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Short range EM wave transmitter and receiver

Discussion in 'General Electronics Discussion' started by ah8688, Oct 10, 2012.

  1. ah8688

    ah8688

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    Oct 10, 2012
    Hi guys,

    I am looking to put a simple rig together that will transmit EM waves between and transmitter and receiver, with a 10-15cm gap between them. The aim is to put different substances between the 2 (water and gas to start) to look at each materials attenuation factor with different wavelengths.

    The EM wavelength needs to be around 1GHz, but I hope to use a signal generator so the frequency can be varied.

    So far, I have gathered that I need to connect a Signal Generator to an Antenna as the transmitter, and use a Spectrum Analyser to pick up the transmission.
    Apparently the antenna and receiver can be wire coils, although I have not confirmed this.

    How would I connect and Signal Generator to an Antenna coil, and how would I receive a signal and get it to a spectrum analyser.

    I realise this is quite a complex problem, but any help would be greatly appreciated.

    Many thanks,
    Ali
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    Hi Ali
    welcome to the forums :)

    just put a dipole antenn on the output of the generator and another one on the input to the spectrum analyser. The dipoles should be a 1/2 wavelength at 1GHz
    300 -1000 = 30cm / 2 = 15cm ( for a 1/2 wavelength)
    that part is really simple

    I dont know if you will measure any difference with such a small distance
    most of the RF signal is just going to go around an object so small and get to the receiving antenna

    Dave
     
  3. ah8688

    ah8688

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    Oct 10, 2012
    Hi Dave,

    Thanks, that's really helpful. Yep I figured that would be an issue. I spoke to someone in my department who thought I'd be able to isolate the antennas so the EM waves could only travel through my chamber.

    Another issue I've discovered in the last 24 hours is that water apparently de-tunes antennas. Any truth in this, and is there and easy way to fix the problem?

    Really appreciate all the help,
    Ali
     
  4. rob_croxford

    rob_croxford

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    Aug 3, 2010
    you are correct. The issue is that the dielectric constant is different for different mediums. For instance the dielectic constant of air is 1 wheras for water it is arround 80. This will make you antenner seem "longer" which in turn will detune it :)

    I think this is correct anyway i dont use such things much but i remmeber this from an issue a friend had while at UNI.
     
    Last edited: Oct 11, 2012
  5. ah8688

    ah8688

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    Oct 10, 2012
    So unless the antenna is actually in contact with the water, I shouldn't have a problem?
    As in, have both the transmitter and receiver in air, and let the microwaves cross the air-glass and glass-water interfaces, then any detuning that occurs will be rectified at the other end...

    Does this seem logical, or have I missed something obvious?

    Cheers,
    Ali
     
  6. rob_croxford

    rob_croxford

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    Aug 3, 2010
    I believe you are correct. The issue my friend had was with ice building up on the atenner itself due to being in a cold enviroment. Once the ice was removed i believe things returned to normal.
     
  7. ah8688

    ah8688

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    Oct 10, 2012
    Ok great - One final thing...

    I've read today a project of transmitting and receiving EM waves with a coil antenna as opposed to a dipole antenna. Would this be possible, and what complications would there be? The reason I am keen to use a coil is to keep the whole rig compact.

    Once again, thanks for all the advice!
    Ali
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    well the coil in the tank circuit will radiate, a proper dipole antenna is just more effective/efficient

    D
     
  9. ah8688

    ah8688

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    Oct 10, 2012
    Ok thanks - by radiate do you mean transmit waves in all direction, whereas the dipole is able to aim waves in a specific direction?

    I think I'm starting to get a better idea of the issues I'll have to overcome with this task, and for that I really appreciate the help...

    Cheers,
    Ali
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    no :) a dipole is omni-directional
    the only directions a dipole doesnt radiate from is from the end tips

    Dave
     
  11. ah8688

    ah8688

    6
    0
    Oct 10, 2012
    Ok great thanks... The last thing (I hope!)...
    Could you see diffraction/ refraction/ reflection in the water being a problem, if my wavelength is, say, 20cm?

    Thanks once again,
    Ali
     
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