This is a short description + notes of a meeting which occurred on 
2001oct10 to discuss the feed circle layout for the eVLA antennas.

Those attending: Ed Szpindor, Peter Napier, Vivek Dhawan, Bryan Butler,
Rick Perley, Bob Hayward, Jim Ruff, Jim Jackson, and Mark McKinnon.

These notes were taken by Ed Szpindor, Bryan Butler, and Vivek Dhawan.  
Last amended by BJB on 2001oct15.



Introduction - Butler gave a review of why we were having this meeting, 
  and some history.  The reason for this meeting was to try to nail 
  down, as much as possible, the layout of the feeds around the feed 
  circle for the eVLA.  We attempted to do this by discussing a number 
  of issues related to this (see comments on these specific issues 
  below).  The history is that the original feed circle layout was 
  superseded in 1999 based on input from Butler's WVR memo (details were
  worked out by Jack Campbell and Jim Ruff at that time).  Since that 
  time, there has been sufficient confusion about which is the 'right' 
  layout that a need is perceived to finalize it (at least the strawman
  layout), and make this known to the rest of the project.


We then went straight into discussion on the issues that Butler had sent
out earlier.  Discussion did not progress linearly - rather we bounced 
around alot.  The notes below are collected under the question for which
they are most relevant.

The 8 questions we had were:
1. What is the current best estimate of the diameter of the L-band feed?
2. Is having all of the high frequency feeds next to each other on the 
3. What is the 'vertical layout'?  i.e., do the phase centers of the 
   feeds need to be co-planar, and what implications does this have 
   (e.g., do the tops of the low frequency feeds 'shadow' others)?
4. Maintenance and accessibility issues, including: should we consider a
   VLBA-style 'feed room' instead of the currently designed 'feed cone 
   in segments'?
5. Where *is* the A-rack, physically, currently, and do the C- and/or 
   Ku-band feeds really need to go above it?  Also, in this same vein, 
   do C- and Ku-bands need to be adjacent?  If they are not, what is the
   penalty we pay (for having a longer piece of waveguide)
6. What are the implications of dichroics?  What are the possible useful
   combinations considered to be?
7. How does all of this affect the transition plan?
8. How does 3mm affect things (at least in the sense that we don't 
   design it out)?

notes on them follow:


1. What is the current best estimate of the diameter of the L-band feed?

Szpindor- Sri has provided scale factor of 0.8333, which means the 75" 
  feed is now 62.5" in diameter.  The length is TBD, will get from Sri.
Napier - Length scale factor should be smaller that 0.8333, because it
  goes, to first order, like diameter squared (so it might be as short
  as .65 times the old length).
Many - what does diameter mean specifically?
Szpindor - Sri said it is O.D., will clarify with Sri if this is 
  electrical or mechanical O.D.

2. Is having all of the high frequency feeds next to each other on the 
   elevation axis the right approach?  Does this have implications for 
   the current feed ring sections?

Dhawan & Butler - beam divergence for feeds which are across the
  feed circle from each other is roughly 50m at 2km altitude.  this
  results in an expected phase error of roughly 10 deg during summer
  daytime (median - from Butler's WVR memo).  Compare this to the
  'electronic' noise, and it's likely not acceptable.  However, if
  necessary, the Ku-band feed + Rx could be moved to the other side
  of the feed circle and have acceptable phase variation from the
  sky separation (only a few degrees).
Napier - another important point is that if a feed is across the
  feed circle from K-band, then the K-band Tsys goes up by about
  30 K (because of spillover).  This would cause WVR performance to
  degrade.
All - it appears to be the right approach, because it has little
  impact for the feed cone design and fabrication, and has advantages
  both for the WVR and for the common high-frequency IF.

3. What is the 'vertical layout'?  i.e., do the phase centers of the 
   feeds need to be co-planar, and what implications does this have 
   (e.g., do the tops of the low frequency feeds 'shadow' others)?

