. 1B can be determined arbitrarily.

If the direction of one null point is appointed as above described, the amount of relative phase shift at point q.sub.1 is determined by adjustment of the phase shifter L.sub.31 in accordance with the distance between the third element antenna #3 and point q.sub.1. At this time, if an electric field E(I.sub.2) that cancells fields E(I) and E(I.sub.3) were created by the second element antenna #2, a null point would be formed in the direction of .gamma..sub.1.

The electric field E(I.sub.2) may be formed by the reverse phase component of field E(I.sub.1) and the reverse phase component of field E(I.sub.3). The phase of the electric field E(I.sub.21) created by the exciting current I.sub.21 flowing through the phase shifter L.sub.21 can be varied by varying the amount of phase shift effected by the phase shifter L.sub.21. Accordingly, it is possible to cause the fields E(I.sub.21) and E(I.sub.1) to have equal amplitude and opposite phases by suitably selecting the amount of phase shift of the phase shifter L.sub.21 by taking into consideration the distance between the second element antenna #2 and the wave front O.sub.1 -Q.sub.1. Similarly, it is possible to cause the field E(I.sub.22) created by the current I.sub.22 flowing through the phase shifter L.sub.22 to have the same amplitude and the opposite phases at the field E(I.sub.3) by suitably selecting the amount of phase shift of the phase shifter L.sub.22.

However, since the second element antenna #2 is crossed at the feed port so as to reverse the phase of the current, except the effect of crossing, it is sufficient to cause the fields E(I.sub.21) and E(I.sub.22) to have the same phases as the fields E(I.sub.1) and E(I.sub.3) respectively.

When one null point direction is determined in a manner described above, the second null point direction would be determined automatically.

More particularly, considering the direction of .gamma..sub.2, the wave front in this direction becomes O.sub.1 -Q.sub.2. At this time, the amount Q of the phase shift is shown in FIGS. 1B and 1C such that the Q corresponds to the difference (l.sub.2 '-l.sub.1) between the distance l.sub.2 ' between the center 0.sub.2 of the second element antenna #2 and a point q.sub.2 ' and the distance l.sub.1 between the center O.sub.2 and point q.sub.1 '. If the field E(I.sub.22) shown in FIG. 1B is rotated (lagged) by 0, the field E(I.sub.1) becomes a position which is just opposite to field E(I.sub.22) as shown in FIG. 1C. Then the phases of the fields E(I.sub.21) and E(I.sub.22) are also lagged by 0. Because the field E(I.sub.21) created by current I.sub.21 and the field E(I.sub.22) created by current I.sub.22 are formed by the electric wave radiated by the same second element antenna #2, so that, with reference to direction .gamma..sub.1, direction .gamma..sub.2 results in a phase lag corresponding to the difference (l.sub.2 '-l.sub.1). For this reason, considering the field for the direction .gamma..sub.2, both fields E(I.sub.22) and E(I.sub.21) shown in FIG. 1B rotate in the clockwise direction by .theta.. Under these conditions, field E(I.sub.3) rotates by a phase lag corresponding to the difference between the distance 0q.sub.2 between the center 0.sub.3 of the third element antenna and point q.sub.2, and the distance Oq.sub.1.

Comparing .DELTA.O.sub.1 O.sub.3 q.sub.1 and .DELTA.O.sub.1 O.sub.2 q.sub.1 ', since O.sub.1 O.sub.2 =O.sub.2 O.sub.3 =d and .angle.q.sub.1 O.sub.1 O.sub.3 =.angle.q.sub.1 'O.sub.1 O.sub.2, O.sub.3 q.sub.1 =20.sub.2 q.sub.1 '=2l.sub.1. Similarly, comparing .DELTA.O.sub.1 O3 q.sub.2 and .DELTA.O.sub.1 O.sub.2 q.sub.2 ', since O.sub.1 O.sub.2 =O.sub.2 0.sub.3 =d, and .angle.q.sub.2 O.sub.1 O.sub.3 =.angle.q.sub.2 'O.sub.1 O.sub.3, O.sub.3 q.sub.2 =20.sub.2 q.sub.2 '=2l.sub.2. As a result, the phase of the field E(I.sub.3) in the direction of .gamma..sub.2 with respect to direction .gamma..sub.1, would lag by an angle corresponding to O.sub.3 q.sub.2 -O.sub.3 q.sub.1 =2l.sub.2 -2l.sub.1 =2(l.sub.2 -l.sub.1). As above described, a phase lag of electrical angle .theta. is caused by the difference l.sub.2 -l.sub.1 so that the field E(I.sub.3) would lag by an electrical angle 2.theta., in other words, would rotate 2.theta. in the clockwise direction when represented by the vector diagram.

