US2789286A

US2789286A - Dual frequency antenna arrays

Info

Publication number: US2789286A
Application number: US320633A
Authority: US
Inventors: Thomas A Marshall
Original assignee: Individual
Current assignee: Individual
Priority date: 1952-11-14
Filing date: 1952-11-14
Publication date: 1957-04-16
Anticipated expiration: 1974-04-16

Description

April 16, 1957 T. A. MARSHALL mum. FREQUENCY ANTENNA ARRAYS Filed Nov. 14, 1952 \INVENTORQ mam/2am "1.6

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ATTORNEYS- ,7 2,789,286 Ice Patented Apr. 16, 1957 DUAL FREQUENCY ANTENNA ARRAYS Thomas A. Marshall, San Diego, Calif.

Application November 14, 1952, Serial No. 320,633

g 4 Claims. (Cl. 343-815) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to antennas and more particularly to antennas for the reception of high frequency signals such as are utilized for television transmission.

' Television signals are broadcast at present over a number of frequency ranges or channels. It is desirable to provide television receivers adapted to receive signals transmitted on any of the channels used. The strength of signals obtained on any channel, however, is controlled in the first instance by the antenna which intercepts the signals and translates them to the television receiver. Antennas used for reception of signals from distant transmitting stations at present are of several types. The best of these types have either a high gain at a single frequency or a low gain at two or more frequencies. In the instant invention, the shape, number, and relative position of the active and inactive elements of the antenna are so disposed that extremely high gain is realized at two separated frequency bands with a fair gain at intermediate frequencies.

One of the objects of the invention is to provide a broad picture frequency response antenna which enables the array to be operated in or near resonance with the picture frequency.

It is an object of the invention to provide a new and improved television receiving antenna which provides a large voltage at its output terminals.

Another object of the invention is to provide an antenna array having a high gain on a low frequency channel and a high gain on a high frequency channel.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following description.

Fig. 1 is a diagram of the field pattern of a dipole at a frequency three times the fundamental of the antenna frequency;

Fig. 2 is a diagram of the field pattern of a V-shaped dipole;

Fig. 3 is a perspective view of the elements in an antenna embodying the invention;

Fig. 4 is a diagrammatic sketch of a modified form of the invention; and

Fig. 5 is a diagrammatic sketch of a further modified form of the invention.

The operation of all types of antennas, no matter how simple or complex their construction, is based upon the principles of the fundamental folded antenna whose physical length is equal to approximately one-half the wavelength of the signal to be transmitted or received. A convenient equation for computing the length of the fundamental folded antenna is where L is the length of the antenna in inches and F is the frequency in megacycles per second. The constant in the equation has been determined experimentally and makes allowance for the capacitance effect at the ends of the antenna in the television range of frequencies.

For receiving signals from a nearby source at several frequencies, a folded dipole cut to the proper length is fairly effective. The ability to receive signals from a distant station may be enhanced by a series of directors and reflectors or in yagi arrays which are among the best antennas for fringe areas. Yagi arrays, however, function for a single frequency only. It is obviously impracticable to have one antenna for each of the several channels that may be received in many localities.

A simple folded dipole properly cut for channel 4, which has a picture frequency of 67.25 me, will also gather in signals on channel 11 having a picture frequency of 199.25 me. This is so because the length in inches for a folded dipole for channel 4 equals and a long antenna consisting of a number of half-wave dipoles will provide considerable power gain over a halfwive dipole. Hence, the picture frequency for channel 11 is approximately equal to three times the fundamental reconant frequency of channel 4 and will be received on the same antenna. The calculated length of a dipole for channel 4 is 82.37 inches which is close to that of a three half wave dipole for channel 11 at 83.41 inches.

The field pattern for an antenna operating at a frequency three times the fundamental is shown in Fig. 1 wherein arms 10 and 12 of the folded dipole are arranged in a straight line.

If the antenna of Fig. 1 is modified so the free ends of the dipole are protruded forward to an angle of approximately 72 degrees or 144 degrees across the V-portion, the field patterns of the two

end sections

14 and 16 will combine to produce a bi-directional'lobe as shown in Fig. 2. An antenna having such a symmetrical V-shaped element has sufficient gain for nearby stations and the configuration produces a broad response. For fringe areas, however, additional elements are required to gather in the weak signals available. 7

In Fig. 3, a useful gain on channels 4 and 11 is obtained when element 18 is added to dipole 20. The length of element 18 is slightly less than that of dipole 20. Both elements are operated at or near resonance so there is maximum energy transfer between elements. This relationship is unlike that obtaining in yagi arrays which have straight elements with the parasitics operated off resonance.

