US2352977A - Self-compensating video antenna - Google Patents

Description

y 1944- M. w. SCHELDORF SELF'COMPENSATING VIDEO ANTENNA Filed Sept. 18, 1942 2 Sheets-Sheet l t TRANS/717727? 91 r .w E w 0% b5 Tim eW ct v.. A TLV .l rfl w s b y 1944. M. w. SCHELDORF 2,352,977

SELF-COMPENSATING VIDEO ANTENNA Filed Sept. 18, 1942 2 Sheets-Sheet 2 Fig.5.

Irwverfibor: Marvel W. Scheldorf,

10 fi aJMAM His Attorney.

Patented July 4, 1944.

SEIF-COMPENSATING VIDEO ANTENNA Marvel W. Schcldorf, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application September 18, 1942, Serial No. 458,766

7' Claims.

My invention relates to antennae for radio apparatus and particularly for apparatus utilizing a wide band of ultra high frequency oscillations,

such as employed in television. While my inventhe antenna system disclosed in the abovementioned prior application.

It is an object of my invention to provide an improved antenna system which radiates horizontally polarized waves having a field pattern such that uniform radiated signals are provided for a given distance in a neighboring area and for greater distances in one or more given directions.

It is a further object of my invention to provide an improved high frequency antenna array which has uniform impedance characteristics over a wide band of frequencies on either side of the mean operating frequency to which it is tuned.

In an antenna array wherein high impedance radiating elements are connected by an open quarter wave impedance-inverting coupling line, serious unpredictable terminal conditions are caused by the discontinuities at the line ends. It is a further object of my invention to provide an improved antenna of the above mentioned type in which the discontinuities at the ends of the coupling line are eliminated.

In accordance with the present invention a plurality of pairs of coplanar angularly disposed radiators are connected to feed points spaced a quarter wave length apart on a coplanar coupling line, the ends of the line being terminated in short-circuited quarter wave length sections. A transmission line is connected to the feed point of one of the pairs of radiators. For transmission this pair of radiators becomes an antenna and the other pair becomes a driven reflector, or director. The shape of the radiation pattern is controlled both by adjusting the degree of mismatching between the impedance of the reflector and the coupling line to vary the ratio of currents flowing in the antenna and reflector, and by adjusting the angle between the radiators of each pair.

The features of my invention which I believe appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which Fig, 1 is a diagrammatic sketch of an antenna array embodying my invention, Fig. 1a illustrates apparatus which may be used with the antenna array of Fig. 1, and Figs. 2, 3, and 4 are graphs prepared from data derived from certain antennae showing their radiation characteristics.

Referring to the drawings, the numerals l0 and I I designate respectively an antenna array in accordance with my invention and a transmitter for supplying oscillations thereto, being arranged, for example, to supply oscillations for television transmission in the frequency range from 50 to 108 megacycles. The band width may be of the usual order of six megacycles. The antenna I0 comprises four radiators or conductors I! to 95 arranged in pairs l2 and la and I4 and I 5. The pairs of radiators are substantially coplanar and parallel and are spaced apart by a quarter wave length at the mean operating frequency for which the system is designed. The individual conductors of each pair are angularly disposed, the angle formed between conductors I2 and I3 being equal to the angle formed between conductors it and I5. For reasons that will be explained later, each of the radiators has a relatively low ratio of length to cross-section and radiators [4' and I5, as shown, are of a difierent length than radiators l2 and H3.

The pairs of radiators are connected to a coupling line it at feed points I 1 and It. The coupling line It is illustrated as an open-wire transmission line having a length equal to three-quarters of a wave length of the mean operating frequency for which the system is designed.

High frequency energy is transferred between the high frequency transmitter II and the array In through a concentric transmission line. is. This line is connected to the coupling line is at the feed point I! through a quarter wave openwire matching line 20 and is provided with a quarter wave length shield 25 at its upper end to permit connection of the balanced antenna load to the single ended transmission line 19. This outer shield 25 is grounded to the outer conductor of the line I!) at a point 26 one quarter of a wave length from its end and the end of'the outer conductor of line I 9 is ungrounded. In this way an impedance is produced between the ends to be novel are set forth with particularity in the of the two conductors of line i9 balanced with respect to ground and matching the balanced impedance of line 20.

