What materials are transparent to radar? Weatherproofing a radar gun

[Jack_O]

I’m currently working a project to produce a roadside speed camera which displays vehicle speed to drivers to improve speed awareness.

A Doppler radar gun operating at 24.1GHz (1.24cm) is being used as the source and detector. There is a weather tight plastic box (opaque, at least to visible light) to put this in, also have some transparent 10mm acrylic. The idea is to cut a hole in the box and use the acrylic as a window. The problem is I have no idea if the acrylic is transparent to the radar frequency being used and I haven’t been able to find any information online.

If you have any knowledge in this area please let me know!

 

[jedishrfu]

I don't think acrylic will absorb too much of the radar beam. It seems really absorbant materials have embedded metal particles,

http://en.wikipedia.org/wiki/Radar-absorbent_material

anyway, I found this company that makes lens for radar and microwave devices:

http://www.sdplastics.com/rexolite.html

 

[Jack_O]

Thanks, think I will experiment with the materials and see if the signal still gets through. There's a limited budget so it would be better not to have to order in specialist materials.

I'll update the thread with my findings.

 

[Aero_UoP]

I wish you won't succeed. We don't need more speeding tickets... we can get one already :P
lol kidding mate ;)

 

[Baluncore]

I suspect that you should use either a very thin window or a quarter wave window at 24.1GHz, about 3mm. The quarter wave transformer should eliminate the partial reflections at the air-window interfaces.

 

[Jack_O]

I suspect that you should use either a very thin window or a quarter wave window at 24.1GHz, about 3mm. The quarter wave transformer should eliminate the partial reflections at the air-window interfaces.

Could you explain the reasoning behind the quarter wave window?

I've been read the article about EM penetration depth on wikipedia, but this has led to more questions than answers really (e.g. how well does acrylic reflect radar and what is the complex refractive index of acrylic at 24GHz?). I suppose the problem is the equations are very general and the specific values for the available materials don't appear to be readily available. Not had a chance to test yet due to the weather conditions here in the uk.

I wish you won't succeed. We don't need more speeding tickets... we can get one already :P
lol kidding mate ;)

The point of the camera is raise speed awareness and improve safety culture, it won't take pictures or have any way of issuing tickets. Of course there is a simple way to avoid speeding tickets ;)

 

[Baluncore]

Could you explain the reasoning behind the quarter wave window?

My idea was like the quarter wave coating on optical surfaces, where the coating has a refractive index close to the geometric mean of the optical material and the environment. If the reflection from the entry interface cancels the reflection from the exit interface then the reflections would disappear.
However, in this case since it is air on both sides without a centre layer maybe it should be a half a wavelength thickness adjusted for the velocity in the window material. I have not thought about it.

 

[Baluncore]

http://www.ece.umd.edu/class/enee381-1.F2004/lectures/lecture16.htm

"the thickness of the dielectric layer be a multiple of a half-wavelength in the dielectric at the operating frequency.
Such a dielectric layer is referred to as a half-wave dielectric window."

 

[Jack_O]

Definitely some good thoughts, I may include in a technical write up.

Had a chance to test the materials this morning, to my surprise the detector continued working with all of them. The first test piece was 10mm 'armoured' clear acrylic normally used in the windows on yachts. The second was 6mm clear 'cast acrylic sheet' made by Altuglas and the final piece was 1mm Clear HIPS (high impact polystyrene). I don't have anyway of measuring the exact drop in signal but as long as it works all is good.

The source and detector is an MPH industries DS4, with a maximum output of 20mW, in case anyone is curious.