US2650304A

US2650304A - Television antenna

Info

Publication number: US2650304A
Application number: US114955A
Authority: US
Inventors: Schlesinger Kurt
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1949-09-10
Filing date: 1949-09-10
Publication date: 1953-08-25
Anticipated expiration: 1970-08-25

Description

8- 25, 1953 K. SCHLESINGER 7 2,650,304

TELEVISION ANTENNA Filed Sept. 10, 1949 2 Shetg-Sheet 1 FIG. I FIG. 2

INVENTOR. Kurt Schlesinger BY Atty K. SCHLESINGER 2,650,304 TELEVISION ANTENNA Aug. 25, 1953 2 Sheets-Sheet 2 Filed Sept. 10, 1949 FIG.

INVENTOR. Kurt Schlesmger Patented Aug. 25, 1953 OFFICE TELEVISION ANTENNA Kurt Schlesinger, Maywood, Ill., assignor to Motorola, Inc., Chicago, 111., a corporation of Illinois Application September 10, 1949, Serial No. 114,955

8 Claims. (Cl. 250-337) This invention relates generally to antennas for use in the very high frequency range and more particularly, to antennas which are responsive over a plurality of bands in the very high frequency range without individually-tuning or orienting for the various channelsin the frequency bands. l l

Televisioncommun-ication is presently carried on in two separate frequency bands inthe very high frequency range. These bands are generally referred to as the low television band and the-high television band with'the low band ineluding frequencies from 54 to- 72 megacycles and '16 to 88 megacycles and the high band covering frequencies, from L74 to 216 megacycles; The low band therefore actually includes two separate frequency ranges and extends over 34 megacycles whichis a very large percent of the average frequency of the low band. It is very difficult to provide a single antenna system having tuning means for an antenna which is naturally responsive in one range of frequencies for tuning the antenna to provide reception of a wide band of lower frequencies without substantial loss of power.

A feature of this invention is the provision of tuning means for antennas of either the inductive or capacitive type in which an element having reactance of opposite sense to that of the antenna is coupled to the antenna for tuning the same for operation at lower frequencies with high response over'the entire range of the low band and which is also effective throughout the high frequency band. l

. Most presently used systems provide separate antenna elements which are of such configuration to provide optimum response in the two bands. These systems have'been large and bulky and have not generally'been suitablefor indoor installation and particularly cannot be enclosed in the receiver cabinet Other antennas have been used inwhich the antenna elements are adjustable so that the length can be made an optimum value for the station being received. This is undesirable as it is necessary to make adjustment of the antennaelements each time the receiver is tuned to a different station; These available antenna systems are also directional in their characteristics and when the same antenna elements are used for receiving a plurality of stations, the position of the elements must be adjusted for the various stations or'a compromise must be made so that theresponse is not optimum for all of the channels.

It is therefore an object of the present invention to provide an improved antenna system for operation on two separate-frequency bands each of which includes a relatively wide range of frequencies.

A further object of this invention is to provide a smaliantenna which can be enclosed in a receiver cabinet and which provides efficient signal pickup on a plurality of channels in a wide range of frequencies without tuning or orienting the antenna for the individual channels.

l Another objectof this invention is to provide minimum power loss.

A further feature of this invention is the provision of an antenna system having reactive signal collecting means of such dimensions to provide eflicient pickup at high frequencies and reactive tuning means coupled to the collecting means for tuning the system for operation at lower frequencies, with one of said reactive means being inductive and the other capacitive, and with a circuit coupled to the inductive means providing a double resonant circuit to increase the band width of the antenna at the lowerfrequencies.

Another featureof thisinvention is the provision of a loop antenna having such dimensions to provide efiicient pickup at frequencies in one frequency band and capacitive means coupled to the loop for tuning the loop for efficient pickup at frequencies in a lower frequency band, with resonantcircuits coupled to theloop to provide double resonance in both frequency bands to increase the frequency range over which eflicient signal pickup is obtained.

Still another feature of this invention is the provision of loop antenna systems including tuning means for providin efiilcient pickup over two separate frequency bands, with a plurality ofsystems being coupled together to provide increased received power.