Ruff - Current design was obtained by scaling the new K-band feed 
  phase center to Q, Ka, and Ku and placing those feeds appropriately,
  then placing all of the lower frequency feeds such that the tops of
  their apertures were level with the Ku feed (actually, I cheated
  here - this was not discussed in the meeting, but in discussion 
  afterward - but is still noteworthy).
Napier - A feed may be adjusted vertically as long as the phase center 
  is within 10 wavelengths from the secondary focal point (it can be
  compensated for by the transverse travel of the sub-reflector).
  The relationship between the necessary movement of the sub-reflector 
  (dY1) for a given vertical movement of a feed phase center (dY2) is:
   dY1 = dY2 / [1 + (M^2/2)]  ,
  where M is the effective cassegrain magnification (roughly 9 for the
  eVLA antennas).
Napier & Dhawan - How does the phase center location vary with frequency
  across the bands (especially for those where bandwidth is 2:1)?  This
  may be an important consideration, and is one we have not worried
  about previously.
Ruff - volunteered to evaluate the effects of shadowing on the current 
  feed layout and feed sizes, but needs phase center information.
Napier - Sri must have provided phase center information to Jack 
  Campbell previously (this may be what Jim was using?).
Szpindor - We can estimate the phase centers, but recommend waiting for 
  Sri to provide real number - if he can get them in a few days.
Napier - Use inside wall of feed (from the top of the grooves) projected
  to the edge of the sub-reflector to evaluated shadowing, because the 
  diffraction does not start to occur until some distance from the feed 
  aperture.
Napier - it is possible to move the feeds horizontally as well as 
  vertically.  Small horizontal movement is possible with a small hit 
  in efficiency.  Rule of thumb is 1 dB loss in efficiency for movement
  from ideal position less than 1 feed radius.

4. Maintenance and accessibility issues, including: should we consider a
   VLBA-style 'feed room' instead of the currently designed 'feed cone 
   in segments'?

Ruff - wedge design will facilitate maintenance accessibility by 
  removing the common-wall side panels and all common-wall side support 
  structure of each wedge when it is connect to another wedge (the inner
  radius support will also be removed).  The design + material is strong
  enough to support the feeds and receivers such that the existing floor
  and feed cone support structure can be removed.  This will open the 
  feed cone segment so that the feeds and receivers are accessible from
  the inside of the feed cone like the VLBA.  We should be concerned 
  about receiver designs - especially the location of card cages.  In 
  the final enclosure design, it would be nice to get rid of the 
  'windows' on the outside of the enclosure, because they are a nuisance
  for RFI shielding.  All monitor data should be accessible from a 
  laptop, so LED readouts should become obsolete (this may not be 
  realizable though).  We should be concerned about crowding in the area
  of the 4 high frequency receivers.  Ruff will work with Mertely to 
  ensure maximum maintainability.  Feed cone mock-up will help to 
  resolve some of these issues.  It will be scheduled when feed and 
  receiver designs are ready.  May end up being constructed from angle 
  iron instead of wood.
Napier - it is possible to consider increasing the length of the feeds 
  if necessary to accommodate increased accessibility of the receivers.

5. Where *is* the A-rack, physically, currently, and do the C- and/or 
   Ku-band feeds really need to go above it?  Also, in this same vein, 
   do C- and Ku-bands need to be adjacent?  If they are not, what is the
   penalty we pay (for having a longer piece of waveguide)

Jim - currently located under the position of the new S-Band horn.
All - C-Ku band feeds do not need to be positioned adjacent on the feed
  circle, because there are many alternate approaches that will 
  facilitate the existing design without a significant performance hit 
  during integration. (see #7 below)

6. What are the implications of dichroics?  What are the possible useful
   combinations considered to be?

Napier - Rule of thumb is that the separation of the feeds must be more 
  than the diameter of the higher frequency feed.
All - With the current layout, this precludes the X-C and C-S
  combinations.  This was not considered to be critical, *but*, we need
  input from the scientists about what combinations are deemed useful,
  if any.

