1
\$\begingroup\$

I'm working with radio tagged animals. Fish tagged with 150 MHz tags. I'm interested in localizing these tag positions.

Ideally, I'd like to build a few stations that can return bearing estimates of the tag location. I was wondering how I could do this.

Initially (and knowing very little about radio), I thought 2 orthogonally oriented Yagi antennas could make such a station. Knowing the radiation patterns of the antennas, the difference in received levels could produce a bearing estimate. However, I'm also aware that closely placed Yagi antennas will interfere with each other, so that idea sounds unfeasible.

I was hoping to get some advice on how I could go about building a station that returns bearings of a radio transmitter. Perhaps there are more suitable antennas for this purpose, or perhaps there are alternative solutions than using antenna arrays.

additional info based on comments so far

Here im specifically working with river fish in shallow (fresh) water. In deep water, we use acoustic transmitters and do time difference of arrival positioning. Stations would be above water on the river banks, stationary (unattended), and distances to the fish would be max 200 meters away, so im interested in small spatial scales here.

I already write models for the underwater acoustic transmitters... But in shallow water (a meter or less), this tech works poorly. Radio transmitters are already widely used in fishes (tracking river movements)... But they are traditionally just tracked with handheld yagi antenna.

The radio tags used give out short pulses.. around 200ms long every few seconds.

\$\endgroup\$
9
  • 1
    \$\begingroup\$ What research have you done? Try:cdn.rohde-schwarz.com/us/campaigns_2/a_d/… \$\endgroup\$
    – glen_geek
    Jan 31, 2023 at 14:10
  • \$\begingroup\$ VHF direction finding... 8 antennas with PIN diodes switcher? VOR navigation. \$\endgroup\$
    – Antonio51
    Jan 31, 2023 at 14:28
  • \$\begingroup\$ rtl-sdr.com/… \$\endgroup\$
    – Antonio51
    Jan 31, 2023 at 15:10
  • 2
    \$\begingroup\$ How far away are the fish expected to be and at what depth are they going to be approximately? You are going to struggle getting a strong signal due to the attenuation of the 150 MHz signal in water. Using an online calculator I am getting a skin depth less than 1 mm for seawater at 150 MHz. If this is the case you are going to lose around -4.3 dB of power per mm and its unlikely(depending on the fishes distance) your tag's signal will be above the noise floor of your receiver. \$\endgroup\$
    – CMH12
    Jan 31, 2023 at 16:09
  • \$\begingroup\$ What is the duration of the signal from the tags? Pings of a few mSec? Repetition rate? Continuous? Are the receiving sites manned or unmanned? All this will determine what options you have for the DF design. \$\endgroup\$ Jan 31, 2023 at 16:36

3 Answers 3

2
\$\begingroup\$

Being out in the open, Time Difference of Arrival (TDOA) is pretty easy.

A single station with two antennas placed less about a quarter wavelength apart with some pin diodes and an oscillator will produce a tone proportional to the angle and a null when the signal is perpendicular.

I built one of these back in college to find rockets using this guide on radio direction finding, and it works really well. enter image description here

One improvement to make on the simple two antenna version is when the signal is perpendicular, the system can not detect whether the signal is directly in front or in back. There are several methods to over come this, including making another set of antennas that are perpendicular to the first set.

\$\endgroup\$
4
  • \$\begingroup\$ Will this work underwater? \$\endgroup\$
    – RussellH
    Jan 31, 2023 at 16:58
  • \$\begingroup\$ @RussellH It'll work as well as the radio transmitters that are in the water. \$\endgroup\$
    – Aaron
    Jan 31, 2023 at 17:00
  • 1
    \$\begingroup\$ @150MHz water (even fresh water) is lossy as hell, and salt water is orders of magnitude worse. Radio generally sucks under water (Which is why most under water stuff is sonar based), but if you can hear the carrier then yea, it will work. \$\endgroup\$
    – Dan Mills
    Jan 31, 2023 at 18:14
  • \$\begingroup\$ Very interesting! I already do TDOA for acoustic tags. I will dive into this on the weekend and let u know if i have any questions. \$\endgroup\$
    – RTbecard
    Jan 31, 2023 at 23:06
1
\$\begingroup\$

Look up how the radio hams do 'Foxhunting' which is basically a competition to locate a small intermittently operating transmitter.

The antennas they use for the 2M band should scale well enough for the purpose.

Usually a dipole (Look for the null, it is shaper then the maxima) or yagi, steered by the operator.

There is another approach, you can do 'doppler DF' with 4 verticals electronically switched into a FM receiver, this can be made to give you the classic spy movie circle of leds indicating relative bearing, but multipath can be a real issue.

\$\endgroup\$
2
  • \$\begingroup\$ Will this work underwater? \$\endgroup\$
    – RussellH
    Jan 31, 2023 at 16:58
  • \$\begingroup\$ Great! The doppler df with switching antenna sounds promising! Ill take a deep dive into this on the weekend. \$\endgroup\$
    – RTbecard
    Jan 31, 2023 at 23:14
0
\$\begingroup\$

Look up 'doppler direction finding'. It's an established technology, and used by the popular 'Lojack' vehicle recovery system. If I remember, a simple, stationary, antenna array can give a bearing without referencing complex antenna patterns. Multiple antenna arrays in different areas can triangulate the transmitter. If you have one array, make it mobile, and 'home in' on the signal directly.

\$\endgroup\$
1
  • \$\begingroup\$ Will this work underwater? \$\endgroup\$
    – RussellH
    Jan 31, 2023 at 16:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.