US2279130A

US2279130A - Radio antenna system

Info

Publication number: US2279130A
Application number: US384521A
Authority: US
Inventors: Bruce Malcolm
Original assignee: Individual
Current assignee: Individual
Priority date: 1941-03-21
Filing date: 1941-03-21
Publication date: 1942-04-07
Anticipated expiration: 1959-04-07

Description

April 7, 1942. M. BRUCE RADIO ANTENNA SYSTEM Filed Ma rch 2i, 1941 jwnzan M Cay,

Wzass Patented Apr. 7, 1942 STTS 10 Claims.

The present invention relates to radio antenna systems, and associated tuning and coupling means, for the radiation and collection of electromagnetic waves. More specifically, the invention relates to antenna systems for vehicles, having particular though by no means exclusive application to aircraft.

The problem of radio communication with vehicles especially two-way communication, has been complicated by difficulties in providing an adequate antenna system for the vehicle. This is particularly true in the case of airplanes, where any external antenna, such as a trailing wire, exposed rods, or wires mounted on and insulated from the aircraft structure, if of a size to be eifective, offers serious mechanical difficulties as well as materially reducing the speed of the craft through increasing its aerodynamic resistance.

In the present inventors prior United States Letters Patent No. 2,235,139, issued March 18,

1941, there is disclosed an antenna system for aircraft which obviates the necessity of adding external elements such as wire, rods or the like, to serve as the radio antenna. Instead, the metallic structure of the craft is employed as the antenna, one portion of the craft, such as an elevator or other pivotally mounted airfoil member, being insulated from the remainder of the craft at supporting points, and the-mutually insulated portions coupled to the radio equipment in a manner which provides a series tuned antenna system which may be operated over a wide range of frequencies substantially independent of the equivalent electrical dimensions of the aircraft structure.

While the system of said application provides an effective self-contained antenna which is entirely satisfactory from the electrical standpoint, the adjustment of the system generally being such that a current loop occurs in the vicinity of the point of coupling to the structural portions of the craft, nevertheless in certain instances there may be mechanical difficulties in the way of insulating one or more airfoil members attheir supporting points, especially where it is desired to install the system in a craft already constructed in the conventional manner. The object of the present invention is therefore to provide an improved antenna system for aircraft and other vehicles employing the metallic structure of the craft, or a portion thereof, as the antenna, operation over a substantial frequency range being achieved without insulating or otherwise disturbing any structural portion of the craft.

'More particularly, an object of the invention is to provide for vehicles a novel and improved antenna system employing the metallic structure of the vehicle as the radiating and collecting portion of the system and operative as a resonant antenna system at substantially any, desired frequency of operation in no way.limited by the equivalent electrical dimensions of the vehicle structure or portion thereof utilized as the antenna.

Another object of the invention is to provide a novel antenna coupling system particularly though by no means exclusively adapted for use in conjunction with vehicles the metallic structure of which is employed as the radiating portion of the antenna system, whereby coupling may be effectively provided to the said antenna system in the vicinity of a voltage loop thereon.

In accordance with these and other'objects, a feature of the invention involves the provision of reactive means, preferably disposed adjacent one end of the vehicle and having at least its free or outer end exposed to space and unshielded by the metallic structure of the vehicle, wherein the vehicle structure and the reactive means comprise an antenna system in which the vehicle structure constitutes substantially the entire radiating portion thereof, said reactive means being adjustable to provide values of reactance which enables the system as a whole, in respect to its equivalent electrical length, to be made substantially equal to an even number of quarter-wavelengths of the selected operating frequency, and which likewise enables optimum coupling to the system element, for example a rod, and "an inductive member in series with said capacitive member. Through adjustment of the inductive member, the valve of the reactance may be varied over wide limits, enabling suitable adjustment of the antenna system to be attained for operation at frequencies in no way limited by their relation to the equivalent electrical length of the particular vehicle structure or portion thereof which is utilized as the effective radiating portion of the system.

