Salt/Brackish Water RF Transmitter and Construction

Discussion in 'The Projects Forum' started by DMHerp, Aug 25, 2012.

  1. DMHerp

    Thread Starter New Member

    Aug 25, 2012
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    Hey everyone,

    I'm new to the site and relatively new to the field of RF. I'm a wildlife biologist and the majority of my work deals with radio tracking of animals. Until now, the majority of my work has been done with terrestrial animals working with transmitters that use 150-164 MHz range. However, I'm starting to transition to more aquatic animals---specifically, animals that live in salt water.

    I'm interesting in continuing to use radio tracking to monitor these movement patterns, but due to signal attenuation in salt water at these high frequencies it may be nearly impossible to do. From what I've read, I understand that the lower the frequency, the better it will transmit through salt water. Specifically, would it be feasible to build a transmitter that 1) works the VLF range 3-30 KHz and can transmit signal at least 2-3 kM to be picked up by a receiver and antenna on land, and 2) how exactly does the antenna on the transmitter send signal---i.e. does signal emit from the very end of the antenna, or along the entire length?

    Meaning, in theory if there is a fish one-foot underwater that has an antenna attached to it that is 12 inches long, 2 inches of that antenna will be above the water surface---will this then transmit impeccably even if it's in the VHF range of around 150 MHz or will the saltwater around the majority of the transmitter whip antenna attenuate the signal?

    Thanks for any help! If the answers to these problems are obvious please excuse me---like I said I'm a biologist by trade---not a RF guy or an engineer.

    -David
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    A couple of things:
    The radiation pattern of an antenna can be visualized as a three dimensional solid, kind of like a large donut. The shape of this donut will be larger in some directions and smaller in others corresponding to how much of the RF energy is concentrated in the favored directions.

    Second thing is that high frequencies use short antennas for the short wavelength. Low frequencies use very long antennas for very long wavelengths. Certainly you know that frequency times wavelength equals the speed of light. The graph of this relationship is a hyperbola. This is what you get when things are in inverse proportion to each other.
     
    Last edited: Aug 25, 2012
  3. DMHerp

    Thread Starter New Member

    Aug 25, 2012
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    Papabravo,

    Thanks for your quick reply. However, other than the visualization of radiation patterns which I looked up after reading your reply (thanks!), I'm not quite certain how to interpret what you posted as an answer to my questions. If I'm just confused by the way you were writing, please explain it again---like I said, I'm new to this.

    -David
     
  4. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    I think he said your antenna will be rediculous. 100 Mhz needs a 1 meter antenna. Scale that to 30 KHz and the antenna is 3,333 meters long.

    I have bearely heard of this technology but the last I heard, signals from crustaceans (or is that cetaceans?) were only sent when the animal surfaced. Satellite link, GPS, that sort of thing.

    Please excuse an amateur for butting in.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    An efficient radiator of RF energy is the quarter wave vertical antenna. To construct such an antenna you have to know the wavelength of the signal you are using. For example.
    Code ( (Unknown Language)):
    1.  
    2. For 154 MHz the wavelength is given by:
    3.  
    4. 300 /154 ≈ 1.95 Meters
    5.  
    6. one forth of that is ≈ .487 Meters
    7.  
    8. BUT
    9.  
    10. for 30 Khz the wavelength is given by
    11.  
    12. 300 / .03 ≈ 10,000 Meters
    13.  
    14. one forth of that is 2,500 Meters or just over a mile and a half
    15.  
    I was trying to point out the impracticality of building an antenna for low frequency work such as you propose.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Underwater RF communications are extremely difficult and a very specialized field. Although I am not a big fan of wiki-pedia, this article provides some idea of the complications.

    http://en.wikipedia.org/wiki/Communication_with_submarines

    You aren't likely to run into many people with expertise in this area.
     
  7. DMHerp

    Thread Starter New Member

    Aug 25, 2012
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    Thank you everyone for your responses. You've definitely helped me get a better understanding of how RF works in general and in aquatic environs.

    -DMHerp
     
  8. dataman19

    Member

    Dec 26, 2009
    136
    29
    Most marine tracking is done with a data logger that tracks the animals movement. The info is stored in a capsule that is programed to pop off and float to the surface. It then logs into the server and downloads the animals movement data.
    ..
    No one attaches a 100 meter antenna wire to even a wale (the wale would get really annoyed, and stressed).
    ...
    There is a whole set of transponders on the GPS satellite network that are used for aquatic signal transponder relays. Incidentally, the GPS signals do in fact penetrate significantly into sea water at reasonable depths. So GPS receivers can still operate at some depth, but often the animal (i am speaking about wale tracking now) goes down to depths where the GPS signal is lost. But the tracker picks it up again when the animal comes back closer to the surface.
    ..
    How do you think they track all those wales and sharks across the globe (with little midgets in diving suits??? Just poking fun, no harm/insult intended).
    ..
    You need to obtain a few marine trackers - they aren't that much. And if you are grant funded, they can easily be justified.
    ..
    Dave
    Phoenix, AZ
     
  9. DMHerp

    Thread Starter New Member

    Aug 25, 2012
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    Hi Dave,

    Thanks a lot for the input. I've looked into using GPS units quite a bit and am familiar with their use. However, do you know of any specific companies that build reasonably priced GPS units for tracking purposes? The ones I've seen are all in the $2,000/unit price range, which I'm not able to justify at this time. I've got some grants for the work---but not quite THAT hefty :)

    Thanks,
    -David
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,126
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    I don't know about tracking per se, but I've dealt with US Global with good results. They might at least be able to point you in the right direction.
     
  11. DMHerp

    Thread Starter New Member

    Aug 25, 2012
    6
    0
    Thanks Wayne,

    That site does look promising, however one of the requisites of the tracking I want to do is that it is relatively long-term. I'm looking for ways to track these animals for around a year minimum. Be it VHF, Acoustic, or GPS. GPS would be ideal if I can find a unit that is relatively cost-effective (preferably under 1,000 USD/unit) and can log data for around a year.

    That being said, I'm also looking into different attachment options for VHF tracking to increase transmission seeing as it is by far the cheapest.

    -David
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Speaking from a position of blissful ignorance, it seems you might be reinventing the wheel a bit? Haven't similar animal trackers been used before? Maybe your problem is you're trying to do it cheaper? Just trying to understand the situation.
     
  13. DMHerp

    Thread Starter New Member

    Aug 25, 2012
    6
    0
    Just trying to look into all possible avenues. There are companies that sell GPS devices perfect for my needs such as Telonics and Sirtrack but they are out of my price range. Just trying to exhaust all options by looking into companies that produce units not made for Wildlife that could be modified to fit my needs. Hence, what lead me to this forum.
     
  14. wayneh

    Expert

    Sep 9, 2010
    12,126
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    Ah, OK, so we're looking at a reverse engineering project to save money. It might help the pros here if you could post some details about the "perfect" devices that you'd like to emulate. Pictures tend to stimulate creative ideas.
     
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