The direction of maximum directivity of the antenna array in is along the axis of the coupling line It and towards the right as viewed in Fig. 1. Since the line l6 between the feed points l1. and I8 is a quarter wave long, the currents in radiators l4 and I lag those in radiators l2 and I3 by approximately 90.

If satisfactory wide band operation is to be secured, and reflections avoided, the reactance presented to the matching line should be substantially zero over the entire operating band and the resistance should be substantially constant over this range. Zero reactance across the feed point I! is obtained by utilizing the impedanceinverting characteristic of the quarter wave line It between feed points I! and it. Thus, if at the mean operating frequency of the system the apparent reactance and resistance of the conductors l2 and I3 connected across the feed point I! is equal to the apparent reactance and resistance of the conductors l4 and I5 connected across the feed point IS, the quarter wave section of line It transforms the reactance and resistance characteristics of the radiators I4 and IS SO that the reactance appears in an opposite sign at the feed point II. The reactances of the two pairs of radiators are thus self-compensating and these antenna elements appear as a pure resistance across the matching line 20.

It is readily apparent that for variations of the mean operating frequency this compensating effect is maintained to a practical degree; for, while both pairs of conductors vary in reactance with variations in frequency, the variations of the conductors l4 and is are inverted by the quarter wave line so that they compensate for the variations in the conductors I2 and I3.

As is pointed out in my above mentioned copending application, the terminal impedance of the two pairs of radiators consists of two essentially separate impedances, (1) the self-impedance of the radiators which would exist if there were no other radiators in the vicinity and (2) the mutual impedance due to the field produced by the other antenna element. The magnitudes of the self impedances of the radiators are the same when the lengths are equal. However, one element has positive mutual resistance and the other element has negative mutual resistance of essentially the same values. By "mutual resistance is meant that change in the resistance encountered by a radiator in radiating energy into space which is caused by the presence of another radiator. Likewise, one element has positive mutual reactance and the other, negative mutual reactance. It can be shown, therefore, that if the pairs of radiators are of equal length, they do not provide the proper transmission load at their feed points and cannot be balanced against each other to compensate for their resistance and reactance variation. In order to proportion their net reactances so that both units have the same natural frequency f0, preferably near the center of the band, which is a condition required for compensation, it is necessary to lengthen the antenna radiators l2 and I3 and to shorten the radiators l4 and 15. Also in order to provide satisfactory compensation over a wide band by reducing reactance variation with changes in frequency, both of the pairs of radiators are constructed so that they have relatively large cross-sections as compared to their length which requires that both lengths be reduced in-order to maintain resonance, often to a degree such that both lengths are considerably less than one-half wave length.

While the radiators of the antenna may be adjusted in a self-compensatory manner, as above outlined, to eliminate reflections which would otherwise arise because of the presence of these radiators across transmission line l6, unpredictable terminal conditions are still present on the system across the terminals II because of the end effects of the transmission line It. To reduce these undesirable effects to a negligible value. sections 2| and 22, each a quarter wave in length, are added to transmission line l8 and these sections are short-circuited at points 23 and 24.

, The low impedances at these short circuit points are inverted by the quarter wave length sections 2! and 22 so that they appear as high impedances at the feed points I! and I 8 so as not to disturb the impedances of the radiators connected across the lines at these points. The continuous character of the lines passing the points I! and I8 eliminates any discontinuity which would exist without the presence of the line extensions 2| and 22. Also since the short-circuit points are at ground potential, the line as a whole may be supported at these points without difficult insulation. thus eliminating the capacitive loading effect of the insulators normally used in antenna systems.

If a single pair of half wave dipoles, the radiators H and I5 for example, is arranged in the form of a V with legs disposed at an angle 0, it is well known to those skilled in the art that the radiation pattern of such an antenna is dependent upon the angle 0. With the legs of the V at the horizontal field strength pattern is not exactly circular but instead, for a pair of radiators having relatively large cross sections as compared to their length, has a pattern illustrated by the curve marked 90 in Fig.2. The field strength in the direction of the bisector of the angle 0 is less than in a direction at right angles thereto. The manner in which the characteristic pattern of such a single V antenna varies with changes in the parameter 0 is illustrated by the remaining curves of Fig. 2. As the value of 0 is reduced,

the field strength along the bisector of the angle increases. Conversely, as the radiators are spread further apart, the field strength along this bisector is reduced, becoming zero when 0=180. Also, it is observed that, when 0=70, the pattern is'most nearly circular.

