US2678392A - Mounting choke - Google Patents

Description

May 11, 1954 BLITZ 2,678,392

MOUNTING CHOKE Filed June 12, 1952 F'IG. 2

A v INVENTOI? l. 2/ DAN/EL BLITZ ATTORNE'V Patented May 11,1954

MOUNTING CHOKE Daniel Blitz, Boston, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass, a corporation of Delaware Application June 12, 1952, Serial No. 293,225

4 Claims.

This invention relates to chokes for the mounting of radio equipment such as antennas to be mounted with their outer surfaces flush with a conductive sheet such as the skin of an aircraft.

In mounting antennas flush with the skin of aircraft, it is often desirable that current at radio frequencies flow between the antenna and the skin so that it would not be desirable to insulate the antenna from the skin. Furthermore, even if insulation were introduced at this point, it would be likely to accumulate dirt and metallic particles. This would reduce the impedance of the joint at radio frequencies. If an attempt is made to connect the antenna to the skin directly, the vibration of the aircraft is likely to cause the contact of the antenna with the skin to become intermittent. This has the effect of amplitude modulating the current flowing between the antenna and the skin to generate spurious sig nals which can interfere with the performance of the equipment. A permanent fastening, such as riveting, can minimize this difficulty but then the antenna is not easily removed for servicing.

These difliculties are overcome while still maintaining an aerodynamically smooth outer surface by the use of the mounting choke of this invention. This mounting choke forms with the antenna a shorted half wave transmission line at the operating frequency with its open end at the outer surface where it presents a virtual short 9 circuit to the antenna current. The mounting, which is in the form of a bracket, is located in this transmission line at a point approximately a quarter wave away from the shorted end at the operating frequency, so that there is a minimum flow of current at the point where the bracket is attached to the antenna frame, so that the resistance through the contact point is not critical. Thus, the mounting of the invention forms a choke at the operating frequency. Conductive material collected at the joint between the antenna and the skin has no effect as there is already a short circuit at this point. Any looseness or vibration at the fastening point has no effect as good contact is not required here. In fact, an insulating gasket can be inserted between the bracket and the frame. The gap between the skin and the antenna may be filled with a waterproof resilient dielectric, such as cork, to prevent accumulation of conductive material at the point of attachment.

The foregoing and other advantages, objects, and features of the invention will be better understood from the following description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a bottom view of a pair of antennas mounted in a conductive sheet with a choke structure of the invention;

Fig. 2 is a section taken along the line 2-"? of Fig. 1; and

Fig. 3 is a detail of a lower left corner of Fig. 2 showing a choke of the invention.

