We have researched and obtained advice from various sources on the different plastics available for the SCIGN radome and base plate. We have decided that both the hemispherical dome and the base plate and ring parts are best made by the injection molding process.

Before machining of the molds can be started, we must select the plastics to be used for each part. There are a great many plastics to choose from that can be used for injection molding. We first discuss some considerations and general properties of the resins we are considering using, and follow that with a comparison of properties and characteristcis of the several plastics we've zeroed in on as top choices for the radome itself.

There are two main options for the base plate and ring parts:

• use the same plastic as we use for the dome, and
• use one of the other resins listed below, or else a higher flow 'nylon' resin such as a type 66, with 14% glass reinforcement. (see footnote #1)

General Considerations:

Of course we want to make radomes that are as nearly transparent to the GPS signal as possible. By making the dome hemispherical, we reduce geometrical asymmetry, and by making the wall of the dome thin and uniform in thickness we further reduce any asymmetries that would scatter the GPS signal. Domes are needed to provide some measure of protection from the weather and from vandalism. We have seen severe corrosion of choke rings within 2.5 years of operation in the field, as well as 'chalking' of the fiberglass dome that houses the Dorne-Margolin antenna elements (indicative of change in the resin properties with UV degradation). We will be performing some tests to see if these surficial changes in antenna are associated with changes in the apparent phase center of the antennas, but since tests were not performed when the antennas were new, we will not be able to quantitatively assess these problems.

We are in the process of checking all of the antennas that have been delivered for future deployment at SCIGN stations. Visual inspection, data collection on a short baseline, and checking the short baseline results for variance in phase center of each antenna will be performed. Despite these precautions, however, the general concerns about changing performance through time of the GPS antenna itself can be reduced by emplacing a radome to protect the antenna at least to some degree, from the weather and from sunlight. Furthermore, domes are needed in the SCIGN array for protection from possible vandalism.

Previous plastics used in various radome-type GPS antenna covers include lexan, acrylic, foams, ABS, Xenoy, and a variety of other polycarbonates and alloys. Fiberglass covers, and other inventive materials such as coated fabrics have been used in operational or prototyping, as well as plastics. We recognize from these experiences that opacity or low transmissivity of the GPS signal can occur from things like metallic additives, used in fiberglass and plastics either as a fire retardents or colorant. We also recognize that some of the materials used in the past are insuffienctly resistant to strong impacts, and feel that for the new injection-molded dome, the plastic selected must be as strong or nearly as strong as the lexan thermo-formed dome prototypes we had made in late 1997. Key parameters for evaluating this are 'notched impact' and 'tensile elongation' values that we list below for Lexan, Xenoy, and Geloy. The material must also resist degradation from weather and ultra-violet (UV) radiation over a long time interval (5-10 years) as well as possible. Whereas polycarbonates have long molecular structures that tend to break down into shorter molecules with time, UV stabilizing additives are available to slow this degradation. Other plastics, such as polycarbonate-PBT alloys or ASA-PVC alloys considered here, are inherently less susceptible to UV degradation. Longer polymers are stronger but less resistant to UV degradation, so there is a trade-off between weather resistance and strength (see simple table below).

We also believe that moisture absorption by the plastic is important, and the plastic selected should have low absorption values. The outer surfaces of the dome should shed water droplets well, so that water tends not to accumulate either on the outside (as a result of precipitation) or inside (as a result of condensation). Water droplets on the dome surfaces could certainly effect propagation of the GPS signal, as would accumulated dust. An additional factor is how well the plastic flows into the mold when molten (its viscosity), though we have tried to design the parts so that relatively viscous resins should be able to flow adequately into the mold. Generally speaking, longer polymer resins are more viscous and do not flow as well.

General Product Information:

From product guides and data sheets provided by GE Plastics (cross-checked with web page information from BASF, Dow, and other sources for comparable resins):

						UV stability	Strength
						------------	--------

  Lexan - polycarbonate (PC)			good (U-grade)	best


  Xenoy - PC alloy with polybutylene		better		better
	  terephthalate (PBT) [or PET]

  Geloy - acrylic-styrene-acrylonitrile (ASA)	best		good
	  alloy with polyvinyl chloride (PVC)

Samples Available for Testing (as of 2/4/98):

Number	Type	Grade	Appearance	Sample	Comments
------	-----	------	----------	------	--------
1. 	Lexan	143 	clear		plaque	medium flow
2. 	Lexan	HF1130	clear		plaque	higher flow than #1
3. 	Xenoy	2230EU	black		plaque  stronger than #4 or #5 
4. 	Xenoy	2730EU	green		plaque	(weaker than #3)
5. 	Xenoy	1731	black		plaque  low water absorption 
6. 	Geloy	XP2003	white		plaque	(#6 and #7 are of the
7. 	Geloy	XP2003	lt. tan		circle	   same material)

Note A: all samples are of UV stabilized grades of these resins

Note: We want the dome portion to be white so as not to absorb sunlight, and we want it opaque to visible light so as to be less obvious what's inside (for vandalism concerns). These plastics are available with coloring additives - for example Lexan can be made opaque and white, Xenoy and Geloy are available in white (and both are always opaque to visible light).

