US2474480A - Antenna system - Google Patents

Description

June 28, 1949. G. P. KEARSE 2,474,480

' ANTENNA SYSTEM Filed ma 18, 1948 mama June 2a. m

corpora poration of minoh Application my is. an. Serial No.

1 Claims. (on. mass) The present invention relates to antenna systems and deals specifically with the problem of providing a television antenna designed'for high gain unidirectional performance throughout all of the thirteen television channels.

In introduction, it may be well to point out that throughout the history of radio unccasing at titttw short of achieving the desired result, and it has tempts have been made to provide antennas cahas been greatly complicated by the advent of commercial television, primarily due to the fact that a broadcast signal covers only a few'kilocycles of the ether spectrum, while television reception from even a single station requires an an antenna system adapted to respond to two separate signals, each having a far greater spectrum width than a broadcast signal.

The magnitude of the task involved begins to become apparent when it is remembered that the videoor picture signal of a single television channel covers approximately five hundred times the ether spectrum width of a signal in the broadcast band, and the frequency modulation signal which supplies the sound to the television receiver is about fifteen times as wide. These figures in themselves are staggering, but the full significance of the problem is not appreciated until it is remembered that, to be successful from a commercial standpoint, a television antenna must not only respond to the signals within a single channel but must respond with an equally high degree of performance to any of the thirteen channels presently in use. Further, these thirteen channels are not continuous in the ether spectrum, since the first six channels occupy the frequency range of from 44 to 88 megacycles (with a break from 72 to '76 megacycles. for non- Government fixed and mobile applications), while the other seven channels function at a considerably higher frequency and occupy the range of from 174 megacycles to 216 megacycles. The design of a successful television antenna presupposed that it should reject other signals, yet the presence of the standard frequency modulation band (88 to 108 megacycles) between theztwo groups of television channels even further complicates the problem of satisfactory electrical design.

When the extreme range ofvfrequ'encies to which the antenna must respond is considered, it will be apparent that the conventional methods of broad banding an antenna stmcture, as by become more or less universal practice to providea television receiver with a more or less conventional dipole that is deliberately mismatched to the transmission line. This gives a very poor gain at resonance, but the degree of mismatching is greatly reduced on either side of resonance. with the result that a fairly uniformresponse is achieved across a range of frequencies wide enough to reasonably cover one television channel (8 megacycles) Such an arrangement, however, falls far short of offering a full solution to the problem, since it necessarily fails to produce anything even approaching high gain performance, is incapable of operation over the entire range of 44 to 216 megacycles and does'not yield'a bidirectional radiation pattern over all of this range. In addition, it is noted that even if satisfactory response over the entire spectrum could be achieved, a conventional antenna of this type would fail to reject the signals in the standard frequency modulation band, which could cause interference with the television signal.

It is accordingly the primary object of the present invention to provide a television antenna system, wherein a single antenna array and conventional twin conductor transmission line may be employed for high gain performance throughout all of the present television channels, and

wherein the different components of the antenna are so designed and related as to achieve high gain performance, surpassing the performance of even a resonant dipole and so designed as to discriminate against frequency modulation signals between the upper and lower groups of television channels. l

A further object of the invention resides in the provision of an antenna system having the performance characteristics noted above, yet having a pronounced unidirectional pattern, extending substantially unchanged to the full upper and lower limits of the present thirteen frequency bands. The importance of a unidirectional pattern in a television receiving antenna cannot be overstressed, since if the antenna tends to pick up signals other than those emanating directly from the transmitting station, the time interval between signals picked up directly and those refiected will cause reflected signals to produce multiple images or ghosts, as they are called, and

lowering the Q of the circuit by decreasing the L/D (length to diameter) ratio fall hopelessly to-back and front-to-side ratio be extremely high to reject unwanted signals.

a further object of the invention is to provide 3 an antenna with the electrical characteristics noted, yet so designed that it is inherently matched to a conventional 300 ohm twin lead transmission line without the need of resorting to quarter-wave sections or other matching devices.

Theimportance of this feature isreadlly apparent when it is remembered that conventional receivers, as now manufactured, are almost invariably matched to a 300 ohm line, and that useof any type of matching section between the line and the antenna necessarily causes the circuit to become frequency sensitiveand thusnullifies any I broad banding characteristicslthat the, antenna itself might possess.

