US2147808A - Antenna - Google Patents

Description

ANTENNA Original Filed March 17, 1936 2 Sheets-Sheet l FIGJ.

INVENTOR ANDREW ALFORO ATTO RN EY Feb. 21, 1939. ALFORD 2,147,808

ANTENNA Original Filed March 1'7, 1936 2 $heets Sheet 2 INVENTOR AND/FEW ALI-0R0 ATTORNEY Patented Feb. 21, 1939 ANTENNA Andrew Alford, New York, N. Y., assignor to Mackay Radio and Telegraph Company, New York, N. Y., a corporation of Delaware Original application March 17, 1936, Serial No. 69,292. Divided and this application November 19, 1936, Serial No. 111,642

4 Claims. (Cl. 250-33) This invention, which is a division of my copending application Serial No. 69,292, filed March 17, 1936, relates to antenna structures for radio communication and pertains more particularly to bi-directional antennas adapted for short wave communication.

An object of the present invention is a bi-directional antenna which has a nearly constant input impedance.

Another object of the present invention is a bi-directional antenna which may be conveniently installed on a ship, and which will require only relatively simple arrangements for eiiiciently transferring the power from the antenna to the transmission line, and from there to the receiver when this antenna is used for receiving, and from the transmitter to the transmission line and from there to the antenna when the latter is used for transmitting.

In the past, two types of long wire antennas have been used; the terminated antennas and the open-ended antennas. The terminated antennas have always been unidirectional, while the open-ended antennas have always been bidirectional, unless provided with suitable reflectors.

The terminated antennas, when properly terminated, have input impedances which remain nearly constant as the frequency is varied and for this reason they are well suited for use at a number of different frequencies.

The open-ended antennas have input impedances which vary with frequency within rather wide limits and consequently they are not suited for use at a number of difierent frequencies except when used in conjunction with relatively complicated matching devices.

Thus, generally speaking, the terminated antennas are superior to the open-ended antennas in those instances in which the constancy of input impedance is one of the primary requirements.

Now, there are applications which require bidirectional rather than uni-directional antennas. For instance, aboard a ship, which sails back and forth approximately along the great circle between two ports, antennas should preferably be, in certain instances bi-directional rather than unidirectional, so that the ship may communicate with both ports.

Under these circumstances, in the past, the open-ended type antennas would have been used, since the terminated antennas of the prior art were uni-directional unless provided with objectionable switching arrangements and transmission lines extending from the transmitter or receiver, as the case may be, to both ends of the antenna. Since in practice it is often found that a transmission line from the translating device to the far end of the antenna cannot be installed, particularly on a ship, it is clear that in the past it would have been necessary to use lei-directional antennas having end impedances which vary with frequency over a considerable range, an undesirable characteristic where communication is to be carried out at several diiierent frequencies.

In accordance with present invention it is possible to construct a oi-directional antenna which has a nearly constant near-end impedance Within a considerable range of frequencies.

The above mentioned and further objects and advantages of my invention, and the manner of attaining them will be more fully explained in the following description taken in conjunction with the drawings.

In the drawings Fig. 1 shows a simple form of antenna constructed in accordance with my invention.

Fig. 2 shows a modified form of antenna embodying my invention.

Fig. 3 shows the antenna of Fig. 1 installed on a ship in the immediate vicinity of a stay and means for detuning the stay, and

Figs. 4 and 5 show other arrangements for detuning stays.

In Fig. 1, reference numerals ii and I2 indicate two inclined wires joined together at their far ends by jumper l3 and connected to a concentric tube transmission line 55 through an aperiodic coupling device i which matches the impedance of the antenna to the surge impedance of the transmission line i5. A resistance to the value of which is equal to the surge impedance of the antenna is connected between the outer end of line l2 and the ground. Seventeen (i?) is a translating device, that is, either a receiver or a transmitter, i8 is a tower, and IS, 22, 2| are insulators.

The operation of the antenna shown in Fig. 1 may be described as follows: electric currents generated in transmitter l'i proceed through transmission line F5 into matching device I4. Emerging from l i these currents proceed in the form of a travelling wave toward jumper I3. After passing through jumper i 3 these waves proceed along wire l2 toward the terminal resistor l6. When the value of resistor R3 is properly chosen, that is, when it is equal to the surge impedance of wire 12, the waves travelling along [2 are not reflected at I6.

