US2611871A - Antenna detuning system - Google Patents

Description

Sept. 23, 1952 A. ALFORD ET AL 2,611,871

ANTENNA DETUNING SYSTEM Filed Aug. 28, 1947 4 Sheets-Sheet l INVENTON. Ana raw J g BY K ep 1952 A. ALFORD ET AL 2,611,871

ANTENNA DETUNING SYSTEM IN V EN TOR.

Am/rw l/ni BY Henry Jim Sept. 23, 1952 A. ALFORD ET AL 2,611,871

ANTENNA DETUNING SYSTEM Filed Aug. 28, 1947 4 Sheets-Sheet 3 x 5o ohms x +187 06m; I x= 17/84 ohms FIG. 31

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IN VEN TOR.

And/ ew My "7 $431K BY i /4M7.

Sept. 23, 1952 A. ALFORD ET AL ANTENNA DETUNING SYSTEM Filed Aug. 28, 1947 4 Sheets-Sheet 4 INVEN TOR. Andrew 4226M y Henry s/Qsn( Patented Sept. 23, 1952 ANTENNA DETUNI'NG SYSTEM Andrew Alford, Cambridge, and Henry Jasik, Roslindale, Mass.; said Jasik assignor to said Alford Application August 28, 1947, Serial No. 771,084

11 Claims.

Our invention relates to antenna systems and more particularly to systems comprising a radiating element energized by a source of radio frequency power and another radiating element that is parasitically excited by the first element.

Radiating antenna towers for broadcast stations are preferably installed on ground of good conductivity and usually in the vicinity of a large city. Economic and other factors combine to reduce the areas suitable for broadcast antenna tower installations so that in many cases there are only one or two relatively small areas available for a number of broadcast stations. Under such conditions the radiating towers of one broadcast station have to be erected in the vicinity of towers of another broadcast station. Since many broadcast antennas must be designed to produce directional radiation patterns with deep nulls in the direction of other co-channels or adjacent channel stations in other cities, great care is taken to adjust such antenna systems so that the areas served by other stations are free from interference. In the presence of neighboring towers, induced currents in neighboring towers may result in strong reradiation in the direction of the co-channel or adjacent channel stations in other cities causing interference and thus defeating the tedious and expensive task of design and adjustment of the primary radiating system.

In accordance with our invention, the distribution of currents induced in neighboring towers may be controlled so that the reradiaticn from these towers is substantially reduced. This control of induced current distribution is accomplished by providing at the base of a tower in which current is induced a net work having an inductive impedance at the frequency of the induced current.

The provision for controlling the distribution of the induced currents in neighboring towers is one object of our invention.

Another object of our invention is to reduce the amplitude of the current induced in neighboring radiators.

A still further object of our invention is the control of the phase distribution of the induced currents.

Other objects and advantages of our invention will be apparent from the particular description thereof made in connection with the accompanying drawings illustrating a few of the embodiments of the invention in which;

Fig. 1 shows one embodiment of our invention;

Fig. 2 shows the efiect of varying base reactanceon the distribution of induced currents ina tower 0.49 wavelength high at the frequency, of the induced current;

Fig. 3 shows the effect ofvarying base reactance on the distribution-of induced currents ina tower 0.375 wavelength high as the frequency-of the induced current;

Fig. 4 shows the effect. of varying base reactance in the distribution of induced currents in a tower 0.705 wavelength high at the frequency of the induced current;

Fig. 5 shows another embodiment of our inven-' tion;

Fig. 6 illustrates one form'of network which may be used in the arrangement of Fig. 5;

Fig. '7 shows still another embodiment of our invention.