In a 70 double V antenna of the type described above, when the pairs of radiators are electrically identical and excited 90 apart with equal currents, the radiation characteristic is about in the form of a cardioid, such as is shown by curve 30 of Fig. 3. More explicitly, the characteristic is the product of the cardioid curve 30 and the radiation pattern of a V antenna in which 0=70 and is obtained by multiplying a particular radius of the curve 30, for example the radius OA, of Fig. 3 by the corresponding radius 0A of the curve marked 70 of Fig. 2. Since the latter curve is essentially circular, if the slight difference from a circular pattern is neglected, the radiation pattern of such a double V antenna may be represented by the cardioid of Fig. 3. It will be readily appreciated that with such an antenna characteristic, radiation is obtained in three directions only, and but a negligible amount of radiation is obtained in the fourth direction, even in the vicinity of the antenna itself. In order to alter this pattern and to obtain response over a general area in the vicinity of the antenna, while favoring in particular a certain direction or directions, the ratio of the current in the two branches of my antenna is varied. This result is obtained preferably by varying the characteristic impedance of the line l6. This may be accomplished by changing the spacing between the conductors of line l6, for example, or in any other manner well known to those skilled in the antenna art. Thus, in a practical installation, the

an arrangement is illustrated in Fig. 1a where the rigid conductors 22 are supported bya shortcircuiting member 24 comprising the two threaded metallic transverse portions 21 and a turnbuckle 28. A similar arrangement may be employed for the short-circuiting member 23. As the characteristic impedance of line I6 is altered by adjusting turnbuckle 28 to vary the spacing between the conductors, a mismatching of the impedance of the line with that of the radiators at points l8 occurs and the current in radiators Ill and I5 is increased or decreased depending on the change of the characteristic impedance of line Hi.

The efiect of alteration of the characteristic impedance of the line l6 may be illustrated by reference to curves 3| and 32 of Fig. 3 with particular reference to a circular pattern for each single V element. Let K equal the ratio of the currents in the two branches of the antenna III, that is, current in branch l2 and i3 divided by current in branch 14 and I5. As mentioned above, when K equals 1, the radiation pattern shown by curve 30 is obtained from the antenna 60. When K equals .6 or 1.667, the radiation pattern shown by curve 3! is obtained. As the ratio is increased, the variation from the cardloid pattern becomes more pronounced, curve 32 showing the pattern when the value of K is .3 or 3.333. The limit of the variation of this parameter is reached when the current in one of the pairs of radiators is zero. Under such conditions, the characteristic pattern of the antenna, being the pattern of only one branch, is substantially circular.

In Fig. 4 I have shown how the results of variation of K and may be combined in the antenna of Fig. 1 to produce a desired field pattern. The particular pattern desired for an antenna depends, of course, upon the physical conditions of an area to be served by a station. Where it is desired that signals of a given minimum strength be provided for a given distance in the neighboring area and for a greater distance in a single given direction, a field pattern, such as shown by curve 33, is used. To produce this characteristic pattern, the antenna ill of Fig. 1 uses an angle of 110 between the conductors l2 and i3 and the conductors I4 and i and a current ratio of K equal to .3, that is, the current in radiators i2 and I3 is .3 of the value of the current in radiators l4 and I5. On'the other hand, where it is desired to provide signals for a local area and 'for three given directions, the pattern of curve 34 is employed. This pattern is obtained from the antenna of Fig. 1 when the angle 0 is equal to 80 and the value of K is .3. In each of the above cases, the final pattern is the product of the basic curve shown in Fig. 3

.and the curve corresponding to the current ratio.

In general, it is necessary to determine the correct length of the individual radiators, the angle between the radiators, and the ratio of the current flowing in the branches of any given form or system by actual operating tests. Thus, the actual physical dimensions for correct compensation of the reactances oi the two branches of the antenna cannot be calculated exactly because of the many interdependent quantities involved, most of which are not known with any degree of certainty. On the other hand, the most desirable angle of separation of the conductors in the two branches and the most desirable ratio of current in the two branches depend largely on the geophysical aspect of the area to be served by a particular broadcast station.

