Aerial system

May 5, 1953

2,637,814

W. A. JOHNSON

AERIAL SYSTEM Filed Sept. 1, 1949

<img file="US2637814A_D0001.tif" />

Patented May 5, 1953

2,637,814

UNITED STATES PATENT OFFICE

2,637,814 · AERIAL SYSTEM

-William Arthur Johnson, Farnborough,.England, assignor to the Minister of Supply in His Majesty’s Government of the United Kingdom Of Great Britain and Northern Ireland, London,England

Application September 1,1949, Serial No. 113,574 In Great: Britain 'September 3,1948

Claims.

This invention-relates to suppressed or dragless aerials, particularly for aircraft and-is especially concerned with the annular slot variety of such aerials.

An annular slot aerial consists, as its name implies, of an annular slot in a metal surface and this necessarily involves a central-metal disc requiring support. A method of doing this is to provide a cavity in the¹‘form of a shallow cylindrical box at the back of the surface and to support the disc-on a column projecting· from the bottom of the cavity. The slot may then be current fed by a concentric cable, the·inner conductor being passed through the column and connected to the centre of the disc and the outer conductor connected to the-'back of the cavity so that there is a radial current flow from the centre of the disc towards the slot and a potential difference is established across-its edges.

According to the present invention, however, the slot is fed, or energy’is taken from the slot, by inductive coupling with the magnetic¹ field of ' the cavity. The inductive coupling may be achieved by means Of one or more coupling loops consisting or each consisting of a. conductor extending radially from the central column ·just behind the •disc and then downwardly’orrearwardly· to the back of the cavity so as to embrace most of the magnetic field of the cavity. The column may be hollow and contain a variable matching condenser through which the loops are fed from the inner conductor of a concentric cable taken to the back of the cavity.

An annular slot aerial has a¹. more* or less circular polar diagram:in azimuth· and· is vertically polarised, assuming the surface is 'horizontal. The vertical distribution approximately follows -a cosine law for slots whose diameter :.is -less than a wavelength, and varies with the «radius of the slot, for larger slots. The polar .diagram of an aerial in accordance with· the invention is more or less circular depending nn thei number of feed loops. Two .loops are-found :to give sufficient symmetry : for most . practical purposes, and four loops give almost uniform· polar diagrams.

A feature of the invention consists in taking advantage of the asymmetry Of the · polar diagram when the aerial¹ is fed by¹ a single loop to provide a directional aerial of the· “split”'type.

(Cl. 250—33)

Two coupling loops are provided· at'radially opposite points‘in the cavity and the loops are separately fed. ¹ Thus’by switchingofromone to the other the sense of the asymmetry·is reversed ⁵ and a spilt beam polar · diagram is obtained.

Alternatively, or in conjunction- with· this «switching; the annular· slot may be short circuited-at the appropriate points.

•Reference is-now 'directed·to the accompany•to ing drawing illustrating one-practical embodiment of the invention. -In the drawing:

Figure 1 is a'· diagrammatic cross· section of an • aerial in accordance with the invention and

Figure 2 is a plan- view or back .view of the 15 aerial shown in Figure :1.

'Referring to Figure --.1,:.- the · annular . slot is formed between a frame part 2 secured around a circular ..hole in the.metal. skin..3 of.an aircraft, preferably the under .surface, of the main plane, 20 and a metal disc 4. The disc-4 is secured to the central boss 5 «.of a shallow metal spinning -6 having an . annular channel .7 and . a flange 8 whereby it is secured to the frame 2. It will be understood that, the;parts 3 and 2, 2 and 8, and 4 25 •and· ·5 are secured together -in close metallic contact by means such as rivets x and that there is no space-between them as in the drawing.

The slot I is,fed or energy is taken from the slot . by inductive coupling with the magnetic ₃₀ field of the cavity. In the embodiment being described four coupling loops fed in phase are employed : at - 90° spacing ; in order to obtain a : particularly uniform polar, diagram. Each, loop a—b—c—d consists of a rod a—b including a series condenser C, a part,c—d of one side wall ⁰ of the channel 7 and a part e?—a of the bottom of said channel 7. The four , rods a—b are extensions of the inner conductors of the respective co-axial cable terminal fittings S which are . connected . in .parallel -by a branched system of ^υ quarter-wave and-half-wave transformers comprised of co-axial cables as . indicated in the drawing. Each of the series condensers C comprises a mushroom head on the rod a—b and the ₄₅ adjacent surface of the , disc 4. -A pre-set adjustment for the capacity.of the.condensers may be provided in. association.with.the·fixing means for the fittings 9 whereby the mushroom head may be moved closer to or further from the disc 4. 50 The embodiment of the invention which is

2,637,814 illustrated in the drawings was designed to operate over the wide band of about 300 to 500 mc./s. (55 to 100 cm.) and has the following dimensions:

Outer diam. of cavity---------------cm- 61

Inner diam. of cavity----------------cm— 34

Depth of cavity---------------------cm__ 4.27

Width of slot_______________________cm__ 3.2

Distance b—c ______________________cm— 7

Capacity of condenser C------------μμ fd— 5

Mean diam. of slot (D)--------------cm- 58

The calculated constants of a structure of the above dimensions are given in the following table from which it can be deduced that the system is resonant at a wavelength of 81.4 cm.

stituting with the juxtaposed portion of the conductive sheet a series-matching condenser.

2. An annular slot antenna comprising a conductive, substantially plane, sheet having a cir5 cular opening, a conductive backing part defining an annular channel which has an outer diameter substantially equal to the inner diameter of the opening, means for securing the backing part concentrically to the conductive sheet, a cir10 cular disc secured to the central portion of the backing part in the plane of the conductive sheet to form in conjunction with the conductive sheet an annular slot, a coaxial cable terminal fitting secured to the backing part, an extension of the 15 inner conductor of the terminal fitting insulated from the backing part and extending through

<td> Wavelength in cm..............</td><td> 55</td><td> 60</td>

<td> D/Wavelength.________________ Radiation Resistance, ohms— Reactance of Slot, ohms________ Reactance of Cavity, ohms_____</td><td> 1.055 34.4 -9.81 61.0</td><td> 0.967 32.3 -10.7 35.2</td>

70 75

An aerial in accordance with the invention for operation at other frequencies may be designed by scaling the above dimensions up or down in proportion to the wavelength. In practice it may be desirable to cover the slot with low loss dielectric and to compensate for the introduction of the dielectric by widening the slot. For example, if the slot in the above embodiment is covered with Λ in. thick “Perspex,” then reducing the diameter of the disc 4 gives sufficient compensation.

A modified application of the invention is to feed the coupling loops independently through switches or phase changing means so as to vary the shape of the polar diagram. By switching from one feed system to another a directional diagram of the twin lobe “split” type is obtained.