US3516936A - Lossy material for antenna decoupling - Google Patents

Description

June 23, 1970 v A'. EQHENDERSON ,5 3

I LOSSY MATERIAL FOR ANTENNA DECOUPLING e: 3.0 )TAN 8 :05

Y MAGNITUDE OF 7| 1 AS A FUNCTION OF 72 1 Filed July 27. 1967 0 5 0 E N A L P 0 Z U S O T m L T A v M .l R G O N S N I 0 m m x E A R y ALVIN E. HENDERSON .INVENTOR.

t r I BY REGION 2 e TAN 8 MAGNETIC SOURCE (z o) ATTORNEY FIG.I

United States Patent US. Cl. 25263.2 2 Claims ABSTRACT OF THE DISCLOSURE A moldable lossy material that can be bonded to any surface for accomplishing RF attenuation between two RF sources in the same ground plane, and consisting of a mixture of a modified epoxy, filler-hardener, graphite, hydrogenated naphtha and carbon The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

When two antennas are mounted in a common ground plane, there is a certain degree of mutual coupling. Since for some applications this coupling imposes various restrictions upon the performance of the antenna system, it is desirable to reduce this interaction and the accompanying leakage of energy from one antenna to the other. Previous decoupling methods, such as ribbed structures and modulated reactive sheets, achieve decoupling by radiation of the energy in the surface wave causing deterioration of the antenna azimuthal pattern. These, as well as cavity structures, and the milling of attenuation slots along antennas which weakens the structure, have proven to be costly and inadequate. The use of the lossy material of the present invention to provide RF attenuation between two RF sources on single ground plane overcomes the disadvantages of the previous decoupling methods.

It is an object of the invention to provide a practical means for reducing the mutual coupling and leakage of energy between two flush-mounted antennas in a common ground plane.

Another object is to provide a moldable lossy material for accomplishing RF attenuation between two RF sources on a single ground plane.

A further object is to provide a decoupling compound that can readily be formed into complex shapes or forms for decoupling between plural antennas.

Other objects and many of the attendant advantages of this invention will become readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 shows coordinate axes used in the derivation of the TM surface mode that produces coupling between flush-mounted antennas in a common ground plane.

FIG. 2 illustrates various typical mode solutions.

FIG. 3 shows an azimuthal polar pattern for a single slot with a slab of lossy material of the invention covering one side of the antenna.

In a preferred embodiment of the invention the lossy material for antenna decoupling is made as follows: To 75 grams of modified epoxy having a specific gravity of 1.12 to 1.14 and an epoxide equivalent of 180-200 is added 75 grams of a graphite colloidal mix of 50 parts by weight of graphite and 100 parts by weight of hydrogenated naphtha. Then add 7 grams of carbon having a resistivity (ohm-inches) of 0.032 when 100% compressed is added and the ingredients mixed to a smooth slurry for approximately minutes. To this mixture is added 75 grams of filler-hardener having a specific gravity of .97 to ice 1.03, an amine value (Meg HClO /gm.) of 14.3 to 16.5 added thereto and hand mixed for approximately 10 minutes. The resulting compound can then be molded. For example, this compound was poured into a mold preheated to 160 F., cured for 12 hours at room temperature and post cured for 4 hours at F. to form a flat sheet /a inch thick.

This material was used for reducing the mutual coupling and leakage of energy between two flush-mounted slot antennas in a common ground plane by introducing a thin slab of the lossy material between the antennas. Theoretical equations are derived below to describe the surface mode of propagation, and experimental measurements given by way of example show that this method of decoupling using the lossy material of this invention produced 19 db of decoupling without any appreciable effect on the antenna azimuthal beamwidth. Theoretical attenuation or decoupling is 22.1 db.

The radiatoin field about a slot mounted in an infinite flat ground plane has an amplitude that is constant with the azimuthal angle about the slot. As the ground plane becomes finite, however, amplitude variations are observed in the radiation field. The slot sets up two modes, one that radiates through free space, and another that propagates loosely along the ground plane. The latter the surface modeproduces the coupling between flushmounted antennas in a common ground plane.

