- a) Science shall determine one or more alternate solutions acceptable for RF. This can be done by buying samples and testing or discussing with observatories
- b) FE should be in the loop to check the mechanical charactreistics of the material and estimate which and how the solution proposed represents a valid choice for mpunting or may pose possible problems.
- c) get one or more of the acceptable samples of the material and mechanical test it in one of the Antennas at the ATF.
A thin RF-transparent membrane (Goretex) will cover the aperture through which the RF beam enters the cabin at the vertex hole. The RF transparent membrane will be tilted at an angle of 5 degrees from a plane perpendicular to the boresight axis. The orientation of the 5 degrees inclination shall be settable in steps of 45 degrees in relation to the elevation axis. The change in orientation may be achieved by unbolting and refastening the mechanism from inside the receiver cabin. The mounting of the membrane shall not vignette the free optical space.
The characteristics of this membrane are defined by ALMA. The baseline specification is:
- Type Radome Material (Goretex))
- Fabric Type: RA 7956 / RA7957 The difference between RA7956 and RA7957 is the thickness.
- Material 100% fluoropolymer
- Thickness up to 1.5 mm
The characteristics will be confirmed by ALMA and the Radome material will be provided by ALMA as specified by the Statement of Work. The membrane shall be protected by a remotely operated metallic shutter, when the receiver is not in use and in case of precipitation (snow, rain, hailstones). Precipitation water entering through the vertex hole in the BUS and reaching the membrane and the space above the cabin shall be evacuated by opening and/or drainage pipes. The drainage system shall be effective also with the Antenna pointing at Zenith.
ALMA Memo No. 309
discusses measurement of Goretex RA 7956 / RA7957:
"Goretex radome material RA7956/7957 has been used in several applications at the CDL of the NRAO. To characterize its properties, careful measurements have been made of the dielectric constant, Eps_r , which was found to be between 1.2 and 1.3, somewhat higher than previously determined values. As some concern exists about the future availability of thin Goretex sheets, the product is not recommended for any future designs or for use in ALMA."
On 2006 March 23, Jeff Zivick reported that he had "contacted the manufacturer (W. L. Gore & Associates) and they have confirmed that the GORE radome material, RA7956, is available today and they have no plans to discontinue its
production." This removes the availability concern mentioned in Memo 309.
ALMA bought the goretex in 2 different POs, in 2001 and 2003. A quote
identifies price ($3100/sheet) and quantity (5 sheets) for 2001. This material was shipped to VA for fabrication of the prototype RF Membranes. A datasheet
Several unattributed plots apparently show performance, see the Memo for the best determined data.
The Science IPT will next develop science requirements, which should include specs on transmission loss, polarization effects, scattering, phase effects... I don't think science should
set requirements on UV resistance, mechanical properties, thermal properties and resistance to wind but will try to make suggestions.
Masao Saito points out: 'on the RF membrane or its substitute. Thermal conductivity: the goretex made from fluoropolymer has a low thermal conductivity of 0.24 W/(m*K). (http://www.matweb.com/SpecificMaterial.asp?bassnum=O1900&group=General
'Given the ambient and cabin temperature of -20 and 20 deg, the heat loss from the membrane window with a thickness of 1.0 mm (< 1.5 mm according to Antenna spec) is about
0.24 * (20 - (-20)) * (0.75/2)^2 * pi / (1.0e-3) ~ 4.2 kW.
'This is not negligible because HVAC capacity of the prototype antenna is close to 10 kW, I believe.
'If you consider the material for the window, please take the thermal effect into account as well as the transmission efficiency. If the material has a larger thermal conductivity and thin, the heat loss from the window is significant and possibly over the HVAC capacity. As a result, the cabin air cannot be maintained as specified and the large heat transfer to the BUS may also affect the surface accuracy.'
It was decided that this is a contractor responsibility.
In April 2006, Masao informed us of a new material:
I recently contact with a sales person in Japan Goretex. They told me they developed a prototype material slightly different from what ALMA
plans to adopt.
The differences are
- surface is smooth rather than poromeric, so resistant to dust accumulation
- a higher tensile strength by a factor of 3, guessing less fluttering when attached.
- a little more loss than RA7956
- minimum thickness of 1 mm
If you are interested in this new material, I got a sample and will ask Hasegawa-san to bring a piece together with measurement data taken by
Sugimoto-san and Osaka prefecture Univ. group.
Subsequently Masao wrote:
Measurement results demonstrated that properties of new material are
disappointing. Its transmission loss of 1 mm thick sheet in power ranges
from 3 to 18 % (~0.1 - 0.9 dB) between 200 and 1000 GHz.
The maximum loss of RA7956 is 4.5 % (0.2 dB) in the same freq range.
I don't think this is usable for ALMA.
Requirements and options: note by Richard Hills
A detailed note in PDF format is attached here: https://safe.nrao.edu/wiki/pub/ALMA/RfMembrane/RF_Membrane.pdf
Other Observatory Experience
Hills mentioned in this month's Calibration Group telecon that the woven fabric with mm spacing used at the JCMT now has some loss and polarization characteristics which might
be improved but that with a 60' aperture there were requirements for strength and durability which might differ from ALMA requirements. I also noted that h e mentioned that the JCMT membrane had good UV properties.
Chris Walker at the U of A "Arizona Radio Observatory"
has been using a material called "Zote Foam" as a dewar window. It's evidently very strong, and although it's relatively thick they claim the loss even at 1 THz is unmeasurable. They used the material on the 7-pixel 345 GHz DesertSTAR
receiver windows for the HHT. The material is ZoteFoam
PPA-30 (1 inch thick). Chris must have data on this material, maybe he could loan us a little sample for study.
The company Zotefoams http://www.zotefoams.com/uk/company.asp
) makes all kinds of foams that are being used in radio applications. EVLA uses their products to build highly shielded boxes. They have low loss materials that we could use on the ALMA antennas.
Further information on this from the SMA, from Jim Moran:
there is a door in front of mirror 3 just behind the hole in the center of the main reflector that we open when the weather is good for observations, so the radiation gets a clear shot.
And from Peter Schilke, at APEX:
Concerning the Gore Tex cover, this is a sad story. The material we have (same as you) has, contrary to your statement, quite good submm transmission - I have measurements somewhere, which I could dig out. It also doens't seem to affect the 800 GHz transmission significantly. We wanted to replace it, because it's lost tension and we get bad standing waves when we look to the west (where the wind comes from), since it flutters in the wind. Apparently, this specific material isn't manufactured any more by Gore Tex. The replacement materials they sent
DOES have bad submm performance. One of our engineers dug out some old material, which was good as well, but is also not manufactured any more. They could do it again, but since it would be a special production run, it would be a ridiculous price - one square meter sheet 3000 Pounds (we're dealing with Gore UK), and minimum order 10 sheets or so. There is one other material that's OK, but that's very thin - somebody suggested to use two sheets of it in layers, and pump them full of air to keep the tension, but I don't know if that would be feasible. So we're kind of stuck, but would be very much interested in any solution you come up with. Foam, in the past, wasn't very good, because of the inhomogeneity, but maybe modern one would be fine.
Comment from R. Laing:
Just a note about Goretex membranes before I forget. Apparently the membrane on APEX is insufficiently stiff, and can vibrate in windy conditions. This causes some nasty standing-wave problems at low frequencies (specifically 86 GHz).
The Cassegrain hole in the dome-enclosed CSO telescope is open, although some instruments mount there.
- 28 Oct 2005