7. How does all of this affect the transition plan?

Napier - Suggested that the 1st time an antenna comes into the barn for 
  EVLA modification, it will receive a complete new feed cone.  A new 
  L-band feed will be installed (with old L-band Rx if necessary, which
  would mean fabrication of a bit of waveguide transition) and all 
  existing feeds and receivers will be moved to their assigned positions
  in the new feed cone design.
All - This would include moving the Ku and C-band feeds+Rx apart from
  each other and away from the A-rack.  The A-rack could stay where it 
  is, and have short lengths of waveguide (or cable) from the Rx's to 
  it, or it could be moved under the Ku feed+Rx and then only the C-band
  would need the extra cabling/waveguide.
Hayward - The existing A-rack has approximately 5 feet of waveguide to 
  the C-band feed.  Adding a bit more should not have a significant 
  effect.
Szpindor - Keeping the A-rack in its current position centered under 
  the new home for S-band will keep waveguide runs short, because the 
  nominal distance to Ku and C band feed centers is about 33" measured 
  from the center of the S-band feed.  Assuming the S-band feed goes in 
  last, the A-rack may not have to move or move very little. If S-band 
  feed goes in before the both the Ku and C-band receiver, we may be 
  able to move the A-rack to the center of the feed circle.  Flexible 
  waveguide and low-loss coax may be options to mitigating costs or 
  one-time use parts during the transition.
All - We need some guidance from the scientists on how much performance
  hit is allowable in the Ku- and C-band systems during the transition.
Napier - This transition plan means that the availability of the new
  L-band feeds is critical.  The design and fabrication of the prototype
  and first real L-band feed system may well be a critical pacing item 
  in the schedule.  Will have Gene Cole try to lay out a real transition
  plan with this in mind and work back to see whether (and by how much)
  the L-band design needs to be accelerated.
Napier & Ruff - This also means that the structural design for putting
  everything together has to be done pretty quickly, including how to
  support the L-band behemoth feed.  The first EVLA upgrade to an 
  antenna is scheduled to happen in April 2003, and everything must be 
  in place by that time - meaning that Jim probably needs a pretty final
  design within a year or so.  Jim thinks this is feasible, as long as 
  he gets final feed dimensions from Sri early enough.

8. How does 3mm affect things (at least in the sense that we don't 
   design it out)?

Napier - the 3mm feed must be essentially right on the elevation axis,
  since the sag compensation is critical.
Napier - Could add 3mm near the centroid of the upper right quadrant in
  the space there, then add a pick-off mirror above the Q-band feed.
Ruff, Napier, & Hayward - Could put it in the same dewar as Q-band, and 
  have both look through the same hole in the enclosure.  This may be
  difficult since it would involve a retro-fit to the current Q-band
  systems, but is a possibility.
Ruff - Direct the beam with reflectors for both Q and W-band frequencies
  through the feed cone and split using a flip reflector to the 
  appropriate receiver.
Many - Utilize freed space on feed circle by the reduced size of 
  L-Band feed.
Napier & Butler - Address 3mm by identifying these options.  Add this 
  to project book.


Miscellanea:

·Ruff and Szpindor to meet in the morning of 10/11/01 to discuss most 
 efficient ways to produce a L-Band prototype by 4/03.
·Napier - feed placement angles must be accurate to a 20th of the angle 
 subtended by the sub-reflector  - approximately 1°.
·Ruff to compare VLBA and GBT L-band manufacturing techniques. 
 (Conductive Epoxy vs. welding)
·McKinnon - GBT feed tested for susceptibility to RFI - found to be very
 good, even with no conductive epoxy or complete weld (it was only spot
 welded at the quadrants).  It was unclear whether the welding was
 considered necessary for structural strength.  Contact: Mike Stennes.

specific questions for Sri (Szpindor will do this?):

·what are the feed diameters for all of the bands?
·what, specifically, does he mean when he specifies a feed diameter?
·where are the phase centers for all of the bands, and how do they
 vary with frequency within a given band (especially for the 2:1 feeds)?
·are there methods to reduce off-axis cross-pol at band edges, perhaps at
 the interface between the feed and the polarizer?
·when can he come out here and spend some time working on this?