The reference numeral It designates the skin of an aircraft or other sheet of conductive material into which is fitted a frame it carrying a pair of horn type antennas if that may be associated with the radio equipment of the aircraft. The horns l2 and associated equipment are arranged within a metallic case it that also serves as a shield for the equipment. This case It is attached to the frame it and to the skin it! by means of a mounting bracket it, best seen in Fig. 3. It can be seen from Fig. 2 that the bracket id is formed integral with the lower edge of the case It. A flange 15 formed on the lower edge of the case 13 is attached to the skin it by rivets It or by other well-known means. The bracket proper Ill is formed as a second flange coming out from the case at a point located a distance equal to a half wave length at the operating frequency upward from the outer surface of the skin iii. This bracket i l bends downward parallel to the case I3 for a distance equal to a quarter wave length at the same frequency and then turns inward to form a flange ii to which the frame I i is attached by a bolt and nut 18. A gasket 2!) of insulating material is shown inserted between the flange ii and the frame it. This gasket is optional. In addition, a strip 2! is shown inserted into the joint for the purpose of keeping out moisture and foreign substances that might lodge at the point between the sides of the transmission line at a high impedance point, spoiling the action of the choke by changing its transmission characteristics. This strip may bemade of cork or any other resilient waterproof insulating material. The use of such a strip is not essential to the operation of the invention, but is desirable. The physical length of the transmission line must be reduced to maintain the same electrical length with the insulating material introduced. When the radio equipment is operating and the horns :2 are radiating or receiving radio frequency energy, currents at the operating frequency will flow in the frame I l, and it may be desirable, as pointed out above, that these currents also flow in the skin It]. It was pointed out above that the bracket It comes out from the case I3 a distance equal to a half wave length from the outer surface of the skin Ill, and turns downward to meet the frame II. The space between the case I3 on one side and the bracket 14 and the frame it on the other side forms a shorted half wave length section of transmission line which offers a virtual short circuit to the currents at the operating frequency and permits them to flow freely between the frame II and the skin Hi. It was pointed out above that the bracket is a quarter wave length long from the point where it comes out from the case I3 to the point where it is attached to the frame II. This section of the structure forms a shorted quarter wave length section of transmission line so that there is a minimum flow of current at the operating fre-- quency through the junction of the flange ll and the frame ll. Thus the junction need not be a good electrical contact and any variation in its conductivity during use will not aifect the operation of the equipment. For this reason, the gasket 2!] may be of dielectric material, if desired. However, if foreign substances, such as moisture or particles, are allowed to accumulate across the transmission line between case it on one side and bracket I l and frame it on the other side, its electrical length is altered. This is prevented by the cork strip 2!. When the space between the case l3 and the bracket 14 and frame H is filled with cork, as shown in the drawing, the effective Wave length of the operating frequency within the space is reduced and the dimensions of the parts of the mount are reduced over what they would be in the absence of such a strip. The electrical lengths of the parts of the mount need not be exactly the half and quarter wave lengths specified. It is sufficient if the dimensions approximate these optimum dimensions sufficiently closely to result in a relatively low impedance path between the outer surface of the antenna frame and the skin of the craft and a relatively high impedance at the point of attachment.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A mounting for a radio frequency energy radiator that is to be mounted flush with the surface of a sheet of conductive material comprising a bracket having a J-shaped main portion with the long arm approximately half as long as a wave length at the operating frequency of the energy to be radiated and the short arm approximately a quarter as long as a wave length of the operating frequency to provide a virtual short between the conductive sheet and the outer surface of the radiator, and a low current flow at the point where the radiator is connected to the bracket.

2. A mounting for a radio frequency energy radiator that is to be mounted flush with the surface of a sheet of conductive material comprising a bracket having a J-shaped main portion with the long arm approximately half as long as a wave length at the operating frequency of the 4 energy to be radiated and the short arm approximately a quarter as long as a wave length at the operating frequency to provide a virtual short between the conductive sheet and the outer surface of the radiator, a low current flow at the point where the radiator is connected to the bracket, and resilient dielectric material inserted between the arms of the bracket.

8. In combination a radio frequency radiator, a sheet of conductive material and means to mount said radiator with its external surface flush with the surface of said conductive sheet comprising a bracket having a J-shaped main portion with the long arm of a length equal to approximately half a wave length at the operating frequency of the radiated energy and the short arm of a length equal to approximately a quarter of a wave length at the operating frequency with means to attach the end of the long arm to the conductive sheet, mean to attach the short arm of the bracket to the radiator at a point located a distance equal to approximately a quarter wave length at the operating frequency from the outer surface of the conductive sheet so that the arms of the bracket and the part of the radiator between the point of attachment of the bracket to the radiator and the outer surface of the radiator is a shorted half wave length section of transmission line at the operating frequency, thus presenting a short circuit to currents at the radiated frequency flowing between the outer surface of the radiator and the conductive sheet, and the point of attachment located at a point where there is a minimum of current seeking to flow between the radiator and the mounting bracket.

4. In combination a radio frequency radiator, a sheet of conductive material and means to mount said radiator with its external surface flush with the surface of said conductive sheet comprising a bracket having a J-shaped main portion with the long arm of a length equal to approximately a wave length at the operating fre quency of the radiated energy and the short arm of a length equal to approximately a quarter wave length at the operating frequency with means to attach the end of the long arm to the conductive sheet, means to attach the short arm of the bracket to the radiator at a point located a distance equal to approximately a quarter wave length at the operating frequency from the outer surface of the conductive sheet so that the arms of the bracket and the part of the radiator between the point of attachment of the bracket to the radiator and the outer surface of the radiator is a shorted half wave length section of transmission line at the operating frequency, thus presenting a short circuit to currents at the radiated frequency flowing between the outer surface of the radiator and the conductive sheet, and locating the point of attachment where there is a minimum of current seeking to flow between the radiator and the mounting bracket, and resilient dielectric material inserted between the arms of the bracket.

No references cited.

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