Radio waves for short distance application

Replies

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  Ashraf HZ

  Its quite difficult to know the distance if the object is the one emitting the radio waves. In proximity detectors and laser range finders, the detector is the one emitting the waves, and the CPU (whatever it may be) will determine the distance from the time it takes to get the reflected wave back.
  
  I believe you are following the concept of doppler shifts then? If thats the case, you'd want an omni directional antenna at the moving object, and on the detector side.. preferably a directional antenna pointing at the object. The source of the radiowaves is simply using an radio circuit, quite readily available online. So how would you connect the detector to the PC? Well, if I were you.. I'd buy one of those USB spectrum analyzers. Alternatively you can use the input port of your sound card, but the accuracy will not be as good.
  
  So some things you'll need to research on is which frequency you should use that is suitable. This will also determine the design of the antenna.
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  Temk

  Thank you for your advice,
  
  the reason why I want an object to be an emitter is that I need to get not only the distance but the object's rotation parameters as well.
  
  So if you imagine a simple rod, I want to define in space coordinates of both tips. For that I thought to place two transmitters in the object - first on the one end and second on another. Those two would transmit a signal at different frequencies.
  
  And last thing in this system - two detectors, one X detector, another - Y detector that are receiving those signals, differentiating for both tips.
  
  I know, this complicates it, but is this still possible?
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  Ashraf HZ

  Interesting! I suppose the rotational parameters can be figured out entirely by software. The hardware would only be concerned with the distance of a particular "node".
  
  Yep, certainly possible. Though, the two detectors will need to have frequencies far apart to avoid possible interference, due to overlapping doppler shifts.
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  Temk

  I'll keep this in mind. Thank you so much!
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  Ashraf HZ

  Your welcome!
  
  One more thing which I completely missed. If you are indeed planning to use a spectrometer, you can see the signal strength (via the amplitude of the main lobe) of an incoming wave. Since the strength varies with distance, you can somewhat roughly determine the distance. However, this method is somewhat inaccurate and may require a lot of calibration and proper antenna design.
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  Temk

  Thanks, but accuracy is vital in this application.. Ideally, I would like to get the coordinates within mm or tenth of mm
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  reachrkata

  From what I read in the internet, if you are referring to tracking methods that are usually employed in Wildlife, it is not very accurate and can only give a general sense of location. This method is suitable for large objects.
  
  One way would be to use GPS on your object and make it transmit its co-ordinates to you. This too however gives an accuracy of a few meters only, though its vastly better than the previous method.
  
  Can you elaborate on the exact application and why you need mm accuracy?
  
  -Karthik
  😁
• 

  Temk

  thanks for your reply,
  
  wow, my object changes its coordinates only within 1 meter approximately, so GPS is not a solution at all..
  
  I need to model this object's behaviour on PC as sharply as possible
  
  To be exact with an application :
  Imagine that you have to model wrist behaviour when person is sitting, but you are not allowed to use any wires