US3771157A

US3771157A - Ferrite broadband semi-notch antenna

Info

Publication number: US3771157A
Application number: US00268850A
Authority: US
Inventors: P Stang
Original assignee: Lockheed Aircraft Corp
Current assignee: Lockheed Martin Corp
Priority date: 1972-07-03
Filing date: 1972-07-03
Publication date: 1973-11-06
Anticipated expiration: 1990-11-06

Abstract

A ferrite-loaded semi-notch antenna, especially suitable for flush mounting in the leading or trailing edge of an aerodynamic surface of an aircraft, and which is capable of functioning in both transmit and receive modes. Combines a short, folded dipole, with an active notch. The electrical length of the antenna is increased over conventional dipole and/or notch antennas by means of end loading in the form of ferrite rings encircling the ends of the dipole. A part of the energy radiates from the conducting surface surrounding the notch, and the remaining (major) portion radiates from the elements located within the notch.

Description

United States Patent [1 1 Stang Nov. 6, 1973 [54] FERRHTE BROADBAND SEMI-NOTCH 2,636,987 4/1953 Dome 343/708 ANTENNA 1 Primary Examiner-Eli Lieberman [75] Inventor. Paul F. Stang, Saugus, Calif. y g C. Sullivan et a1. [73] Assignee: Lockheed Aircraft Corporation,

Burbank, Calif. 57 ABSTRACT [22] Filed: July 3, 1972 A ferrite-loaded semi-notch antenna, especially suitable for flush mounting in the leading or trailing edge [21] Appl 268850 of an aerodynamic surface of an aircraft, and which is capable of functioning in both transmit and receive [52] US. Cl 343/708, 343/747, 343/787, modes. Combines a short, folded dipole, with an active 343/789 notch. The electrical length of the antenna is increased [51] Int. Cl. H01q 1/28 over conventional dipole an r notch n a by [58] Field of Search 343/705, 708, 789, means of end loading in the form of ferrite rings encir- 343/747, 787 cling the ends of the dipole. A part of the energy radiates from the conducting surface surrounding the [56] References Cited notch, and the remaining (major) portion radiates from the elements located within the notch.

14 Claims, 2 Drawing Figures 1 FERRITE BROADBAND SEMI-NOTCH ANTENNA BACKGROUND OF THE INVENTION I-Ieretofore it has been common .to employ ferrites in conjunction with electric antennas for increasing the efficiency and decreasing the size of the antenna structure. Two such antennas are disclosed in US. Pat. Nos. 2,748,386, entitled Antenna Systems, and 3,295,137, entitled Shortened Folded Monopole with Radiation Efficiency Increased by Ferrite Loading. However, it is impossible to flush mount such antennas on an aircraft so as to obviate drag penalties. Toovercome the drag problem it has been proposed, heretofore, to employ slot or notch antennas since their flushmounted configuration is aerodynamically desirable. One such prior antenna is disclosed in US. Pat. No. 2,607,894, entitled Aerial System. However, conventional notch antenna design parameters require structures which are unacceptably large for use in small aircraft. Therefore, where it is desired to employ a structurally small antenna which can be flush mounted in virtually any type of aircraft, there has not, heretofore, been an entirely satisfactory answer. This is particularly so where it is desired to have an antenna which will function efficiently for both transmitting'and re-' ceiving.

SUMMARY OF THE INVENTION There is provided by the present invention a transmit/receive antenna comprising a physically short, folded dipole having its electrical length increased by means of end loading by ferrite toroids or rings encircling the ends of the dipole radiator. The entire assembly is flush-mounted in a notch made in the leading (or trailing) edge of a conductive aerodynamic surface or the empennage of the aircraft. Both the dipole and the surface surrounding the notch are effective to transmit or receive the radio-frequency (RF) energy. The antenna is matched to a coaxial feed cable, over a broad frequency band, by means of tunable capacitor and inductance'elements integral with'the antenna. The tuning section is a parallel circuit which broadens the bandwidth of the antenna, and is tuned to the center frequency. One end of the antenna is grounded, thereby protecting it against lightening Strikes. In a typical construction the *antennamay be operated in the frequency range of 100 to 300 megahertz. The frequency dependance of the ferrite used greatly increases the antennas effective bandwidth.

