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SMA Near-Field Microwave Radio Antenna Holography

Technical Details (pdf)

The surface smoothness of the SMA antennas is measured by near-field holography at 232.4GHz using a phase-locked test signal source mounted on the Subaru Telescope building. The complex beam pattern (amplitude & phase) of the antenna under test is measured using on-the-fly mapping with a vector voltmeter back-end. A second antenna in the array provides the phase reference. The resulting beam pattern is Fourier-transformed to produce amplitude and phase distribution (complex illumination) on the aperture. The aperture phase is then corrected for systematic patterns due to the near-field measurements, bore-sight pointing offset and defocus leaving behind phase errors attributable to surface deviations (from a paraboloid). This gives a surface error map from which a list of panel adjustments are derived. The panels are then adjusted accordingly and remeasured. The repeatability of the measurements is 8 microns RMS over timescales of several weeks. Typically 3-4 iterations of this procedure are needed to achieve 15 microns RMS starting from the roughness level of ~60 microns when the antenna is first released from the swing template. The results on antenna 4 are shown below.

Results on Antenna 4

Pictures show the surface error maps for antenna #4. This antenna was adjusted during December 2000 and the surface has been under periodic monitoring measurements since then. The maps show:

(1) the surface before and after adjustment,

(2) short-term repeatability (1 month),

(3) long-term surface stability (7-months), and

(4) average surface since last adjustments (Dec 2000) and the end-to-end repeatabilty over this 7-month time base.

The last two maps exclude the outer most 10-cm of the 300-cm radius dish to avoid edge effects. The average surface rms is 13 micron. The stability of the panel-panel deviations indicate further improvements to the surface are possible. For the inner 275-cm radius region, the surface rms is 12 micron, which is the SMA surface smoothness specification (residual diffraction effects still persist in the outer 25-cm radius region). Finally, the antenna was transported from one pad to another during the 7 months under discussion, implying the surface will not be affected by array reconfigurations.

Swing template mechanism for assembling an SMA dish
(5) Swing template

Summary of latest holography results on all antennas

1.3mm holography Technical Memo (pdf)

Surface improvement of Antenna 4
(1) Surface improvement of Antenna 4

Surface error repeatability
(2) Short-term (1-month) stability

Long term surface stability of Antenna 4
(3) Long-term (7-month) stability

Surface error average and repeatability
(4) Long-term average and repeatability
Last modified: Tue May 24 17:11:55 2005