Nearfield Systems Inc.
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1. How accurate is the open-ended waveguide probe model included in the NSI 2000 software?
2. Is the NSI 2000 software capable of controlling a beam steering computer (BSC) during a scan?
3. How is the peak FF value determined in the NSI2000 software?
1. How accurate is the open-ended waveguide probe model included in the NSI 2000 software?
The OEWG probe model is based on a NIST algorithm (Yaghjian-1983). The model provides a far-field prediction of an open-ended waveguide probe with an accuracy of approximately 0.15 to 0.4 dB over +/- 60 degrees in Elevation and Azimuth.
Click here to see a comparison of the NSI2000 OEWG model with measured probe data.
2. Is the NSI 2000 software capable of controlling a beam steering computer (BSC) during a scan?
Yes, with open-loop (output only) control. The standard NSI 2000 software does not support BSC handshaking, but it can be added as an option. The standard open-loop capability can be configured to output a bit pattern (typically up to 8-bits, TTL output - more bits may be provided with customization), which may be used by the BSC to control pre-defined beam states.
The open-loop timing may be verified using the NSI 2000 inner-loop timing (ILT) display capability. The ILT display shows all beam running at the defined switching rate so that amplitude, phase and SNR may be verified at a fixed XY position in the high energy region of the antenna. If there is a timing or interface problem, the measured data will exhibit erratic behavior.
Full handshaking with the BSC requires a custom interface and typically involves technical coordination between NSI and the customer to define the hardware and software interfaces. Contact the NSI sales department for a quotation.
3. How is the peak FF value determined inthe NSI2000 software ?
In the NSI 2000 software there is a result that is referred to as 'far-field peak' when the far-field is computed. This value is related to gain and is described more fully in the article written by Allen Newell, Far-Field Peak in NSI Programs.
4. Does the NSI software integrate over the measured radiation patterns to obtain the directivity (ie the usual way of obtaining directivity)?
NSI2000 does calculate directivity in the “usual” surface integral way. Some extra detail re. polarization: The NSI 2000 program uses the output from the spherical wave expansion code after converting from theta phi components to the polarization components specified in the far-field menu. This data array always covers the complete sphere and so the integration of the pattern to obtain the radiated power is always over the full sphere regardless of the far-field spans in the far-field menu. As defined by international standards, both gain and directivity are total power quantities and are defined in terms of the total power per unit solid angle and therefore should be independent of the polarization components used to represent the field or the polarization of the antennas used to measure the field. In almost all uses of gain standards however, the quantity of most value is the partial gain for the main component of the antenna. Since in most cases the cross polarization of the antenna is small at the peak of the beam, the actual difference between the total power gain and the partial gain is very small. In the NSI software, the gain values are always partial gain values and use only the peak far-field values for the main component. To be consistent, the calculated directivities also use only the peak far-field values for the main component and the total radiated power is calculated by integrating only the main component pattern. You can see how this affects the directivity by changing the polarization sense from linear to circular. The peak far-field values will change, and generally the directivity will also change.
We do have a script that would calculate total power directivity, and you should see no change in the results when you change the polarization sense. The script was developed to check the directivity calculations and we could modify it to calculate either total power directivity or partial directivity. The current version calculates total power directivity and if that quantity is desired, the script can be used. For partial directivity, the NSI 2000 results are correct.