It is therefore an object of theinvention to provide a novel and efficienty flush-mounted transmit/receive antenna suitable for mounting in small mobile or airborne vehicles.

Another object of the invention is to provide a novel and improved antenna of the flush-mounted type which is the functional equivalent of a folded dipole and/or a notch antenna but of smaller physical dimensions.

Still another object of the invention is to provide a novel and improved antenna which is inherently immune to lightening strikes.

Yet another object of the invention is to provide a novel and improved aircraft antenna suitable for both transmitting and receiving and which is of unusually wide bandwidth and small physical size.

These and other objects of the invention will become better understood upon consideration of the accompanying specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.--l is a side elevation view, partially broken away, showing the details of construction of one embodiment of the invention.

FIG. 2 is a perspective view illustrating an embodiment of the invention suitable for installation in the vertical stabilizer of an airplane.

DESCRIPTION OF THE PREFERRED EMBODIMENT connected to the central conductor 7 of coaxial cableconnector 8, which is mounted in wall 2. The outer (viz., shield) conductor of connector 8 is directly connected to wall 2. A tunable inductance assembly, indicated generally at 9, is slidably mounted in wall 3 and extends outwardly therefrom so that its outer terminus l 1 slidably engages the dipole element 5. This assembly (9) comprises a hollow conductive tube 12 having a pair of flattened finger portions 13 surrounding the dipole element 5 and which are clamped thereto by means of screw fastener 14. The other end of tube 12 is mounted on, or otherwise secured to, an insulating flange 15 which in turn is carried on plate 16. Plate 17 is located on the opposite surface of wall 3.

Plates

16 and 17 straddle an elongated slot 18 in wall 3, thereby permitting assembly 9 to be moved back and forth with respect to one end of element 5.

Plates

17 and 18 are clamped to either side of wall 3 by a pair of

screw fasteners

21 and 22 or other suitable means. Rod 19 has an exterior threaded portion 24 which threadedly mates with an interior threaded portion of insulated flange 15. Rod 19 is coaxially disposed within and spaced apart from tube 12. Thumbwheel 25 is attached to rod 19 for rotation therewith, and permits the rod to be moved in the direction of arrow 26 by suitably rotating the thumbwheel 25. Rod 19, which is made of metal or other conductor, plus tube l2-comprises a coaxial tuning capacitor for incorporation into the antenna matching network to be described hereinafter. By sliding the entire assembly 9 in direction of arrow 27, the center frequency of the antenna may be established.

The tunable capacitor and inductance assembly (9) will match the antenna to the input impedance of a 50 ohm transmission line over a broad frequency band. Broad tuning is obtained by adjusting the location ofthe cylindrical capacitor 9 along the dipole member 5 so as to match the mid-band frequency. It is thereafter fixed in place with respect to the base (e.g., wall 3). Fine tuning is then obtained by means of the capacitor (12, 19) only. It will be appreciated by those versed in the art, that the location of capacitor 9 with respect to dipole member 5 may be fixed as a design parameter and, therefore, need not be made adjustable.

Dipole element 5 carries a plurality of ferrite rings at either end, the outer pair of which, in each instance, is provided with set screws to retain the rings in position on the tubular dipole element 5. For example, rings 31-34 are secured to element 5 by set

screws

35 and 36. Similarly, the ten rings 37-46 are secured to the opposite end of element 5 by means of set

screws

48 and 49. Each ring or toroid has an axial bore slightly larger than the diameter of the dipole element. As is well known to those versed in the art, and as is discussed in detail in the aforementioned U.S. Pat. No. 2,748,386, the length-to-diameter ratio of the ferrite rings should be 8:1 or greater in order to take advantage of the heneflts of the high effective permeability.

Referring to FIG. 2 there is shown, by way of example, another embodiment of the invention installed in the vertical stabilizer 52 of a small aircraft. As can be seen, a notch is cut into the leading edge 51 of the stabilizer 52 and enclosing walls 53-55 of metal or other conductive material define the notch.

