Detecting Signal Direction

Discussion in 'General Electronics Chat' started by GFTMC, Jan 20, 2008.

  1. GFTMC

    Thread Starter Member

    Jan 20, 2008
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    So first - Yes, I know this has been discussed before. I have used the search function.

    Now, to my problem - I have a fairly large TV antenna (A pole with a metal comb on it), and I want to use it to detect the direction of a signal.

    I know that I can use a rotating screen, and that it can hide the signal direction, but this is liable to cause interference with the communications, so it is less favored.

    So what I want to know: Will such an antenna re-emit the Radio waves it transmit, so that four smaller antennas (Or satelite dishes) will be able to recieve the weaker re-transmitted signal from its oposite direction?

    In other words, if I have this configuration:
    [​IMG]
    Will each small antenna (The ones with the circle on top) Recieve a signal (Provided that wthout the main one they cannot), and if yes, will it be stronger when the small antenna will be in the direction the signal is coming from?


    If the answers to all of these question is no, then is there is another way to recieve direction without interfering with the connection?
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    First, let me say that I am not an expert on antennas.:rolleyes:
    Does your original signal come from somewhere else, or are you transmitting it from the main (Yagi) antenna? It is not clear to me whether you want to transmit the signal, or hope to receive a signal that is incoming to the Yagi and reflect it to one of the dishes, or maybe something else entirely.
    You may find it helpful to Google "yagi antenna gain", making sure you understand what "yagi" means, and what "antenna gain" means.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,142
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    Right! Yagi and Uda were the names of the inventors, and no, antennas such as the one you describe do not reflect any noticable amount of the incident energy which is quite small to begin with.

    The Yagi-Uda is already highly directional in the forward direction and somewhat less so in the reverse direction. The favored direction is perpendicular to the elements and toward the direction of the smallest element.

    Basicaaly you rotate the antenna until you maximize the input signal and that is the direction it is coming from.
     
  4. GFTMC

    Thread Starter Member

    Jan 20, 2008
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    Well, I cannot rotate the antenna itself. It is too big for that.

    What I want to do, is to detect a signal from a variable position transmitter (A walkie talkie, to be precise), And once I get it I want to detect what its direction.

    So, you have said it doesn't reflect enough for me to be able to recieve the reflection. Any idea concerning that? Also, if I can make this a normal Di-pole, will it reflect enough to be recieved by dishes or other dipoles around it? and lastly, can I use reflectors on the main antenna without interfering with the signal?
     
  5. rwmoekoe

    Active Member

    Mar 1, 2007
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    Papabravo has just stated it for you. Your needs of detecting where the (variable position walkie talkie) signal comes from (its direction) is best obtained through the use of your yagi antenna. Just insert an elecrtric rotator at the base of the yagi antenna at the end of your antenna pole, rotate it while reading the signal strength. note when it is the strongest, that will be the direction of the signal.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Antennae do not "reflect" energy.

    If you want to use fixed Yagis to find direction, use multiple Yagis pointing in different directions and compare the signals from them.

    If you only have one Yagi, you might want to explore alternative ideas for tracking the little radio.
     
  7. rwmoekoe

    Active Member

    Mar 1, 2007
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    i remember back then when i was still active in the 2m band amateur radio in my hometown. we used to have the annual competition of signal hunting. a portable yagi with several elements worked best. but i have tried succesfully using a plain coil antenna (the short coil antenna covered in rubber hose, used in a handy talkie).
    the trick is by detecting the 'dead signal', that is where the signal comes the weakest, when pointing the short antenna. (we're using the dead zone of the antenna to locate the direction of the signal). only, it doesn't tell where the signal is actually from, the bottom of the coil, or the top. so, given two different location with each separate direction's inrterpolated on the map, you pinpoint the exact location of the signal source.
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    At VHF frequencies there is also the issue of polarization. A horizontally polarized yagi will receive signals from other horizontally polarized antennas but will do relatively poorly on vertically polarized antennas.

