Boresight Chamber Assembly Apparatus

This invention relates to measurement apparatus for antenna patterns in general and more particularly to a near field measurement technique for providing a boresight error.

A boresight only measurement facility is a special class of near field measurement systems in which the primary manifestation is in the measurement of the phase distribution of a near field pattern. An antenna system with zero boresight error with respect to the near field measurement plane would have a constant phase throughout the collimated region of the near field for the sum mode and 180 degree step function for the difference mode. Thus a boresight error manifests itself as a tilt of the phase front and can be determined by a mathematical relationship.

Essentially, the boresight error is the difference between an assumed boresight direction or a mechanical boresight direction and the electrical boresight direction determined from the radiation of the antenna. The use of near field measurement techniques for field antenna pattern determination was developed many years ago. The near field measurement technique became practical over 20 years ago with the advent of phase/amplitude receivers and with the development of the theoretical basis for probe compensated measurements, sampling and digital computation. The near field measurement techniques have been accepted in the industry and have been used by the military for measuring antenna patterns. The technique has become the NBS standard for gain calibration of microwave antennas.

Near field measurement systems are used not only to determine far field patterns of antennas but are also used to detect fault elements of phased array antennas, phase align phased antennas, focus antennas and boresight antennas. Many of these require the use of only the near field data without the need for far field calulation.

Essentially, all measurement techniques for measuring the far field radiation pattern of an antenna can be used to measure the boresight error of an antenna. These techniques include the elevated outdoor far field range, the ground reflection far field range, the slant far field range, the anechoic chamber far field range, point source compact range, the line source compact range, point source lens range, the planar surface near field range, cylindrical surface near field range and spherical surface near field range. Each of these ranges is capable of complete far field antenna pattern measurement and thus boresight measurement. These techniques as indicated above, require extensive equipment including expensive antenna mounts, receivers pattern recorders and are specifically designed for high accuracy boresight measurement. There are, of course, existing in the prior art several apparatuses specifically designed for high accuracy boresight error measurement which include the far field difference pattern null seeker, far field sum pattern beam straddler and point source, lens assembly. The first two techniques are adapations of the far field antenna pattern measurement technique and thus require extensive equipment and normally a large outdoor or anechoic chamber facility. The third is an adaptation of the point source compact lens technique.

In any event, a near field measurement system has great advantages when compared to other techniques for measuring the boresight error. In such a system one can achieve high accuracy in a relatively small size with an attendant increase in reliability and utilizing accepted techniques. As will be explained, the present system is a near field measurement system which is fully automated and is capable of performing many measurements in a short time. The system can measure a phase distribution with approximately 30 near field measurements in one minute.

A boresight chamber assembly apparatus for measuring the boresight characteristics of an antenna, comprising a rectangular housing having an aperture of a given diameter located on a side wall and adapted to accommodate an antenna to be tested, an antenna structure having a base secured to a plate said plate being of a larger diameter than said aperture to enable said antenna to be positioned into the hollow of said housing when said plate is inserted over said aperture, a movable RF probe located in the hollow of said housing and adapted to move parallel to the radiating surface of said antenna, a source of RF energy coupled to said probe for enabling said probe to emit RF radiation and means coupled to said probe to move the same to a plurality of separate postions along a line each parallel to said radiating surface of said antenna, and processing means coupled to said antenna and said RF source for providing outputs indicative of any phase difference between the same with a separate output for each of said separate positions whereby the boresight error of said antenna can be derived from said outputs.

FIG. 1 is a block diagram of a boresight chamber assembly apparatus according to this invention.

FIG. 2 is a perspective plane view of a test chamber employed in this invention.

FIG. 3 is a partial sectional view depicting the test chamber together with an antenna system secured thereto.

FIG. 4 is a block diagram showing a control/display for the boresight chamber assembly.

Referring to FIG. 1, there is shown a block diagram of. the boresight chamber assembly according to this invention.

Essentially, there is shown a test chamber 17 which consists of a shielded enclosure. The test chamber 17 contains an inner frame 16 which has the components secured thereto. Essentially, the test chamber is approximately 24 inches high by 24 inches deep and 30 inches wide. The test chamber is a structurally rigid unit which contains an inner U-shaped frame 16 The inner frame 16 is enclosed by suitable sheet metal to thereby basically form the test chamber which is a rectangular box. Test chamber 17 is completely lined with an RF absorbing material 19 to prevent any radiation from reflecting from the chamber walls and entering the antenna under test and from leaking out of the test chamber. It is, of course, understood that there are many ways of fabricating such a test chamber.

