X band transparent material

On Feb 17, 2019, Clive Arthur wrote (in article ):

Avoid plastics that absorb water, even when weathered. I think that automobile radars (77 GHz) use polypropylene windows. Nylon and PVC are not all that good.

As a practical matter, stock little electronics boxes are only made of a few materials.

.

Joe Gwinn

Polypropylene is probably the best commonly availble material to use at 10Ghz. Heres a handy chart

Cheers

Right, Polypropylene. We use to put poly-pro windows on the low temperature Far-infrared probes. (~50 to 500 um wavelength) to block the room temp 'light' but allow the longer wavelengths to pass.

George H.

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Clive

Get the stuff with UV absorbent in it--the plain stuff degrades quite rapidly in sunlight, as all boaters with polyprop lines will know.

(Unlike nylon, it floats and doesn't stretch when it gets wet, which is why it's popular. I used to use polyprop with a nylon jacket, which was the best of both worlds.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

That's the stuff that is very hard to tie knots in. I learned how to splice rope in order to use polypropylene.

If you really want to know what works outside, ask in a ham group. Those guys have tons of real world experience.

Rick C.

Me too. ;)

Cheers

Phil Hobbs

Am 17.02.2019 um 19:53 schrieb Clive Arthur:

Hello Clive,

the thickness of the plastic must be d = Lambda_0/sqrt(Epsilon_r) for optimal transmission.

regards Leo

Nope. It needs to be an odd number of quarter wavelengths thick, so that the round-trip reflection from the back surface can cancel the front surface reflection.

0.25*Lambda_0/sqrt(Epsilon_r) is the ticket. But a quarter wavelength in polyprop at 10 GHz is pretty thick--around 3cm/(4*sqrt(2.2)) = 5mm, so you might be better off aiming for something nearer 0 mm.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Try the magnetron window from a microwave oven. I know it's 2.4Ghz instead of 10GHz, but it might work. They're usually made from mica. For example: More:

Also try the plastic covers used by DBS satellite TV dish horn covers. I couldn't find the material but it might be worth asking. These operate on Ku band(US) at 11.7 to 17.8GHz and Ka band at 18 to 31Ghz.

Most waveguide vendors also sell pressure windows. More:

Design and Comparison of Waveguide Windows

It depends on how much air or vacuum pressure you need. For very low pressure, I used to use 2 pieces of Kapton tape back to back.

If you want to test if your proposed enclosure is fairly RF transparent, just stuff a plastic sample into a microwave oven, with a glass of water to act as a load, and let it cook for a while. If the plastic gets warm, hot, melts, or catches fire, it's not suitable. While this test is at 2.4Ghz, instead of 10Ghz, it's generally close enough. In general, you want the lowest loss tangent at or near the operating frequency.

This article has nothing to do with your project but does include a useful chart of various plastics and their dissipation factor at 1GHz on Pg 8:

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Hunters Bend works well in PP. Unfortunately too few people know it exists and it doesn't get included in books of knots, because they all copy previous ones and it wasn't invented in the golden age of sail... in fact not until the 1960s! It's very strong because no cord loops around fewer than two others (unlike the sheet bend). It's easy to undo, like a bowline, and cannot collapse when it has an assymetric strain or hits a pulley, like the reef knot.

Amazing that such a significant discovery could have been so delayed.

... contains a terrible depiction of how to tie it.

Clifford Heath.

Am 19.02.2019 um 03:45 schrieb Phil Hobbs:

Nope.

phi = 360*d/Lambda_0*sqrt(e_r) Reflection=0 for sin phi-->0

--> phi=0, 180, 360 ...

Example: e_r=2.8 f=12 GHz Lambda_0=25 mm d=7.5 mmm //

Reference:

regards Leo

I'm surprised that nobody has mentioned FR4. A thin panel such as 0.8mm will have fairly low loss but it will be reasonably rigid if suitably mounted. I have used 1.6mm thick FR4 as a window for a rugged patch antenna at 2.4GHz.

At a much higher price, a sheet of low dielectric constant Duroid might work well.

John

Also on the same website a chart from Emmerson-Cummin. It shows five materials in the lowest loss column, PP, Teflon, Polypropylene, CPE and Fused Quartz,

When we were doing ultrasonics in water, we used FEP Teflon as a window to pass the high frequency sound through (660kHz). One of our customers after doing many experiments changed the window to PTFE Teflon. His results were very poor after this. At the time it was not apparent what he did and it took a while for all concerned to finally figure out it was the PTFE that was the problem. A change back to FEP Teflon and all was normal again.

Mikek

Of course only when they are from a good manufacturer. When you buy an LNB with "0.1dB noise factor" you know the manufacturer does not care, and they might well have used any plastic they could buy cheaply for the cover. Often the LNB performs a bit better when the cover is removed, and often this really does not matter in DBS. It will be different when transmitting (as the plastic may melt, for example).

I once had a satellite LNB with Kapton cover, apparently a manufacturer that cared about performance (although in general the LNB sucked, due to IF breakthrough sensitivity).

Problem: birds sit on the feed holding arm, see themselves mirrored in the Kaptan, and start picking at it. After a short while, the cover develops a hole. Seen at several installations of said LNB. Replaced the cover with thin Teflon film with much better results, the birds don't pick at it as it is opaque.

And note that the panel can be quite stiff while remaining transparent, if a ridged shape is used (e.g., said FR-4 with slots milled/molded in it). The average thickness seen by the wave is less, and so as well, the loss. Downside: it only works one way, so if you need polarization other than linear (parallel to the ridges), this kinda sucks. :-)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/

It's an odd number of _half_ wavelengths because the sign of the reflection coefficient is opposite, right. I was thinking of AR coatings, where the reflection coefficients at the inside and outside are opposite.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

I bought some "0.1dB noise figure" type of Ku band LNB front ends on eBay. I don't have any way to actually measure the noise figure, but they were considerably better than the stock LNB's supplied by DirecTV. I've also bought and used C-band LNB's. All of these came with some kind of plastic cover. Some of them crumbled into dust after a summer of UV exposure. Some survived. I complained to the various vendors, who immediately mailed me replacements that seemed to last. I didn't ask what they were made from. For a temporary cover, I used cellophane wrap and a rubber band.

I guess I just assumed that LNB manufacturers would use low loss plastics.

I haven't had that problem. I did have a bad case of overheating causing the LO to drift off frequency. The temporary solution was to send someone out to the dish farm and have them spray cold water on the LNB. The fix was to replace the LNB with a better one that used a PLL for the LO.

Good idea. When I used Kapton, it was on the end of a waveguide with a horn attached. The bird would have to crawl inside the horn in order to see its reflection. That didn't happen so the Kapton survived.

I just realized that radar speed trap detectors probably have an RF transparent window on their plastic box. However, Google image search showed a large number of photos of the drivers side of the package, and not one picture showing the RF window on the other side. This might be worth pursuing.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558