Additional elements

22 and 24 may be added to provide still more gain. The length of all elements ahead of the dipole is preferably about 0.987 times the length of the dipole. The optimum spacing between the various forward elements is approximately 0.104 times the wave length of the low frequency channel. Element 26 may be added to the rear of dipole 20 to provide additional gain. The optimum spacing between dipole 20 and rear element 26 is approximately 0.094 times the wavelength of the low frequency channel. The free ends of element 26 are placed close to the free ends of dipole 20 in order to provide coupling since there is little gain, through the action of the rear element, on high frequencies unless such coupling is present. Element 26 must be cut slightly longer than dipole 20 to allow for adjustment of the free ends. It has been found that an increase in length of 4% over the length of the dipole works very well. The spacing between the V-sections is a compromise for obtaining optimum gain and best picture performance on both channels. The gain of or 82.37 inches the-above'described antenna on channel 4 was found to be 2.0'over a fiveelement'yagi -array'designed-"for'thatchanconductors 28 and 30are, of course, insulated. from the otherelements andfrom'the' supportingmembers.

' 'Thesizeandf-spacing ofthe tubing for dipole 20 depends upon the impedance of the transmission line; Tubing having an outer. diameter of /4 inch for theparalleled portion and an outer diameter. of ym inch for the active arms with .a center-to-center separationof parallel and active arms off'2 /2' inches. has been foundto give a good match to 300 ohm transmission line. The active and parallel arms of dipole 20 are preferably aligned in a vertical plane. The diameter of

elements

18, 22, 24 and 26 may conveniently be inch. The horizontal plane including theinactive elements. should .be centered. between. the ac: tive,-and..parallel.arms.ofdipole 20.; this.refinement results. in an. appreciable, gain.

Fig. 4. shows the spacingarrangement for a dual frequencyarray, for-channels 2 and 7. The proportions are similarto. thoseof theantenna shown inFig. 3 and corresponding.elementshave been given like numbers. The length of thedipole corresponds totheoptimum length for channel 7 whichis 9.6 inchesfor a folded antenna that canbe operated. on its third harmonic. The array may bedesignedtogive ahighergain on one channel than on r the other. This may be desirable in case the transmitting station...on,one:channel is..much.more powerful than the stationontheother channel. .Arraysmay bereadily designedforothercombinations of channels such as 3, and 8,3 and:9 4 and.12,.and-.5.fand.13.

In Fig. .5, a tetra-channel array. is shown. It. is derived by, combining .twodual frequency arrays. Dipole 30 is resonant to.channels. 5 and 13, and dipole, 32.is.resonant 'tochannels 4 and 11 Each. dipole is provided with a separate transmissionline whichis connected to an antenna transfer switch located at the receiver. A tetra-channel arraytwill provide highengainover all four channels than aya giantenna on-any single channel.- The gain at one frequency may: be.increased,,at the expense of gain on the other channels, by adjusting the relative lengths and spacings of the inactive element. When dipole. 30 is in use, theforwardelements are the

shorter tubes

34 and 36 while. the rear elements38, 4t 42 and dipole 32, acting as -an.in active element, are all longer than the dipole in use. When dipole 32 is connected tothe receiver, the Ifiye active elements aheadof'itall contributeto an increase in gain.

Obviously many modifications and variations of the 6 present invention are possible in the light of the above teachings. It. istherefore to beunderstood that within 4 the scope of the appended claims the invention may be practiced otherwise than'asspecificallydescribed:'

What is claimed is: 1. An antenna for television reception comprising a folded dipole the arms of which diverge forwardly at an angle of approximately 144 degrees to each other, a plurality of elements located zforwardly of said dipole each 7 having arms diverging forwardly at about the same angle of the-dipole-arms, and an element located rearwardly of said dipole. and. having; arms diveringrforwardlyeat a smaller angle than said dipole arms, said elements and said dipole lyingin a common pla'nej 2. A television antennaecomprisinga-folded dipole having the two sides in a symmetrical V shape with an ineluded? angle. vofii144 degrees-camil 'adapted'lto" resonate; at one television frequency and at harmonics thereof, a plurality of forward elements each having two sides substantially parallel with the sides .of said dipole, the forward elements'havingn length slightly. shorterth'an said dipole and having a distancebetween said dipole an'dthe nearest forward" element and"betweenforward' elementsv of ap proximately 0.1 times the wave "length ofithe low frequency to be' received.

3. The invention definedin' claim 2comprising'in addition a V-shapedrearelement cutjslightly'longeri than and disposed:rearwardlyof said'dipole, said rear element having -anincl uded angle'of lessthan 144 degrees whereby coupling between thefree-errds of said rear element and said dipole is accomplished, the-distance between the apiees of said dipole: and-said rear element approximately equal -to 0.09" times the wave-length of the" low frequencyto'be reeeived.

4. A tetra channel-antennaarray comprising-aforward dipole having the free end's; protruded forwardly in =ah'orizont-al 'plane to' form a VV-shaped 14'4-degree angle; a rear References Citedin the file-of this patent V UNITED STATES PATENTSI 2,040,079.- Carter May-12,1936 2,204,175 .Carter June. 11, 1940 2,352,977 Scheldorf July. 4, 19.44 234128.08 Carter--- .Mar..25,- 1947v Epstein Nov. .21., 195.0

-OTHER -REFERENGES 'J'FITMfg. Co. publication cover, Long Ranger Y'agi TV Arrays. Received .in Patent Office April.2.0}19'5lj 250-33 TV;

Noll: Antenna. for. Television Receiversfi May 1945', Radio News, pages '42 andl30.