A further object, and one of considerable importance from a commercial standpoint, resiciles" dipole Ill. 'The'dipole II and the reflector I9 then function to provide an emcient unidirectionalarray At frequencies in the upper six chanhels (174 megacycles to 216 megacycles) of the.

television spectrum, the dipole Ill functions as therefiector for the high frequency dipole l3, so

that here again an efficient unidirectional effect attained. Thus, by the expedient of utilizing in the provision of an antenna having the functional characteristics heretofore discussed, and at the same time being of a. physical construction such that it is readily capable of mounting on a single rotatable mast and designed in such a manner that it may be commercially manufac taxed at reasonable. cost, and when installed is rugged, durable, capable of withstanding high winds and heavy ice loading, yet having its total weight suiliciently 'low' that it may safely be mounted on any reinforcing. The present invention accomplishes the objects noted by the provision of an antenna system which uses, in combination, a folded dipole connected to the twin leads of a conventional 300 ohm transmission line, together with a reflector spaced from the dipole, and a second folded dipole positioned on the opposite side from the reflector and joined to the first by an additional length of transmission line. The desired results heretofore outlined are achieved by'the simple expedient of designing the center dipole for low frequency operation, and using a second dipole to the rear of the low frequency dipole 28.

suited to operation over the upper group of fre-', quency channels; but calculating the length of the transmission line and the second dipole so a that, at low frequencies, the circuit of the second dipole acts as a quarter-wave section, presenting substantially infinite impedance to the low frequency signals and thus preventing the second dipole from interfering with the performance of the first. The manner in which this is accomplished is best illustrated with reference to the I drawing of this specification, wherein:

Figure 1 is a diagrammatic view of an antenna system as contemplated by these teachings; and

Figure 2 is a perspective view or the preferred commercial embodiment of the invention as presently manufactured.

In the diagrammatic illustration of Figure 1 the dipole III is designed to operate over the low frequency group of channels and-is adapted to be connected directly to the twin leads of the transmission line at the points II and I2. The high frequency dipole I3 is also of folded form and is designed to operate over the upper group of frequency channels.

The ends of the folded dipole it are also con-.. nectedto the transmission line at the points II and I 2 by conductors II and I5, but these conductors include looped sections l6 and I! so that they are somewhat longer than the actual distance between the dipoles II and I3, and the length of the sections it and I1 is so calculated that the electrical length between the points ll and I2 and the midpoint ll of the high frequency dipole is a quarter-wave length of the mean frequency at which the low frequency dipole ll operates. Thus the impedance of the one folded dipole as a reflector for the other, it is not only possible to attain high gain performance throughout the entire television spectrum, but it is also possible to accomplish unidirectional characteristics through the entire frequency range by the use of only three elements. v In the commercial embodiment of the invention illustrated in Flgure2, the entire array is mounted on a single fitting 2| at the top of a mast 22. A cross'arm 23 extends both forwardly and rearwardly from the mast andcarries a back fitting 24 to support the reflector 25, and a front fitting 26 to support the high frequency dipole 27, The low frequency dipole 28 is mounted directly on the fitting 2|, and a twin lead 300 ohm transmission line 29 or any suitable radio frequency transmission line is joined to the ends of the dipole 28 at the points 3| and 32. The pair of forwardly of the low frequency dipole.

conductors extending between these points and the second dipole is in this case formed of an additional length of transmission line which extends in a loose loop 23 to the ends 34 and 35 of the high frequency dipole. The transmission line is positioned parallel with the mast 22 but spaced .,Best results are obtainedby providing a low frequency dipole 78 inches in length with a 3.25 inch center-to-center distance between itsupper and lower spans. Th'e reflector is 98 inches long and is spaced 38.5 inches behind the dipole 28. The high frequency dipole is 29 inches long with 1.5 inch center-to-center distance between its upper and lower spans and is spaced 13.75 inches A 300 ohm twin lead transmission line is utilized as the lead in, and it hasbeen learned that with the spacing and the dimensions of the parts given as above, it is unnecessary to form dependin sections, such as the sections l6 and I I of Figure 1,

and that excellent results are achieved by merely allowing the section of line-33 to be suspended in a rather loose loop between the points 3 l-32 and the points 31-", since in this particular construction the parts are so dimensioned that, at low frequencies, the electrical length between the points 3l-42 and the fitting 26 causes the circult of the dipole 21 to have almost infinite impedance.