Walls

53 and 55 have been shaped to conform to the geometry of the stabilizer 52. A non-conductive or dielectric cover 56 encloses the notch and is faired into the leading edge skin of the stabilizer 52 so that the aerodynamic properties of the vehicle remain unaltered as a result of installation of the antenna. Other elements shown in FIG. 2 are similar to like parts in the embodiment of FIG. 1. The notch defining structure (53-55) is secured to, and is electrically contiguous with, the outer conductive surface of the vehicle (52). Thus, the frame (53-55) is grounded to the vehicle. The antenna may be enclosed in a fiberglass fairing 56 conforming to the exterior contour of the vehicle skin. If desired, the notch portion of the antenna may be filled with dielectric foam once the desired adjustments have been completed. The dipole (5) is preferably oriented in accordance with the desired polarization of the radiated energy.

Operation of the invention may be better understood from a consideration of generally similar antennas. Resonance of a simple notch antenna may be obtained either by varying the physical dimensions of the notch alone, or by inserting a network which increases the excitation of the notch area and makes the conducting surface into a radiating element. In the first case, the

notch is the radiating element; in the second case, most of the radiation emanates from the surrounding conductive surface with only minor radiation from the notch.

In the present invention, only a part of the conducting surface functions as a radiating element while most of the energy radiates from the elements which are located within the notch area. The antenna resembles a short, folded dipole, but its electrical length is increased by the addition of the ferrite loading rings at the input end, the grounded end, or both. The dipole actually comprises a folded monopole (e.g., element 5) with a ground plane (e.g., wall 2) to provide the reflected image. A tunable inductance-capacitance (L-C) section, matches the antenna impedance to the impedance of the coaxial feed cable. The portion of the dipole element (5) between the transmission line (8) and the tuning assembly (9) comprises the matching section, and the remaining portion (viz., the portion between the tuning assembly 9 and the opposite end (4)) comprises the excited section, of the antenna. The

notch antenna, but smaller in, size than a rectangular or annular slot antenna. The cutout size of the notch is a function of the type ferrite material employed for rings 31-34 and 37-46 and, therefore, depend on the effective permeability Ut The length of the dipole memberS and number of encircling ferrite rings (31-34 and 37-46) will bring the structure into resonance with the desired frequency of operation. The tuning assembly 9 comprises a parallel circuit which broadens the bandwidth and is tuned to the desired operating center frequency. The ferrite rings effectively shorten the electrical length of the dipole element 5 or the notch and act as reactive loading devices. These loading devices (31-34 and 37-46) change the current distribution along the antenna (1 and 5) and, therefore, produce a more efficient radiator than an unloaded radiator with the same physical dimensions. A high Q ferrite should be selected to keep losses to a minimum.

In a practical implementation of the invention various types of ferrite material may be used for the loading rings including those identified as magnesium-iron TT105, nickeliron T112430 and Ferroxcube type IVD. Magnesium ferrite and nickel ferrite are similar in frequency sensitivity and therefore allow operation over the same band. A nickel-zinc type ferrite material, having the following characteristics at 25 C, has been found to be especially suitable for use in the'practice of the invention: 7

Initial permeability 40 at 1 MHZ Maximum permeability 1 15p Saturation Flux Density 2400 Gauss Residual Mangetism 750 Gauss Coercive Force 4.7 Oersted Temperature Coefficient 0.1% max/C Between 25C to 50C Currie Point 450C Lem Factor at 50 MHz 170 X 10'' Although there is a slight reduction in the power transfer efficiency resulting from the use of ferrites, which efficiency reduction is attributable to magnetic losses, the advantages outweigh the losses. Specifically, the ferrite-loaded semi-notch antenna of this invention is simpler in construction, has a greater bandwidth and is of considerably smaller size than either simple notches or recessed dipole antennas. Furthermore, it allows fixed tuning and matching over practical bandwidths without expensive or complex matching devices.