In Fig. 1 is shown one embodiment of our invention. In this figure numeral I refers to a source of RF power energizing a metallic tower 2, which is supported above ground 3 by insulators 4. Electromagnetic waves radiated by tower 2 induce current of amplitude I in a neighboring tower I0 supported by the insulators I4. The amplitude and the phase of the induced current at different levels along tower I0 depend on distance R between towers 2 and Ill and on the height H of tower III in terms of the free space wavelength radiated by tower 2. When height H is in the neighborhood of a half wavelength,'the current in tower III has a large amplitude and is distributed as in diagrammatically shown in Fig. 1 by dashed line I5. In this illustration the length of horizontal line I6, I1, is made proportional to the amplitude of the total vertical current flowing in tower III at level I9, which corresponds to level of line I6, IT. The current flowing half way up tower I0 is proportional to length of line 2|, 22. As dashed line I5 indicates, the maximum amplitude of induced current in this case occurs just below the center of the tower.

In accordance with our invention, the distribution of the induced current in tower III may be modified by connecting reactor 24 between ground 3 and bottom 23 of the tower Ill. Typical effect of varying the value of reactance 24 is shown by Fig. 2 in which curves 30 to 4| indicate current distributions corresponding to different values of the reactance 24. These curves have the same meaning as curve l5 in Fig. 1.

Each of the twelve curves shown in Fig. 2 corresponds to a different value of reactance. The approximate value of reactance 24 in ohms for each case is indicated by the value of X written next to each curve. Thus, for example, the cur'-' rent distribution in accordance with curve 33 was obtained with network 24 having a reactance zv=j87 ohms. Similarly, the current distribution shown by curve 36 was obtained with network 24 having a very large value of reactance, X=+i1400. This particular value of reactance produces the same result as if network 24 were not connected to the base of the tower. Curve 36 in Fig. 2 is, therefore, identical with curve l in Fig. 1.

Inspection of Fig. 2 shows that there are two types of current distributions, type I and type II. Type I distribution has no minima between the lower and upper ends of the tower. Type II distribution has a distinct minimum somewhere along the tower. When a current distribution of type I is present, the current at all points along the tower flows in the same direction at a given instant and the radiation produced by the current flowing in the upper portion of the tower reinforces the radiation produced by the current flowing in the lower portion of the tower. On the contrary, in a distribution of type II the current flowing in the upper portion of the tower flows in a direction opposite to that of the current in the bottom portion of the tower. Therefore, radiation from the top portion of the tower is at least partially cancelled by the radiation from the bottom portion of the tower.

While the amplitudes of cuirents in distributions of type II are comparable in magnitude with the amplitudes of currents in some distributions of type I, the fields reradiated by certain distributions of type II are substantially lower than those reradiated by distributions of type I. This substantial difference in reradiated fields is due to the cancellation of the field of the upper portion of the tower by the opposing field reradiated by the bottom portion of the tower. The type II current distributions which result in the minimum of reradiation are those similar to the distribution shown by cLu-ve 33. Such distributions have a minimum of current substantially onethird of the distance up the tower. An important feature of such distributions is that the area enclosed by the portion of the induced current distribution curve above the current minimum is substantially equal to the area enclosed by the portion of the current distribution curve below the minimum. For example, area 42 enclosed by line 43 and the portion of curve 33 above current minimum 44 is substantially equal to area 45 enclosed by lines d3, 46 and the portion of curve 33 below current minimum 44. In this diagram line 43 has the same meaning as line [8 in Fig. 1, and line 46 corresponds to the bottom end of the tower. ,The equality of areas en closed by the portions of the current distributions curve above and below the current minimum is important because these areas are proportional respectively to the fields reradiated by the upper and lower portions of the tower. When there is a reversal of the phase of the induced current at the current minimum, the equality of said areas results in the upper portion of the tower reradiating a field equal in amplitude and opposite in phase to the field reradiated by the bottom portion of the tower. These two reradiated fields therefore cancel in the directions of the horizon and partially cancel in directions above the horizon.

It will be observed that the distribution of induced current shown b curves 32, 83 are sub stantially different from those encountered in towers energized by a source of radio frequencypower at the bottom end such as, for example, tower 2 in Fig. 1. In fact, the current flowing in an energized tower of uniform cross section is substantially sinusoidally distributed and the distance between the top of the tower and the first minimum is equal to a free space half wavelength, except for an end correction. The current distribution shown by 'curve 33 of type II in Fig. 2, has a minimum which lies only of a half Wavelength below the top of the tower. This unusual distribution is due to the fact that the induced currents are, in effect, caused by the action of a large number of generators uniform- 1y distributed along the tower, rather than by one single generator at the bottom of a fed tower.

