RF direction-finder?

Discussion in 'The Projects Forum' started by dave_kish, Oct 25, 2007.

  1. dave_kish

    Thread Starter New Member

    Oct 17, 2007
    7
    0
    Hey guys,

    I need to be able to determine the direction of a fixed-location transmitted RF carrier signal (in the range of ~350kHz). The signal will be sent by a dummy transmitter beacon - no data sent, no tranceiver for 2-way communication.

    My reference point is a mobile robot, which will use the direction of the RF signal to orient it's servomotors toward the signal.

    I've seen similar things done with sound waves, but triangulation was used. It's going to be tough, if not impossible, to triangulate an RF signal strictly because of it's speed.

    I've also thought about a series of RF power sensors set up in a circle on the robot, to detect the strongest direction of transmission. But power sensors are VERY expensive, from what I've found.

    Any other suggestions?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Use a Yagi antenna for the direction of the transmitter. Adding elements to the antenna will increase it's directionality. Gonna be bulky if your RF source is down around 30 meters. The ARRL website should have articles on constructing Yagis.
     
  3. dave_kish

    Thread Starter New Member

    Oct 17, 2007
    7
    0
    Thanks for the prompt reply.

    The problem is that our transmitter will have no idea of where the reciever is. It is broadcasting it's carrier, and it's the robot's job to find out where that beacon is.

    From what I've read about Yagi antennae, it's used to direct a transmission in a certain direction, not recieve from a certain direction. Please let me know if I'm mistaken, as I'm fairly new with the concept of a directional antenna.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Antennas work in both directions. As a receiver, the Yagi would show a stronger signal if pointed at the transmitter. That said, the Yagi might be impractically large (or small) depending on the transmitter frequency.
     
  5. dave_kish

    Thread Starter New Member

    Oct 17, 2007
    7
    0
    Ah, I see. Did some further research on Yagi antennas, and they're far too big for any practical use in my project.

    I've come across loop antennas, such as the one mentioned here: http://www.longwave.de/smlloop.pdf

    Would that work? A loop antenna would give me 2 possible directions (in the forward and reverse directions), but I'm sure I can figure out a way to determine between the two.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Loops should work, but resolving the direction may call for triangulation.
     
  7. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    #1
    ~350kHz is way too long a wavelength for yagis , dipoles or anything of that sort but it is perfect for a ferrite core antenna which is highly directional.

    #2
    If you are going to use a directional antenna, be aware that it is *way* more effective to look for the minimum or null on the prependicular than to search for the maximum which is difficult to pinpoint.

    #3
    A directional antenna of any type gives you the direction from where the transmission originates and that is all you need if all you want is, well, the direction. You only need to triangulate from another source if you also want to know the exact point of origin (i.e. distance).
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    We had a clunky old 1950's vintage weather balloon tracker when I was in the Navy. It used a rotating tube reflector spinning around a fixed vertical whip antenna. The signal was blocked when the reflector was pointed away from the xmitter, and augmented when the reflector was pointed toward the xmitter.
     
  9. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    I vaguely remember making a setup for "Foxhunting" many years ago that used 2 whip antennae which were alternately switched at around 1khz or so. When there was a signal strength difference between the antennae, the 1 khz switching frequency would be heard in the receiver, when the signals were the same (i.e both antennae an equal distance from the transmitter) the tone was mostly nulled out. The trick was working out whether the signal was coming from in front, or behind.
     
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