For this reason there is no primary reflected wave on I2 which, travelling through I3, could cause standing waves along II. Consequently wire II acts in approximately the same manner as though it were terminated into its surge impedance at the point of junction with jumper In operation wire II induces currents in wire I2, and vice versa wire I2 induces currents in wire II, this results in the formation of a certain amount of standing waves on both wires. The amount of these standing waves, however, is much less than one might expect. Thus, for example, I find by actual experiment at frequencies of the order of 10 to 20 megacycles that when wires II and I2 are the order of 1 wavelengths long and are placed a foot or two from each other the amount of reflected wave on portion III of wire II adjacent to the coupling device I I is only about 10%. This result is somewhat surprising particularly in View of the fact that one might expect some reflected waves due to reflection at the junction points of IS with wires I I and I2, in addition to the reflected waves produced by the mutual inter-action of wires II and I2.

Because of the relative absence of reflected waves in portion I II of wire II, the input impedance of the antenna at It is nearly independent of the frequency and consequently matching device I4, capable of matching two fixed impedances, namely the impedance of the antenna and the surge impedance of I5 at various frequencies within a certain band, is all that is required for properly terminating transmission line I5. When line I5 is so terminated transmitter I I will work into nearly the same impedance at all frequencies within the band in which matching device If is operative.

When it is desired to carry on communication between two points separated by a distance greater-than about 400 miles and when a hori- Zontal reflecting surface such as ground or sea is located at a distance of a fraction of a wavelength below the lower end of the antenna it is found that the following table gives approximately the relation between the length of each of the wires II and I2 in wavelengths and the angle to which they should be inclined to the horizontal.

The values of 6 for Z/A not given in the table may be interpolated or extrapolated.

In a few cases when the lower end of the antenna is located at a considerable height above the reflecting surface it may be found that the .manner which will be obvious to one skilled in the of the tower. The various parts and devices which were already described in connection with Fig. 1 have been designated by the same members in Fig. 2 in order to save repetition. In Fig. 2 wires III and I I2 are continuations of wires II and I2. Jumper I3 in Fig. 2 connects the ends of III and H2 instead of the ends of I I and I2.

The operation of the antenna of Fig. 2 is quite similar to the operation of the antenna in Fig. 1.

The gain which is obtainable with the antenna of Fig. 2 is somewhat greater than that obtainable with the simpler structure of Fig. l. The interaction between wires II, I2, III, and H2 is also somewhat greater than the interaction between wires of the simpler structure of Fig. 1. For example, when wires II, I2, III, II2 are made each about two wavelengths long and the distance between II and I2, and III and H2 is 6 feet, I find that any variation in the antenna impedance at I4 is but a small percentage of the variation which would occur with an open-ended antenna.

The angle at which wires II and I2 as well as the angle which wires I I I and I I2 should make to the horizontal may be determined from the foregoing table. Wires I I and I2 need not be of the same length as wires I II and H2. In some cases, however, it may be necessary 'to install phase correcting means at the point of junction of wires II and III and wires I2 and II2. The operation of such phase correcting means has been described in my copending application, Serial No. 18,995, filed April 11, 1935, Patent No. 2,127,198, dated August 16, 1938.

Fig. 3 shows the antenna which was discussed in connection with Fig. 1 installed aboard a ship. In this figure numbers I0 to El refer to the same parts and apparatus which have already been described in connection with Fig. 1.

In practice, when an antenna is installed aboard a ship it is often found that stays, low frequency antennas and other wires or metal masts may interfere with the proper operation of the antenna. Thus, for example, stay 30 in Fig. 3 may sometimes interfere with the proper action of the main radiating wires I I and I2 of the antenna.

The mechanism of the interfering action is briefly as follows: the radiating portions of the antenna located in the vicinity of a stay produce an electric field which has a component along the stay. This component of the electric field proluces an electromotive force which sets up currents in the stay. The magnitude of the current so produced in the stay depends on two factors; namely, (1) the magnitude of electromotive force along the stay and (2) the self impedance of the stay. When high frequency currents flow through a long conductor some energy is always radiated into space. In this respect a stay does not differ from an antenna wire. Since the phase of the radiation from a stay may have any relation whatsoever to the phase of radiation from the inducing antenna, this parasitic radiation from the stay may either increase 'or decrease the field produced by the antenna in a given direction. The more sharply directional is the inducing antenna, the greater is the probability that a stay or any other haphazardly placed wires in the vicinity of the antenna will decrease, rather than increase, the total radiation in the desired direction.

