CONSIDERATIONS FOR UPGRADING A PRE-EXISTING NEAR-FIELD SYSTEM
Antenna Measurement Techniques Association Conference
October 4-8, 1993
Nearfield Systems Incorporated
1330 E. 223rd Street #524
Carson, CA 90745 USA
In the past, various companies have installed
large permanent Near-field antenna measurements systems. In many
instances, a test range has been constructed for a particular
project or purpose. the conclusion of the project, the range
may become dormant or under-utilized. In addition, a dormant
range quickly becomes a potential source for spare parts. These
factors combine quickly to render the once functioning range unless.
With the current industry emphasis on cost
reduction, minimizing new capital purchases, and utilization of
existing resources, an upgrade of a dormant test facility is a
preferable path. NSI has recently upgraded an existing Near-field
antenna measurement system at Hughes Space and Communications
Co. Hereinafter referred to as Hughes S&C. This paper focuses
upon the design considerations undertaken during the upgrade procedure.
Keywords: Near-field, Scanner, Interface, Receiver, Software, Upgrade
The advent of low cost, portable Near-field
Measurement System technology has drawn many companies into this
method of antenna metrology. This was a completely new method
of testing for some of these companies. However, a few had already
invested substantial capital to develop near-field measurement
technology in-house. These were typically larger permanent facilities
built for projects that could justify the expenditure. However,
after project completion, some ranges become dormant due to lack
of work. The focus of this paper is to examine the issues encountered
during process of upgrading and restoring a dormant near-field
Upgrading an existing range provides an extremely
cost effective method of increasing the test capability. If the
main structure is largely intact, the facility can be upgraded
and made functional for as little as 10 to 20% of the cost to
build a new facility. Replacement of worn or missing mechanical
parts and the interfacing to existing hardware is readily achievable.
NSI was contracted to restore a system at Hughes
S&C to operation, installing new hardware as required and
implementing NSI's PC-based Data Acquisition and Processing software.
Full documentation for the system was included in addition to
a detailed operators manual.
Following is a list of major considerations
involved with Near-Field System upgrades. Several of these considerations
will be discussed.
-Assessment if Project Scope
-Scanner Mechanical Integrity
2. RANGE DESCRIPTION
The facility under examination is a Vertical
Near-Field range located at Hughes S&C in El Segundo, California.
This system possesses a vertical scanner with a 12' by 12' scan
area housed inside an indoor anechoic chamber approximately 20
feet wide by 30 feet deep by 20 feet high (see Figure 1). In
addition, the chamber also houses a Compact Range located at the
end opposite the Near-Field Scanner. An HP 8530A Microwave Receiver
is shared by both the Near-Field range and the Compact Range,
Probe position information is provided by an HP 5501 Laser Interferometer
System (see Figure 2).
The Near-Field range was originally built to
support the Intesat V program with its last usage occurring sometime
in the 1985 - 1986 time frame. After testing was completed, the
range was not used with some of the equipment taken elsewhere.
See Figure 3 and 4 for block diagrams.
3. ASSESSING THE PROJECT SCOPE
In order to determine the scope of this project,
a detailed inspection of the facility was performed. In this
particular case, the scanner itself was largely intact. This
included the scanner drive mechanism (lead screw), structure,
motors, HP 5501 Laser Interferometer, RF cables, and AUT Positioner
(AZ over EL rotators). NSI provided the following hardware upgrades:
- New Servo Amplifier for X and Y Axis Motors
- New X and Y axis motors
- Indexing Switches
- 486/33 PC based Controller/Processor System
- NSI Data Acquisition/Processor Software
including interfaces for the existing HP 5501 Laser Interferometer
and interface to HP 8530A Microwave Receiver.
- C-Band Probe with motorized Roll an Z translation
drives and motor controller.
- Hand-held keypad controller
4. ASSESSING THE PROJECT SCOPE
A close inspection of the scanner itself was
performed. The X and Y Axes lead screws were connected via a
flex coupling to separate DC Servo Motors. The initial plan to
use the existing motors changed after discovering that the original
motors were worn more severely than originally estimated.