Plan of Action - testing of these samples:

• We will place 2 antennas side by side and track for 24 hours on both with no covering on either antenna.
• Then, we will begin sequentially placing the plastic samples over one of the antennas, collecting about 4-8 hours worth of data for each of the samples. The sample will be mounted over the DM antenna element using a metal cylinder. Photos of the set up for this testing will be put onto the web.
• We will make all of these data available on our anonymous ftp site so that others in SCIGN can analyze the data themselves if so inclined.
• We will look at gross characteristics of the data to see, for example, if signal to noise ratios change, phase breaks occur more frequently, etc. when any of the samples are mounted.
• Ken Hurst of JPL has an interest in evaluating these data in order to attempt to discern the refractive index for each of the plastics at GPS frequencies. His idea is to look for critical angle of incidence on ascending and descending SV's.
• The purpose of testing, from our point of view, is mainly to provide a 'pass/fail' criteria for these plastics. We want to make sure that we do not go into dome production using a plastic that happens to have an absorption band at or near GPS frequencies. Furthermore, if Hurst's testing succeeds, we should be able to make more accurate physical models of the domes' effects on phase center position. Other tests we've considered making of completed dome assemblies are more empirical in nature (see what effect occurs with dome either on or off), yet still very important.

Plan of Action - testing of the 'first article' pieces:

In about May or June, we should have some prototype pieces of these injection molded parts to evaluate. We will need to do dome on - dome off type tests, as well as jumping and banging on them, etc. to see whether or not we've made the right decisions.

It is certainly possible to revert to another plastic at that stage, but might involve some re-working of the molds' plumbing (used for cooling) for example. Hopefully the plastics we're interested in have similar enough values for shrinkage so that the parts would still fit together properly if we were to switch to a different plastic for final producation.

Key Properties Comparison:

Physical Properties	SI 	Lexan	Lexan	Xenoy	Xenoy	Geloy
(Test Data from GE)	units	143	HF1130	1731	2230EU	XP2003			
-------------------	-----	------	------	------	------	------
Melt Flow Rate		g/10m	10.5	25.0	-NA-	-NA-	4.5
Mold Shrinkage	      in/in e-3	 5-7	 5-7	5-8	6-9	3-5
Water Absorption	 %	0.15	-NA-	(0.08)	0.14	0.11	
Tensile Elongation	 %	 130	 110	120	120	40
Flexural Strength	 MPa	  97	  93	 93	 86	 68
Compressive Strength	 MPa	  86	-NA-	(46)	-NA-	-NA-
Notched Impact		 J/m	 801	 640	668	801	961

Acknowledgments and Information Sources:

Jason Reed and others at GE Plastics were very helpful in sending product literature and samples. Mark Hurley and John Knight at Molding Corporation of America provided guidance, particularly concerning the general properties of various classes of plastics and their performance both during molding and afterwards. Paul Mooney at Dorne-Margolin, Bob Watkins at Ashtech, and James Stowell at Leica all provided information on the plastics they use for GPS antenna radomes and covers. Others, too numerous to name, also offered advice, assistance, and shared insights gained from their prior experience. The Western Plastics Exposition in Long Beach (Jan. '98) was a great experience, as people in the plastics industry gave me lots of great advice ( here are my photos from that day). The companies whose web pages are listed below provide an incredible amount of useful information through these pages, and our efforts have been accelerated greatly by having these resources so readily available.

Web Sites -

• GE
• Dow
• Bayer
• BASF
• Allied Signal

1. Glass-reinforced nylon has excellent weatherability and is ~2x stronger than the pure polycarbonates such as Lexan; it also is less viscous. One drawback is that having glass reinforcement may make the material less transparent to microwaves (one of GE's customers reported that in their tests of a different plastic, with and without 40% glass reinforcement, they observed a factor of 5 less microwave transmission in the glass reinforced material).

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