A still further feature of this invention is the provision of a dipole antenna system having the arms thereof dimensioned to provide efficient pickup at relatively high frequencies and inductivetuning means for the dipole to provide efile cient'pickup in a band of lower frequencies, with a reactive circuit coupled to said inductive tuning means to provide double resonance in the band of lower frequencies to increase the response throughout this band.

Further objects, features and advantageswill be apparentfrom a consideration of the following description taken in connection with the accompanying drawings, in which:

Fig. 1 illustrates a tuned loop antenna;

- and inductance l3.

Fig. 10 illustrates a dipole antenna constructed a in accordance with the invention; and

Figs. 11 and 12 illustrate the coupling of a plurality of tuned antenna systems.

In practicing the invention, antennas of the loop or dipole type are constructed ofsuch dimensions to provide efiicient signal pickup in the high frequency television band. Tuning systems are provided for the antennas to provide efficient operation in the low frequency television band. It has been found that for an inductive (loop) antenna the tuning should be done entirely by capacity, and for a capacitive antenna (dipole) the tuning should be done entirely by inductance. The tuning is arranged to provide maximum response near the center of the low frequency band with the response obtained by the main tuning element providing efficient pickup over only a central portion of the band. A tuned circuit is then coupled to the inductive element (either the loop antenna or the tuning means for the dipole) to provide double resonance in the low frequency band so that the band width is increased and efficient pickup is provided over the entire low frequency band. A plurality of antenna elements may be coupled together to provide increased pickup power.

Referring now to the drawings, in Fig. 1 there is illustrated a loop antenna 10 with tuning means I l coupled thereto. The sides of the loop have a length substantially equal to one quarter of a wave length in the high frequency band to provide good response in the high band. This provides an antenna of a size which can be conveniently provided within a television receiver cabinet. The tuning means is provided for tuning th antenna to provide efficient pickup in the low frequency band. The tuning means II is shown schematically to include capacitance l2 It is known that tuning can be obtained by various combinations of capacity and/or inductance to provide maximum response over a desired frequency range. The optimum power which can be obtained from an antenna varies directly with the square of the signal available and varies inversely with the band width. However, when tuning means are coupled to antennas the power derived by such antennas may be considerably reduced. When using an inductive antenna such as the loop IU of Fig. 1, with tuning means as indicated at H, the resulting power is governed by the following formula:

In the above formula P is the power available from the antenna, P0 is the optimum power obtainable, L1 is the inductance of the loop and L2 is the inductance of the tuning means. It is apparent from the above that for maximum derived power L2 should be infinite. Therefore an inductance should not be used in the tuning unit 4 H for tuning the loop and capacity only should be used for tuning an inductance antenna.

The above considerations also apply'to a dipole antenna such as shown in Fig. 2. In this figure a dipole antenna I5 is illustrated having arms l6 and I1 which are connected to a tuning unit l8. This tuning unit is'also illustrated as including inductance l9 and capacity 20. The antenna elements are of such dimensions to provide efllcient pickup at one frequency as for example the h gh television band and the tuning unit I8 may be used for tuning the system for operation at a lower frequency. The optimum power from the dipole is governed by the same factors as the loop, that is the power varies directlyas the square of the signal and inversely as the band width. The power which may be derived from the antenna is again reduced by the use of tuning means with the power available being indicated by the following formula:

.dipole. It is apparent from this formula that for a minimum power loss C2 should be zero.

Therefore for capacitive antennas, such as dipoles. the tuning should be done'entirely by an' inductance.