In a typical construction the antenna may comprise a inch diameter aluminum conduit, 15 inches in length, for the dipole radiator. The depth of the notch is approximately 7% inches. Two ferrite rings encircle the dipole conductor near the feed point. Similarly, eight ferrite rings are located at the opposite end of the antenna. A construction of these dimensions will operate in the frequency range from megahertz to 230 megahertz. Without any ferrite loading on the antenna, the minimum voltage standing wave ratio (VSWR), or resonance, occurs at 280 MHz. The addition of two ferrite rings at the input area and eight ferrite rings at the grounded end of the antenna, shifts the resonance to MHz. The concurrent change in minimum VSWR will be from approximately 1.521 to very near 1:1. The ferrite loading tends to create a more uniform current distribution along the antenna element, and hence increases the effective height and radiation resistance for a given length of the antenna. The improved RF current distribution makes a series-loaded antenna, constructed in accordance with the invention, superior to an unloaded antenna of equal physical dimensions.

From the foregoing it will be seen that the present invention provides an antenna meeting the objectives set forth hereinabove and which is readily adapted to installation in small aircraft to replace simple notch, slot, helix, or dipole antennas, used heretofore, thereby overcoming the limited effectiveness and/or the excessive drag of such prior devices.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to preferred embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the antennas illustrated may be madeby those versed in the art. For example, the orientation of the notch structure with respect to the airframe may be altered in accordance with specified aerodynamic parameters and radiation patterns. Furthermore, the notch dimensions may be increased or decreased in accordance with particular mechanical and electrical design requirements. Such modifications may be made without departing from the spirit of the invention; therefore, it is intended that the invention be limited only as indicated by the scope of the following claims.

What is claimed is:

1. An antenna comprising:

means defining an elongated discontinuity in an electrically conductive surface; an elongate conductive radiating element disposed within said discontinuity in spaced relationship thereto and having its major axis extending along the longitudinal axis of said discontinuity; ferrite means adapted to series load said conductive radiating element for increasing the inductive reactance thereof; and

means for electrically interconnecting said discontinuity defining means and said conductive element whereby said ferrite-loaded antenna element functions in connection with said notch as the electrical equivalent of a center fed folded dipole.

2. An antenna as defined in claim 1 including:

an inductive-capacitive tuning means connected between said discontinuity defining means and said conductive radiating element for determining the operational center frequency of said antenna.

3. An antenna as defined in claim 1 wherein said discontinuity defining means comprises:

a generally U-shaped conductive frame defining a notch having its open end flush with said conductive surface.

4. An antenna as defined in claim 1 wherein said conductive radiating element comprises:

a tubular dipole conductor having first and second ends, said first end being driven and said second end being directly connected to said discontinuity defining means.

5. An antenna as defined in claim 4 wherein said ferrite means comprises:

a ferrite ring coaxially disposed with respect to said driven end of said dipole conductor. 7

6. An antenna as defined in claim 4 wherein said ferrite means comprises:

a ferrite ring coaxially disposed with respect to said second end of said dipole conductor.

7. An antenna as defined in claim 4 wherein said ferrite means comprises:

a first ferrite ring coaxially disposed with respect to said driven end of said dipole conductor;

and a second ferrite ring coaxially disposed with respect to said second end of said dipole conductor. 8. An antenna as defined in claim 1 wherein said in- 1Q terconnecting means includes:

a two-conductor transmission line having one of its conductors connected to said discontinuity defining means, and the other of its conductors connected to said elongate conductive radiating element.. 9. An antenna, comprising: a generally U-shaped conductive frame defining a notch; a conductive sheet secured to the periphery of said frame and extending outwardly therefrom for radiating and receiving radio-frequency signals; an elongate ferrite-loading antenna element disposed within said frame and having first and second ends, said first end comprising and input end closely spaced from said notch and said second end being connected to said frame; and a two-conductor transmission line having one of its conductors connected to said input end of said antenna element, and the other of its conductors connected to said frame whereby said ferriteloaded antenna element functions in connection with said notch as the electrical equivalent of a center fed folded dipole. 10. An antenna as defined in claim 9 including: an inductive-capacitance tuning means connected between said frame and said antenna element for determining the operational center frequency of said antenna. I '11. An antenna as defined in claim 10 wherein said tuning means includes:

a coaxial capacitor having a fixed outer sleeve connected to said antenna element, and an insulated inner cylinder electrically connected to said frame. 12. An antenna adapted to be mounted in a cutout in the external surface of an aircraft, comprising:

a generally U-shaped conductive frame defining a notch mounted in said cutout and having its open end flush with said exterior surface; a ferrite loaded dipole element located within said frame; means grounding one end of said dipole element to one end of said frame;

a two-conductor transmission line having one of its conductors connected to the other end of said dipole element and the other of its conductors connected to the other end of said frame whereby said ferrite-loaded antenna element functions in connection with said notch as the electrical equivalent of a center fed folded dipole; and

a dielectric window covering said cutout.