As shown in FIG. 1B, since the electrical angle between field vectors E(I.sub.1) and E(I.sub.3) is .theta., when the field E(I.sub.3) rotates 2.theta. in the clockwise direction, the field E(I.sub.3) would allocate a position which is symmetrical with respect to E(I.sub.1). Consequently, field E(I.sub.3) would lag by .theta. with respect to field E(I.sub.1). On the other hand, since field E(I.sub.21) lags by .theta. with respect to the vector shown in FIG. 1B, field vectors E(I.sub.21) and E(I.sub.3) would become just opposite. Thus, where a null point is selected in the direction of .gamma..sub.2, E(I.sub.22) would cancel E(I.sub.1) and E(I.sub.21) would cancel E(I.sub.3).

As above described, when the direction of one null point is determined, for instance in the direction of .gamma..sub.1, the direction in which the other null point is formed is also determined. More particularly, when it is selected in FIG. 1A that O.sub.2 q.sub.2 '-O.sub.2 q.sub.1 '=l.sub.2 -l.sub.1, the second null point would be formed in a direction corresponding to electrical angle .theta., in FIG. 1B. However, at the time of determining the first null point direction .gamma..sub.1, it is possible, independently on .gamma..sub.1, to vary as desired the direction of vector E(I.sub.3) shown in FIG. 1B by varying the amount of phase shift imparted by the phase shifter L.sub.31. In short, according to this invention, when the number of the element antennae is 3 it is possible to establish null points in any two directions.

The reason that the directions of these null points do not vary over a wide frequency band will now be described. Generally, in a transmission line having a definite length, the phase of the current flowing therethrough lags in proportion to the frequency. However, in the arrangement shown in FIG. 1A, the phase lags of waves originating from the divider D.sub.s and passing through respective phase shifters, respective dividers D.sub.1, D.sub.2, D.sub.3, respective element antennae to reach wave fronts O.sub.1 -Q.sub.1 and O.sub.1 -Q.sub.2 respectively are equal and proportional to the frequency. Since the connection for the second element antenna #2 is crossed at the feed port to cause the currents to have opposite phases independently of the frequency, except the effect of crossing, the phases of the waves at the two wave fronts would lag which maintaining the same phase relations that the file E(I.sub.1) is equal to E(I.sub.21) or E(I.sub.22) and E(I.sub.3) is equal to E(I.sub.22) or E(I.sub.21). For this reason, in said direction E(I.sub.1) cancels E(I.sub. 21) or E(I.sub.22) and E(I.sub.3) cancels E(I.sub.22) or E(I.sub.21) independently of the frequency.

The principle of this invention will be described hereunder with the aid of mathematical equations with regard to a general case wherein the number of element antennae is n.

FIG. 2A is a block diagram showing a linear antenna array of such case which comprises n element antennae #1 through #n which are arranged on a straight line with an equal spacing d. Each element antenna may comprise a dipole antenna. A portion bounded by dotted lines comprises a feed network which characterizes the invention and is constructed as follows. More particularly, it includes first dividers D.sub.1 through D.sub.n corresponding to respective element antennae #through #n. The common ports of these dividers are connected to the feed ports of respective element antennae #1 through #n, directly or through lines, amplifiers, frequency converters, balance-unbalance circuit and the like. Each of the first dividers, D.sub.1 through D.sub.n includes split ports of a number equal to a binomial coefficient expressed by the following equation (1).

