US2490782A - Antenna testing shield - Google Patents

Description

Dec. 13, 1949 D. E COLLUP ANTENNA TESTING SHIELD Filed April 5, 1946 Patented Dec. 13, 1949 UNITED STATES TENT ortica ANTENNA TESTING SHIELD Doyle E. Collup, United States Army, Fort Worth, Tex., assigner to United States of America as represented by the Secretary of War Application April 5, 1946, Serial No. 659,742

(Cl. Z50- 33) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 7 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention concerns a shield which is fitted over an antenna (particularly of the transpondor type mounted on an airplane or Yother vehicle) to prevent radiation during test of this antenna from reaching other antennas in the vicinity.

An object of the invention is to provide a shield (preferably non-circular) which surrounds the antenna being tested and which is asymmetrically located with respect to it, the shield also including within it radiation-absorbing cloth for the purpose of absorbing the radiation from the antenna.

Another object of the invention is to provide within the shield, a radiator for triggering the transpondor and for taking oi the transpondor reply.

Still another object of the invention is to provide a shield for testing a transpondor antenna, the shield being so constructed as to minimize symmetrical reections from it to the antenna.

Fig. 1 shows a generally cross-sectional view of the shield in position over the antenna of a vehicle.

Fig. 2 is a cross-section taken on the line 2-2 of Fig. 1.

Fig. 3 is a bottom plan view of the shield.

In the drawings, Ill represents a portion of the body of an airplane or other vehicle, to which is attached a streamlined casing I I. Within casing Il is located a transpondor antenna I2.

The antenna test shield itself includes a casing I3 made of electrically-conducting material and preferably of rectangular cross-section, within which there is another casing I4 of the same shape as casing I3. Casing I4 is made of a good high-frequency dielectric having a low dielectric constant, low absorbing power, and low reflecting qualities. A suitable material of this character is polystyrene.

Casing I4 serves to support, and is covered with, radiation-absorbing space cloth I5. This cloth is preferably a thin rubberized fabric having a deposit of graphite on its surface, and having an impedance of 377 ohms per unit square which is equal to that of atmosphere. The distance between cloth I5 and outside casing I3 is on all possible sides made to be substantially M4, where i is the wave-length of the radiation emitted by the antenna. This distance is capable of some variation, particularly since the wave-length of the antenna may not be xed but may vary within a certain range. Where there is such variation, A should be taken as the mean wave-length within the range. Considerations of practical construction may also necessitate some adjustment of this distance.

The radiation from the antenna I2 sets u-p currents in the space cloth I5 which are substantially absorbed by such cloth. These currents cause some radiation toward the antenna I2 as well as some toward the casing I3. The vwaves which are radiated toward the casing are, by reason of the M4 spacing of the casing from the cloth, returned to the cloth in opposite phase to the waves which are radiated from the cloth toward antenna I2. Thus the two sets of waves largely tend to cancel each other.

Antenna I2 must be asymmetrically located with respect to casing I3 in order to minimize the eiect of stray reiiections by causing these to be returned to the antenna by the shield in random phase.` This result is effectively and preferably accomplished by making the shield of rectangular cross-section and placing the antenna within the shield at a location which is asymmetrical with respect to at least the long axis of the shield. However, it is possible to accomplish reasonable reduction of the effect of stray reflections with cross-sections of the shield other than rectangular.

In order to prevent radiation from antenna l2 from reaching other antennas, the former must be surrounded by metal tothe fullest extent possible.

Within casing i4 there is an additional casing I6 made of a material like that of which casing I4 is constructed. Casing I6 has an opening 2| into which the streamlined casing II is inserted when antenna I2 is to be tested. Casing I6 is so shaped as to t closely around casing II and thus to position the antenna properly with respect to casing I3.