As will be explained in greater detail, the test chamber essentially contains an input RF connector 1 which is coupled to an input RF power divider 2. Directed from the power divider 2 is a reference signal cable 3 and a measurement signal cable 4. Located within the test chamber are coil semi-rigid cables 5 which cables are connected to a near field probe antenna 8.

The probe antenna 8 can move in the vertical direction under control of a probe positioning unit 7 which will be described in greater detail. Located within the test chamber is an antenna seeker and gimbal system 9 hereinafter referred to as LRU 9.

The LRU 9 is associated with a matched load impedance 12. Located adjacent the test chamber is a manual control/display unit. The unit is also enclosed in a rectangular frame assembly 30 and contains a phase comparator 11 which as will be explained is coupled to the test chamber via a coaxial cable 31. One input of the phase comparator 11 is connected to the output of an RF power divider 10 which receives an input from the test chamber via cable 32. The test chamber also includes a module 13 which is a power supply for furnishing bias to electronic circuitry also included in module 13 and as will be explained. The manual control/display unit also has an output RF connector 14 which enables connection to an RF power meter 25. There is a data/control connector 15 which has one output coupled to a digital multimeter 23 and an input emanating from a computer 24. The digital multimeter, as will be explained, supplies input signals to the computer 24.

Coupled to the test chamber via the input coaxial connector 1 is an RF source 22. The source 22 supplies approximately 20 dBm of RF power at a desired test frequency which frequency is within the range of antenna operation. The RF power from the RF source 22 enters the test chamber through the input coaxial connector 1 which may be a standard steel type N connector. The input power is divided by the input power divider 2 into two paths which, as will be explained, are both directed B to the phase comparator 11 contained in the manual control/display unit. One path from the power divider is the reference path 3 an the other path is the near field measurement path 4. The power divider is a conventional component and many examples of such dividers are commercially available from many different sources The ratio of the power division between the reference path 3 and the near field path 4 is such that the two signals arriving at the phase comparator have approximately the same amplitude. As will be explained, the phase comparator does not require that the two levels be the same as the exact signal level is not critical. The power division ratio from divider 2 is determined using the near field gain of the seeker antenna and associated gimbal system (LRU 9). The seeker antenna and the associated gimbal system (LRU 9) are secured to the test chamber 17 via an externally supplied mounting plate. The test chamber has an aperture for receiving the LRU 9 which is mounted on the side of the test chamber as shown in FIG. 1. The power division ratio is also determined in accordance with the gain of the near field probe 8, the insertion loss of the scissor arm cable assembly 6 and the spatial loss between the probe 8 and the LRU

The amplitude and phase reference path 3 is coupled to the comparator 11 by means of the coaxial cable 31 which extends directly from the input power divider 2 to the phase comparator 11. The near field measurement path begins at the input power divider 2 and extends by way of the coaxial cable 4 to the probe postioning unit 7. The signal passes through three, three coil semi-rigid cable 5 which is incorporated into the scissor arm assembly 6 on the probe postion unit 7. The probe position unit 7 is used to move the near field probe 8 vertically in a plane parallel to the phase of the LRU 9. The probe positioning unit 7 is a self-aligned configuration which utilizes preloaded linear bearings. Such devices are well known and are utilized for linearly advancing a mechanism by means of an lead screw 32 which is associated with guides. The guides are normally made of stainless steel and the lead screw 32 is also stainless steel. The lead screw is mounted using high grade threaded bearings. As indicated, such probe positioning units as 7 are commercially available and have been employed in a great many devices. The three point scissor arm assembly 6 is necessary to allow the signal to arrive at the near field probe 8 without introducing amplitude or phase variation as the probe is moved throughout its vertical travel. The scissor arm assembly utilizes two or three turn coils of semi-rigid coaxial cable at each of the three scissor arm points. These coils are used rather than conventional rotary points to provide better high frequency performance and thus improved dependability of phase measurement. The technique of using semi-rigid coils in lieu of rotary points is also well known and has been employed in other systems. Thus the output of the scissor arm assembly signal path is applied to the input of the near field probe 8. The near field probe 8 is a linearly polarized low gain open ended wave guide type antenna positioned approximately three wave lengths from the front of the LRU 9 under test. This broad beam width probe 8 will radiate energy to all elements of the antenna or LRU 9 under test. Therefore, a single scan will provide a complete test of each element in the LRU 9. The near field measurement path signal is radiated from the near field probe 8 and received by the LRU 9. The LRU 9 has two output ports both of which become energized upon receiving the signal radiated from the near field probe. One port is the difference port and the other port is the sum port.