where P.sub.i represents the number ports, and i represents the number of the corresponding element antenna counted from the right-hand end as viewed in FIG. 2A and i=1, 2 . . . n. Thus, the total number S of the split ports is shown by ##EQU2## D.sub.s represents a common power divider having S split ports and phase shifters .phi..sub.ij are connected between the split ports of the common power divider D.sub.s and the split ports of the first power dividers D.sub.1 through D.sub.n respectively. The first suffix i of the phase shifter symbol .phi..sub.ij represents the number of the corresponding element antenna, whereas the second suffix j represents the number of the split port of the corresponding first power dividers. The common port of the common power divider D.sub.s is termed a feed port A. In this feed network, for the purpose of alternately reversing the phases between the feed port A and the feed ports of respective element antennae #1 through #n, the connections of the common ports of the first power dividers D.sub.1 through D.sub.n are alternately crossed and then connected with the feed ports of respective element antennae #1 through #n. As the power divides D.sub.1 through D.sub.2 and D.sub.s, it is used a power divider having a high degree of isolation between respective split ports.

It is sufficient to construct the feed network such that the exciting currents transferred to the feed ports of respective element antennae from the feed port A through respective phase shifters would have the same amplitude with respct to each branch path for the reason described hereinafter but their phases are delayed by the phase shifters. Actually, it should be taken into consideration the phase delay between the input and output of the power divider, but for the convenience such phase delay is treated by including in the amount of phase shift afforded by the phase shifter. The power divider is generally provided with a number of split ports, but in the following description such dividers such as D.sub.1 and D.sub.n having only one split port is also called a power divider.

The reason that the directivity can be synthesized over a wide frequency band in the linear antenna array shown in FIG. 2A will now be described. Suppose now that respective element antennae #1, #2, . . . #n are supplied with exciting currents I.sub.1, I.sub.2. . . I.sub.n respectively as shown in FIG. 2A and that these element antennae are disposed along the x axis of a polar coordinate system at an equal spacing d with the first antennae element #1 positioned at the origin. Then the array factor D(.theta.,.phi.) can be shown by the following equation; ##EQU3## where =2.pi./.lambda., .lambda. is wavelength.

putting now

the equation (3) can be modified as follows. ##EQU4##

As has been pointed out hereinabove, since it is not always necessary that the element antennae are dipole antennae, we can express the directivity of a commonly used radiation element as f(.theta., .phi.). Then the superposed directivity F(.theta., .phi.) is given by

In order to attain the object of this invention, that is to synthesize a desired directivity pattern over a wide frequency band, it is necessary to determine the exciting currents I.sub.i (i=1.about.n) such that the array factor D(.theta., .phi.) will have null points in a given direction independently of the frequency.

The equation (5) has (n-1) roots since it is a complex coefficient polynomial of order (n-1). In other words, the array factor D(.theta., .phi.) may have (n-1) null points. To obtain the exciting current I.sub.i (i=1.about.n) described above, I use the relation between roots and the coefficients of the algebraic equation shown as (5). Thus, when the roots of equation (5) are denoted by t.sub.1, t.sub.2, . . . t.sub.n-1 respectively, there are the following relation. ##EQU5##

Generally, equation (7) can be rewritten as follows ##EQU6## where i=(1.about.n-1).

By the relation shown in equation (4), since the roots t.sub.1, t.sub.2, . . . t.sub.n-1 are located on a unit circle of the complex coefficient plane, they can generally be shown by

where k=1.about.n-1.

Accordingly, equation (8) can be rewritten ##EQU7## where i=1.about.n-1.

As can be noted from equation (10) when the direction (.theta..sub.k, .phi..sub.k) of a null point of the directivity pattern is designated, current I.sub.i varies with the frequency, that is .beta., so long as I.sub.i is given, the null point directions of the directivity pattern which is designated at first can be maintained over a wide band.

The term (=1) .sup.n-i on the right-hand side of equation (10) shows that the phase is reversed at alternate element antannae in addition to the fact that the phases of the exciting currents of respective element antennae are shifted by the phase shifters. To accomplish this, where the power dividers D.sub.1 through D.sub.n and D.sub.s are of the type that divide a signal into the same phase, the connections of the common ports of the dividers D.sub.1 through D.sub.n to the feed ports of respective element antennae #1 through #n are alternately crossed. When the connections of the common ports of the power dividers D.sub.1 through D.sub.n are alternately reversed, the following equation (11) may be used in which the term (-1).sup.n-1 in equation (10) has been excluded. ##EQU8## where i=1.about.n-1.