Extending between the sides and near thelower end of casing I4 there is a partition I1 which is also made of a material like that used in this casing. Located in the space between partition I'I, the bottom of casing I3 and casing I4 is a radiator I9 which, in its simplest form but not necessarily, is oi the M4 type. Radiator I9 1s used to trigger transpondor antenna I2 and to take oi the transpondor reply. Radiator I9 is so located with reference to antenna I2 that the former will not have any appreciable reaction on the impedance of the latter, and so that the selfimpedance of the former has a value which matches that of its connecting cable 24 and which remains substantially constant over the range of frequencies radiated by antenna I2. Ordinarily, the attenuation between radiator I9 and antenna I2 should be at least I2 db. Ordinarily, these requirements will be satisfied only if the radiator is located between antenna I2 and space cloth I5. If desired, radiator I9 need not actually be iixedly situated within casing I3, but may instead be separate from this casing and simply inserted into it through an opening in the bottom of the latter when antenna I2 is to be tested.

In the top 22 of the casing I3, lugs 2I are provided. These lugs pass through slots 23 in the body I and serve to support the shield on this body. A handle is also provided for convenience in using the shield.

While because of limitations of size and practical construction, this testing shield is especially suitable for use with antennas which are part of ultra-high-frequency systems, it could be adapted for use with systems employing other frequency bands.

The invention has been specifically described in connection with the testing of a transpondor antenna mounted on an airplane or other vehicle. However, it is capable of use in the testing of one antenna at a fixed location in the presence of other antennas at the same location.

I claim:

1. A shield for preventing radiation originating from a first antenna from reaching other antennas in the vicinity of the rst antenna, this shield comprising a box-like casing made of electrically-conductive material, within such casing an opening into which is inserted the first antenna, such opening being so situated as to place this rst antenna asymmetrically with respect to the casing, and also within the casing, means for absorbing radiation from the first antenna, these means being separated from substantially all the sides of the casing by a distance substantially equal to M4 where A is the mean wave length of the radiation from the first antenna.

2. A shield, as described in claim 1, in which the casing has a rectangular cross-section and in which the first antenna is placed asymmetrically with respect to at least two opposite sides of the rectangle.

3. A shield for preventing radiation from a rst antenna of the transponder type from reaching other similar antennas in the vicinity of the rst antenna, Ythis shield comprising a box-like casing made of electrically-conductive material, within such casing a rst opening into which is inserted the rst antenna, such opening being so situated as to place this first antenna asymmetrically with respect to the casing, also within Such casing a second opening into which may be inserted a radiator for the purpose of triggering the first antenna and taking oi the transpondor reply, a connecting cable for the radiator, the second opening being so situated as to place the radiator where its self-impedance matches that of its connecting cable and remains substantially constant with varying frequency of the radiation from the first antenna and where this radiator will not have any appreciable reaction on the impedance of the rst antenna, and also within the casing, means for absorbing radiation from the rst antenna, these means being separated from substantially al1 the sides of the casing by a distance substantially equal to M4 Where Aris the mean wave length of the radiation from the rst antenna. I

4. A shield, as described in claim 3, in which the casing has a rectangular cross-section and in which vthe first antenna is placed asymmetrically with respect to at least two opposite sides of the rectangle.

5. A shield, as described in claim 3, in which the means for absorbing radiation from the rst antenna include a rubberized fabric having a deposit of graphite on its surface and having an impedance of 377 ohms per units square.

6. A shield for preventing radiation from a irst antenna of the transpondor type from reaching other similar antennas in the vicinity of the first antenna, this shield comprising a box-like casing made of electrically-conductive material, within such casing a iirst opening into which is inserted the first antenna, such opening being so situated as to place this iirst antenna asymmetrically with respect to the casing, also within such casing a radiator for the purpose of triggering the iirst antenna and taking off the transpondor reply, a connecting cable for the radiator, the radiator being so situated that its self-impedance matches that of its connecting cable and remains substantially constant with varying frequency of the radiation from the rst antenna and where this radiator will not have any appreciable reaction on the impedance of the iirst antenna, and also within the casing, means for absorbing radiation from the first antenna, these means being separated from substantially all the sides of the casing by a distance substantially equal to M4 where A is the mean wave length of the radiation from the first antenna.

7. A shield, as described in claim 6, in which the casing has -a rectangular cross-section and in which the rst antenna is placed asymmetrically with lrespect to at least two opposite sides of the rectangle.

DOYLE E. COLLUP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,847,872 Hand Mar. 1, 1932 2,293,839 Linder Aug. 24, 1942 2,296,678 Linder Sept. 22, 1942 2,412,562 Crawshaw Dec. 17, 1946

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