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.

 

[BrianR]

I can't give a direct answer except to point out that inflatable radomes are around. I have seen a number, a couple quite large, like 70ft dia.

At least it can be treated as a thin wall design which simplifies things but I would try the radome people like Norton.

 

[lighterup]

Thanks Brian, but the inflatable is probably not an option, since weight and portability are the main concerns with this system. The entire system is packed into two cases, and hand-carried or dropped from a helicopter to the operations area.
See this site:

http://www.syrres.com/stc/products_lcmr.htm

 

[MacGyverS2000]

How can a blow-up radome not fit your criteria of weight/portability? It takes minimal space when deflated, and weight is maybe a few pounds...


Dan - Owner

http://www.Hi-TecDesigns.com

 

[lighterup]

For one thing, on battery operation, the power to run the air pump along with the radar electronics is limited, unless you pack larger, and you guessed it, heavier batteries. The radome structure would need to be sealed to minimize leakage, which can be a challange on sand, requiring more stuff to be packed into the hand portable cases. Did you look at the picture of the system? It is pretty simple, and it is hard to get lighter than the nylon shroud presently employed. We are looking into a cotton canvas radome material, which is kind of in the middle of the triboelectric series and not nearly so apt to accumulate surface charge, but mechanically it has limitations.

 

[IRstuff]

Have you considered embedding a loose wire grid in the nylon?

Or the same with something like Tyvek?

TTFN

 

[VEBill]

Make the top and bottom plates and the baggie, I mean radome, a bit bigger so that the radome doesn't get so close to the antenna array. If necessary, add bag stiffeners made from RF-tranparent plastic.

I'd also investigate why the DC-grounded antennas aren't protecting the LNA inputs from the static. I would have thought that they would...

 

[lighterup]

VE,
We thought the same thing. The inputs are pretty well protected by grounded stub input filters and limiter diodes, but the outputs of the LNAs are vulnerable to the overvoltage of the discharge. These antenna columns have approximately 5' of cabling between them and the primary power supplies, so there is some impedance between the local ground and system ground, thus allowing ground plane charging. I have had success putting 1N5907s across the bias supply filter caps for the LNAs, but would like to find a non-electronic solution until we can re-spin the board.

 

[lighterup]

IR,
I do not understand the loose wire grid in the cover material. Would that still be radiolucent?

 

[VEBill]

The length of the cabling shouldn't matter. You only need to address the peak voltage across the LNA device in question (in other words, local ground). Putting it another way, the protection scheme across the devices should be designed to work irrespective of external grounding.

If the static discharge is getting into the LNAs through the power supply (as you've indicated), then fixing it should be fairly easy.

If you're not perfectly clear on the exact failure mode, then it might be well worth doing a detailed analysis to see how the 'sparks' are getting in.

If it is getting into the device via the power supply, then you can fix that (easily), then you don't have to worry about the radome any more...

 

[IRstuff]

Depends on the spacing and resistance, so something with 2-3 inch gaps should be reasonably transmissive. And the wires, or carbon fibers, can be quite thin, like 40+ AWG.

You could try buying an ESD labcoat with embedded wiring just to test it out:

http://www.esdproducts.biz/ESD_Grounding/Garment_Table/Lab_Coat/lab_coat.html

http://www.vidaro.com/StaticControlESD/index.html

http://www.sullivanuniforms.com/Merchant2/merchant.mvc?store_code=SU&screen=PROD&product_code=KK18

TTFN

 

[lighterup]

VE,
The static is definitely coming from the radome. The reason it causes the ground-to-power bus voltage to exceed the absolute maximum for the LNA device is that the impedance of the twisted pair back to the power supply system ground has some finite inductance which resists the dissipation of the many coulombs of charge injected onto the antenna elements (local ground plane) for several hundred nanoseconds while it flows through the output pin lead wire of the LNAs. I can short the power supply connector and inject charge onto the ground plane and the charge dissipation time is reduced to several nanoseconds. We first encountered this phenomenon out at Fort Huachuca Az during MIL 810F sand tests,which are performed in 40 MPH winds. The local humidity was a whopping 7% that day, and if it had been dark, I'll bet we could have seen the little lightening bolts going off inside the radome.