For coupling the antenna system to the radio equipment ofthe vehicle, tuning and coupling means may be employed in series between the vehicle structure and the external reactance, with the system adjusted in such a manner that a current loop appears within or in the vicinity of the coupling unit when the antenna system is tuned to resonance. In case it is desired, howhaving the antenna system coupled to the inner or shielded conductor thereof in the vicinity of a voltage loop thereon and having the outer or shielding conductor short-circuited upon itself to prevent any appreciable shift in the voltage and current distribution on the inner conductor through loading by the antenna. With such a coupling system, it becomes practical to couple to an antenna system in the vicinity of a-voltage loop thereon even though the radiating portion of the system may be of such character as to introduce a substantial loading effect, by reason of possessing relatively large values of capacitance per unit length, as for example, where the metallic structure of an airplane or other vehicle is utilized as the radiating portion of the antenna system.

In the accompanying drawing which illustrates the invention as applied to aircraft, Figure 1 is a view showing in schematic fashion the tail portion of an airplane having an inductance and a capacitive member mounted at one end of the'fuselage structure in accordance with the invention, with coupling to the antenna system provided in the vicinity of a current loop, and Figure 2 illustrates an alternative embodiment in which the coupling system of the invention is employed, with coupling to the antenna system provided in the vicinity of a voltage loop thereon.

The fuselage of the airplane is indicated at 6, and it is contemplated that this will be either of all-metal construction,in accordance with general practice in the larger craft, or having a metallic structure as in smaller airplanes, the invention being applicable to either type. The drawing also shows, merely for illustration, the rudder 8 and the vertical fin l0.

The radio equipment with which the craft is provided is indicated generally at I2 and may be located at any convenient point in thecraft. The equipment may comprise high frequency apparatus such'as a transmitter or receiver for radio communication, direction finding apparatus, or other apparatus relating to the radiation or collection of electromagnetic waves. Such equipment is schematically indicated in the drawing by the inductance M which may be taken to To enable the radio equipment to operate with maximum effectiveness over its entire. range of operating frequencies, it is necessary that a resonant antenna system be provided. Since, in the case of aircraft or other vehicles no connection to earth is available, the antenna system must have an equivalent electrical length such that a half-wavelength or multiple thereof of the operating frequency or, in other words, an even number of quarter-wavelengths, may appear thereon.

In order that the metallic structure of the aircraft itself may be employed as the radiating portion of a resonant antenna system operative at frequencies whose half-wavelength or multiple thereof isnot equivalent to the electrical length of the portion of the aircraft structure utilized,

there is provided in accordance with the invention means for altering the equivalent electrical length of the system as a whole, without requiring as in the embodiment of the aforesaid application, electrical discontinuity in said structure through the use of insulation between structural portions.

7 To this end there is employed at or adjacent one end of the vehicle structure an adjustable reactance arranged so as to be at least in part external to and unshielded by the metallic structure of the vehicle. This reactance preferably consists of a capacitive member or area, which maybe in the form of a streamlined metal cap or shell I6, and an adjustable inductance l8 disposed within a housing 20 of insulating material secured at one end to the aircraft fuselage and supporting at its other end the shell IS. A lead 22 connects the outer end of inductance to the shell. The reactance thus may be small in size and light'in weight, and of a shape which will add negligible wind resistance and hence will have but slight efiect on the speed of the craft.

For operation of the structure of the craft as the radiating portion of a resonant antenna system with coupling to the system provided in the Vicinity of a current loop thereon, the arrangement of Figure 1 is employed. In this embodiment the antenna tuning and coupling network, indicated generally at 24 and located as closely as possible to the point of connection to the antenna system, is connected in series between the end of the fuselage 6 and the inner end of the inductance I 8. The tuningand coupling unit may comprise a variable reactance, here illustrated as a parallel-connected inductance 26 and variable condenser 28, coupled to the radio equipment by a transmission line 30 together with

coupling coils

32 and 34. A lead 36 from oneside of the tuning and coupling reactance passes through an insulating bushing '38 to the inner end of the inductance I8, while the other side of the tuning and coupling reactance is connected to the fuselage, preferably, in the case of metal-clad structure, by passing the conductor through an opening in the fuselage structure and connecting to the exterior surface as at 40, to facilitate the distribution of the radio frequency energy over the outside surface of the craft.