In Fig. 3 curves 4'1, 8, to show induced current distributions in a tower 0.375 wavelength high at the frequency of the induced current. The inductive reactance which results in a preferable current distribution of type II with the minimum of current one-third way up the tower has a value of the same order of magnitude as the reactance which results in the preferable current distribution in Fig. 2. In a tower 0.375 Wavelength high the type II distribution having a current minimum onethird way up the tower results in a lower amplitude of current along the tower than the current amplitudes obtained with distributions of type I along the same tower.

In Fig. 4 curves 50 to 53' show the induced current distributions measured in a tower 0.705 wavelength high. In this case too, distributions of type I and type II are observed, but the induced currents in distributions of type II are of the same order of'magnitude as the currents observed in distributions of type I. Furthermore, when a distribution of type II is achieved, there are two current maxima along the tower and these maxima are separated by a distance nearly equal to a free space half wavelength. Even though type II distribution in this case results in somewhat lower value of reradiated field its advantage over current distributions of type I is relatively small, particularly when the ratio of tower height to the average tower diameter is less than 40.

The control of reradiation by addition of a reactive network coupled between the lower end of a tower and ground is effective when the tower height is not greater than seven-tenths of the wavelength and preferably less than .55 of the wavelength at the frequency of the induced current.

The best value of reactance depends to some degree on the height of the tower. For towers 0.49 wavelength high the best value of reactance was found to be in the neigb-horbhood of i8? ohms. For towers 0.375 wavelength high the preferable value of reactance was found to be approximately 7'65 ohms.

The preferable value of reactance for a tower of a given height is that which results in a fourrent distribution of type II with the portion of the current distribution curve above the current minimum enclosing an area substantially equal to the area enclosed by the portion of the dis-- tribution curve below the current minimum.

Our tests show that there is a convenient method for adjusting the value of reactance for a low value of reradiatlon. This method comprises coupling of a current indicating device substantially one-third of the way up the tower and adjusting the value of reactance connected .between ground and the bottom end of the tower until the current indicator reads maximum current. This method is preferably applied to towers lower than .55 wavelength at the frequency-of the induced current. v 4

When distance i R; between the inducing antenna such as; for example, antenna 2 in Fig. 1, and the'antenna-in which current is induced such-as tower H) in Fig. 1, is .greater than one wavelength, the preferable value of base reactance may also-be determined by making-measurements of the-field strength along the line joining the two towers. Along thisline and in the vicinity of tower Hi there is a standing wave pattern resulting from an interference of thewaves traveling in opposite directions. The field strength along the line joining the tower exhibits maxima and minima. These maxima and minima are separated by distancsapproximately equalfto a free space half wavelength. The maxima are obtained at points where the reradiated field adds in phase to the direct field; the minima are obtained at points where the reradiated field subtracts from the direct field. One-half of the difference between adjacent maxima and minima is approximately proportional to the value of the reradiated field. This difference between maxima and minima is relatively small in the neighborhood of the primary radiator, such as 2 in Fig. l, but it is relatively large in the neighborhood of the radiator in which currents are induced, such as ID in Fig. 1.

When measurements are made of the field reradiated by a tower one-quarter wavelength high with its base connected directly to ground, it is found that the reradiated field is substantially equal to that given by the well-known formula for the reradiation field of a very thin grounded one-quarter Wave antenna. When similar measurements are made with a grounded tower which is one-half wavelength high it is found that the reradiation field is equal to approximately one-half of the field reradiated by the grounded quarter wave antenna. A tower having a substantial diameter cannot be completely detuned and thereby made ineffective as a reradiator by using a simple short circuit between its base and ground. The best value of base impedance is not zero, but has a finite value as has been explained in connection with Fig. 2.