Since the TM mode requires an inductive reactive surface, it is the natural one for such loose propagation along a flat ground plane. In analyzing a TM surface mode set up by a simple slot in an infinitely conducting ground plane covered by an infinite lossy dielectric slab, the coordinate axes shown in FIG. 1 are used. The slab is excited by a magnetic source at z 0, and the magnetic field above the slab has the form where 'y1=corri plex propagation constant in the x direction 'y complex propagation constant in the z direction The electric field in the z direction is given by The propagation constants are related by Similar equations may be obtained for region 2,

where H., =A cosh 2ac)-e (4) z E, m2 sinh x) e with the propagation constants related by 'Y2 +'Y c The hyperbolic functions are used, since region 2 is lossy. The electric field E varies as the sinh x), since it must be zero at the boundary for x=0. Making use of the remaining bounadry condition that at x=t, where t is equal to the dielectric thickness,

the following transcendental equation is obtained '1 J or, since 6 is complex, Eq. 7 may be written as (T1 1m tanh "/2 (8) and using Eq. 3 and 6 (71' 1 2'( j tan 2) }+('Y2 where e "=I'elailVC dielectric constant tan 5 =loss tangent k =propagation constant of region 1 (i.e., free space) AI2B (ta P1 44061 01 e 10) The relative power at any point may be written as relative P =e- (11) From the mode solutions that give '7 a may be solved in terms of k a and B by Experimental measurements were made of the coupling and of the antenna pattern in the azimuthal plane. Coupling measurements were made with and without the lossy slab between the antennas. The lossy slab used was 0.125 in. thick and made of the compound of this invention. The dielectric constant was 7.0 with a loss tangent of 0.9. Coupling was down 19 db, as compared with a theoretical 22.1 db.

FIG. 4 gives a polar pattern in the azimuthal plane of one of the two antennas in the test setup. The half-power beamwidth is approximately 168 deg. wide.

Experimental measurements corroborated the theoretical equations that were derived to describe the surface mode of propagation and to demonstrate how the magnitude of the coupling varies as a function of the dielectric parameters. The use of lossy material, as disclosed herein, for decoupling antennas in a common ground was the only method out of several various other type methods also investigated that did not result in deterioration of the antenna azimuthal pattern.

Obviously many modifications and variations of th present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In an antenna system wherein a plurality of antennas are mounted in a common ground surface, a moldable lossy material for providing RF attenuation between RF sources mounted between the antennas for decoupling same, the lossy material consisting substantially of:

(a) seven parts by weight of modified epoxy having a specific gravity of 1.12 to 1.14 and an epoxide equivalent of 180-200,

(b) seven parts by weight of graphite colloidal mix of parts by weight of graphite and parts by weight of hydrogenated naphtha,

(c) one part by weight of carbon having a resistivity of 0.032 ohm-inches when 100% compressed added to said modified epoxy and said graphite colloidal mix, and the three ingredients mixed to a smooth slurry for approximately 5 minutes,

(d) seven parts by weight of filler-hardener having a specific gravity of 0.97 to 1.03 and an amine value (MegHClO /gm.) of 14.3 to 16.5 added to said slurry and again mixed for about 5 minutes,

(e) approximately two parts by weight of hardener having a specific gravity of 1.00 to 1.20 and an amine value (MegHClO gm.) of 14.3 to 16.5 mixed with said slurry and filler-hardener mixture for approximately 10 minutes to form a resulting moldable lossy dielectric compound.

2. A compound as in claim 1 molded to form a desired configuration in mold preheated to 160 F. and cured for 12 hours at room temperature, then postcured for 4 hours at F.

References Cited UNITED STATES PATENTS DOUGLAS J. DRUMMOND, Primary Examiner U.S. Cl. X.R.

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