P.Napier, 21 February, 1998

A meeting of the MMA antenna working group was held by phone on 28 January, 1998. Attendees included from NRAO J.Cheng, J, Kingsley, J. Lugten, J. Mangum, P. Napier, J. Payne, W. Shillue; from OVRO J.Lamb and D. Woody; from BIMA M. Fleming and J. Welch.

Agenda:

(1) Time schedule for MMA antenna work.

(2) Plans for face-to-face meeting in February.

(3) Antenna design ideas from M. Fleming.

(4) Discussion of major antenna issues.

(1) P. Napier briefly reminded the group that the dates for key antenna activities are: Preliminary Design Review-July 98, Critical Design Review-Jan 99, Issue RFP-June 99.

(2) It was agreed that, because of the many complex issues needing discussion, the next group meeting will be a face-to-face meeting in Tucson on 25 February and morning of 26 February.

(3) Matt presented a number of ideas concerning various aspects of the antenna:
PANELS. MAZAK makes a mill suitable for machining 30 in wide panels. There is a reasonable panel layout for a 10m diameter antenna using panels of this width.
BACKUP STRUCTURE. The CFRP BUS is supported on a cone with a sliding ring on a pin at its tail end. At the vertex the BUS is connected to the vertex using triangular fingers with radial flexibility to allow for differential expansion. Concerns were expressed that this method of BUS support would allow gravitational deformations and wind pointing errors. Also, access for the large receiver package into the cabin would have to go through the cone.
CABIN AND EL STRUCTURE. Steel with provision for large diameter el. drive ring.
ELEVATION DRIVE. Friction drive ring and roller is preferable to direct drive because of difficulties with latter. J.Payne pointed out that NRAO 12m has operated with direct drive without problems, but agreed that having the 12m enclosed in an enclosure gives it a more benign environment.
AZIMUTH DRIVE. Friction drive using a hardened outer race of a sealed azimuth bearing (roller, crossed roller or 4 point ball) as the drive ring.
YOKE. Steel platform supporting CFRP A frame.
BASE. Focuses weight of structure to center foundation with locating cone. 6 point contact to foundation.
FOUNDATION. Precast foundation to reduce labor skill for installation.
TRANSPORTER. Consider a single rail for loadbearing. A giant forklift that lifts from yoke.

(4) John L. led a discussion of the key issues which must be considered in developing and evaluating our concepts for the 10 m antenna. John's e-mail, issued shortly after the meeting, containing an updated list of these issues is included below. Several members of the group felt that discussing these issues in isolation, without tying them to particular antenna designs, would not be useful. This is because the quality of a total antenna design depends on the interplay of all the various features used in the design, and the correct solution for an issue on one design may not be the correct solution for that issue on another design. We will try to make progress on these issues at the February meeting.
 
 

Antenna design issues to be resolved prior to PDR (July 98) 20jan98 jbl

rev 1 30jan98
 
 

Antenna Design

mount configuration -- Kingsley*, Fleming, Lamb, Woody, Cheng, Emerson

transporter size (also road width), weight, maneuverability in tight array
cabin size
truss-like design vs beam/plate design
foundation size and concept (overconstrained mount good or bad?)
elevation range vs structural performance
cable wrap accessibility, temp control
availability of temp reg rack space below azim wrap
 

metrology -- Lamb*, Woody, Welch, Holdaway, Payne, Lugten
generally, automatic correction vs optional correction
generally, laser metrology vs ref structure + sensors
generally, radio offset vs near IR guiding or offset
resonant frequencies of reference structures
sun shielding, wind shielding, temperature inhomogenieties
must we preserve access to intersection of axes?
is BUS metrology necessary? if so, how?
are separate pointing bearings (bearings which are not
substantially loaded) useful? what geometry is best?
 

azimuth bearing -- Woody*, Lamb, Fleming, Kingsley, Cheng, Lugten
sealed, lubricated vs open, dry --maintenance
sealed bearing concerns
stiffness, uniform support, turning friction, ...
wheel & track concerns
thrust loads of mis-aligned wheels, stiffness, ice/dirt on
track (environmental robustness), servo response to torque
variation (smooth tracking)
 

drives -- Fleming*, Woody, Kingsley
direct vs friction roller vs geared
servo performance (fast switch perf., stiffness, heat generation)
max torque
safety (runaway, speed limit)
 

receiver cabin -- Cheng*, Fleming, Lamb, Woody, Lugten
traditional homologous geometry vs vlba style box vs ...
structural performance
cabin volume and shape
door size, ease of access
affect on mount geometry
 

BUS design -- Cheng*, Lamb, Woody, Kingsley, Fleming
all cfrp bus vs cfrp tube and steel node (vs all steel)
weight, grav performance, thermal performance
tripod/quadrupod geometry (leg attach radius, triangle/flat bottom),
structural performance (thermal, wind, grav load, nutator loads)
environmental protection -- UV shielding, ice, blowing grit, ..
panel size, support, adjustment access
 

Optical Configuration
update strawman optical parameters -- Shillue*, Lamb, Lugten, Napier, Emerson
make sure we can accomodate the various proposed ant/receiver
optics options: off-axis feeds, selection mirror, off-axis
subreflector, movable dewar, ...
look at aberrations: coma, astigmatism, curv. of field, despace,
decenter, tilt of secondary