Question about RF absorbers and radar

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In summary: If you want to maximize performance, use a high-gain horn (which, however, will have a smalll field of view) and choose a receiver/detector with lowest noise figure and best sensitivity. These are often traded off against cost for products like radar detectors where competition is fierce and selling price is important.If the surface errors are less than one tenth of the wavelength, then they will not be visible to the radiation, so at that wavelength it will appear to be a mirror.
  • #1
GlennGlenn
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Hi everyone , new to forum, but not new to forums. I am a radar detector hobbyist and have some knowledge of how radar works. Specifically and to be on point and productive, here's my question(s):

First, are there any materials readily available to general consumers that will readily absorb X, K and Ka band radar? For example, I am looking for a material such as a flexible foam or sheet material that will absorb 33.8, 34.7 and 35.5 ghz radar ? Ka is considered wide band and X and K are less wide. I have searched and done my homework, but many of the foams only absorb a certain band of K Or Ka radar. They don't do the whole spectrum. Eccosorb is one such material.

Second, regarding a radar horn ( receiver) , would it be possible and/or beneficial to either polish the interior of the horn with a Dremel to reduce attenuation of the signal the horn receives OR gold plate the interior of the horn to retain the signal captured. Thoughts? I am not an EE and just a hobbyist with some questions. Thanks in advance.
 
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  • #3
Student100 said:
Hey, how does this look?

I wouldn't mess with the horn.

nice stuff
I dread to think what the price per metre2 would be tho

cheers
Dave
 
  • #4
Thanks for the link, looks good. I wonder if it'll absorb higher Ghz such as Ka 33 .8/34.7/35.5?

The reason I ask is nearly 30 years ago a guy from CO developed a car bra that absorbed radar and I was wondering if it would work. He covered the mirrors and the front of the car.

As far as the horn, from the factory they are not terribly polished/finished . I understand that a smooth surface transfers RF better than an unfinished one. Is this accurate?
 
  • #5
You will also need to install a thick radar absorbent windscreen to hide the seats and your head.

As an anarchist, you might consider driving more slowly to save fuel and speeding tickets.
That will bring the Government down by reducing revenue from speeding fines.
 
  • #6
Baluncore said:
You will also need to install a thick radar absorbent windscreen to hide the seats and your head.
Seemed to work for this guy:http://youtu.be/9E-14fIoa7A
 
  • #7
Sorry, but I do not have bandwidth sufficient for youtube at my end of the Earth.
 
  • #8
Baluncore said:
You will also need to install a thick radar absorbent windscreen to hide the seats and your head.

As an anarchist, you might consider driving more slowly to save fuel and speeding tickets.
That will bring the Government down by reducing revenue from speeding fines.

Absolutely. Speed is a decision we make.
 
  • #9
sophiecentaur said:
Absolutely. Speed is a decision we make.


More on topic, any thoughts from an experienced EE about the horn and polishing it to make it smoother and can it possibly make a difference?
 
  • #10
GlennGlenn said:
More on topic, any thoughts from an experienced EE about the horn and polishing it to make it smoother and can it possibly make a difference?
Polishing is quite unnecessary.

If the surface errors are less than one tenth of the wavelength, then they will not be visible to the radiation, so at that wavelength it will appear to be a mirror.
 
  • #11
GlennGlenn said:
More on topic, any thoughts from an experienced EE about the horn and polishing it to make it smoother and can it possibly make a difference?
Do not touch the horn. Assuming it is from a commercial supplier, it will have been designed and tested to work properly. At Ka band, removing just 0.5 mm from each wall changes the dimensions by 0.1 wavelength--enough to change the performance. Nothing a non-pro (hobbyist) has the skills for will improve performance, while many many things will degrade it.

If you want to maximize performance, use a high-gain horn (which, however, will have a smalll field of view) and choose a receiver/detector with lowest noise figure and best sensitivity. These are often traded off against cost for products like radar detectors where competition is fierce and selling price is important.

BTW, your horn will reflect microwaves back to the transmitter, defeating much of the benefit of foam. This is a general property of antennas. Your vehicle will be stealthier with no radar receiver at all...
 
  • #12
marcusl said:
BTW, your horn will reflect microwaves back to the transmitter, defeating much of the benefit of foam. This is a general property of antennas. Your vehicle will be stealthier with no radar receiver at all...
This is quite true. Any mismatch at the mixer's RF port will reflect incident RF.

But there is another way to use a horn, and it takes advantage of the reflection feature. If instead of driving the mixer diode with RF you drive it with a square-wave audio frequency, then the horn can be made to switch between reflect and absorb and so modulate the reflected wave with that audio frequency. When that frequency enters the observers receiver it will be detected and interpreted as a strong doppler component, masking your real speed. The receiver will search for the maximum doppler frequency present in-band and find your generated audio doppler frequency. If you calculate it right, to give say 250 km/h, it will be obvious that something is wrong with their equipment. If that audio was moved every two seconds by say 50 km/h then it will confuse the operator even more. Don't forget that you will need to select different audio frequencies for different microwave bands, which will require multiple tuned cavities, or multiple horns.