 

[VEBill]

Voltage protection should be local and directly (as close as possible) across the device being protected. Once this is done, then the instantaneous common mode voltage relative to some distant ground should be a non-issue.

But, you might also want to think about the 'RF Noise' issue of all these static discharges. Will your system still work with all this static occuring right at the input?

It seems like you've got the worst possible radome material when it comes to static. How about tossing it into the dryer with a sheet of 'Bounce' or similar consumer static control product? It seems like anything would help.

 

[MacGyverS2000]

Can you go with a hybrid inflatable system? Maybe use a standard material, but the supports themselves are inflatable. That would require a very small air pump that eats a minimal amount of energy compared to blowing an entire structure up. Or maybe use an air canister that is refilled when the unit is at base, or a larger pump to refill the canister only when the unit is at home base and power concerns are at a minimum.

Nylon just seems like the worst way to go in a dry environment with a lot of ionising-particulate circulation.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[lighterup]

That's an excellent idea Dan. We were kicking around a polystyrene leaf spring structure which would mount between each of the 24 columns to hold the radome away from the antenna elements, but an inflatable balloon might be even more effective, if we can figure out how to keep it from getting punctured by the surface mount stuff on the boards. Another idea we are looking at is to tie bungee cords axially around the structure between each of the columns to pull the radome into intimate contact with the antenna elements, as we have found out the hard way that the antenna columns facing into the prevailing winds do not fail because the radome tends to stay discharged by the ground plane of the column boards. It is the columns in the +/- 90 degree quadrants which tend to flap in the breeze that build up enough charge to do damage.

 

[MacGyverS2000]

Another option might be to use a few semi-rigid horizontal rings stacked vertically every couple of inches, like a plastic dryer vent hose. The rings themselves should present a minimal radar return issue, especially if you choose the correct material, but provide enough stiffness to the radome to keep the correct overall shape. A couple of stiff vertical sticks to tie each ring to should also prevent the middle rings from pushing off center and towards the antenna elements during heavy winds.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[Higgler]

Beech aircraft adds/paints antistatic coating on their radomes. The system we used was 2-18 ghz and it had minimal effect on performance. This coating was on the outside of the radome on their aircraft. It's a simple paint coating.

This is the prime reason why I suggest always using grounded center conductor antennas. You could switch to a folded dipole antenna which has typically wider bandwidth and the two arms are connected, i.e. center conductor to ground.

also, the famous quarter wave short = open circuit.
If your antenna is a printed circuit, you could add a quarter wave short on the distribution systems to short out the center conductor.

kch

 

[lighterup]

Hig,
As I mentioned in the original post, the elements are an integral part of the ground plane. The problem is that the small impedance between the local antenna system ground and the main system ground causes ground charging when massive charge dump onto the plane occurs. We have actually observed this phenomon an oscilloscope attached between the power and ground pins during the sand tests.

 

[VEBill]

"The problem is..."

...lack of voltage protection ACROSS the victim device (locally). Distant grounds, and the associated inductance to get there, are not 'the problem' - they're just a real world given. It's a red herring to even mention it.

Once the radome is sorted, then you'll probably find that human static discharge during set-up and take-down handling will be the next problem... You might as well respin the PCB to add the protection diodes. And just be glad that the susceptibility is - apparently - just on the power supply side and not in the RF side where adding protection might be much more difficult.

 

[Higgler]

1) that has decent surface charge mobility in an insulating flexible dielectric? Sounds contradictory.

If your antenna is not circularly polarized, and is linear polarization, then following IRstuff's suggestion you can metallize your radome in the opposite linear polarization. It's possible to metallize about 75% of the radome surface and still have great rf transmission properties in your primary linear polarization. Normally you'd think to add thin wires as suggested by IRstuff, oriented cross polarized covering 1-5% of the radome surface, but a recent analysis by a co-worker showed that having much more metal, or just the right thickness metal to non metal ratio can provide minimal thru loss over a pretty wide bandwidth. I was very surprised at this result, but I believe it.
Is your antenna linear polarization? If so, you could try a metal tape experiment, or wire experiment with very little effort.

Actually, what is your plan? Is this a very high priority item with funding requiring detailed analysis and a 9 month project with modifications of 462 pieces of hardware? Just trying out my psychic thoughts.

kch
PS: Has anyone noticed that the word your is often typed you, must be a human factors natural oops, I do it alot.