It is apparent from the above that in either a loop or dipole system which is tuned to provide maximum response at a frequency below the natural frequency of the antenna, the system can be represented as a condenser and inductor in series with the voltage picked up. This is shown in Fig. 3 in which 25 indicates signal voltage, 26 the condenser, and 21 the inductance. In the case of a loop antenna, 21 is the inductance of the loop and 26 is the tuning condenser. In the case of a dipole antenna, 26 is the capacity of the dipole and 21 is the tuning inductor. As shown by the basic power formula, the power varies inversely withband width and if the tuning is such that the complete low frequency band is provided, the power will be substantially reduced. It has been found that increased power can be obtained by tuning the antenna system for optimum pickup over only a portion of the band and increasing the band width by the use of another tuned circuit coupled to the antenna system. This secondary circuit may be most easily applied to the inductive element of the antenna system and provides a double tuned circuit which equallzes the response over the desired band. This is illustrated in Fig. 3 in which the condenser 28 and inductance 29 are coupled to the inductance 21 of the primary antenna circuit. The secondary circuitis coupled to a critical fraction of the inductance 21 to provide the desired overall response. The output impedance is symbolized by resistor 30 and the ratio of the inductance 21 to the capacity of the condenser 28 may be such that the impedance is 300 ohms to match the standard balanced line, as used for connecting the antenna system to the receiver with which it is used.

In Fig. 5 there is illustrated the response curves of the system illustrated in Fig. 3. Curve A shows the response provided by the main tuning elements, that is the tuning condenser of the loop or the tuning inductorof the dipole. Curve B shows the response curve when the secondary or equalizing circuit is used to provide a double response which broadens the band to cover the mo em.

noted that the maximum pickup obtained with the secondary circuit is not substantially less thanthe pickup obtained by the single resonant system; and the average gain over the entire low frequency band is more unite-rm than that obtained' by a single resonant system. i This circuit therefore acts as an equalizer;

In Fig. 4, there is illustrated a Sohmatio circuit generally similar to. that of Fig. 3 with the antenna system being represented by Signal voltage 25, capacitor 26' and inductance 21 in series. The secondary circuit or equalizer of Fig. 3 is replaced by a, balanced circuit including eondensers 3i and 32 and inductor 33 so that a balanced output is: provided to match the balanced input of standard television receivers. The secondary circuit is tuned to the same frequency as the primary antenna circuit and the ratio of inductance to capacity of the secondary circuit is adjusted to providethe desired output impedance.

In Fig. 6, there is illustrated a structural embodiment of a loop antenna system in accordance with the invention. The antenna is provided on an insulating board 40 on which the conducting elements 4| and 42, which form the loop, are supported. The V-shaped elements and 42 have

sides

43, 44, 45 and 46 which are of a length substantially equal to one. quarter or a wave length in the high frequency band. For operation in the high television band, around 200 megacycles,

the length of each side of each of the elements should be about 14 inches. The two elements 4| and 42 are interconnected by

balanced transmission lines

41 and 48, the conductors of which are transposed at the center so that a continuous circuit is provided through the conducting elements 4| and 42 and through the transmission lines 4-! and 48; The width of the conducting elements is selected to provide the desired loop inductance and. distributed capacity for the frequencies involved. A conducting strip may be provided on the. insulating board 40 connecting the corners of the conducting elements 4! and 42,

so that these points are maintained at a refer- 'ence potential.

For tuning the loop antenna of Fig. 6 for P- eration in the low frequency band, capacitor is provided. An inductor 52 is connectedin series r with the capacitor 5! but the value of the inductor 52 is such that it has little effect for low band operation. The purpose of this inductor will be described in connection with the band operation. Capacitor 5| is connected across I 'the loop. This is accomplished by the

condensers

53 and 54 and inductor 55 which are connected to the adjacent ends of the

sides

43 and 46 of the loop.

For a better understanding of the operation of the loop in the low frequency band, reference is made to Fig. 7 in which the various elements vof the loop are represented by equivalent circuit elements. L43, L44, L45 and L46 represent the inductances of the

sides

43, 44, 45 and 46, respectively of the loop. L56, L57, L59 and Lao represent the inductances of the

conductor

56 and 51 of the isfactory operation.