Experiments with antenna systems constructed as'described above have shown that they are not only efllcient from an electrical standpoint, but are also highly satisfactory by reason of their mechanical design. It ,will be readily seen that an antenna of this type may be easily constructed assessoiently assembled. It follows that it is well suited to commercial sale, and may be installed in any upp r and lower limits of operating frequencies, 3 and that the'front-to-back and front-to-side ratios are quite high, so that the system responds well to signals coming directly from the transmitter but failsto respond to reflected signals andthus avoids ghost images on the screen of the l a transmission line; a reflector spaced from the television receiver.

The form of the invention illustrated and described herein is the preferred embodiment of these teachings, in the form now commercially manufactured. It is shown as an illustration of the inventive concept, however, rather than by way of limitation, and it is pointed out that various modifications and alterations may be indulged in within the scope of the appended claims.

Having thus described the invention, what 1 I claim as new and desire to-protect by United,

States Letters Patent is:

1. .In an antenna system, the combination, with transmission line, of a first a radio frequency folded dipole directly joined to the ends of the transmission line; a reflector spaced .from the first dipole at one side thereof, and a second folded dipole shorter than the first and on the oppo- 1 site side of the first; and means for isolating said second dipole from the first dipole at the resonant frequency of the first dipole comprising a pair of conductors joining said seconddipole to the transmission line; said conductors being of length greater than the spacing-between said first and second dipoles.

2. In an antenna system, the combination, with a radio" frequency transmission line, of a first folded dipole directly joined to the end of the transmission line; a reflector spaced from the first dipole at one side thereof, and a second folded dipole shorter than the first and on the opposite side of the first; said second dipole also being joined to the on line by a pair of conductors and the second folded dipole is substantially equal to a shorted quarter wave length at the resonant frequency of the first dipole.

5. In an antenna system, the combination, with a radio frequency transmission line, of a first dipole. directly joined to the ends of the transmission line: a second folded dipole at substantially the same horizontal level and on one side of the first dipole and joined to the transmission line by a pair of conductors of such length that the sum of the lengths. of the conductors and the second folded dipole is substantially equal to a shorted quarter wave length line at the resonant frequency of the first dipole.

6. In an antenna system, in combination, an untuned radio frequency transmission line, a first folded dipole directly joined to the ends of the first dipole at one side thereof and a second folded dipole shorter than the first joined to the 1 transmission line by a pair of conductors of such length that the sum of the lengths of the conductors and the second folded dipole is substantially equal to a shorted quarter wave length line at the resonant frequency of the first dipole; the

'dipoles being at substantially the same horizontal level and spaced apart from each other where-- by one may act-as a reflector for the other.

7. A television antenna having a supporting frame consisting of a vertical mast with a horizontal cross arm extending in opposite directions from its upper end; a first folded dipole mounted across the top of said frame, and a radio frequency transmission line directly joined to said first folded dipole: a reflector mounted on one end of the cross arm at substantially the same horizontal level as the first dipole and spaced parallel with and at one side of said first folded dipole;

a v second folded dipole shorter than the first mounted on the other end of the cross arm par- 1 allel with and at substantially the same horizonfor isolating said second dipole from the first dipole at the rwonant frequency of the first dipole comprising an additional loose loop of transmission line joining said second folded dipole to the on line and the first folded dipole.

4. In an antenna system, the combination, with a radio frequency on line, of a first dipole directly joined to the ends of the transmission line; a reflector spaced from the first dipole at one side thereof, and a second folded dipole on the opposite side of the first dipole and joined to the on line by a pair of conductors of suchlengththatthesmnofthelenithsoftho Number Name Date 2,039,295 Carter May 5, 1936 2,255,520 Schuster Sept. 9, 1941 2,268,640 Brown Jan. 6, 1942 2,297,329 -Scheldorf Sept. 29, 1942 2,352,977 scheldorf July 4, 1944 2,452,073 Schivley et a1. e Oct. 26, 1948 FOREIGN PA'I'ENTS Number Country Date 520,628 Great Britain Apr. 80, 1940 7 OTHER REFERENCES 7 Publication: FM 8; Television, Feb. 1948 p. 51,

tal level as the reflector and the first folded dipole and spaced from said first folded dipole on the side opposite the reflector; and means for isolating the said second dipole from the first dipole at the resonant frequency of the first dipole comprising an additional loose loop of transmission line joining said second folded dipole to the transmission line and the first folded dipole.

'- GEORGE P. KEARSE.

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

UNITED STATES PATENTS (see Collins antenna).

Publication: Amphenol Television Antenna Model 114-005, Manual file 14.7, American Phenolic Corp., Chicago 50, Ill. March 1, 1948.

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