Thus, according to the antennae of this invention, the exciting current shown by equation (11) is given by the feed network shown in FIG. 2A.

The principle of operation of the feed circuit network will now be described with reference to FIG. 2C which shows the feed network section for the i-th element antenna #i shown in FIG. 2A. It is possible to explain the principle generally, because i represents any one of l through n. In FIG. 2C the description of the invention is taken for short under the condition that each port is matched with the reference impedence Z.sub.O. An S-matrix element (an element of a scattering matrix) between the common port s and the split port k of the common power divider D.sub.s is denoted by S.sub.k,s.sup.(i) (which is equal to S.sub.s,k.sup.(i) due to the reciprocity theorem). Then, the split ports are sufficiently isolated as above described, so that the incident wave a.sub.k and the reflected wave b.sub.k at the port k are shown as follows. ##EQU9##

On the other hand, the incident wave a'.sub.k and the reflected wave b'.sub.k at the split port of the first power divider D.sub.i are expressed by the following equations. ##EQU10## where L.sub.i,k represents the electrical length of a transmission line which comprises a phase shifter .phi..sub.ik and .beta.=.lambda./2.pi. (.lambda.: wavelength). Denoting the S-matrix element between the split port k and the common port i of the power divider D.sub.i by S'.sub.ik (which is equal to S'.sub.ki due to the reciprocity thereom), the incident wave a'.sub.i and the reflected wave b'.sub.i at the port i are expressed by ##EQU11## Due to the definition of S parameter, where a load having a reference impedance is connected to the port i

so that the exciting current I.sub.i supplied to the i-th element antenna #i can be derived out as follows from equations (12), (13), (14) and (15). ##EQU12## where i=1.about.n.

According to this invention, the exciting current shown in equation (11) which is necessary for synthesizing a directivity pattern over a wide frequency band can be obtained by the exciting current shown by equation (16).

From equation (16), since ##EQU13## it is possible to put equal the exciting currents expressed by equations (11) and (16) thus obtaining the following equation. ##EQU14## where i=n.about.n-1.

Accordingly, when the S-matrix elements S.sub.i,k ' and S.sub.k,s .sup.(i) and the transmission line length (electrical length) L.sub.i,k of the phase shifter are set such that the equation (18) always holds irrespective of the variation in the frequency, it becomes possible to realize a directivity pattern having (n-1) null points in the desired or designated direction over a wide frequency band with the circuit construction shown in FIG. 2A. To this end, it is sufficient to satisfy the following design conditions.

wherein i=1.about.n and k=1.about.p.sub.i. Because, the following equation can be obtained by substituting equation (19) into equation (18) ##EQU15## where i=i.about.n-1 It is clear that this equation can hold. In other words, it is possible to realize the aforementioned directivity by using a design parameter L.sub.i,k. By designing the power divider so as to hold equation (19) it is possible to always hold equation (18) irrespective of the variation in frequency. Although, in addition to the condition shown by equation (19) there are conditions S.sub.i,k ' and S.sub.k,s.sup.(i) that always satisfy equation (18), but in order to systematically process the design conditionsof the power divider and the phase shifter under completely isolated conditions, in other words, in order to design them by maintaining the power dividing condition and the phase shifting condition at completed independent relations, equation (19) is sufficiently practical.

Equation (20) shows that the line length L.sub.i,k of the phase shifter is determined as a function of a null point direction (.theta..sub.k, .phi..sub.k), and that a definite phase deviation may be permissible.

When L.sub.i,k is determined such that the exponent of equation (20) can be expressed as

the equation (20) holds irrespective of the variation in the frequency, that is .beta., so that the directivity pattern can be satisfactorily synthesized over a wide frequency band by designating (n-1) null directions.

Having completed the description of the theory a preferred embodiment of this invention will now be described.

