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Is there any way to modulate/change a radio wave reflection?

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  1. Jul 28, 2017 #1
    Hi all,

    Is there any way to impart a transformation upon a radio wave's reflection from an object, such that the reflected wave is different from the original signal in some way (frequency, phase, etc.)? Not sure what this would require (maybe a constant and active coordinated interference signal from the target object?), or whether it's possible at all. The idea here would be to devise an object that could be uniquely identified in a radar system by changing the reflected signal somehow (thus making the reflection distinguishable from other reflections coming from other objects). Anyone know of a way to achieve this?

    Thanks!
     
  2. jcsd
  3. Jul 28, 2017 #2

    CWatters

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  4. Jul 28, 2017 #3

    berkeman

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    You can certainly modulate the direction of a cube corner reflector arrangement at audio frequencies using a fairly standard voice coil driver (like what audio speakers use). This will impart an audio bandwidth AM signal on the radar return, which could contain an encrypted code. The bandwidth is low, and there is plenty of AM noise in a radar return signal to compete with this information, however.

    Can you say more about the application?
     
  5. Jul 28, 2017 #4

    tech99

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    There are several ways of doing it. You can use an antenna as the reflector and switch between a short or open circuit between its terminals, for instance, maybe at audio frequency. Another way is to use a square law diode at the centre of the antenna so it re-radiates second harmonic. And yet a third way is to apply a local oscillator to the diode so it acts as a mixer and re-radiates a new frequency.
     
  6. Jul 29, 2017 #5

    davenn

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    yeah, that is really easy .... civilian or police radar does that all the time

    the frequency of the reflected radio wave is proportional to the speed of the object that is reflecting the signal
     
  7. Aug 1, 2017 #6

    sophiecentaur

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    If we are talking about Radar, then there are active reflectors available which will give a much better return than a simple mast height radar reflector. Post #2 referred to IFF systems. They work pretty well, so I have read. If this is a Marine environment, having anything electromechanical in a location where the reflector is fitted will be subject to a pretty harsh environment.
    You need, also, to be thinking about how the source 'radar' unit works. A 'wobbling' passive reflector will be hard to detect, requiring some clever mods to the standard demod.
    Give us some more details about this project. Is it low cost? Is it a "sky's the limit" sort of thing?
     
  8. Aug 1, 2017 #7

    Nidum

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    In short range applications you can use the incident radio signals as an energy source . This energy is used to activate a transponder circuit which then emits an all new coded radio signal on a different frequency .

    This is the way that one type of store anti theft tag device works .
     
  9. Aug 1, 2017 #8
    Hi all,

    Thanks for the replies! General application here would be the ability to track a specific "tagged" object at short ranges, possibly indoors with lots of multipath, etc. Hence the need to isolate the reflections from the "tagged" object. Upon some further research (as @Nidum pointed out), this is basically how RFID works. I just want to combine this with radar functionality. So assuming you had an RFID-type setup using modulated backscatter to ID a target object, would a passive tag/reflector be feasible at any significant range (i.e. 5-10 meters)? If not, would an active tag (something more akin to the solutions brought up by @tech99) still be compatible with time-of-flight ranging? Or would some sort of indeterminate delay be introduced?

    Bit of a side tangent here, but I was reading about some early IFF systems here after @CWatters brought it up. The page talks about German pilots in WW2 rolling their aircraft inverted in order to change the polarization of the radar return, hence distinguishing them from Allied planes on German ground radars. Can anyone explain or point to some resources on what's going on here? Why would changing the aircraft's attitude also change the polarization?

    Thanks!
     
  10. Aug 1, 2017 #9

    tech99

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    The antennas used for airborne radar were dipoles, so banking the aircraft rotated the antenna.
     
  11. Aug 1, 2017 #10

    sophiecentaur

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    targets do have different amplitudes of reflection, depending on polarisation but flipping 180° wouldn't effect the polarisation the reflection. They would have to roll to 90° which is not an attitude that they could sustain for long. Bearing in mind the sensitivity and resolution of WW2 radar systems, I would think that any fancy polarisation system would have been difficult to implement. (It could have been a bit like the Carrots Help you See Better in the Dark story that the Allies spread about when they were ahead with their radar technology. :smile:
     
  12. Aug 1, 2017 #11

    tech99

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    You need top apply the Radar Equation to find the path loss. If the target is being switched on and off., that introduces another 3dB loss.
    As an alternative to the Radar Equation, you can consider it as two paths, go and return, using the Friis Formula, which might be easier. You need to know antenna gains including the target.
     
  13. Aug 2, 2017 #12

    Baluncore

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    During WW2, the “Right-Left wing waggle” was used by fighter ground controllers to identify their controlled fighter aircraft. The German ground controllers would call “Rolf-Lisel machen” when they became unsure about which blip on their PPI was their fighter.

    The radar cross section changed with orientation of the aircraft, which caused flutter of the reflected signal seen on the radar. It was nothing to do with the orientation of any polarisation. A night fighter does not need to fly upside down to identify itself to it's controller. Instrument flying is hard enough at the best of times without disorienting the pilot.
     
  14. Aug 2, 2017 #13

    Baluncore

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    The problem with close targets is wavelength and bandwidth. For 5 to 10 metres range you would probably do better using an ultrasonic signal than a microwave signal. The lower velocity of sound yields an advantage in resolution. So, if you transmit an ultrasonic chirp that is received and returned by a microphone and radio link, (transposer), you could measure the range to the target accurately. When you multiply a linear chirp by it's delayed reflection you get a fixed tone determined by range. Multiple longer paths can be rejected as they give a higher frequency beat than the direct acoustic wave. That is an application for an FFT.
     
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