Facility specifications

Positioning equipment

Modes of operation

1. Full sphere, automatic mode. Step and scan with programmable increments. Either of elevation or azimuth axes may be chosen as the scan axis. The other axis is the step axis. 
2. Partial sphere, automatic mode. Limits on both axes are programmable.  
3. Point sequence, automatic mode. According to programmed table of points (azimuth, elevation).  
4. Point sequence, manual mode. Rotation about both axes simultaneously is possible. 

Full sphere, automatic mode. Step and scan with programmable increments. Either of elevation or azimuth axes may be chosen as the scan axis. The other axis is the step axis. 

Sampling accuracy
Azimuth and elevation axes, reproducibility: ±0.002° 
Absolute accuracy of angular readout, azimuth and elevation: ±0.001° 
Automatic scanning. Sampling positioning accuracy: ±0.002° 

Positioning
Maximum speed: 3.3°/s, both axes 
Normal speed range during measurements: .0.5 - 3°/s (programmable)
Acceleration: 0.02 - 2°/s2 (programmable) 

Antenna positioner
Vertical axis. Deviation from 'true vertical': ±0.002° 
Horizontal axis. Deviation from 'true horizontal': ±0.002° 
Horizontal axis. Height above swing arm: 3.4m 
Accuracy of intersection: ±0.05mm 
Maximum vertical load on flange: 250kg

Probe tower
Measurement distance: ~6m  

 

High frequency equipment

Signal source: Scientific Atlanta frequency synthesiser SA2180.
Frequency range: 0.1-18 GHz, with extension to 40 GHz 
Power output: +10 dBm, levelled 
Frequency stability: 5 x 1010 day ageing rate 

Receiver: Scientific Atlanta microwave receiver SA1795.
Frequency range: 0.1 —40 GHz 
(amplitude-phase receiver with four channels for simultaneous measurement of two polarisations)

Probe antennas: DTU-ESA Facility's own conical horns with orthomode transducers for dual polarisation 0.8 - 30 GHz. Nonorthogonalities of the two ports are calibrated out before each measurement. Probes are mounted on precision 'probe frames' with levels for indication of polarisation reference. Front plate of frame is interchangeable for easy change of band.

 

Software  

Software consists of programs for making automated spherical near-field measurements and programs for data processing and plotting, The data processing software is based on the SNIFTD software which transforms the measured near-field to the corresponding far-field.

 

Measurement accuracy  

Measurement accuracy has been studied in depth. The influence of single sources of inaccuracies has been assessed theoretically and experimentally, and error budgets have been prepared. Statistical data have been determined experimentally by repeated measurements of the same pattern, but with measurement parameters such as distance, speed, scan type etc. varied in a systematic manner. Typical accuracy values are summarised in Table below.

Source	Accuracy	Infuence on directivity, dB	Influence on gain, dB
Reflectivity level (> 1GHz) Multiple reflections Antenna tower pointing Measurement distance Axes intersection Amplitude drift Amplitude noise Amplitude non-linearity Phase drift Phase noise Phase shift in rotary joints Channel balance amplitude Channel balance phase Probe polarization ampl. Probe polarization phase Transformation SGH gain Mismatch correction Cable variations	< -50dB ±0.2% ±0.05° ±2mm ±0.1mm ±0.06% ±0.12% ±0.5% ±0.15° ±0.05° ±0.1° ±0.5% ±0.2° ±0.5% ±0.2° ±0.01dB ±0.08dB ±0.05dB ±0.02dB	±0.03 ±0.015 ±0.001 - ±0.003 ±0.005 ±0.01 ±0.043 - - - ±0.003 ±0.001 - - ±0.01	±0.03 ±0.015 ±0.001 - ±0.003 ±0.005 ±0.01 ±0.043 - - - ±0.003 ±0.001 - - ±0.01 ±0.08 ±0.05 ±0.02
Root Sum Square (RSS)		±0.06	±0.11
Standard deviation (1s)		±0.03	±0.06

The accuracy values listed in the table are typical and obtained as an average over several measurements. The actual values will depend on the measurement frequency and the size of an antenna under test.