In addition to being able te scatter energy by radiation 2. stay may dissipate a substantial portion'of the energy by turning it into heat. This occurs when the internal resistance of the stay is 4 fairly large in comparison with its radiation resistance.

When the internal resistance of a stay is very high so that the major portion of the energy which it picks from the antenna is dissipated as heat, the small radiated portion does not affeet the directional characteristic of the antenna as seriously as in the case when the internal losses in the stay are relatively small and when most of the picked up power is radiated. Thus, a high resistance stay tends to decrease the total radiated power without distorting the directional characteristic of the antenna while a low resistance stay tends to distort the directional characteristic without changing the total radiated power.

Since both the power radiated from, as well as the power dissipated in a stay, is proportional to the square of the current which is induced in it, it is clear that the effect of a stay on an antenna may be controlled by controlling the induced current. It has already been pointed out that the magnitude [of this current depends among other things on the self impedance of the stay. By making the self impedance of a stay as large as possible the induced current in it may be reduced to a minimum.

If a stay may be cut and insulators inserted at frequent intervals, the self impedance of each section may be made so high that the stay becomes entirely inactive either as a radiator or a dissipator. This procedure is well. known in prior art.

When, however, a stay can not be cut and broken up by insulators the problem of eliminating the effect of the stay on an antenna is much more difficult. In the first place, since the self impedance of a stay of a given length varies with frequency and at a given frequency may have any value lying between the radiation resistance, which may be of ohms, up to 10 or 15 times the radiation resistance, it is clear that, in general, a given stay may not interfere with an antenna at one frequency and still cause a considerable distortion of the directional characteristic at another frequency.

Let us, therefore, assume that at the particular frequency at which antenna of Fig. 3 is to be operated, stay 30 has a low self impedance so that it picks up and scatters a considerable amount of power which would normally be radiated by the antenna.

Under these circumstances the induced current in the stay will be distributed in the form of standing waves. This distribution of current is diagrammatically indicated in Fig. 3 by the dotted wavy line 2.

Since the position of current maxima and minima with respect to the ends of a stay depends on the relation of terminal impedances to the surge impedance of the stay, the distance of the first current maximum from one of the ends may have any value between minus wavelength and plus wavelength depending on the terminal impedance. When a stay is terminated by a good insulator the first current maximum occurs approximately at A; wavelength from the insulated end. When, as in Fig. 3, a stay is terminated by a very large metal object such as funnel 34, without insulation, the first current maximum occurs but a small fraction of a wavelength from the end of the stay. For this reason if we imagine that stay 30 were cut at a point J the impedance of the short por tion of the stay between J and funnel 34 as seen at J would gradually increase to a maximum as J is moved from 34 toward a point A; wave from- 34. This maximum impedance seen at J when looking into J-34 is usually of the order of 1500 ohms. If then at J, which is located at a frac tion of a wavelength from a current maximum, there were connected an auxiliary wire 3| there would be directed a substantial portion of the current into this auxiliary wire. The self impedance of the whole stay would, of course, change and the effect produced by varying the length of 3| would be approximately the same as the effect which would be produced by altering the length of stay 30 when 3| is disconnected. Thus, the self impedance of the stay may be controlled by varying the length of the auxiliary wire 3|. By controlling the self impedance in this manner the current in the stay may be reduced to a small fraction of what it is when the stay has a very low self impedance. The wire 3| may be terminated by an insulator 32, or may be connected directly to ground or to a large metal object. In the latter case the wire 3| should preferably be about wavelength shorter or longer than when it is terminated at an insulator.

It sometimes happens that after a stay has been detuned at one frequency, it acquires a low self impedance at another frequency at which it may be desired to use the antenna. In such a case an additional degree of freedom is required. Fig. 4 illustrates how this additional degree of freedom may be secured in practice. In this figure 3| and 35 are two auxiliary wires connected at two ponts J1 and J2 to stay 39. By adjusting the length of these wires one can detune stay 30 at two frequencies. The points of junction of the auxiliary wires with the stay may either coincide or be separated by a substantial distance.