It will also be apparent that various modificat'ons may be made in the construction of antenna arrays utilizing the principles of my invention. In some cases, it may be desirable to use conductors of unequal diameters for the radiators. Therefore, while I have illustrated what I believe to be a preferred embodiment of my invention, I do not wish to be limited thereto since many such modifications may be made and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An antenna comprising two parallel pairs of angularly disposed conductors, said pairs of conductors having similar impedance characteristics at their inner extremities. a transmission line connected between said pairs of conductors at their inner extremities, the distance between the points of connection of said line and said pairs of conductors being equal to a quarter wave length at the desired operating frequency, said line having an impedance characteristic similar to that of said pairs of conductors, and means for varying the impedance of said line to adjust the current distribution in said pairs of conductors, whereby the field pattern of said antenna may be adjusted to a desired configuration.

2. In combination, a source of oscillation, a transmission line having a length equal to threequarters of a wave length of said oscillation, said line being short-circuited at both ofits ends, an antenna comprising two substantially coplanar and parallel pairs of angularly disposed conductors, said pairs of conductors having similar impedance characteristics at their inner extremities, means for connecting said inner extremities to said line at feed points spaced respectively a quarter wave length from each other and a quar ter wave length from said ends, the impedance of said line being the same as the impedance across said extremities, and means for connecting said source to said line at one of said feed points.

3. In combination, a source of high frequency oscillation, a transmission line having a length equal to a multiple of a. quarter wave length of said oscillation, said line having low impedances connected across both of its ends, an antenna comprising two pairs of conductors disposed in the same horizontal plane, both of said pairs of conductors being connected to said transmission line at feed points spaced respectively a quarter wave length from each other and a quarter wave length from said ends, meansconnecting said source of oscillation to one of said feed points, and means for varying the surge impedance of said line to vary the ratio of the currents set up in said pairs of conductors by said oscillation for adjustingthefield pattern of said antenna to a desired configuration.

4. An antenna comprising, two substantially coplanar and parallel pairs of angularly disposed conductors, said pairs of conductors having similar impedance characteristics at their inner extremities, a transmission line substantially coplanar therewith, said line having a length of three-quarters of a wave length at the desired operating frequency and being short-circuited at both of its ends, means for connecting said inner extremities to said line at points spaced respectively a quarter wave length from said ends, and means for adjusting the ratio of the currents in said pairs of conductors to obtain a desired field pattern for said antenna.

5. The combination, in a high frequency antenna system, of a V antenna unit comprising a pair or angularly adjustable radiating elements, a V director unit comprising a pair of angularly adjustable conductors substantially coplanar to said antenna unit and spaced therefrom by substantially one quarter wave length at the mean operating frequency of the system, an impedanceinverting line interconnecting said units, and means for adjusting the impedance characteristic of said line to adjust the ratio of currents in said radiating elements and said conductors, the directions of farthest radiation of said antenna being adjusted by varying the angle between said angularly disposed radiating elements and conductors and the intensity of radiation in the vicinity of said antenna being adjusted by varying the ratio of the current in said radiating elements with respect to the current in said conductors.

6. In an antenna system having two substantially coplanar and parallel pairs of angularly disposed conductors whose inner extremities are connected by a transmission line having a length equal to a quarter wave length at the operating frequency of said system, the method of adjusting the field pattern or said antenna to a desired configuration which comprises, adjusting the angle enclosed by said conductors to obtain directivity in desired directions and adjusting the ratio of currents in said pairs of conductors to obtain radiation for a desired distance in the vicinity of said antenna.

7. An antenna for providing uniform radiated signals fora given distance in the vicinity of said antenna and for a greater distance in a given direction comprising, two substantially coplanar and parallel pairs of angularly adjustable radiators, said pairs of radiators having similar impedances at their inner extremities, a transmission line substantially coplanar therewith and having its longitudinal axis extending in said given direction, said line being short-circuited at both of its ends and having a length equal to three-quarters of a wave length at the operating frequency of said antenna, said inner extremities being connected to said line at points spaced a quarter wave length apart and a quarter wave length from said ends. said radiators being disposed at equal angles to said line and the distance of radiation 01. said signals in said given direction being determined by adjustment of the angles between said radiators, and the distance of radiation of said signals in the vicinity of said antenna being determined by adjustment of the ratio of the currents flowing in said pairs 01 radiators.

MARVEL W. SCHELDORF.

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