The lead screws could be turned easily by
hand showed no evidence of damage. However, oxidation has built
up requiring cleaning and lubrication. The scanner support rails
at the top and bottom were also cleaned and lubricated.
During the scanner checkout phase, two parameters
were examined in detailed. The Z-Plane planarity of the probe
travel was measured and the Dynamic Servo Performed for both the
X and Y axes turned. Servo Error is a measured parameter examined
for both directions of scanner travel in either axis. This Servo
Error parameter represents the difference between the predicted
position vs. The actual position measured by the Laser Interferometer.
This parameter is optimized to tune the Servo Performance during
the integration of the Servo Amplifier, Scanner and Software.
The X axis traveled at a speed of 3 inches per second (ips) and
the Y axis traveled at a speed of 5 ips.
This does not affect the accuracy of the RF
results since the receiver triggering is based upon the actual
position measured from the Laser Interferometer. The positioning
error due to the Laser Interferometer is 1.0 mil for the Y axis
traveling at a velocity of 5ips and 0.6 mil for the X axis traveling
at 3 ips.
The planarity of the scanner Z-plane was measured
with a Theodolite sighting from the side of the scanner and measuring
a 7 by 7 grid of points. The years of dormancy, which probably
included some earthquakes, induced distortions. The RMS Z-planarity
error could be reduced to 10 mils if the last 2 feet of the X
axis were not used. Of course, NSI software performs Z-plane
correction to minimize the effects of Scanner distortion. Realignment
of the scanner would occur in the next phase of facility improvements.
6. SAFETY ISSUES
Operational safety is of prime concern. Three
(30) "Kill" switches were added to the scanner to provide
emergency shutdown of the scanner. There were none previously
provided. The switches locations were: 1) On the Operators Console;
2) On the scanner near the Laser Head; and 3) On the back wall
of the chamber. Two (2) sets of limit switches were implemented
for both the X and Y axes. The innermost set of switches provided
indexing functions as well as software triggered stops. The outermost
switches acted autonomously to kill the Servo Amp on the occasion
7. EQUIPMENT INTERFACES
The interfaces that were required included:
- PC to HP 5501 Laser Interferometer
- PC to Servo Amp
- PC to Hand Held Controller
- PC to HP 8530A Receiver
These are described in the following paragraphs.
PC/LASER INTERFEROMETER INTERFACE
The PC was connected to the HP 5501 Laser
interferometer receivers. The receivers for both the X and Y
axes were connected to a Laser Quadrature Converter unit (LQC)
developed by NSI which eliminated the need for the large original
HP interface rack unit which had been accidentally surplused by
Hughes. The LQC was then connected to a DSP board in
the PC. This combination of hardware allows the direct reading
of the information from the interferometer receivers by the PC.
PC/SERVO AMPLIFIER INTERFACE
The PC was connected tot he Servo Amplifier
via a DAC card. The DAC card provided an analog voltage to the
Servo Amp to drive the DC servo motors. A Watchdog Timer Circuit
was also included in the interface between the PC and the Servo
Amp which shuts down the Servo Amp power in the event of computer
failure. The Limit switches to kill the Servo Amp were directly
wired from the switches to the amp.
The software indexing limit switches were
directly connected to the PC via a digital I/O board.
PC/HAND HELD CONTROLLER INTERFACE
The RS-232 port of the PC was used to drive
a hand held controller terminal. It functions as a remote terminal
allowing the operator to walk inside the chamber and move the
scanner precisely to any point. The controller features a numeric
keypad with additional function keys and a two line LCD display.
PC/HP 8530A RECEIVER INTERFACE
This system sued a GPIB board used in NSI's
standard systems. The event trigger to the Receiver is taken
directly from the computer through a digital I/O board.
PC/PROBE ROLL/Z STAGE INTERFACE
The Roll and Z translation stages for the
C-band probe are driven by low cost stepper motors. The PC is
connected to these stages through an NSI Antenna Range Controller
(ARC) Box. This box converts the PC's TTL signal's into the appropriate
control for the stepper motors.