For operation of the antenna system of Figure 1 at some selected frequency, adjustment of the reactance of the tuning and coupling unit 24 will be coordinated with the value of the reactance of the externally mounted coil I8 and capacitive member l6 so as to provide resonant operation with optimum coupling between the radio equipment and the antenna system. I This 'requlres that two conditions be complied with,

first, that the equivalent electrical length of the antenna system as a whole be substantially equal to an even number of quarter-wavelengths of the selected operating frequency, and second,

that at said frequency a current loop appear within or in the vicinity of the tuning and coupling unit 24. Since the equivalent electrical length of the effective radiating portion of the antenna system, that is, the metallic structure of the vehicle, is fixed for any given position of the external reactance relative to said structure, the electrical characteristics thereof cannot be altered. Accordingly, for any selected operating frequency, only a standing Wave or portion thereof app opriate to that frequency can appear on the vehicle structure. The remainder of the standing wave, must therefore appear in the tuning and coupling unit 24 and in the external reactance. At the same time, the adjustment of the external reactance, as determined by the tuning of inductance I8, must be such that a current loop is caused to appear within or in the vicinity of the coupling unit 24, in order that optimum coupling to the antenna system be achieved.

In other words, while there may be a variety of adjustments of external reactance and tuning unit which together in conjunction with the equivalent electrical length of the metallic structure of the vehicle, comprise an antenna system of resonant length, nevertheless these adjustments must be limited to such as will also cause a current loop to appear at or in the vicinity of the tuning and coupling unit 24. Since the external reactance may through suitable adjustment of inductance l8 be varied over a wide range of values which may be made capacitive, inductive, or incertain cases purely resistive, the desired current and voltage distribution in the tuning and coupling unit for optimum coupling may readily be obtained while permitting the system as a whole to be adjusted substantially to resonance. These adjustments, while necessary to permit operation in the desired manner, are nevertheless not excessively critical in view of the relatively high capacitance per unit length of the radiating portion of the antenna system and consequent rather broad resonance characteristics thereof, with the result that exact.

resonance in tuning need not be achieved. As a result, the vehicle structure may be made to serve effectively as the radiating and collecting portion of an antenna system which is operative at frequencies in no way limited by the equiv- {alent electrical length of the said vehicle strucure.

Where the operating frequency is such that the equivalent electrical length of the vehicle structure, for example, the fuselage of an airplane does not greatly exceed a quarter-wavelength or an odd multiple thereof, the antenna system may, through adjustment of the external reactance and the tuning and coupling net-work, be tuned to resonance with a half-wavelength appearing thereon. In cases where the equivalent electrical length of the vehicle structure is such that the antenna system, comprising the vehicle structure, the external resistance and the tuning and coupling means, necessarily exceeds a half-wavelength of the operating frequency, then the system is preferably adjusted to be resonant at the next higher odd number of half-wavelengths in order that the antenna system may be terminated by portions having voltage loops thereon which are opposite in phase. Alternatively, the antenna system may be arranged with the tuning and coupling net-work 24 exposed to space, together with the external reactance, for example, by enclosing the net work components in the insulating shell 20 together with the adjustable inductance it, such arrangement permitting operation. when the system is tuned to an even number of halfwavelengths of the operating frequency as well as to an odd number of half-wavelengths.

In certain cases it may be found desirable to operate the antenna system in the manner of an end fed antenna, with coupling to the antenna system being provided in the vicinity of a voltage loop thereon. For such operation the arrangement illustrated in Figure 2 may be employed to advantage. In this embodiment of the invention, a coupling system is employed which makes it possible to couple to a vehicle antenna system, comprising the vehicle structure and an external reactance, in the vicinity of a voltage loop on said antenna system, even though such structure generally has a substantial loading effect because of its relatively high capacitance per unit length. The coupling system is not limited to such applications, however, being well adapted to couple to any continuous metallic structure for use of the said structure as a portion of an antenna system in instances where no ground is available against which such structure may be operated. This results from the fact that any shift of the voltage distribution in the coupling system, with consequent reduction of voltage in the vicinity of the coupling point and corresponding decrease in coupling, is effectively prevented.