transmission line 41 n th conduct rs 5% and seen that resonant, circuits are provided including the loop Portions; in which the received sig-f nal is induced.) The si nal i du d in e o is represented by thagenerator symbols 58 in series with each side of; the loop. The signals induced all of the sides, are added so. that the total signal is four times. that induced ineach loop side. As previously stated, the condenser tunes the resonant circuits to. the center of. the low frequency" band and the output thereoi would be such as shown in curve A of Fig. 5. The secondary circuit is connected across a poption of the inductanceof the loop circuits, being connected acros inductances L43 and L46. At the frequencies involved thestrip 49 at reference potential may he assumed to have no impedance so that n efiect the

sides

43 and 46 of the loop are directly connected in series. The

condensers

5,3 and 54 and inductor 55. provide a double tuned circuit which produces ares-ponse as indicated in curve B of Fig," 5. The output istaken off across; inductor 55 as indicated by resistor R300. It; is well known that to provide a double tuned circuit the two branches must be tuned tothe same frequency. That is, the primary circuit including the inductances of the sides of the loops and the conductors of the transmission lines and the capacity of condenser 5i must be tuned to the same frequen y as the secondary circuit including condensersiii and and inductor 55. The couplin inductances L4: and L46 must have the. value to provide the desired 'band width.

Also the ratio of inductance tocapacity of the secondary circuit determines the load impedance. To provide satisfactory tuningv over the low frequency television band (5d to 818 megacycles) the following values were used: l

Loop sides 43, 44, 451 and It is to be pointed out that these values are merely representative and other values mayprovide sat- For an explanation of the operation of the loop on the high frequency band, reference is made to Fig. 8. As previously stated, the sides of the loop have a length substantially equal to one quarter of a wave length in the high Irequency band. More specifically, the length may be equal to one quarter of a wave length in the center of the band which, in the high television band, is about megacycles. At this frequency, the capacitor SI and the inductance 52 become ineffect arelatively large capacity and form a fective electrical length thereof is one quarter of a wave length. Each of the conducting elements 4| and .42 forms. in effect, a half wave dipole for high frequency Operation. The transmission line 48 conne ts the c nduc in l ments and 42 in series so that the signals therefrom are added in phase to provide high gain. The response of of the low frequency band.

the antenna in the high band is indicated by the curve C in Fig. 9.

Although the gain of the antenna is very high in the high frequency band, it may be desirable to make the gain throughout the band more uniform. This may be accomplished by the provision of a' secondary or equalizer circuit represented in Fig. 8 by the inductors 80 and Bi and the condenser'82. For operation in the band from 17 4 to 216 megacycles, the inductors 80 and 8| should have values of the order of 0.005 microhenry and the condenser 82 should have a value of the order of 4 micromicrofarads. This secondary circuit is ineffective when operating on the low band because the inductors 80 and BI would be negligible and the impedance of the condenser 82 would be very large. Similarly, the secondary circuit for low band operation is ineffective when operating in the high frequency band as the

capacitors

53 and 54 become substantially short circuits and the impedance of the inductors 55 is relatively high. The double tuned response is shown in curve D, Fig. 9.

In Fig. 10 there is illustrated a dipole antenna system embodying the invention. The

dipole elements

62 and 63 may be provided on any suitable insulating support and may have increased widths as they extend away from each other to providebetter response over a relatively wide range of frequencies. The antenna elements are of such dimensions that the antenna will provide efficient pickup at the highest frequency in the high frequency band. If it is desired to mount such an antenna in a receiver cabinet, the ends "of the elements may be bent at right angles to conserve space. The bent down ends load the antenna and the response thereof is not substantially reduced. The dipole antenna is tuned by an inductance formed by a section 64 of 300 ohm transmission line connected to the dipole 1

elements

62 and 63. The conductors in the transmission line 64 are connected together at the end 65 removed from the dipole arms to form a continuouscircuit. The line 64 is of such length that the inductance thereof is of the proper value for tuning the dipole antenna to the center Coupled to the transmission line 64 is a secondary circuit or equalizer including condensers 66 and 61 and inductor 68. The equalizer operates in the same manner as fully described in connection with Fig.

6 to provide a uniform response over the 7 low band and to provide a 300 ohm output impedance so that standard 300 ohm balanced line can be used to connect the antenna to a television receiver. V

For operation on the television bands referred to, a satisfactory dipole antenna was constructed having an equivalent capacity of the order of micromicrofarads. The stub was a 12 inch length -of 300 ohm line with the secondary circuit being connected about 5 inches from the dipole. The condensers 66 and 61 were micromicrofarads each and the inductor 68 was .7 microhenry.