In Fig. 4 there is shown still another arrangement for the same purpose. This arrangement consists of the auxiliary wire 36 to which there is connected another auxiliary wire 31. By adjusting the length of wires 36 and 3'! as well, as the points of junction J3 and J4 it is possible to detune stay 33 at two different frequencies. All of the auxiliary wires in Fig. 4 may be either terminated by insulators or connected to large masses of metal depending upon which of the various possible arrangements results in greater detuning effect and is simpler to erect.

In Fig. 5 another method is illustrated for reducing the undesirable effects of a stay. This method is particularly useful when the antenna with which the stay is interfering is a receiving antenna.

It has already been pointed out that a high resistance stay has relatively little effect on the directional characteristic of an antenna and that it can merely affect the total radiated or received power. Since the signal to static ratio is not affected when the gain of an antenna is reduced equally in all directions but is usually seriously affected when the shape of radiation characteristic is distorted, it is clear that the presence of a high resistance stay in the neighborhood of a receiving antenna is not nearly as objectionable as is the presence of a low resistance radiating stay.

In Fig. 5 is shown an arrangement which enables one to convert a low resistance stay into a high resistance stay. The arrangement in its simplest form consists of an auxiliary wire 50 and a resistor 5|. The value of resistor 5| is not critical but it should be of the same order of magnitude as the surge impedance of the stay and of the auxiliary wire 50. The length of the auxiliary wire 5!] is likewise not critical. This length may be a small fraction of the wavelength or a whole wavelength or even longer depending on mechanical convenience. The distance between the point of junction of auxiliary wire and stay 38 and the end of the stay, however, should be chosen with reasonable care. The proper position of junction J is fixed by the following considerations. If at some frequency F, point J falls at a current maximum along stay 30 the addition of wire 50 and resistor 51 has little effect and little current is diverted into 50 under these circumstances. On the contrary when point J falls at a current minimum the major portion of the stay current is diverted into wire 50 and hence into resistor 5 I. In this latter case a large portion of the power in the stay is dissipated in resistor 5i and the stay acts as a high resistance stay. When point J is a small fraction of a wavelength from the current minimum the effect of network 56, 5! is still quite considerable. For this reason the effect of this network is not confined to a single frequency but is spread out over a band of frequencies. In fact by using two such networks, one on each end of the stay, it is often possible substantially to suppress the effects of a stay within the whole range of frequencies received by an aperiodic antenna.

So far we have assumed that there is in the vicinity of the stay some large metal object to which resistor 5! may be connected. When this is not the case either an auxiliary counterpoise may be constructed for this purpose or else resistor 5| may be placed between wire 50 and the point of junction J. In this latter case wire 50 will function as a good counterpoise as long as it is roughly 4 wavelength long and is insulated at its far end.

While I have described particular embodiments of my invention for purposes of illustration, it should be understood that various modifications and adaptations thereof, occurring to one skilled in the art, may be made within the spirit of the invention as set forth in the appended claims.

What is claimed is: 1

1. In an electrical system comprising an antenna, a conductor in which currents tend to be induced by said antenna and an impedance surficient to detune said conductor connected in shunt thereto at a point along the conductor removed a fraction of a wavelength of the inducing current, from a standing wave current maximum.

2. In an electrical system according to claim 1, wherein said impedance comprises a second conductor of sufficient length to detune said conductor first mentioned.

3. In an electrical system comprising an antenna, a. conductor in which currents tend to be induced by said antenna, and a shunt connection, including a power dissipating resistance, between a point along the conductor removed a fraction standing wave current maximum, and ground.

4. In an electrical system comprising an antenna, a conductor in which currents tend to be induced by said antenna and a power dissipating resistance connected at one end directly to said conductor at a point along the conductor removed a fraction of the wavelength of the inducing current, from a standing wave current maximum, and a conductor substantially a quarter of said wavelength long connected to the other end of said resistance, said conductor last mentioned terminating in an insulator.

ANDREW ALFORD.

25 of the wavelength of the inducing current from a

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