    At HF frequencies polarization doesn't much matter since skywaves get scrambled by the ionosphere anyway.
     
  9. GFTMC

    Thread Starter Member

    Jan 20, 2008
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    Such as...?

    rwmoekoe - Do you mean to use two different anttenae to pinpoint the exact location?
     
  10. rwmoekoe

    Active Member

    Mar 1, 2007
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    i mean you can use that antenna alone, provided that you can rotate it freely. it should be done practically using a common yagi antenna rotator. the sizes and prices vary, depending on your antenna's size and weight.

    you should also have some kind of a signal strength meter. a portable radio communication transceiver usually has one (at least the mid to high-end ones do). this transceiver is the one you should use as the reciever through the yagi.

    once you got the yagi, the rotator, and the transceiver installed, you're ready to signal hunt.
    rotate the yagi, while noting the signal strength of the huntee walkie talkie. (you should've spotted the huntee signal in the first place, though.)
    while you're rotating the yagi, remember when the signal is the strongest. that direction is where the signal comes from. maybe you should get out of the room yourself and check out for the actual direction the yagi is pointing at right now :).
     
  11. GFTMC

    Thread Starter Member

    Jan 20, 2008
    18
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    That'd be great, only that I don't think I can rotate it.

    If I will be able to add three dipoles on top of it, will each of them recieve the signal differently? (Similar to my initial idea, only that they will be mounted on the Yagi)
     
  12. rwmoekoe

    Active Member

    Mar 1, 2007
    172
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    gftmc,

    far as i know, a plain dipole is unidirectional. that is, it receives signals from all 360 deg. direction equally in strength.
    unless you configure the three dipoles so that they amplify each other or in a way that the signal strength will be directional.

    sorry, but this fact brings the subject about signal reflections that you've mentioned earlier.
    yes signals can be 'reflected'.

    in case of signals with wavelengths longer than a few centimeters, the 'reflection' is actually a kind of resonance interference among conducting objects around the main dipole.
    this is how a yagi is capable of being directional.
    the front elements amplify signals coming from the front, while the rear reflector also prevents or reduce signals coming from the rear direction, while reflecting signal from the front, hence amplify it further.
    the distances, the sizes (diameters and lengths) of the elements' poles, and exact positions of each elements and reflectors are crucial to the frequency, the direction, the bandwidth, and the amplification (gain, actually, in db) of the received signals.

    in case of signals shorter than a few centimeters, the signals can be considered really reflectable. it makes it possible to reflect them using a bowl such as in a parabolic dish antenna we use to catch satelite tvs.

    sorry if i write too long. hope it'll help you.
     
  13. GFTMC

    Thread Starter Member

    Jan 20, 2008
    18
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    First, thanks for helping me with that.

    Second, I have waves about a single meter in size. That means that I cannot use dishes, right? Anyway, thanks for your time. I guess I will need to experiment using the info you gave me. I will post here If I will have any success. I will try to use a 1 metre high dipole and see wether it resonates whith the larger yagi for now.

    That is, If my yagi will even get the signal in the first place.
     
  14. rwmoekoe

    Active Member

    Mar 1, 2007
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    good luck, u can make a good radar there.. ;)
     
  15. Papabravo

    Expert

    Feb 24, 2006
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    This is not true. Horizontal dipoles favor the direction perpendicular to their axis, and there is a null in both directions along the axis. The term for an isotropic response (same in all directions) is omnidirectional. Even a yagi is not unidirectional (response in one direction) because it has gain in the reverse direction, just not as much as in the forward direction.
     
  16. rwmoekoe

    Active Member

    Mar 1, 2007
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    thx papabravo, you're right.
     
  17. Angelwaves

    New Member

    Jun 11, 2010
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    i'm not questioning the motives for such a thread, but what are the legalities of signal hunting in most countries?
     
  18. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That thread is over two years old. Radio direction finding is hardly a questionable activity. Many wildlife biologists use the technique to track animals. Amateur radio clubs use RDF in fox hunts.
     
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