The difference port contains the pattern information relating to the difference pattern null which determines the elevation boresight direction. When determining the elevation boresight error, the difference output port is connected to the directional coupler 10 or RF power divider. The through port of the coupler 10 is applied to one input terminal of the phase comparator 11. The sum output port as shown in the FIG. is terminated in the matched load impedance 12 when not in use. Should it be desired to measure the sum response of the antenna, then the sum output port is connected to the coupler 10 and the difference port is terminated with the matched load impedance 12. The coupler 10 is included to allow the external RF power meter 25 to monitor the amplitude of the receive signal. The power meter 25 may be the HP-436A which is an RF power meter manufactured by the Hewlett-Packard Company of California.

The coupler 10 is connected to a stainless steel type N coaxial connector 14 to facilitate the connection of the power meter to the unit. The ratio of the power divider 10 is approximately 20 db with the largest output directed to the input of the phase comparator 11. The phase comparator 11 compares the received signal output of the LRU 9 to the reference signal from cable 31. Two dc analog signals are produced by the phase comparator. One signal is proportional to the amplitude and the other signal is proportional to the phase of the output of the LRU 9 with respect to the reference signal. By using a phase comparison technique, it is not necessary that the phase of the input signal remain constant since the performance of the LRU 9 is determined by the phase difference between the RF input and the LRU 9 and not the absolute phase. The analog outputs from the phase comparator 11 which are the phase and amplitude signals are directed to the electronic module 13 where they are buffered by conventional amplifier techniques and are then coupled to connector 15 to be applied to the digital multimeter 23. The digital multimeter 23 is a conventional component manufactured by many companies such as the Hewlett-Packard Company and will provide digital output signals relating to amplitude and phase. The output signals directed to connector 15 may also be appropriately scanned to drive suitable display meters whereby a display of amplitude and phase can be obtained directly.

Such meters can provide direct digital readouts which correspond to amplitude and phase. The computer 24 which has outputs coupled to the connector 15 which output signals operate to control the probe position motor to enable movement of the near field probe vertically in the plane perpendicular to and centered on the face of the LRU 9. Essentially, the probe positioning unit 7 contains a drive motor which drives the lead screw 32 based on the magnitude of voltage applied to the motor. The computer will generate a specified number such as 30 of output digital signals which are converted to analog signals by means of a digital-to-analog converter contained in module 13. These analog signals will then drive the probe 8 to approximately 30 equally spaced points along the surface of the LRU 9. At these thirty equally spaced points, the outputs of the phase comparator 11 are sampled and recorded in the computer via the digital multimeter 23. These thirty data points are then used to calculate the boresight direction of the LRU 9 using a conventional algorithm which the computer 24 is programmed to solve and as will be explained. The calculated boresight direction is compared to the specified direction to determine the boresight error of the LRU 9. The digital multimeter 23 and the computer 24 as indicated are connected to the system through a standard multipin connector 15.

The connector 15 is connected via internal cabling to the electronic assembly 13. In regard to the above noted operation, the boresight error manifests itself as a tilt of the phase front and can be determined by the following equation: ##EQU1## Where .DELTA..phi. is the phase difference in degrees across the collimated region L of the antenna and .lambda. is the wavelength. For the case of small boresight error and for an antenna aperture L of 13.9 .lambda., this expression reduces to approximately

This expression shows that the phase must be measured to an accuracy of 5 degrees for a boresight error of 1 milliradian. The following error budget for phase measurement is easily achievable with off-the-shelf components.

Thus it is estimated (based on a total phase accuracy of .+-.0.7 degrees) that the accuracy of the proposed system is about .+-.0.14 milliradians. The repeatability of the system is less as the two largest errors (Phase change in Rotary Joints and Mechanical Accuracy) are in most part repeatable.

Thus as one can see from the above expression and FIG. 1, the system allows one to rapidly scan the near field of the antenna (thirty different phase readings at thirty different points) and, therefore, compute the boresight error for the antenna based on the thirty different points. The system utilizes conventional components all standard off-the-shelf units. It is, of course, understood that the computer 24 can be replaced by a mechanical control to thereby mechanically move the probe 8 into a desired number of positions.

