US2888676A

US2888676A - Folded television antenna

Info

Publication number: US2888676A
Application number: US457889A
Authority: US
Inventors: Joseph C Spindler
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1954-09-23
Filing date: 1954-09-23
Publication date: 1959-05-26
Anticipated expiration: 1976-05-26

Description

' y26 ,1959 J.C.SPINDLER 2,888,676

' FOLDED TELEVISION ANTENNA Filed Sept. 23, 1954 2 Sheets-Sheet 1 I? 20 I9 I? I2 6l 6| 7 ll FIG. 1 w 8 I21 8 6| Receiver 4,

i I V I 60 I8 22 23 I5 l 3 l K M v 6O 6O g7 3: 30 24 4? 45 42 1 I L l2 8- H l JOSEPH C. SPINDLER INVENTOR.

HIS ATTORNEY.

May 26, 1959 Filed Sept. 25, 1954 Freq.

Antenna of Fig.1 in

Vertical Plane.

Horizontal Radiation Pattern J'. c. SPINDLER FOLDED TELEVISION ANTENNA Antenna of Fig.1 in

Horizontal Plane.

Horizontal Radiation Pattern 2 Sheets-Sheet -2 Antenna of Fig.2 in

Horizontal Plane.

Horizontal Radiation ttern 1 20l Me FIG. 5

JOSEPH C. SPINDLER IN V EN TOR.

l gamma His ATTORNEY.

United States Patent )fiice Patented May 26, 1959 2,888,676 FOLDED TELEVISION ANTENNA Joseph C. Spindler, Chicago, Ill., assignor to Zenith Radio Corporation, a corporation of Delaware Application September 23, 1954, Serial No. 457,889 6 Claims. (Cl. 343744) This invention relates to a television antenna and more particularly to a television antenna for receiving signals throughout a plurality of separate bands of frequencies spaced from one another in the very-high-frequency (VHF) and ultra-high-frequency (UHF) spectrum.

To reproduce satisfactory television pictures with a conventional television receiver, it is necessary to employ a receiving antenna which is capable of receiving signals throughout a wide frequency band. With the present allotment of television broadcasting frequencies in the VHF and UHF spectrum, a television antenna should be capable of receiving signals efficiently over a frequency spectrum extending from approximately 54 megacycles to 900 megacycles.

At the present time, suitable antennas for use inside the home have consisted of indoor antennas which are positioned at optimum receiving points in the vicinity of the television receiver. Such indoor antennas generally require adjustment in orientation and/or spacing of the receiving elements when the receiver is tuned from one channel to another. In addition, known types of indoor antennas are generally unsightly and detract from the decor of the room.

In order to obviate the above difficulties, antennas which may be hidden within the confines of the television cabinet have been proposed and are frequently built into commercially produced television receivers. Built-in antennas, however, are ordinarily extremely limited in the frequency spectrum over which they efiiciently receive signals. Furthermore, since such antennas are usually highly directional and are mounted in a fixed position within the confines of the receiver cabinet, orientation or adjustment for optimum reception on individual channels is not feasible. Furthermore, the various commercially available built-in antennas generally provide efficient operation only when mounted in a single plane or in a particular orientation within the cabinet. This imposes an undesirable limitation on cabinet design.

It is an object of the present invention to provide a new and improved television antenna of the built-in type which is operable throughout a plurality of separate bands of frequencies spaced from one another in the VHF and UHF spectrum. 1

It is a further object of the present invention to provide a novel antenna structure which operates efficiently when mounted in either a horizontal or a vertical plane.

It is an additional object of the present invention to provide a television antenna which is substantially omnidirectional over a wide band of frequencies.

It is yet another object of the present invention to provide an antenna which can be conveniently enclosed within a television cabinet and yet overcomes many of the disadvantages of prior art built-in television antennas.