' In Figs. 11 and 12 there are illustrated arrange- ;ments for connecting a plurality of loop antennas such as disclosed in Fig. 6 together to form an array. In Fig. 11 the two antennas I0 and H are connected in parallel to a single equalizer including condensers I2 and 13 and inductor 14.

For operation with loops as described, the condensers 12 and 13 should have a value of about 18 micromicrofarads and the inductor 14 should have a value of theorder of 1 microhenry. It is essential that the loops be oriented in the same loop, said members being formed of relatively wide strips of conducting material with each side manner and that the lines connecting the loops to the equalizer have equal lengths so that the signals are'exactly in-phase. It may be desirable to make one of the tuning condensers 5| variable so that the two loops can be accurately tuned to the same frequency. Similarly in Fig. 12 three

loops

15, 16 and 11 are connected in parallel and again the lines connecting the loops to the equalizer circuit must be of equal length.

It is seen from the above that antenna systems of'both the loop and dipole type have been provided, which produce high gain over two spaced frequency bands. By the use of double tuned circuits relatively wide band widths are obtained in both bands. These antennas are of such size that they can be enclosed in a television receiver cabinet so that separate-installation is not required. Alternatively the antennas may be used for outdoor or attic installations. For such installation it may be desirable to provide two separate antennas for the two bands, with the anfrequency band and in a second frequency band of lower frequency than said first band, with each of said bands covering a relatively wide range of frequencies, said system including a pair of V-shaped members positioned to form a square thereof having a length substantially equal to one quarter of a wave length at a frequency within said first frequency band, a transmission line including a pair of spaced conductors connected to the ends of said members, said transmission line having the conductors thereof transposed so that a continuous series circuit is provided through said members and said conductors of said transmission line, and tuning means coupled to said transmission line at substantially 1 the center thereof for tuning said antenna system for operation in said second frequency band, said tuning means providing a' low impedance between the conductors of said transmission line at frequencies in said first frequency band.

2. An antenna system for operation in a first frequency band and in a second frequency band of lower frequency than said first band, with each of said bands covering a relatively wide range of frequencies, said system including a pair of V-shaped members positioned to form a square, said members being made of relatively wide strips of conducting material with each side thereof having a length substantially equal to one quarter of a wave length at a frequency within said first frequency band, a transmission line including a pair of spaced conductors connected to the ends of said members, said transmission line having the conductors thereof transposed so that a continuous series circuit is provided through said members and said conductors of said transmission line, condenser means coupled to said transmission line for tuning said antenna system for operation msaid second nequencyband, and a circuit to adiacen t ends of members providing a double tuned circuit for increasing the responseofsaid arrtenna sys'temover a-wide range of frequencies in said second frequency band.

3; an antenna system for operation in a first frequency band and in er-second frequency band of lower frequency than said first band, with each of said bands covering a relatively wide range of frequencies, said system including a pair of V-shaped members positioned to form a square loop, said members being formed of relatively wide strips of conducting material with each side thereof having a length substantially equal to one quarter of a wave length at a frequency within said first frequency band, a transmission line including a pair of spaced conductors connected to the ends of said mem'bers said transmission line having the conductors thereof transposed so that a continuous series circuit is provided through said members and said conductors of said transmission line, tuning means coupled to said transmission line at substantially the center thereof for tuning said antenna system for operation in said second frequency band, said tuning means providing a low impedance between the conductors of said transmission line at frequencies in said first frequency band, and a circuit coupled to adjacent ends of said members providing a double tuned circuit for increasing the response of said antenna system over a wide range of frequencies in said second frequency band.

4. An antenna system for operation in a first frequency band and in a second frequency band of lower frequency than said first band, with each of said bands covering a relatively .wide range of frequencies, said system including a pair of V-shaped members positioned to form a square loop, said members being formed of relatively wide strips of conducting material, with each side thereof having a length substantially equal to one quarter of a Wave length at a first frequency within said first frequency band, a transmission line including a pair of spaced'conductors connected to the ends of said members, said transmission line having an electrical length substantially equal to one half of a wave length at said first frequency, said conductors being transposed and being connected to said members in such manner that a continuous series circuit is provided through said members and said conductors, and tuning means coupled to said transmission line at substantially the center thereof for tuning said antenna system to a frequency within said second frequency band, said tuning means providing a low impedance between said conductors of said transmission line at frequencies in said first frequency band so that a portion of said transmission line forms a quarter wave shorted stub connecting said V-shaped members in series.

