Nearfield Systems Inc.
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1. What issues should I be aware of when considering near-field as an option for antenna testing?
2. Why is the planar scanner scan plane center offset from the chamber center line?
5. What are the advantages of stepper motors over servo motors?
1. What issues should I be aware of when considering near-field as an option for antenna testing?
NSI's near-field systems are currently in use world-wide for a variety of antenna measurement applications. A short tutorial on the basics of near-field theory can be found here. For more in-depth study, order a copy of Dan Slater's book, "Near-field Antenna Measurments". Appendix C of Dan's book includes a list of common misconceptions about near-field measurements.
2. Why is the planar scanner base not centered in the chamber?
In order for the scan center to be centered within the chamber the scanner base should be offset to account for the probe tower offset on the x-carriage. NSI recommends that the scanner be arranged in the chamber so that the scan center is aligned with the chamber center. See the pdf slides for a typical arrangement.
3. My chamber is just wide enough to fit the scanner, therefore, how is performance impacted if the center line/plane of the near-field scanner/AUT is offset from the center-line of the chamber?
NSI recommends that the scanner be arranged in the chamber so that the scan center is aligned with the chamber center, however, it is acceptable to have the scan plane and AUT offset somewhat from the center of the chamber for minimizing chamber size and cost.
Asymmetrical side-wall reflections are considered and accounted for in the 18-term range evaluation term: Room Scattering. Other factors that may influence the room scattering error term include AUT directivity, frequency of operation, absorber size and condition, and AUT pointing requirements.
Click here to determine the scanner sizes that will fit your chamber.
4. How does the NSI-700S-90 spherical near-field scanner compare to the SATIMO Stargate 64 or 128 scanner?
The NSI-700S-90 may be considered as an alternative to the SATIMO systems. It is a mechnically scanned system and is therefore slower, however, it allows for arbitrary angular sampling density (the SATIMO system is tied to the fixed probe spacing distance) and therefore allows wider frequency range to 18 GHz and higher. We also believe that it is a much simpler system to maintain, since there is only one probe and a positioner that needs to be serviced. Replacement of the probe by any other suitable version is also a very simple task. Dynamic range limitations of the "scattering modulation technique" may also be an issue for some applications.
5. What are the advantages of stepper motors over servo motors?
The
question of the relative merits of stepper verus servo
based systems comes up from time to time. This write-up
serves to provide some clarity on this.
NSI elected to adopt stepper motor solutions. NSI can interface to and control
servo motors and there are a select few cases where this approach is warranted,
as addressed in: Is
a system with feedback better than an open-loop system?
In order to address the topic certain terms have to be defined. These are,
Maximum Static Positioning Error
Open-Loop Position Repeatability
Steppers motors help us
make our systems very reliable and fast, while maintaining
very accurate control of axis position, implying small following
errors. This can be achieved for ANY load up to a certain
maximum value, at which time a stepper will stall without
damage to the motor. Open loop stepper motors allow us to
avoid positioning problems which result from closed loop
feedback such as dead-bands and hunting.
Servo-based systems require encoder or synchro feedback for
operation. The control system requires tuning, and this tuning
is dependant upon the load. When
the load changes (e.g. when the probe or AUT is changed) the
system needs to be retuned if oscillation or hunting is to
be avoided. Delays have to be included in the timing loop to
provide additional resilience and at a system level, this tends
to slow down servo based systems. This video clip
shows a servo positioner hunting for a final position.
Stepper motors are digital devices with quantized rotation
behavior since they allow motion through a set of discrete
angles providing exact position information which is not subject
to a cumulative error as after each 360 deg rotation the armature
is back to its exact starting position. When we do use position
feedback (i.e. when we add our patented laser systems to our scanners), the scanner
still runs open loop, only now we trigger the receiver from feedback we obtain
from the laser interferometer. This enables us to improve
our positional accuracy, but crucially this means that we don't
run into the closed loop control problems outlined above.
For a closed-loop servo system to approach position accuracy
similar to the encoder resolution, position feedback needs
to be significant and this often leads to control system oscillation
(hunting) with heavier loads. Reducing the feedback stops the
hunting, but sacrifices speed and accuracy. The concept of
a "lock
window" is used for closed loop servo systems as a measure
for when to "declare
positioning success" and this is often an order of magnitude
less than the encoder resolution.
Servo systems also require velocity control
through the use of tachos. Servos have the tendency to accelerate and decelerate
during a scan, which is a critical problem for a system which relies on a predictable
allowed time to take a sample, and as such on a reliably predictable speed. In
a stepper motor based system this problem is overcome by a digital pulse train
that controls the motor's velocity very accurately.
As a final thought; The largest
planar near-field system in the world, the highest frequency
near-field system built to date, and some 370 other systems installed
by NSI all successfully and reliably use stepper motor technology.
6. How accurate is far-field data at Theta 90 degrees, when SNF data is only measured to Theta at 90 degrees?
The answer somewhat depends on the type of antenna, and what energy is ignored or truncated beyond the 90 degree angle. If the antenna pattern is very broad beam, like a dipole pattern with only 10 dB down pattern level at 90 degrees in Theta, the entire near-field pattern at all angles is questionable. If the antenna is somewhat directive, and pattern level is down 30 dB or more at 90 degrees, a good rule of thumb is that the pattern is probably good out to about 10 degrees short of the measured angle. See results below, where we compare results of an X-band SGH at 8.2 GHz where we compare the ‘true’ pattern derived from a full sphere measurement, to the ‘truncated’ pattern from a +/-90 degree Theta measurement. You can see here that the pattern from the hemisphere data set is good to about +/-80 degrees, which is about 10 degrees short of the measured pattern angle for the SNF measurements. The second plot shows the near-field data drop-off is a little more than -30 dB at 90 degrees Theta.