As it does not involve actively transmitting microwave radiation it would be undetectable. It may even be legal in some places. It explains why the police here now measure differential reflection time from speeding vehicles with an infrared laser.
 
  • #13
Baluncore said:
This is quite true. Any mismatch at the mixer's RF port will reflect incident RF.

But there is another way to use a horn, and it takes advantage of the reflection feature. If instead of driving the mixer diode with RF you drive it with a square-wave audio frequency, then the horn can be made to switch between reflect and absorb and so modulate the reflected wave with that audio frequency. When that frequency enters the observers receiver it will be detected and interpreted as a strong doppler component, masking your real speed. The receiver will search for the maximum doppler frequency present in-band and find your generated audio doppler frequency. If you calculate it right, to give say 250 km/h, it will be obvious that something is wrong with their equipment. If that audio was moved every two seconds by say 50 km/h then it will confuse the operator even more. Don't forget that you will need to select different audio frequencies for different microwave bands, which will require multiple tuned cavities, or multiple horns.

As it does not involve actively transmitting microwave radiation it would be undetectable. It may even be legal in some places. It explains why the police here now measure differential reflection time from speeding vehicles with an infrared laser.

Tricky li'l devil, ain't you MrB?
 
  • #14
Baluncore said:
This is quite true. Any mismatch at the mixer's RF port will reflect incident RF.
It is actually true regardless of match. A matched antenna will reflect no less than 50% of the power incident on it. A well-designed and perfectly matched antenna achieves 50% and is called a "minimally scattering antenna," while a more poorly designed but perfectly matched antenna will reflect more than 50%.

This will not, of course, adversely affect your clever spoofer. This device modulates the reflection with a chirp to confuse the police radar
http://www.radarscramblers.com/faq.htm
 
  • #15
marcusl said:
It is actually true regardless of match. A matched antenna will reflect no less than 50% of the power incident on it. A well-designed and perfectly matched antenna achieves 50% and is called a "minimally scattering antenna," while a more poorly designed but perfectly matched antenna will reflect more than 50%.
I do not believe that is true of all antennas.

IEEE Transactions On Antennas And Propagation said:
Vol. 53, No. 9, September 2005, Page 2843.
Absorption Efficiency of Receiving Antennas
J. Bach Andersen, Life Fellow, IEEE,and Aksel Frandsen, Member, IEEE
Abstract—
A receiving antenna with a matched load will always scatter some power. This paper sets an upper and a lower bound on the absorption efficiency (absorbed power over sum of absorbed and scattered powers), which lies between 0 and 100% depending on the directivities of the antenna and scatter patterns. It can approach 100% as closely as desired, although in practice this may not be an attractive solution. An example with a small endfire array of dipoles shows an efficiency of 93%. Several examples of small conical horn antennas are also given, and they all have absorption efficiencies less than 50%.
Full article is at: http://vbn.aau.dk/files/19704710/absorption_efficiency.pdf
 
  • #16
Thank you for that reference, Baluncore. I was familiar only with the older literature that this article refutes.
 
  • #17
GlennGlenn said:
Thanks for the link, looks good. I wonder if it'll absorb higher Ghz such as Ka 33 .8/34.7/35.5?

The reason I ask is nearly 30 years ago a guy from CO developed a car bra that absorbed radar and I was wondering if it would work. He covered the mirrors and the front of the car.

As far as the horn, from the factory they are not terribly polished/finished . I understand that a smooth surface transfers RF better than an unfinished one. Is this accurate?

Welcome to the PF.

Unfortunately, we do not allow discussions of illegal activities. What you are asking about is illegal in many places. Thread is closed.
 

1. What are RF absorbers and how do they work?

RF absorbers are materials or structures designed to absorb radio frequency (RF) energy. They work by converting the incoming RF energy into heat, which is then dissipated. This prevents the RF energy from reflecting back and interfering with the radar signal.

2. What are some common types of RF absorbers?

Some common types of RF absorbers include carbon-loaded foam, ferrite tiles, and lossy dielectric materials. Each type has its own advantages and disadvantages, and the choice often depends on the specific application and frequency range.

3. How are RF absorbers used in radar systems?

RF absorbers are used in radar systems to reduce unwanted reflections and improve the accuracy of the radar signal. They are often placed around the edges of the radar antenna and in areas where there is a high concentration of RF energy.

4. Can RF absorbers completely eliminate radar reflections?

No, RF absorbers cannot completely eliminate radar reflections. They can only reduce the amount of reflection, but there will still be some energy that is reflected back. The effectiveness of RF absorbers also depends on factors such as the type of material used, the frequency of the radar signal, and the angle of incidence.

5. Are there any potential drawbacks to using RF absorbers?

One potential drawback to using RF absorbers is that they can introduce additional losses into the radar system, which can affect the overall performance. Additionally, some types of RF absorbers may not be suitable for use in certain environments or at certain frequencies. It is important to carefully consider the specific application and requirements before choosing an RF absorber.

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