Radome Material Question

[lighterup]

The system is a cylindrical phased array. The antenna elements are dipoles at DC ground potential.
The problem:
1) Due to weight and packaging constraints, must use a flexible, lightweight material as a weather-resistant radome.
2) Due to flexibility requirement, the radome contact with the array elements is intermittant, especially in windy conditions.
3) In low-humidity windy conditions, static charge builds up on the surface of the radome, and can cause catastrophic failure to sensitive LNA components located on the antenna PCB assembly.
Question:
Does anyone know of a radome material (currently using nylon) that has decent surface charge mobility properties as well as decent dielectric loss properties at L-band? I have talked to Gore-Tex about their microwave material, but cannot get anyone except sales folks to return my calls.
Any informatiion is greatly appreciated.

 

[MacGyverS2000]

If the metal tape idea works out, you might be able to combine it with my dryer vent tube radome format to great success.

P.S. I often type "woudl" instead of "would"... no matter how carefully I try to type, I always default back to 'dl'. Very frustrating, especially when other words like "told", "sold", and "should" seem to come out OK the first time around. <shrug>

Dan - Owner

http://www.Hi-TecDesigns.com

 

[lighterup]

Hig,
This system is vertical pol in the orientation of the array cylinder. If you follow the link in the 3rd post, you will see a picture of the system. If you go further and follow the Army link in that piece, you will see a picture of the system without the radome, and if you look carefully, you can see the outline of the individual dipole elements through the PC board material. That is an interesting experiment with the cross-poled wire. We may try that ourselves.
Our next test is scheduled in early August, and we are going to try to "bungee" the radome axially between the individual columns, and if old Dan Bernoulli is kind, hopefully we can maintain intimate contact between the sides of the radome and the antenna elements, keeping the charging to a minimum. Just in case, however, I am taking along an extra set of columns with 1N5907's back-soldered across all the LNA bias filter capacitors.

 

[VEBill]

Static can usually be dissipated with surface resistivity values of as high as single-digit Mohms per square. The values that would affect RF are probably (a guess) orders of magnitude less. I think that the workable range is a wide target.

 

[Higgler]

lighterup,
I saw the photo's. The wires should work unless dissipated static is a localized point source and not globally connected, which is probably is since current doesn't move on an insulator. Hence, I'd guess that individual sections of your radome make static that's isolated from the other areas of your radome. Maybe you'll need this anti-static paint

http://www.all-spec.com/1/viewitem/4000-1/ALLSPEC/prodinfo/w3path=cat

(try it on a piece of dielectric and measure S21 change). Seems simple to spray or brush it on and be done with it. Maybe call Beech aircraft in Wichita KS and ask what they use.

Slight concern using wires that you keep them crosspole if your radome material droops or can change orientation. I'm curious about the rain and it's affects with a wet radome. Normally, you'd want the rain to ball up and roll down the surface, seems like it would wick down and cause problems. A smooth surface can have hydrophobic coatings added to minimize loss due to sheeting. At 2-3 ghz, expect 5-10 dB loss due to a sheet of water, plus pointing error accuracy degradation.

Raytheon Raymarine makes a marine radome 19" diam. x 19" high, although your unit looks larger than that. There are other larger sized and low cost lightweight radomes from Andrew Corp available.

kchiggins

 

[lighterup]

Hig,
I think I will order a sample of the staticide and try it. The problem I see with it is if it stiffens the material when it dries. This system has to be taken down and moved frequently, so the radome gets folded up into a small volume and crunched into a carrying case. If the paint tended to crack, I can forsee problems. Perhaps a thin coat applied in the field in arid locations would work.

 

[Higgler]

Worth a try,
I haven't used the material shown, just found it on the web. I'd do an S21 from antenna to antenna with a sample to make sure it's not too lossy.

I realize from your statement why the array looks so odd, cuz it folds small. Interesting mechanical arrangement.

Maybe a secondary anti-static bag or cloth painted with anti-static coating fitted over the array first may be friendlier than spraying your radome material? At least you won't have to tell your co-workers that you ruined a radome with the anti-static paint until you know it works.

