Antenna
pattern and gain performance of typical RW antennas is generally
evaluated in Anechoic Chambers. Randtron
Antenna Systems Anechoic Chamber facilities
are specially designed and automated, achieving the convenience
and speed required for accurate data gathering and reduction.
In some cases, the chamber is designed for a specific range
of antenna applications. As an example, for electrically small
antennas that have broadbeams and cover multiple octaves, we
use the 12' x 12' x 12' chamber that covers the entire 0.5 to
100 GHz range. This facility is fully automated with positioning
capability in five dimensions so that not only the usual angles,
but also the range distance and transmitter position are controlled.
Patterns, gain and sensitivity are recorded digitally with the
limited power available by electronically tunable millimeter
sources. As a result, this facility is ideally suited for production
testing of low cost, high quantity Radar Warning and similar
antennas.
The absorber material used in all chambers is manufactured by
the Rantec Corporation of Calabasas, California. The back and
source walls of the chamber are lined with large pyramidal absorbers.
The side walls are also lined with pyramidal absorbers in the
critical areas and wedge material in all noncritical reflection
areas. A large access door is available at the receive end of
the 3 large chambers to allow entry of test specimens up to
4' x 6'. By locating the access doors near the working area,
we have minimized the use of walk-on absorber; thus, maintaining
the best reflection level performance.
Our anechoic chambers are equipped with a unique, low reflection
model tower used for antenna mounting when conical patterns
are required in aircraft coordinates. This model tower was custom
designed and developed by Randtron
Antenna Systems to overcome certain deficiencies
in available commercial equipment. The model tower is mounted
on top of a standard Scientific-Atlanta Model 5115 azimuth positioner
and is capable of coverage over a 360 degree range. All positioning
equipment is controlled by Flam and Russell FR8502 positioner
controllers. This equipment can operate stand-alone or remotely
and provide angular digital data to the automated equipment.
All chambers are equipped with very wideband, rotatable, linear
polarized horns, which have the capability of automatic selection
of horizontal, vertical, slant, and rotating linear polarization.
It is not necessary to manually change the transmitting antenna
for any test frequency in the required range.
Transmitters for these anechoic chambers utilize
signal generators that are controlled remotely (with manual
backup capability) and provide operation over a 100 MHz to 40
GHz frequency range. When designing these ranges, we did not
use receivers because experience has shown that the mixers used
in these systems cause inaccurate gain measurements. Instead,
we boost the transmitter output with power amplifiers to provide
adequate signal strength. Antenna measurements are made with
well-matched detectors giving accurate and speedy results.
Absolute gain measurements at Randtron
Antenna Systems are determined using a complete
set of gain standards made to NRL Report 4433 dimensions. The
range of these gain standards is 400 MHz to 100 GHz. These standards
are used to determine the test antenna gain directly (using
the substitution method) or to pre-calibrate antenna to be used
for gain normalization in automated tests.
In the past, antenna pattern data collection
was accomplished using standard Scientific-Atlanta pattern recorders,
such as the SA 1520. These recorders produced rectangular or
polar plots of the radiation pattern in linear (field strength)
or logarithmic (dB) format. However, due to the increasing demands
for production efficiency, accuracy and sophisticated data analysis,
all ranges are now computer-automated. The manual pattern measurement
equipment is maintained for backup and for use on very mature
production programs.
Automation of test ranges has been an on-going
project at Randtron
Antenna Systems.
In addition to improvements over the manual measurement techniques,
automation provides a digital database. This database allows
easy analysis of antenna performance in their intended operation
by applying the end-use system algorithms in the data reduction.
All automated test stations have been developed with programmable
features that provide for present and future test and analysis
requirements.
Since we are involved in designing antennas
for customized requirements, specialized software is used to
provide a wide variety of data reduction formats. The source
code is always available to adapt the data-taking process for
each particular requirement. The automated test stations run
on in-house software that is easily modified. This software
is compatible with standard, commercial antenna measurement
equipment and is designed for operation on PCs using available,
high-level programming languages. Data acquisition time is usually
limited by the speed of the hardware, so personal computer speed
is not critical.
A typical test station is capable of remote
selection--under local computer control--of frequency, power,
polarization, azimuth and elevation position and antenna beam
selection. The equipment can also take readings, under static
or dynamic conditions, of the angle, power and phase received
by any beam.
Antenna measurements at Randtron
Antenna Systems generally
fall into two basic test configurations. The first set-up is
most often required for single element tests. For common Radar
Warning antennas, rotating polarization response radiation patterns
are desired. In this case, the automated test station is configured
around a scalar network analyzer and data is taken "on the fly."
Data from multiple antennas can be collected simultaneously
via additional ports on the scalar network analyzer. The resulting
pattern is also displayed on the monitor in "real time" and
stored on diskettes for off-line analysis. The second common
configuration is required for multi-element phase-amplitude
measurements. For antenna arrays and interferometers, it is
desirable to collect coherent RF data at a single polarization
for many elements in the array over a large frequency band and
large volume in space. In this case, the automated test station
is configured around a vector network analyzer. Generally this
data is taken statically because a high degree of signal correlation
is required between antenna elements. Mathematical operations
on the data can be performed off-line on a compatible computer
in a fashion simulating the real antenna application. The following
list shows some of the typical data reductions we have performed:
-
Determination
of antenna gain, beamwidth, polarization ellipse
and squint
-
Calculation of
antenna directivity and efficiency
-
Volumetric data
plots (3D or contour)
-
Linearization
of interferometer array algorithm
-
Error analysis
of interferometer AOA accuracy
-
Best fit parameters
of quadrant warning antenna system
-
Quadrant warning
amplitude comparison DF error analysis
-
Determine optimum
array feeding and scan capability
-
Elimination of
reflections from data
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