The embodiment of Figure 2 is similar to that of Figure 1 in that an external reactance is employed comprising an exposed capacitive area 42 and an adjustable inductance 44, the inductance in this embodiment however being directly connected in series between the capacitive area and the end of the fuselage 6 by connections 46. The external reactance and the fuselage structure thus comprise an antenna system the equivalent electrical length of which may be varied by adjustment of the inductance 44. Depending upon the equivalent electrical length of the fuselage or such structural portion of the vehicle as may be employed as the antenna, and the selected operating frequency, the external reactance may, by suitable adjustment of the inductance, be made inductive, capacitive, or in certain cases purely resistive, in order that the equivalent electrical length of the system as a whole may be equal to any even number of quarter-wavelengths of the operating frequency. As in the first described embodiment, it is desirable that the capacitive member be as large as convenient, having consideration for weight, size, and other factors dependent on the character of the vehicle, in order that the size of the series inductance may be correspondingly reduced, with consequent reduction of losses in the inductance. If, on the other hand, it is considered desirable for any reason to reduce the size of the capacitive member to some small value, then the size of the series inductance may be correspondingly increased.

The coupling system by which the radio equipment 48 is coupled to the antenna system comprises a coupling unit which, for purposes of illustration, is shown at a resonant quarter-wavelength shielded line, shorted at one end and having the shielding member short-circuited upon itself along its length, or in other words, electrically bonded across points normally of different potential to reduce said points to a common potential and thereby prevent the development of any standing waves on the shielding member. Alternatively, a shielded line a quarter wavelength in length, or odd multiple thereof, may be employed, with the outer or shielding member short circuited to itself at its ends only, to prevent the development of standing waves thereon, and with one end of the inner or shielded conductor shorted to the shielding member. This results in effectively maintaining the shorted end of the inner or shielded conductor at ground po tential under substantially all conditions of operation. Changes in voltage and current distribution on the inner conductor, when an antenna or other load is connected thereto, are thus prevented,xso that uponcoupling an antenna to the inner conductor inthe vicinity of a voltage loop thereon, no appreciable shift in fectively all portions of the shielding member, and particularly the adjacent inner surfaces '52 of the channel. Instead of being formed out of U-shaped channel or strip,the shielding member may for instance be stampedorotherwise-formed out of sheet metal to provide a helically-grooved cylinder .similar to that formed by winding the U-shaped strip. The other portion 54 of the shielding member may be .a metallic shell or cylinder surrounding the portion to complete the shielding. The inner "or shielded element 56 of the line consists of a conductor disposed along the channel of the shielding member and insulated therefrom along its length, except as appears hereinafter. The conductor 56 thus has its adjacent turns electrically shielded from one another by the intervening partitions 58 formed by the sides of the groove or channel in the shielding member 50, while the portion 54 shields the conductor 56 from the surrounding metallic structure of the vehicle. The shielded conductor 56 is shorted at one end by jumper 60 to the shielding

portions

50 and 54, while the free end of the shielded conductor is connected by a short and direct lead 62 through a coupling condenser 64 to the capacitive area 42, which constitutes one end of the antenna system.

For a voltage loop to appear at the free end of the shielded conductor, the equivalent electrical length thereof from the shorted point to the free end may be approximately equal to a quarter-wavelength of the operating frequency or an odd multiple thereof. In certain cases, however, it may prove inconvenient or undesirable to employ in the coupling system a line of substantially the proper resonant length. Accordingly, where a single line is to be utilized for operation of the antenna system over a range of frequencies, means are conveniently employed for adjusting the lineto resonance substantially independent of its length. To this end, a reactive element, which is illustrated as a variable capacitance 66, may be connected across the free end of the line between the shielded and shielding member thereof, the value of said reactance being such that the line may be tuned to resonance at the operating frequency. Thus, in cases where the length of the line is less than one-quarter wave-length of the operating frequency, or falls short of an odd multiple thereof by less than a quarter-wavelength, the reactive element must have a suitable value of capacitive reactance in order for the line to be tuned to resonance and for a voltage loop to appear at the free end of the shielded conductor. If the'antenna system is to be employed for communication at several different frequencies with rapid shift from 'one to another, preferably more than one coupling system of the above type will be employed, each being of such a length that the line may be tuned to resonance over a particular band of operating frequencies.