5. An antenna system for operation in a first frequency band and in a second frequency band of lower frequency than said first band, with each of said bands covering a relatively wide range of frequencies, said system including a pair of V-shaped members positioned to form a square loop, said members being formed of relatively wide strips of conducting material with each side thereof having a length substantially equal to on quarter of a wave length at a first frequency within said first frequency band, a transmission line including a pair of spaced conductors connected to the-ends of said members, said transmission line having anelectrical length substantially equal to one half of a wave length at said first frequency, said conductors being transposed and being connected to said members in such manner that a. continuous series circuit is provided through said members and said conductors, a conducting "strip connecting the apexes of said members to hold the same at a reference potential, a first circuit including a condenser and an inductor connected between said conductors of said transmission line at substantially the center thereof, circuit tuning said antenna system to a second frequency within said frequency band, and a second circuit resonant at said second frequency coupled to adtaicent ends of said members providing a double tuned circuit having a substantially uniform response over the entire range of frequencies in said second frequency band, said first circuit having a low impedance at frequencies in said first frequency band so that a portion of said transmission line forms a quarter wave shorted stub connecting said v-shaped members in series.

6. An antenna system for operation in a frequency band covering a relatively wide range of frequencies including, a pair of V-shaped conducting members positioned to form a square loop, with each side thereof having a length substantially equal to one-quarter of a wavelength and a frequency within said frequency band, a transmission line including a pair of spaced conductors connected to the ends of said members, said transmission line forming quarter-Wave shortage stubs connecting said V-shaped members in series, and a resonant circuit tuned to a said frequency within said frequency band coupled to adjacent ends of said members for providing a double tuned circuit so that the response of said antenna system over said wide range of frequencies is increased.

'7. An antenna system for operation in a first frequency band and in a second frequency band of lower frequency than said first band, with each of said bands covering a relatively wide range of frequencies, said systems including in combination, a pair of V-shaped members positioned to form a square, said members being made of relatively wide strips of conducting material with each side thereof having a length substantially equal to one quarter of a wave length at a frequency within said first frequency band, a transmission line including a pair of spaced conductors connected to the ends of said members, said transmission line having the conductors thereof transposed so that a continuous series circuit is provided through said members and said conductors of said transmission line, condenser means coupled to said transmision line for tuning said antenna system for operation in said second frequency band, and resonant circuit means coupled to adjacent ends of said members including a first portion resonant in said first frequency band and providing a first double tuned circuit for increasing the response of said antenna system over a wide range of frequencies in said first frequency band, said resonant circuit means including a second portion resonant in said second frequency band providing a double tuned circuit for increasing the response of said antenna system over a wide range of frequencies in said second frequency band.

8. An antenna system for operating in a first frequency band and in a second frequency band of lower frequency than said first band with each of said bands coverinea. relativelywide range of frequencies, said system including incombination, first and second elongated conductor-portions having relatively low inductance and each having a length substantially equal to one-half wave length in the higher of said frequency bands, said conductor portions being shaped and positioned to form a loop with the ends of each portion being adjacent the ends of the other portion, a balanced two conductor transmission line connected at each end to adjacent ends of said first and second conductor portions with the conductors thereof being transposed so that a continuous series circuit is provided through said members and said conductors of said transmission line, tuning means coupled to said transmission line at substantially the center thereof for tuning said antenna system for operation in said second frequency band, said tuning means providing a low impedance between the conductors of said transmission line at frequencies in said first frequency band, and a circuit coupled to adjacent ends of said first and second conductor portions providing a double tuned circuit in said second frequency band for increasing the response of said antenna system over a wide range of frequencies in said second fre:

quency band. V e I KURT SCI-ILESINGER.

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