8. COMPUTER SYSTEM
The computer system is the heart of the Near-field
Antenna Measurement system. It controls the Data Acquisition
process with interfaces to motors, lasers and receivers. The
PC that is performing both the data acquisition and processing
tasks is an AST Power Premium 486/33SE model. It is a floor standing
tower model with 8 MB of RAM, a 330 MB hard drive, 1.2 and 1.44
MB floppies and a Zenith 1495 14" monitor. It offered the
most expansion slots of all vendors considered. Performance and
cost effectiveness were important factors. This system provides
a good combination of both.
The computer contains cards for performing
digital input and output functions as well as a DAC card, DSP
card and GPIB card. The system was installed with DOS 5.0.
The DSP card provides high speed processing
capability necessary for the Laser Interferometer and multi-sequenced
scan generator. This allows the measurement of complex scans
when control of the remote hardware is required (see Reference
2, by G. Hindman and Dan Slater)
This system uses the standard NSI Near-Field
Antenna Measurement Software Package, Version 3.3. The menu driven
user interface provides a highly interactive environment and rapid
response. This package also contains the interface support for
DC Servo motor turning, Laser Interferometer Turin as well as
stepper motor capabilities. The software performs bi-directional
scanning and multiple parameter scan set up.2 The software can
support the addition of multiport switches or beam forming networks
at a later time.
Data processing is performed via FFT algorithms
with a choice of either analytical probe models which include
cosine and Open Ended Wavguide (OEWG) models, or NIST formatted
measured files for probe compensation. The data is presented
in a variety of formats including gray scale imaging, contour
plots, 3D plots, E & H plane patterns, and ASCII files. Holographic
processing is another powerful feature allowing the Test Operator
to perform aperture diagnostics by examining the radiated field
at any arbitrary point in X,Y and Z.
10. RF SYSTEM
The RF system used in the Hughes S&C Near-Field
Antenna Measurement system is based on HP 8530A Microwave Receiver.
The receiver is shared between the Compact Range and the Near-Field
range. The HP 8530A provides excellent amplitude and phase accuracy,
dynamic range, acquisition time, and excellent software support.
Switching the receiver between the Compact Range and Near-Field
system can be accomplished in a matter of minutes.
An HP 8511 Frequency Converter is utilized
in addition to an HP 8360 series synthesizer which provides the
signal source to the HP 8530A. The HP-8530A is capable of either
5000 measurements per second or 2500 measurements per second3,
depending on the selection of the automatic gain ranging configuration.
The receiver has an internal 100,000 point storage buffer for
data triggered with the FASTCW triggering mode.
The cost of refurbishing an existing Near-field
facility that has been dormant or is in need of modernization
can be as little as 10% of the cost to build a new facility.
Hughes Space and Communications Co. now as a fully operational
Near-field Range in its chamber with state-of-the-art software
analysis capabilities and NSI is in the midst of upgrading another
Hughes Near-field Range which was not dormant. In conclusion,
the restoration of a dormant range or upgrade of a presently operating
system is a good cost effective alternative for increasing capability
while minimizing capital outlay.
- Hindman, G. Applications of Portable
Near-field Antenna Measurements System, 1991 AMTA Symposium
- Hindman, G. And Slater, D. An Automated
Test Sequencer for High Volume Near-field Measurements, 1993
- Pryst, J. Achievable Measurement Speed
for Antenna and Radar Cross Section Measurements, 1992 AMTA
The author wishes to thank Mr. Jerry Muno of Hughes Space and Communications
Co. For permission to use test results and for his advice during
the alignment procedure of the scanner. He would also like to
thank Mr. Reuben Hall of NSI for his support during the completion
of this project.
Figure 1 - Hughes Space & Comm 12' x 12' Vertical Nearfield Scanner
Figure 2 - Hughes Space & Comm Nearfield Scanner Laser Interferometer
Figure 3 - Hughes Scanner Laser Optics
Figure 4 - Hughes S&C Near-field Block Diagram
© Copyright 1996, Nearfield Systems Inc., All Rights