The coupling unit may be coupled to the radio equipment by a transposed transmission line 68, conductively coupled to the shielded. and shielding members of the coupling unit at points which will provide a proper impedance match between transmission line and coupling unit. A coupling coil 10 inductively couples transmission line 68 to circuit 12 representing the radio equipment.

There may be employed, in place of the above described coupling unit, a resonant concentric or coaxial line, having the antenna coupled to the inner or shielded member in the vicinity of a voltage loop, while the outer member or sheath is short-circuited to itself, for example, by bonding together points normally of different potential to prevent the development of any standing waves thereon. While such arrangement may be successfully employed in the embodiment of Figure 2 for operation in the manner of the invention, the coupling unit previously described is considered preferable in certain respects by'reason of the more effective short-circuiting of the inner surfaces of the shielding member across the relatively short path afforded to radio frequency voltages between adjacent channels thereof.

While the invention has been particularly described as applicable to airplanes wherein the metallic structure of the craft is employed as the radiating portion of an antenna system, the system is equally, applicable to the collection of electromagnetic waves and accordingly operation of the antenna both for transmission and reception of said electromagnetic waves is contemplated throughout the specification. It is likewise to be understood that the invention is in no way limited in its application to airplanes only, but may be employed in vehicles and craft of other forms and types possessing electricallyconductive structure.

The nature and scope of the invention having been indicated, and certain specific embodiments thereof shown and described by way of illustration, what is claimed is:

1. In a vehicle having electrically conductive structure and provided with radio equipment, a self-contained resonant antenna system utilizing a continuous portion of said structure as the antenna for operation at frequencies substantially independent of the equivalent electrical length of the portion of the structure utilized, said system including a reactance mounted adjacent one end of the vehicle structure and electrically exposed to space at least in part, the reactance having a value such that the equivalent electrical length of said system is substantially equal to an even number of quarter-wavelengths of the desired operating frequency, and means for coupling the antenna system to the radio equipment.

2. In a vehicle having electrically conductive structure and provided with radio equipment, a self-contained resonant antenna system utilizing a continuous portion of said structure as the antenna for operation at frequencies substantially independent of the equivalent electrical length of the portion of the structure utilized, said system including reactance mounted adjacent one end of the vehicle and electrically exposed to space at least in part, the reactance comprising a capacitive member and an inductive member connected in series therewith, the reactance having a value such that the equivalent electrical length of the system is substantially equal to an odd number of half-wavelengths of the desired operating frequency, and means for coupling the radio equipment to the antenna system in the vicinity of a current loop thereon.

' 3. In a vehicle having electrically conductive structure and provided with radio equipment, a self-contained resonant antenna system utilizing a continuous portion of said structure as the antenna for operation at frequencies substantially independent of the equivalent electrical length of the portion of the structure utilized, said system including a reactance mounted adjacent one end of the vehicle and electrically exposed to space at least in part, the reactance comprising a capacitive member and an inductive member connected in series therewith, the reactance having a value such that the equivalent electrical length of the system is substantially equal to an even number of quarter-wavelengths of the desired operating frequency, and means for coupling the radio equipment to the antenna system in the vicinity of a voltage loop thereon.

4. In a vehicle having electrically conductive structure and provided with radio equipment, a self-contained resonant antenna system utilizing a continuous portion of said structure as the antenna for operation at frequencies substantially independent of the equivalent electrical length of the portion of the structure utilized, said system including a reactance mounted adjacent one end of the vehicle and electrically exposed to space at least in part, the reactance comprising a capacitive member and an inductive member connected in series therewith, tuning and coupling means connected between the external reactance and the vehicle structure and adapted to couple the radio equipment to the antenna system, the external reactance and the tuning and coupling means having values such that the total equivalent electrical length of the system is equal to an odd number of half-wavelengths of the operating frequency-and likewise such that a current loop appears in the vicinity of the tuning and coupling means.