FIG. 3 is a block diagram showing an embodiment of this invention where the number of the element antennae is 3. The linear antenna array shown therein comprises three element antennae #1, #2 and #3 which are arranged along a straight line at an equal spacing and a feed circuit network 10 bounded by dotted lines.

The feed circuit network 10 comprises a three-way power divider 11 which in response to an input to a common port 11a produces three signals having the same amplitude and the same phase on three split ports 11b, 11c and 11d; first and second two-way power dividers 12 and 13 which in response to the inputs to the common ports 12a and 13a produce signals having the same phase and the same amplitude on two split ports 12b, 12c and 13b, 13c; first to fourth phase shifters 14, 15, 16 and 17 constructed to give a predetermined phase shifts according to the electrical lengths of the transmission lines, and a one-way power dividers 18 and 19.

For example, as the three-way power divider 11 may be used type M3V-50 divider manufactured by ANZAC company, while as the two-way dividers 12 and 13 may be used type MTV-50 divider of the same company.

The common port 12a of the two-way power divider 12 i connected to one split port of the three-way power divider 11 whereas the input ports 14a, and 15b of the first and second phase shifters 14 and 15 are converted respectively to the split ports 12b and 12c of the two-way divider 12. The other two split ports 11b and 11c of the three-way power divider 11 are connected to the input ports 16a and 17a respectively of the third and fourth phase shifters 16 and 17, and the output ports 16b and 17b thereof are connected to the split ports 13b and 13c respectively of the second two-way power divider 13. The common port 13a of the second three-way power divider 13 is connected to the element antenna #2 through an inversion means represented by a symbol X in FIG. 3. This symbol X is used in the description of the invention.

The output ports 14b and 15b of the first and second phase shifters 14 and 15 are respectively connected to the input ports 18a and 19a of the one-way power dividers 18 and 19, while the output ports thereof are connected to element antennae #3 and #1 with a polarity opposite to that of the element antenna #2. It should be understood that although the one-way dividers 18 and 19 are shown in the drawing to clarify their relationship to the principle of this invention, actually they are not necessary.

The element antennae connected to the output port 19b, the common port 13a and the output port 18b may be dipole antennae or antennae of the other type.

With the connection described above, when a signal to be radiated is applied to the common port of the three-way power distributor 11, this input signal would be divided into 3 which are applied to element antennae #1, #2 and #3 respectively through ports 19b, 13a and 18b.

It is advantageous to use power dividers having high degree of isolation among respective split ports as the three-way power divider 11 and the two-way power dividers 12 and 13. In this embodiment, the fact that the equation (19) is satisfied when the number of the element antennae is 3 will be described hereunder.

In FIG. 3, since the elements bounded by dot and dash lines correspond to the power divider D.sub.s shown in FIG. 2A, S.sub.i,k.sup.' and S.sub.k,s.sup.(i), and their products in the case of i=1, 2 and 3 are calculated and shown in the following Table 1.

It will be thus noted that a condition S.sub.i,k.sup.' .multidot.S.sub.k,s.sup.(i) =constant, that is equation (19) is satisfied. To satisfy equation (21), the line length (electrical length) L.sub.31, L.sub.11, L.sub.22 and L.sub.21 of the first to fourth phase shifters are selected as shown in Table 2.

When the line lengths of the first to fourth phase shifters are selected as above described, it is possible to form directive null points in a definite direction over a wide frequency band. However, the invention is not limited to such selection. For example, in the embodiment described above, although the first and second two-way power dividers 12 and 13 were constructed to produce signals having the same phase and the same amplitude, either one or both two-way power dividers may be constructed to produce signals having the same amplitude but opposite phase. In this case, the line lengths of the first to fourth phase shifters are selected correspondingly. Instead of using phase shifters whose amounts of phase shift are determined in accordance with the line length, the other type of phase shifters may also be used in which the amount of phase shift is proportional to the frequency.

Although a phase shift also occurs in a power divider, it is possible to treat it as a portion of the phase shift caused by the phase shifter.

For example, the phase shifts in the three-way power divider 11 between its common port 11a and the splits ports 11b, 11c and 11d are not required to be always equal because such difference can be compensated for by the first to fourth phase shifters 14 through 17.

Thus, with the embodiment shown in FIG. 3, it is possible to provide a three element antennae array having a simple construction and can form null points in the desired and definite two directions over a wide frequency range.

An embodiment of this invention wherein the number of the element antennae is four will now be described with reference to FIG. 4. This modified embodiment comprises 4 element antennae #1 through #4 arranged along a straight line with an equal spacing, one-way power dividers D.sub.1 and D.sub.4, three-way power dividers D.sub.2 and D.sub.3, and transmission line type phase shifters L.sub.11, L.sub.21 to L.sub.23, L.sub.31 to L.sub.33 and L.sub.4, the transmission line lengths thereof being designated by the same reference characters. In FIG. 4, the elements bounded by dot and dash lines correspond to the power divider D.sub.s shown in FIG. 2A.

The power divider D.sub.s is constituted by a three-way power divider 21 having one split port connected to a dummy resistance R and a seven-way power divider 22. (which divides an input power into 7 outputs). Each of the three-way power dividers D.sub.2, D.sub.3 and 21 is constructed to produce on its three split ports signals having substantially the same phase and the same amplitude in response to a signal applied to its common input port whereas seven-way power divider 22 is constructed to produce signals having substantially the same phase and the same amplitude on its 7 shift ports in response to an input signal supplied to the common input ports.

In the embodiment shown in FIG. 4 too, the equation (19) described in connection with the principle of this invention is satisfied. The transmission line lengths L.sub.11, L.sub.21 to L.sub.23, L.sub.31 to L.sub.33 and L.sub.41 of respective phase shifters necessary to satisfy equation (21) are shown in the following Table 3.

Still another embodiment of this invention wherein the number of the element antennae is n will be described with reference to FIG. 5 in which a power divider corresponding to the power divider D.sub.s shown in FIG. 2A is constituted by Wilkinson type n-way hybrid power dividers 23 and directional coupler 24. In this embodiment too, it is possible to form (n-1) null points whose directions do not vary over a range of from low frequencies to high frequencies by adjusting the circuit parameters to satisfy equations (19) and (21).

Although in the foregoing description of the feed network, it was explained that the common ports of the power dividers D.sub.1 through D.sub.n and D.sub.s are matched to a reference impedance Z.sub.o for the sake of the easy understanding of this invention, a problem occurs whether the invention is effective or not when these common ports are not matched to the reference impedance Z.sub.o. Where a dipole antenna is used as the element antenna, it is considered that its input impedance may not match with the reference impedance over a wide frequency band. In addition, the mutual coupling between element antennae should also be considered. However, notwithstanding these facts, the invention is sufficiently effective. Thus, where the number of the element antennae is seven, the total length of a dipole antenna is 100 cm, the diameter thereof is 0.95 cm, and the spacing d between element antennae is 40 cm, the directivity patterns were synthesized by designating the directive null points in 6 directions of 50.degree., 70.degree., 90.degree., 110.degree., 140.degree. and 180.degree. measured from the X axis direction shown in FIG. 2B and the resulting directivity patterns are shown in FIGS. 6A, 6B and 6C which show the directivities in the E-plane and H-plane for 90 MHz, 170 MHz and 222 MHz respectively. These patterns are examples of the results when the frequency bands of the television broadcusting waves utilized in Japan are used. It can be noted that sharp pencil beam shaped directivity patterns can be obtained in which the main beam width has been decrease over an extremely wide frequency band and the side lobe levels have been suppressed. These characteristics were obtained by also taking into consideration the effect of the mutual coupling showing the utility of the antenna of this invention. In other words, FIGS. 6A, 6B and 6C show that the antenna of this invention has an effective operating range over an extremely wide frequency range. The reason that the side lobe and back lobe levels of the directivity patterns shown in FIGS. 6A and 6C for 90 MHz and 222 MHz are not perfectly suppressed is that the radiation patterns are effected principally by the mutual coupling.

To confirm experimentally the directivity pattern synthesis of the antenna of this invention.

I calculated directivity patterns for a 3-element endfire array and manufactured its antenna for experiment.