The above noted equation can easily be solved by a computer but also can be manually solved or be solved in general by ordinary logic circuitry as the operations indicated by the above equation are simple to implement. Based on the components utilized in such a system, the system described can measure the phase differences with approximately thirty near field measurements taken in one minute.

As indicated above, these measurements are taken by the simultaneous movement of the near field probe and the recording of the near field data under control of the computer. Thus once the LRU 9 is mounted on the test chamber, all measurements and mechanical motion can be carried out without manual intervention.

In conjunction with the above noted description, the brief description of the mechanical arrangement will be described

Referring to FIG. 2, there is shown a perspective view of the test chamber assembly. The test chamber is designed to facilitate normal calibration and maintenance procedures. Essentially, the test chamber consists of two components. A first component 40 is a U-shaped frame member having slots as 41 and 42 on the bottom surface and slots as 43 and 44 on the top surface. The U-shaped member has a front aperture 45 into which the LRU 9 is mounted. The U-shaped inner frame member 40 is formed from a suitable metal and is enclosed by a complementary fitting sheet metal member 50 which slides into the slots as shown and is securely mounted to the unit by means of the sealing plates 52 and 53. The slots or grooves as 41 to 44 are cut into the inner frame and are milled to a depth which is equal to 1/4 wave length at the operating frequency. In this manner the grooves act as quarter wave choke sections to effectively seal the test chamber from radiating outwardly. Additional metal textile gasketing and finger stock are used at the mechanic interfaces to substantially reduce the external RF radiation at the frequencies of concern. The inner surface of the frame 40 and the cover 50 are fully lined with RF absorbing material.

As can be seen from the FIG., the U-shaped cover 50 is readily removable from the frame 40. This is accomplished by removing the captive fastening screws from the sealing plates 52 and 53.

Referring to FIG. 3, there is shown a cross section view of the test chamber particularly depicting how the LRU is mounted. As indicated, there is an aperture 45 in the test chamber. The LRU is mounted to a mounting plate 60 via suitable bolts as 61. The mounting plate 60 with the LRU attached to it is then inserted through the aperture 45 and bolted to the test chamber via the mounting plate. The mounting plate also contains an RF shield which prevents RF energy from radiating out.

Shown positioned adjacent the LRU is the probe positioning unit. As indicated, the probe positioning unit consists of a lead screw 62 onto which the probe and housing are movably attached Adjacent the lead screws are guides as 63 and 64 which guides are attached to the top of the frame assembly by means of an adjusting block 66. The guides are made of hardened stainless steel as is the lead screw. The entire unit as shown is available from many manufacturers.

The motor 67 is a dc motor which engages the lead screw 62 to allow the probe to move parallel to the front surface of the LRU.

Referring to FIG. 4, there is shown a panel display which is employed with the manual control/display unit. The panel display can be employed for manual operation and has three meters located on the front. The first meter 70 displays the amplitude of the RF signal while the second meter 71 displays the phase. The meter 72 is calibrated so that it gives a direct readout of the probe position. When the probe is at the center of the antenna, the meter 72 reads zero and when the probe is at the top of the antenna it reads directly in inches from the center as +4.9 as shown in the FIG. When the probe is an equivalent number of inches beneath the center, the meter 72 will read, for example, -4.9. The use of such meters in conjunction with the digital multimeter and RF power meter are well known and many examples of such devices are commercially available. In any event, during manual operation, the travel of the probe is controlled by means of the knob 74 which will activate the motor and cause the motor to move the probe while at the same tim the distance the probe moves is automatically displayed via the meter 72. In the automatic operation of the equipment as shown in FIG. 1, the computer 24 is programmed to allow the test to be automatically conducted. The computer generates a series of digital signals which are converted via a digital-to-analog coverter contained in the electronics package 13 to control the motor and, therefore, to position the probe at a plurality of desired postions. At each of the positions as controlled by the computer, the samples of the amplitude and phase different measurements are taken. The computer software is set up so that it samples the data at fixed intervals and stores this data in memory. At the completion of a scan, the boresight algorithm checks the data for any gross errors. If any gross errors exist, the operator is informed and given the option of halting the test or allow the algorithm to complete its process. At the completion of the process, the operator can then have the computer print out the data and determine whether the LRU under test meets the requirements or not. The computer is programmed to solve the above noted equation as the probe is scanning the antenna.