5. In a vehicle having electrically conductive structure and provided with radio equipment, a self-contained resonant antenna system utilizing a continuous portion of said structure as the antenna for operation at frequencies substantially independent of the equivalent electrical. length of the portion of the structure utilized, said system including a reactance disposed adjacent one end of the vehicle and comprising a capacitive member and an inductive member connected in series therewith, and tuning and coupling means connected between the reactance and the vehicle structure and adapted to couple the radio equipment to the antenna system, the reactance and said tuning and coupling means being electrically exposed to space and having values such that the total equivalent electrical length of the system is equal to an even number of quarterwavelengths of the operating frequency and like- Wise such that a current loop appears in the vicinity of the tuning and coupling means.

6. In a vehicle having electrically conductive structure and provided with radio equipment, a self-contained resonant antenna system utilizing a continuous portion of said structure for operation at frequencies substantially independent of the equivalent electrical length of the portion of the structure utilized, said system including a reactance mounted adjacent one end of the vehicle and electrically exposed to space at least in part, the reactance comprising a capacitive member and an inductive member connected between said capacitive member and the vehicle structure, the reactance having a value such that the equivalent electrical length of the system is substantially equal to an even number of quarter-Wavelengths of the desired operating frequency, and means for coupling the radio equipment to the antenna system, said means including a resonant shielded line comprising a shielded conductor and a shielding member, said line having the antenna coupled to the shielded conductor in the vicinity of a voltage loop thereon, the shielding member of the line being bonded across points normally of difierent potential to reduce said points to a common potential and thereby prevent the appearance of standing waves on said shielding member and likewise prevent a shift in the voltage distribution on the shielded conductor.

'7. A coupling system for coupling radio equipment to an antenna system, comprising a resonant shielded line having a shielded conductor and a shielding member, said. line having the antenna system coupled to its shielded conductor in the vicinity of a voltage loop thereon and having the shielding member bonded across points normally of different potential to reduce said points to a common potential, thereby to prevent the appearance of standing waves thereon and likewise to prevent a shift in the voltage distribution on the shielded conductor.

8. A coupling system adapted to couple radio equipment to an antenna system, comprising a shielding member, a conductor shielded thereby, and a shorting connection between the shielding member and the shielded conductor, the shielding member being disposed with portions physically spaced along its length in direct electrically continuous relation to reduce the shielding member to a common potential at a plurality of points along its length, thereby to prevent the appearance of standing waves thereon and to minimize shifting of the voltage distribution of the shielded conductor upon coupling an antenna system to said shielded conductor in the vicinity of a voltage loop thereon.

9. A resonant shielded line comprising a shielding member, a conductor shielded thereby, and a shorting connection between the shielding member and the shielded conductor, the shielding member being bonded across a plurality of points to prevent the appearance of standing waves thereon and likewise to prevent a shift in the voltage distribution on the shielded conductor when a load is connected thereto.

10. A resonant shielded lineadapted to couple radio equipment to an electrical load, comprising a conductor, a shielding member at least partially surrounding said conductor, and a shorting connection between shielding member and shielded conductor, the line having portions of the interior shieldingsurface of the shielding member which lie opposite portions of the shielded conductor spaced along said conductor, disposed in direct electrically connected relation to prevent the appearance of standing waves on said member and likewise to prevent a shift in the voltage distribution on the shielded conductor when a load is connected thereto.

MALCOLM BRUCE.

Patent No 2,279 ,150.

the same may conform to the record 0 MALCO LN BRUCE It is hereby certified that error appears in the printed specification tent requiring correction as follows: Page 5, first of the above numbered pa d --reactance--; and th t column, line 52, for the word "resistance" rea t should be read with this correction therein that the said Letters Paten f the case in the Patent Office.

Signed and sealed this 19th day of'May, A. D. 1914.2.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents.