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I'm trying to understand the methods and pitfalls for using a circular array of yagi antennas to calculate the angle of arrival of a radio signal. Or any tips on the proper jargon that can help me in my search.

What I want to do: It's becoming more common in wildlife research for folks to set up multiple yagi antennas on a single tower, pointed in different directions, to track animals tagged with VHF transmitters. Most receivers out there do report a signal strength and background noise, often in something like dBm, for each antenna in the array more or less simultaneously, but cannot do time of arrival or anything like that.

Based on some different resources I have come across, how you convert those signal strength readings to a bearing of the the transmitter from the tower, can depend on a few things like:

Whether the radiation patterns of the antennas overlap The type of antenna used (3,5,9 element yagi, gamma match or folded dipole) How the antennas are pointed (i.e. do they cover 180*, 360*, are there dead spots in between, etc.) What I'm trying to understand is:

How does the setup (different bearings of individual antennas) and type of antenna limit the possibility to calculate a bearing? Is there some minimum design specs that are necessary to make this feasible (i.e., detection of the signal on > 1 antenna)? (How) does the approach used to compute the angle of arrival differ with the type and configuration of antennas used (3 vs 5 element yagi, 6 antennas spaced 60* apart vs. 4 spaced 90* apart, 3 spaced 40*, etc.). What can be pulled from the literature vs. needs to be measured in the field (e.g. e.g. antenna beamwidth)? I'm working with a few other wildlife biologists to package up functions that can do some of the calculations to get the AOA, but I want to make sure we're doing it in a way that is correct and can accommodate a few different setups (e.g. 6 antenna, 4 antenna arrays, 5 vs. 9 element yagis. The number of different deployment setups isn't that variable but I want to make sure we understand the constraints and do things correctly.

If there are any resources that are good, especially for folks who may only have an introductory physics background, I'd be grateful to hear about them or if you can tell me what terms I should be searching for in the literature.

Thanks

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  • $\begingroup$ Are you limited to using the signal level/power only? $\endgroup$
    – Envidia
    Commented Apr 28 at 17:45
  • $\begingroup$ yes. Commercially available receivers (or types users construct with instructions like this: docs.motus.org/sensorgnome-v2) for this type of work typically only give you that information, some only provided an index between 1 and 255 $\endgroup$
    – tlyons253
    Commented Apr 28 at 19:24
  • $\begingroup$ Gotcha. This is very doable and once I have some time I can provide an example of what you can do. $\endgroup$
    – Envidia
    Commented Apr 28 at 19:26

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I am not familiar with the literature on animal tracking, but I am very familiar with using a multiplicity of directional antennas for DF. My first instinct is to set up the antenna system in a portable manner in an open field, and record the signal strengths from each of the antennas as you walk a circle around the antenna system with a transmitter in the far-field region. Given the data collected, you can easily construct a mathematical model that will solve for the best estimated direction/bearing and provide a fairly high level of accuracy. As you mention, DF can also be done using time-of-arrival (need pulsed or coded signals), or simply phase measurements from several (3 or more) monopoles. The latter method works very well, and can be accomplished with a simple coherent 4-channel receiver (which can be based on RTLSDR, with some hardware modifications for coherence; I did this back in 2014 when studying LDEs and meteor echoes).

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  • $\begingroup$ thanks, does the distance from the antennas matter when walking the circle (e.g, 150m vs. 300m) ignoring the fact the distance you'd be walking is longer and assuming that you're being detected by multiple antennas at both distances? $\endgroup$
    – tlyons253
    Commented Apr 28 at 20:44
  • $\begingroup$ Yes. You definitely want to be in the far field.In addition you want to be far enough away so that you have a fairly well columnated (think optics) wave as it approaches the antennas. $\endgroup$
    – Scientific Consultants
    Commented Apr 29 at 5:03
  • $\begingroup$ Yes. You want to be in the far field! Also, you want to be far enough away to have a fairly well columnated (think optics) wave as it approaches the antenna system: the angle of approach must be close to equal for all antennas (otherwise the "angle of arrival" as well as "times-of-arrival" do not make sense for the array). This even applies to methods like multiple Yagis without time-of-arrival detection, The minimum distances depend on the wavelengths, and the placement and size of the antennas. You could run some simulations in NEC if you are familiar with that program. $\endgroup$
    – Scientific Consultants
    Commented Apr 29 at 5:14

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