Maybe if you add a plastic disk to the top surface that hangs over 3 inches or so, it might keep the radome from touching the antenna surface and keep the rain off better. The bird poop would miss the radome too if the top disk is large enough. Maybe funnel water collected down thru pipes between your elements. Lots of useless ideas to consider I know.

Let us know how your experiments come out.
I hope your system can be attached to an audio warning system for inbound projectiles to give people a few seconds to take cover. Add some pits or sandbags for more protection. Good luck.

kchiggins

http://www.toyon.com

 

[lighterup]

Thanks for all the food for thought, guys. My mechanical guy tells me that we found a way to cinch the cover really snug without crushing the columns by using a corset cinch with nylon strings and plastic eyes to take up excess material in the radial plane, kind of like a shoe lace concept. This seems to keep the material right up against the antennas in a 30 MPH air stream, which is all we can generate here. MIL-810F calls for 40 MPH, but we will probably go with the radial cinch for our next test attempt. Also, it keeps the surface charge below 500V as seen through our little ACL static locator meter, compared to 1500V with the radome just flapping in the breeze. I think that's going to be more acceptable to the users than lugging around all those bungee cords. ;-)

 

[MacGyverS2000]

Also think about washing the nylon in an anti-static liquid... doesn't Woolite do something like that? Or maybe that was the Bounce sheets I used


Dan - Owner

http://www.Hi-TecDesigns.com

 

[lighterup]

Yes, Mac. But like your household washing, after a couple of weeks out in the warm, dry breezes of beautiful Iraq or Afganistan the effects of the antistatic coating wears off and you are left with the same old problem...and no laundromats within 1000 kilometers. ;-)

 

[MacGyverS2000]

Add it to the ATP (or whatever it's called for this application).

"Radome must be washed on spin cycle using Woolite at least once a week during normal use." ;-)

I've seen army use procedure manuals that actually listed things like "Do not handle board with taco sauce in vicinity." (Gotta love our government). A board was fried because some moron tech handled the boar while eating a taco... sauce spilled on it, shorted it out, magically creating a new caution line in the manual. When the board came back, the taco sauce had dried in place, the tech didn't even worry about cleaning up the evidence.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[BrianR]

I am sure that troops in the field will be able to wash their radome.

 

[IRstuff]

Another option is a antistatic spray:

http://www.mystaticguard.com/products.cfm

which, apparently, you can also make yourself:

http://www.recipegoldmine.com/house/house17.html

TTFN

 

[Higgler]

lighterup,
just saw this in the Plastics solutions newsletter and thought of vous. Maybe they have a thin sheet you could use to dissipate your static.

Mike Oliveto,
Quadrant Engineering Plastics Products
Statically dissipative plastics reduce losses during the handling of electro-sensitive devices and improve safety and reliability of packaging equipment.

Kevin

 

[lighterup]

Thanks Hig. That material is a machineable stock similar to delrin, except it is semi conductive. I think I may try it in an ESD sensitive gluing process for one of the press fixtures used there where we currently use a delrin platten.

 

[BrianR]

In the search for antistatic material don't lose sight of the overriding requirements of mechanical properties, RF loss tangent and water absorbtion. The material must not be hygroscopic and must have a low tan-delta.

The fabric listed in this url might be what you want:

http://www.fabricatedsystems.saint-...egory_edit.asp?ele_ch_id=C0000000000000001282

 

[Higgler]

Good point Brian, fix one problem, create another. Accidental job security?
kch

 

[lighterup]

Thanks Brian. I have contacted them to see if they know where their fabric is in the triboelectric series, shown in this URL:

http://www.school-for-champions.com/science/static_materials.htm

We are also looking for a combination polyester-nylon fabric, which should have pretty good anti-static properties, since these materials have complementary work functions. So far, we are trying to buy COTS without getting involved in a big R&D effort with a supplier, but have not had any great success as yet.

 

[lighterup]

Thanks to everyone who has contributed to this thread. I think Brian has come upon the answer, although it is not within the specific microwave materials that Saint-Gobain markets. They also make a Kevlar composite material used for chem-bio and vapor protection gear which is rated very good for resistance to surface static charge, a very important thing if you are wandering around in a building full of volatile chemical vapors. I have ordered a sample of it to test for transmission loss in our chamber, and if it works out, I think this may be the answer to my original question.
Thanks, Brian.