Loading
NSIFAQsTopicsNearfieldSystems

FAQ Topics

Near-field Systems FAQ

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?

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?

4. What are the advantages of stepper motors over servo motors?

5. How accurate is far-field data at Theta 90 degrees, when SNF data is only measured to Theta at 90 degrees?

6. What is the power handling capability of NSI's near-field systems?


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.

Return to FAQ List

2. Why is the planar scanner scan plane center offset from the chamber center line?

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.

Return to FAQ List

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.

Return to FAQ List

4. 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:


Step Size

Maximum Static Positioning Error

Open-Loop

Feedback

Open-Loop Position Repeatability

Encoder

Tacho

Dead-Band

Tolerance or Lock Window

Following Error


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.

Return to FAQ List

5. 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.

Return to FAQ List

6. What is the power handling capability of NSI's near-field systems?

The ability of a near-field system to handle high radiated power levels is typically limited by the power handling capability of the near-field probe, the probe absorber, scanner absorber and RF components.

In high power applications, an attenuator after the probe is normally used to limit the power into the RF subsystem. The probe power handling is dependent upon connector type. Typical average power handling capacities for various connectors used with an OEWG probe are shown below:

N-type: 200 W
SMA: 50 W
2.9mm: 10 W
Peak power for the above is 3 kW

The average power level determines the amount of heat generated from ohmic losses resulting in elevated temperatures across the device. Dielectric breakdown or arcing may occur at the higher peak power level. For reference, see the table at http://www.microwaves101/encyclopedia/coax_power.cfm, which shows peak power levels with high and low VSWR loading.

NSI's standard absorber is capable of handling an average power of 1 W/inch**2. For higher power requirements NSI can quote and replace the standard absorber with certain types of higher power absorber. The type of high power absorber needed will depend upon the required maximum power density of the AUT radiated power. An estimate of the AUT radiated power density can be calculated by dividing the maximum AUT power radiation by the AUT aperture area. As an example, a 1 m x 1m array radiating 100 watts of average power will have a power density of 100 W/m**2 or 0.065 W/inch**2.

The near-field system may also be used to measure power density by parking the probe in front of a single AUT element and measuring the power output of the probe with a power meter. If the AUT is radiating uniformly, this measurement can be used to estimate the maximum AUT power radiation. As you move away from the array, however, the density starts to vary and in certain areas can become several dB higher. So, this method should be used with caution.

When evaluating high power requirements, the weakest link should always be considered. A wideband near-field system may appear capable of handling high power when used with a low frequency probe that has an N-type connector. Weaker components like SMA or 2.9mm connectors, or other sensitive RF components downstream, however, may be damaged by the high power. Also, consider the power handling capabilities of components at the output of a power amplifer, which could be a concern even if the AUT is a low radiating power antenna.

More information on absorber power handling can be found at the NSI Facilities FAQ page.

Return to FAQ List