As the frequency band was selected a band of from 90 to 220 MHz covering more than one octave and utilized in Japanese television broadcasting systems. The dimensions of the three element array are:

Total length of the dipole antenna=120 cm

Diameter of the dipole antenna=0.95 cm

Spacing between element antennae d=45 cm

Number of the element antennae n=3.

Since the directions of the directive null points are given by n-1=2, two directions of 120.degree. and 180.degree. measured from the radiation direction along the array axis were designated as the null point directions. The results of calculations under these conditions are shown by dotted lines in FIGS. 7A, 7B and 8A, 8B, 8C. These synthesized theoretical values are directivity patterns obtained under the condition of the mutual couplings between the elements. As can be noted from these drawings, even when the radiation mutual coupling presents, the radiation in the designated null point directions was suppressed over a frequency range of about 1.5 octaves (90 to 220 MHz), thus forming small side lobe levels. As shown in FIGS. 7A and 7B, the E-plane directivity patterns for 90 MHz and 220 MHz are substantially the same. It is to be particularly noted that the configuration of the E-plane directivity is substantially the same over a wide frequency range including calculated values of the directivities between 90 MHz and 220 MHz. The incomplete suppression of the directivity in the designated direction is caused by the mutual coupling between element antennae.

The feed circuit network utilized in this experiment was constructed as a combination of a hybrid type two-way power divider and a three-way power divider having the same phase and the same amplitude and the measured degree of isolation of respective power divider was about 30 dB and their measure amplitude error was about .+-.0.1 dB. For this reason, it is thought that equation (19) is substantially satisfied. To synthesize null points in the directions of 120.degree. and 180.degree. from the array axis, the line lengths L.sub.11, L.sub.21, L.sub.22 and L.sub.31 of the phase shifters were designed as follows from equation (21)

L.sub.11 =96 cm (physical length)

L.sub.21 =74 cm (physical length)

L.sub.22 =59 cm (physical length)

L.sub.31 =44 cm (physical length)

The measured values of the E-plane directivity pattern of this antenna are shown by solid lines in FIGS. 7A and 7B respectively, whereas the measured values of the H-plane directivity pattern are shown by solid lines in FIGS. 8A, 8B and 8C respectively. As shown in these figures, the measured values are in good agreement with the theoretical value for both E and H planes.

It should be understood that the invention is not limited to the illustrated embodiments and that various modifications and applications are possible. For examles, although in foregoing embodiment, in the case where the power divider is of the same phase division type, for the purpose of alternately reversing the phase between the feed port A and the feed ports of element antennae, the common ports of the first power dividers were connected in alternate reverse polarities, it is possible to alternately reverse the connections in a power divider, or to reverse the polarity in the transmission lines while constitute phase shifters.

A power divider utilized in a feed circuit network may be designed so as to have a specified amount of phase shift. Such phase shift can be treated by adding a portion of the phase shift into the electrical length L.sub.ik.

Moreover, instead of directly connection the common ports of the first power dividers to the feed port of the element antenna, it is possible to connect the feed ports of element antennae to the common ports of the feed circuit network bounded by dotted lines in FIG. 2A after passing through an amplifier and a frequency converter for producing an intermediate frequency. Using such modified connections this invention can also synthesize the desired directivity pattern of this invention over a wide frequency range. As above described, the number of the directive null points of the antenna of this invention is up to (n-1) where n represents the number of the element antennae, but it is possible to select it such that some of the directions coincide with each other. In this case, some identical transmission lines of p.sub.i phase shifters corresponding shifters corresponding to the i-th element antenna #i may be made common.

Where (n-1) null point directions are the same in a case wherein n element antennae are used, the number of the transmission lines of the phase shifters may be reduced to one for the each element antenna. However, when the number of the transmission lines is reduced to one, the null point can be formed in only one direction thus making it impossible to synthesize the desired directivity. In contrast, according to this invention, as it is possible to form up to (n-1) null points in any direction which do not depend upon the frequency whereby it becomes possible to synthesize desired directivities pattern over a wide frequency band. To make equal all null points is nonsense, but to make equal some null points among (n-1) null points is valuable. In this case, identical transmission lines connected to each divider may be combined into a common line.