Fig. 5 shows another embodiment of our invention. In this figure 54 is a source of radio frequency power energizing tower radiator 55 of broadcast'station A at frequency F1. Radiator 55 induces currents in tower 56 of another broadcast station B. Tower radiator 56 is also energized by a source of radio frequency power 51 at frequency F2 through transmission line 56 and network 59. Network 59 comprises a plurality of reactive elements and is designed to perform the following functions:

(a) Totransmit currents of frequency F2 from source of radio frequency power 51 to tower radiator 56.

(b) To greatly attenuate induced currents of frequency F1 so that they are not transmitted from tower 56 into radio frequency source 51, and

(c) To present to induced currents of frequency F1 3, reactance capable of being adjusted to that value which results in a type II distribution of induced currents with a current minimum substantially one-third of the way up tower 56.

These requirements do not uniquely determine the electrical design of network 59. On the contrary, a number of different networks can fulfill the above stated requirements. Fig. 6 shows a network which can be used at broadcast frequencies between 0.5 megacycle and 1.5 megacycle. In Fig. 6, 60 isthe sourceofhigh frequency power of frequency F2, used-to energize tower radiator 6| through transmission line 62 matching device 63 and network 64. Network 660cmprises adjustable inductors 65, 66; adjustable capacitors 61, 68 and a circuit element 69s Capacitor 68 is so adjustedthat the absolute' value of its reactance is equal to the reactance of'inductor 66 at frequency F1 of the induced current. This adjustment provides for a low impedance path between terminal 10 and ground Tl whereby induced currents of frequency F1 are prevented from reaching radio frequency power source 66. When frequency F2 is greater than frequency F1 circuit element 69 is a capacitor. The reactance of this capacitor is adjusted so that the circuit consisting of the elements 68, 66, and 66 is antiresonant providing for high impedance at frequency F2 between junction 16 and ground H. When frequency F2 is less than frequency F1 circult element 66 is an inductor having a reactance necessary to make the circuit consisting of ele ments 68, 66, 69 anti-resonant at frequency F2, and providing for a high impedance between junction 10 and ground H.

The value of reactance 65 is adjusted for the optimum distribution of the induced current along tower radiator 6| in accordance with the principles already explained in this specification. Capacitor 67 may be adjusted to have a reactance equal and opposite to reactance 65 at frequency F2. With such adjustment of capacitor 61 the impedance at frequency F2 between terminal 12 and ground H is substantially equal to the impedance between the bottom of tower radiator 6| and ground H with network 66 removed. This impedance, when desired, may be matched to the characteristic impedance of transmission line 62 by a matching device 63. This device need not be described here because devices of this kind are well known in the art. I

Still another embodiment of our invention'is shown in Fig. '7 in which 60 isa source of high frequency power of frequency F1 energizing radiator 8|, which induces current in, another substantially parallel radiator 12. Radiator 12 is a balanced dipole provided with a balanced transmission line 73. In the vicinity of radiator 12 is antenna M. The proper functioning of antenna 1 M is disturbed by waves reradiated from dipole T2. In accordance with our invention the dis turbing effect of dipole 12 may be reduced by providing a reactive impedance between terminals.

15, Hand by adjusting the value of this impedance'for a type II current'distributio'n with current minima at points 82, 83 each located substantially one-thirdof the distance from the'center of the dipole and its outer ends. 7

A convenient arrangement for providing the desired value of reactance is shown in Fig. 7. In this figure line 13 is provided with an effective. short circuit at 18. This short circuit is achieved by using a short .circuited half wave section 11.

The value of reactance may be conveniently varied by adjusting the distance between junction 1-8 and terminals 15, 16. The half wave short circuited section 11 allows currents of frequencies other than F1 and its harmonics to flow between terminals 15, 16 and translating device 84.

