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GSM Phone + Metal Box = Directional Reception Tester?

  1. Dec 21, 2009 #1
    I am with a search team who use Mobile Signal to guess the whereabout of missing persons.

    I was told that for 900MHz GSM Signal can go as far as 30km if weather permits.

    I know where the station "B" is, it is about 20km away from my current location "A".

    At "A", I can receive somewhat weak GSM signals.

    Can I build a Metal Box with an opening of say 33cm (equals wavelength of 900MHz Signal), and point that opening to the station, to allow only the signal from the "B", and excluding all others nearby?

    Thanks.
     
  2. jcsd
  3. Dec 21, 2009 #2

    mgb_phys

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    You aren't going to get a good directional fix without a dish many wavelengths across - certainly too big to carry.
    And a GSM phone doesn't give off a signal unless it is making a call or told to do so by the base stations.
    It's also a very low power and intermittent signal compared to a PLB
    If you are linked to a SAR operation you can have the cell provider give you the phone location assuming it still has a link to their network.
     
  4. Dec 21, 2009 #3
    Much appreciated for your input.
    Sadly, we don't have the leisure to have a PLB as we only have the location of the base station for the very last call.
    Hence, we are making wild guesses using things like, base station location, witness report, family's information, etc.
    I am not into the Rescue part (of the usual SAR, Search & Rescue) but very much on the "Search and Recover", just hope to be able to provide an answer to family.
    Thanks.
     
  5. Dec 21, 2009 #4

    mgb_phys

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    If you have some cooperation from the cell provider they might be able to get a fix from multiple towers.
    But a phone on standby doesn't sen any signal until commanded to do so by the base station so you don't have a source to track.
    It's possible the cell company have some test kit that can send a signal to all phones in range to id themselves - they may also have more experience at signal direction finding.
     
  6. Dec 22, 2009 #5
    Unfortunately, no. We do not have contact/cooperation from Cell service provider.

    Chances are the phone's battery is dried.

    It is good to know that they are able to locate a phone if they want to, but I am sure it takes no less than a court order to get this done.
     
  7. Dec 22, 2009 #6
    Some cell phones have antennas that unscrew. I've seen Motorola phones that the antenna connects to the phone through an SMA connector. It seems to me it would be possible to buy or make a yagi antenna for the frequency band of the cell phones and manually move the yagi to find the strongest signal. Also having gain, the yagi should increase your range.
     
  8. Dec 22, 2009 #7
    Thanks.
    I will look into it to see if a yagi antenna would be a practical solution here.
     
  9. Dec 29, 2009 #8
    My finding so far, although a yagi antenna is probable for GSM 900MHz, its directional sensitivity is limited to about 40 degrees vertical and horizontal, which is not very good in terms of directional testing.

    Is there any improvement / modification possible to make a yagi antenna to have a narrower sensitivity, say down to 5 degrees vertical and horizontal?

    Many thanks.
     
  10. Jan 5, 2010 #9
    I haven't tried this but was wondering if you had two yagis connected through a Wilkinson splitter with one yagi having an electrical half wavelength more cable than the other, and if the yagis were oriented in a "V", then the phase difference would be other than 180 deg. except when the source was in the center of the "V". When the yagis are pointed so that the source direction is halfway between the two yagis, you should see a sharp dip in the signal strength.
     
  11. Jan 6, 2010 #10

    sophiecentaur

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    A simple loop antenna has a pretty good Null (actually, two nulls, pointing in opposite directions) in the direction of the axis of the loop. Most DFing used to be done with loops. Going for a maximum requires a large aperture of antenna. However, using a null detection requires a stronger signal. Eventually, the width of the null depends on how far down in the null you can look. Using two Yagis, as described above, could give better performance but your directional resolution is still limited by SNR.
    I am a bit confused, though. If you use a cell phone as your detector, this will locate a base station - not another cell phone.
     
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