In accordance with the invention, a television antenna operable throughout a plurality of separate bands of frequencies spaced from one another in the VHF and UHF spectrum comprises a pair of substantially U-shaped conductive radiating elements each having leg and base portions of substantially equal length and with corresponding end portions in closely spaced mutual juxtaposition to constitute a quadrilateral loop having an effective electrical length equal to one-half wavelength at a particular frequency in the lowest one of the frequency bands. The effective electrical length of each of the longer sides of the loop is substantially twice that of each of the shorter sides. Electrical signahtranslating means are coupled to a pair of the corresponding end portions.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawings,v in the several figures of which like reference numerals indicate like elements and in which: I

Figures 1, 2 and 3 are diagrammatic representations of various embodiments of the invention;

Figure 4 is a perspective view, partially cut-away, of the embodiment of Figure 3 in a commercial environment; and

Figures 5a, 5b and 5c are idealized graphical representations of the radiation patterns of different embodiments of the present invention.

As shown in Figure l, a receiving loop comprises a pair of substantially U-shaped conductive radiating elements 11 and 12 which may be formed of electrically conductive material such as copper or aluminum. The thickness of the individual radiating elements is extremely small compared to their length. Element 11 comprises

segments

13, 14 and 15 of which

segments

13 and 15 are substantially parallel and are spanned by segment 14. Element 12 comprises

segments

16, 17 and 18 disposed in a pattern corresponding to that of the segments of radiating element 11. Corresponding

end portions

19 and 20 of radiating elements 11 and 12 respectively provide terminals to which electrical signal translating means such as a television receiver 21 is connected. Corresponding

end portions

22 and 23 of elements 11 and 12 are collinearly disposed in closely spaced mutual juxtaposition.

The length of each of the longer sides, which are comprised of

segments

13, 16 and

segments

15, 18, respectively, of the loop is preferably twice that of each of the shorter sides which correspond to

segments

14 and 17 and in any event is related to that of each of the shorter sides in a ratio between one and one-half to one and two and one-half to one. The overall physical length of the loop is equal to one-half wavelength at a predetermined frequency in the lowest of theVHF bands.

In a practical antenna for use throughout the entire commercial television frequency spectrum, the loop is con-. structed to have an overall physical and electrical length equal to one-half wavelength at a frequency of 67 megacycles, which has been found to provide the best compromise for optimum reception throughout the low VHF, high VHF and UHF bands of the television spectrum. It has been experimentally determined that optimum reception is achieved with an antenna structure which is resonant at the geometric mean frequency of each of the three bands of frequencies of television transmission.

The antenna structure of the present invention has an effective length which makes it resonant at a frequency near the geometric mean frequency of the lowest VHF television band; higher harmonics of this fundamental frequency fall near the geometric mean frequencies of the other television bands, as illustrated in the following table:

Geomet- Resonant Har- Freq. in ric mean freq. of monic me. freq. antenna Fundamental Low VHF Band 54-88 69 67 High VHF Band 174-216 194 201 .Brd UHF Band 470-890 646 603 9th While 69 megacycles is the geometric mean frequency of the lowest VHF band, the receiving efliciency of an antenna tuned to that frequency begins to drop off near 58 megacycles and therefore a compromise frequency of 67 megacycles is preferably utilized to improve the reception of the lower frequencies. In general a variation of plus or minus 10% in resonant frequency about a frequency of 69 megacycles may be tolerated, without critical defection from the optimum receiving conditions.

In the succeeding explanation, it will be shown that the antenna embodied in Figure 1 is effective in receiving television signals when it is mounted in either a vertical or a horizontal plane. For convenience, the operation of the antenna when mounted in a vertical plane will be explained first, followed by a description of its operation in a horizontal plane. To avoid encumbering the operational explanation of this embodiment only the horizontal radiation pattern is discussed; the vertical radiation pattern is of insufiicient interest to be considered since the television transmitters are normally situated in substantially a horizontal plane about the television receiver. Since television signals are propagated as horizontally polarized waves in accordance with present broadcasting standards, the operational explanation contemplates only this type of propagation.

The operation of the antenna of Figure 1 may be most easily understood if the antenna is considered to be a transmitting element rather than a receiving element. It is well known that the receiving pattern of a receiving antenna is identical to that of its radiation pattern when used as a transmitting element, in accordance with the reciprocity theorem. Hence, in Figure 1 the arcuate lines 60 represent the amplitude of the current in a currentfed transmitting antenna identical to the receiving antenna under consideration. In addition, the arrows 61 associated with the current-indicating curves represent the direction of the current flow in the radiating elements of the loop. The illustrated current distribution curves repreesnt the condition in which a signal having a frequency of 201 mes, or the 3rd harmonic of 67 mcs., is being transmitted.

When the antenna is mounted in a vertical plane, and is current-fed as illustrated in the embodiment of Figure 1, the horizontal transmitting radiation pattern may be determined by considering the loop to consist of two dipoles formed by the longer sides of the loop separated in space by a predetermined distance. Under these conditions, since the

shorter sides

14 and 17 are vertically oriented, they are incapable of radiating horizontally polarized signals and the horizontal radiation pattern is unaffected by any radiation from these sides. The longer sides of the loop constitute the only active elements in this array. The maximum energy radiated in a horizontal plane is in a direction along a line perpendicular to and intersecting the center of the plane of the antenna. The energy received at a point along this line is equal to the sum of the energy radiated from each of the longer sides since the electromagnetic fields emanating from these radiators arrive in common phase. At other points equidistant from the center of the loop, the energy received varies as a function of the angle of displacement from the plane of the antenna. The configuration of the radiation pattern of the antenna of Figure 1, when mounted in a vertical plane, is shown in Figure 5a for an operating frequency of 201 mcs. as determined experimentally and is of a figure-8 configuration.

As the frequency of the transmitter is increased, the antenna becomes longer in effective electrical length and additional current loops appear along its length. By again analyzing the behavior of the transmitting antenna while taking into account the additional current loops, it may be demonstrated that minor lobes appear and increase in size with increasing frequency. If it is desired to determine theoretically the radiation pattern at a particular frequency, this may be simply done by assuming each current loop to be a separate source of radiation and calculating the contribution of each of these sources to the final amount of received energy at any point in a horizontal plane surrounding the antenna. An inspection of the experimentally determined radiation pattern shows that as the radiating frequency increases, the horizontal radiation pattern becomes more omni-directional due to the increased number and size of the minor lobes.

The operation of the antenna of Figure 1 when mounted in the horizontal plane may also be most conveniently discussed if this embodiment is considered as a transmitting antenna rather than a receiving antenna and an operating frequency of 201 mcs. is assumed. In this case, the current distribution pattern along the length of the antenna is identical to that previously discussed in relation to the operation of the antenna when mounted in a vertical position. However, since all the radiating elements are situated in a common horizontal plane, all act as efiective radiators of horizontally polarized signals and the energy received at any point may be determined by linear superposition of the respective contributions of each of the radiating sides. As previously explained, the longer sides of the loop have an effective electrical length of one-half wavelength each, while each of the shorter sides has an eflfective wavelength of one-quarter wavelength.

At a distant point in the plane of the antenna along an imarginary line perpendicularly bisecting the longer sides, no energy is received from

shorter sides

14 and 17 as no electromagnetic field radiates in the direction of a linear conductor employed as a radiating element. The energy radiated from the longer sides is additive, however, with a phase delay of electrical degrees corresponding to the spacial displacement of the two longer sides. The energy contributions may be vectorially added and if it is assumed that each longer side has an energy contribution of one unit, the resultant is 1.414 units of total energy.

For a distant point in the plane of the antenna along an imaginary line perpendicularly bisecting the

shorter sides

14 and 17, the received energy may be easily calculated. Since the currents flow in opposite direc tions in the shorter sides, their radiated fields are out of phase. However, there is a spacial displacement equivalent to 180 electrical degrees between the two

shorter sides

14 and 17 which results in a corresponding delay in propagation of one field with respect to the other, and the combined energy contribution at the receiving point is double that from one side. No energy is received from either of the longer sides of the loop since there is no radiation in the direction of a linear conductor employed as a radiating element. By similar analysis the complete radiation pattern may be calculated, and the resulting pattern is that shown in Figure 5b.

The effect of increasing the operating frequency on the radiation pattern of the horizontally mounted antenna may be analyzed in a manner similar to that previously discussed with respect to the antenna when mounted in a vertical position. The effective electrical length of the antenna becomes longer, and additional current loops appear along the length of the antenna. Considering again these current loops as radiators and taking into account the reversals in the direction of current flow in the adjacent loops, it may be shown that the antenna radiation patterns depicted in Figure 5b for the higher frequencies are realized.

The antenna structure of Figure 1 is effective in receiving television signals not only when mounted in either a vertical or a horizontal plane, but also when mounted in any intermediate plane. The efiective receiving pattern of the antenna in any desired mounting plane may be qualitatively evaluated by a comparison of the radiation patterns of the antenna in both the vertical and horizontal plane at any particular frequency. More rigorous mathematical analysis can be used elfec= tivelyito determine the receiving pattern for the antenna in any position. Y

The analysis of the operation of the antenna of Figureal at a frequency of 201 megacycles evidences the importance of the relationship between the lengths of the longer and shorter sides of this structure. If the longer sides of the antenna are shortened with a consequent lengthening of the shorter sides, the antenna radiates less energy when mounted in a vertical plane since the amount of current flowing in the only elfective radiating elements is reduced. On the other hand, if the longer sides are increased in length with a concurrent decrease in the length of the shorter sides, there is reversed current flow in the longer sides with-a consequent decrease in the electromagnetic energy radiated by the antenna. The optimum radiation pattern is achieved if the longer sides bear a 2:1 ratio in length with respect to the shorter sides; however, this ratio is not extremely critical and eflicient operation occurs when the ratio of the length of the sides falls between 1 /2:1 and 2%:1. It may be similarly demonstrated that this same 2:1 ratio gives optimum reception for the antenna structure of Figure 1 when mounted in a horizontal plane.

The antenna of Figure 2 is smaller than the embodiment of Figure 1 and thus may be confined in a smaller space such as is more commonly encountered in conventional. television receivers. In Figure 2, the quadrilateral loop comprises two U-shaped radiating elements '11 and 12. Element 11 has

segments

24 and 25 connected electrically by a segment 26. Similarly, radiating element 12 has

segments

27 and 28 in substantially parallel coplanar arrangement connected by segment 29.

Terminals

30 and 31 of elements 11 and 12 respectively constitute electrical signal-translating means to which leads 32 may be connected to apply the received signal to the terminals of a conventional television set (not-shown). By overlapping the ends of

segments

25 and 28 there is an effective capacity loading applied to the loop. The amount of overlap and the spacing between overlapping

segments

25 and 28 are adjusted to provide sufiicient capacity loading that the effective electrical length of the loop is equal to one-half wavelength at -a predetermined frequency in the lowest television frequency band.

The operation of the embodiment of Figure 2 is similar to that of the embodiment of Figure 1, and as may be seen from Figure 5c, this embodiment has an almost identical horizontal radiation pattern plane as the embodimentof Figure 1. Any slight differences which may occur between the radiation patterns of the embodiments of Figures 1 and 2 may be due to the amount of capacity loading determined by the spacing and length of overlap of

segments

25 and 28 and the actual reduction in size. 'While the antenna of Figure 2 may be used advantageously in many television receivers, the input impedance of such astructure is approximately 72 ohms and the majority of conventional television receivers have been standardized to have an input impedance of 288 ohms. To avoid the necessity of using an impedance matching network the antenna illustrated in Figure 3 may be employed. In this embodiment, the loop comprises U-shaped radiating elements 1112 which in this case are constructed from ordinary twin-lead transmission line. The

shorter segments

40 and 41 of the loop correspond to the shorter sides of the antennas in the previous embodiments. The longer sides 42 and 43 similarly correspond to the longer sides of the previous embodiments. A desired amount of capacity end loading is introduced by overlapping the leads comprising longer side 43. One conductor of side 42 is discontinuous at its midpoint to provide

terminals

44 and 45 to which a conventional lead may be connected to translate the received signal to the receiver. This antenna has a current distribution similar to that of the embodiment of Figure 2, although in this case each of the individual Length of

segments

13, 14, 15, 16, 17

conductors of U-shaped elements 11 and 12.-carries one-half the total current flowing in eachside. 1 From one viewpoint, the separate conductors of each of the elements 11 and 12 may be considered to be closely spaced parallel conductors which together carry an amount of current equal to that in the conductive elements of the embodiment of Figure 2 but which separately carry one-half the total current. Because of this current distribution for the same total power received, the input impedance of the embodiment of Figure 3 is four times the impedance of the embodiment of Figure 2. Specifically, the impedance of the embodiment of Figure 2 is 72 ohms while that of Figure 3 is 288 ohms. Since conventional television receivers have an input impedance of 288 ohms, the antenna of Figure 3 is preferable for commercial usage.

A practical embodiment of the antenna of Figure 3 is illustrated in Figure 4 which shows a conventional television receiver 50 mounted upon a suitable base 51. Base 51 may constitute a separate cabinet upon which a table model television receiver 50 may be positioned or may constitute the lower portion of a conventional console model television receiver. An antenna 52 of the type shown and described in connection with Figure 3 is positioned in a horizontal plane and may be attached to a suitable supporting structure 53. Supporting structure 53 may be a wooden platform glued into place in the lower section of cabinet 51. Alternatively, antenna 52 may be attached to any inner supporting structure connecting the lower portions of the uprights of cabinet 51. Suitable leads 54 are connected to the

terminals

44 and 45 of the antenna and to the input cincuit of television receiver 50. It has been found expedient to use conventional staples to hold the antenna in the proper horizontal position and proper configuration within cabinet 51.

Merely by way of illustration, the following table sets forth illustrative physical dimensions for the embodiments of Figures l-3; these dimensions are in no way to be construed as a limitation of the invention:

Figure 1 and 18 14.5 inches each. Overall length of the longer sides of the loop 29 inches each.

Figure 2 Overall length of the longer sides of the loop Length of segment 26 of the loop Length of segment 29 of the loop Length of

segments

25 and 28 of the loop 21 inches each. 15.25 inches. 15.75 inches.

11.5 inches each.

Figure 3 Overall length of the longer sides of 21 inches each. I 15.25 inches. 15.75 inches.

This invention provides a television antenna operable throughout a plurality of separate bands of frequencies spaced from one another in the UHF and VHF frequency spectrum and positionable in either a vertical or horizontal plane for the eifective reception of horizontally polarized television signals. Its compactness and simplicity of manufacture greatly exceed that of comparable antennas of the built-in type. Its omnidirectional characteristic make it exceedingly desirable for use in conventional television receivers.

While particular embodiments of the invention have been shown and described, modifications may be made and it is intended in the appended claims to cover all such geese/e modifications as may fall within the true spirit and scope of the invention.

I claim:

1. A television antenna operable throughout a plurality of separate bands of frequencies spaced from one another in the VHF and UHF spectrum comprising: a pair of substantially U-shaped conductive radiating elements each having leg and base portions of substantially equal length and with corresponding end portions of said radiating elements in closely spaced mutual juxtaposition to constitute a quadrilateral loop having an effective electrical length equal to one-half the wavelength at a predetermined frequency in the lowest one of said frequency bands, the effective electrical length of each of the longer sides of said loop being substantially twice that of each of the shorter sides; and a lead-in conductor coupled to a pair of said corresponding end portions for connecting electrical signal-translating means thereto.