The effectiveness of decreasing reradiation from a dipole in accordance with our invention is reduced when the overall length of the dipole is greater than 1.4 wavelengths long at the frequency of the induced current. With fat dipoles having a large average diameter to overall length 7 ratio of the order of 1/20, the overall dipole length should preferably be less than 1.1 wavelengths long at the frequency of the induced current.

While we have described certain embodiments of our invention for the purposes of illustration, it will be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the ap pended claims.

We claim:

1. An electromagnetic wave radiating system having a primary radiator with a nearby ungrounded conductive structure in which currents are induced from the primary radiator, said grounded conductive structure having a height not substantially greater than seven-tenths of the wavelength corresponding to the frequency of said electromagnetic wave in combination with means for causing substantially equal and opposite phase reradiation of said electromagnetic wave in difierent positions of said conductive structure comprising an unenergized impedance connected between ground and the lower end of said conductive structure whereby reradiation of said electromagnetic wave from said conductive structure by virtue of said currents induced therein is substantially, reduced.

2. A combination in accordance with claim 1 wherein said conductive structure forming a second radiator is less than .55 wavelength high.

3. A combination in accordance with claim 1 wherein said conductive structure forming a second radiator is less than .55 wavelength high and the reactance connected between the lower end of the second radiator and ground is inductive.

4. A combination in accordance with claim 3 wherein the inductive reactance between bottom of the second radiator and ground is between 50 ohms and 119 ohms.

5. An electromagnetic wave radiating system having a primary radiator with a nearby ungrounded conductive structure having a height not substantially greater than seven-tenths of the wave length of said electromagnetic wave in combination with means for reducing the reradiation of said electromagnetic wave from said conductive structure, comprising an unenergized impedance connected between ground and the lower end of the electrical conductive structure, said impedance having a reactance of a value X:A at a frequency corresponding to the wavelength of said electromagnetic wave where A is chosen such that an induced current minimum exists at a point substantially one-third the distance of the effective conductive height of the structure from the lower end thereof.

6. In combination, a first vertical radiator energized at frequency F1 a second ungrounded vertical radiator, having a height not substantially greater than seven-tenths of the wave length corresponding to said frequency F1, energized at a frequency F2, a network comprising reactive circuit elements coupled between ground and the bottom of the second radiator said network presenting an inductive impedance to the currents of frequency F1 induced in said second radiator such that a minimum of said induced current exists at a point along said second radiator said point being spaced from the bottom of said second radiator to efiect a substantial balancing out of the re-radiation induced therein from the first vertical radiator.

7. A combination in accordance with claim 6 wherein the network comprising reactive circuit elements freely transmits currents of frequency F2.

8. A combination in accordance with claim 6 wherein the network comprising reactive circuit elements freely transmits currents of frequency F2 and substantially bypasses to ground induced currents of frequency F1.

9. A combination in accordance with claim 6 wherein the second radiator is less than .55 free space wavelengths high at the frequency of the induced current.

10. In combination a first vertical radiator nergized by a first source of high frequency power at frequency F1, a second vertical radiator less than 0.55 wavelength high at frequency F1 and energized by a second source of high frequency power at frequency F2, a network comprising reactive elements coupled between ground and the bottom of the second radiator, said network presenting an inductive impedance to the currents of frequency F1 induced in said second radiator and freely transmitting currents of frequency F2 from the second source of high frequency power to said second radiator said inductive impedance being chosen such that an induced current minimum exists at a point along said second vertical radiator said point being spaced from the bottom of said second radiator to effect a substantial balancing out of the re radiation induced therein from the first vertical radiator.

11. A combination in accordance with claim 10 wherein the network. comprising reactive circuit elements substantially by-passes induced currents of frequency F1.

ANDREW ALFORD. HENRY JASlK.

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

UNITED STATES PATENTS Number Name Date 1,483,860 Von Bronk Feb. 12, 1924 2,096,782 Brown Oct. 26, 1937 2,153,768 Morrison Apr. 11, 1939 2,198,604 Everitt .d Apr. 30, 1940 FOREIGN PATENTS Number Country Date 338,982 Great Britain Dec. 1, 1930

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