13. An antenna as defined in claim 12 including:

a tuning capacitor means connected between said frame and said dipole element.

14. An antenna as defined in claim 13 including:

means for adjusting the capacitance of said tuning capacitor means.

Claims

1. An antenna comprising: means defining an elongated discontinuity in an electrically conductive surface; an elongate conductive radiating element disposed within said discontinuity in spaced relationship thereto and having its major axis extending along the longitudinal axis of said discontinuity; ferrite means adapted to series load said conductive radiating element for increasing the inductive reactance thereof; and means for electrically interconnecting said discontinuity defining means and said conductive element whereby said ferrite-loaded antenna element functions in connection with said notch as the electrical equivalent of a center fed folded dipole.

2. An antenna as defined in claim 1 including: an inductive-capacitive tuning means connected between said discontinuity defining means and said conductive radiating element for determining the operational center frequency of said antenna.

3. An antenna as defined in claim 1 wherein said discontinuity defining means comprises: a generally U-shaped conductive frame defining a notch having its open end flush with said conductive surface.

4. An antenna as defined in claim 1 wherein said conductive radiating element comprises: a tubular dipole conductor having first and second ends, said first end being driven and said second end being directly connected to said discontinuity defining means.

5. An antenna as defined in claim 4 wherein said ferrite means comprises: a ferrite ring coaxially disposed with respect to said driven end of said dipole conductor.

6. An antenna as defined in claim 4 wherein said ferrite means comprises: a ferrite ring coaxially disposed with respect to said second end of said dipole conductor.

7. An antenna as defined in claim 4 wherein said ferrite means comprises: a first ferrite ring coaxially disposed with respect to said driven end of said dipole conductor; and a second ferrite ring coaxially disposed with respect to said second end of said dipole conductor.

8. An antenna as defined in claim 1 wherein said interconnecting means includes: a two-conductor transmission line having one of its conductors connected to said discontinuity defining means, and the other of its conductors connected to said elongate conductive radiating element.

9. An antenna, comprising: a generally U-shaped conductive frame defining a notch; a conductive sheet secured to the periphery of said frame and extending outwardly therefrom for radiating and receiving radio-frequency signals; an elongate ferrite-loading antenna element disposed within said frame and having first and second ends, said first end comprising and input end closely spaced from said notch and said second end being connected to said frame; and a two-conductor transmission line having one of its conductors connected to said input end of said antenna element, and the other of its conductors connected to said frame whereby said ferrite-loaded antenna element functions in connection with said notch as the electrical equivalent of a center fed folded dipole.

10. An antenna as defined in claim 9 including: an inductive-capacitance tuning means connected between said frame and said antenna element for determining the operational center frequency of said antenna.

11. An antenna as defined in claim 10 wherein said tuning means includes: a coaxial capacitor having a fixed outer sleeve connected to said antenna element, and an insulated inner cylinder electrically connected to said frame.

12. An antenna adapted to be mounted in a cutout in the external surface of aN aircraft, comprising: a generally U-shaped conductive frame defining a notch mounted in said cutout and having its open end flush with said exterior surface; a ferrite loaded dipole element located within said frame; means grounding one end of said dipole element to one end of said frame; a two-conductor transmission line having one of its conductors connected to the other end of said dipole element and the other of its conductors connected to the other end of said frame whereby said ferrite-loaded antenna element functions in connection with said notch as the electrical equivalent of a center fed folded dipole; and a dielectric window covering said cutout.

13. An antenna as defined in claim 12 including: a tuning capacitor means connected between said frame and said dipole element.

14. An antenna as defined in claim 13 including: means for adjusting the capacitance of said tuning capacitor means.