2. A television antenna operable throughout a plurality of separate bands of frequencies spaced from one another in the VHF and UHF spectrum comprising: a pair of substantially U-shaped conductive radiating elements each having leg and base portions of substantially equal length and disposed in a horizontal plane with corresponding end portions of said radiating elements in closely spaced mutual juxtaposition to constitute a quadrilateral loop having an effective electrical length equal to one-half the wavelength at a predetermined frequency in the lowest one of said frequency bands, the efiective electrical length of each of the longer sides of said loop being substantially twice that of each of the shorter sides; and a lead-in conductor coupled to a pair of said corresponding end portions for connecting electrical signal-translating means thereto.

3. A television antenna operable throughout a plurality of separate bands of frequencies spaced from one another in the VHF and UHF spectrum comprising: a pair of substantially U-shaped conductive radiating elements each having leg and base portions of substantially equal length and with corresponding end portions of said radiating elements in closely spaced abutting mutual juxtaposition to constitute a quadrilateral loop having an effective electrical length equal to one-half the wavelength at a predetermined frequency in the lowest one of said frequency bands, the effective electrical length of each of the longer sides of said loop being substantially twice that of each of the shorter sides; and a lead-in conductor coupled to a pair of said corresponding end portions for connecting electrical signal-translating means thereto.

4. A television antenna operable throughout a plurality of separate bands of frequencies spaced from one another in the VFW and UHF spectrum comprising: a pair of substantially U-shaped conductive radiating elements each having leg and base portions of substantially equal length and with one pair of corresponding end portions in closely spaced abutting mutual juxtaposition and another pair of corresponding end portions of said radiating elements in closely spaced overlapping mutual juxtaposition to constitute a quadrilateral loop having an efliective electrical length equal to one-half the wavelength at a predetermined frequency in the lowest one of said frequency bands, the efiective electrical length of each of the longer sides of said loop being substantially twice that of each of the shorter sides; and a lead-in conductor coupled to said first-mentioned pair of corresponding end portions for connecting electrical signal-translating means thereto.

5. A television antenna operable throughout a plurality of separate bands of frequencies spaced from one another in the VHF and UHF spectrum comprising: a pair of substantially U-shaped conductive radiating elements each having leg and base portions of substantially equal length and disposed in a horizontal plane with corresponding end portions of said radiating elements in closely spaced mutual juxtaposition to constitute a rectangular loop having an eifective electrical length equal to one-half the wavelength at a predetermined frequency in the lowest one of said frequency bands, the effective electrical length of each of the longer sides of said loop being substantially twice that of each of the shorter sides; and a lead-in conductor coupled to a pair of said corresponding end portions for connecting electrical signal-translating means thereto.

6. A television antenna operable throughout a plurality of separate bands of frequencies spaced from one another in the VHF and UHF spectrum comprising: a pair of substantially U-shaped conductive radiating elements each having leg and base portions of substantially equal length and each comprising a pair of closely spaced conductors directly interconnected at both extremities and having a discontinuity in one of said conductors intermediate its extremities to provide terminals; said pair of elements having corresponding end portions in closely spaced mutual juxtaposition to constitute a quadrilateral loop having an effective electrical length equal to one-half the wavelength at a predetermined frequency in the lowest one of said frequency bands, the effective electrical length of each of the longer sides of said loop being substantially twice that of each of the shorter sides; and a lead-in conductor coupled to said terminals for connecting electrical signaltranslating means thereto.

I Television Antennas, by Donald A. Nelson (2nd Edition), March 1951 (page 76). (Copy in Div. 44.)

I. L. Reinartz: Half-Wave Loop Antennas, October 1937, Q.S.T., pp. 274.9.