discussion / Biologging  / 12 March 2024

DIY VHF receivers?

Hi everyone. Looking for an affordable DIY VHF receiver option. I'm working with an engineer to develop an automated audio playback system that will play pre-recorded sounds from a speaker when a tagged animal (currently ocelots, planning to expand to other felids and possibly eventually eagles) is detected to be within range of the playback station (up to, say 400 m or so, if possible). The plan is to include a VHF signal as part of the tag (currently testing with a VHF transmitter from Holohil, and one from Telonics included with an Iridium collar), and use the VHF to trigger the initiation of the playback (e.g., when the frequency is detected with a certain strength). Receivers from folks like Telonics, LOTEK, ATS etc. are prohibitively expensive because I will want to make many playback stations (several stations in the home range of each tagged animal, with several tagged animals per site, and hopefully at several sites) that will be left out in the field for long periods of time. Does anyone have any experience or opinions about the digital USB receivers from radio-tracking.eu (link below)? Or have advice on another solution for a receiver? Or other advice on how one might trigger a playback when an animal of interest is in a radius of up to 400m or so from a speaker? Many thanks! 




Hi @bcwheeler43 , I haven't used the radio-tracking.eu setup in particular, but it looks pretty similar (at least in principle) to Motus (https://docs.motus.org/en/about-motus/quick-reference/station-deployment). My experience with the Motus set up is that it can take a bit to get it all to work, but, there's a fairly active community here (https://sensorgnome.org/) that you might be able to get tips and guidance from. If you are looking for a more off-the-shelf option, have a look here (https://celltracktech.com/pages/digital-radio-products). They might even be able to help you get a system ready to detect tags and then activate your playback...

And in case any of it helps, whilst we didn't use VHF, here's a paper where we used UHF tags we designed to do something similar (https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/2041-210X.13651). See case study 2. All the files are released to open source, although some of the parts might be getting a little harder to find...I am hoping to an update soonish...You might have also come across Breck and co's old paper (https://escholarship.org/content/qt4gv9x4j1/qt4gv9x4j1.pdf) that has a little detail, although not really enough to build a full system.

Anyhow, I think a VHF version would be very useful as so many collars use VHF transmitters. 

Please keep us posted with your progress!

Cheers,

Rob

 

Hi Brandon,

This is hard for me to write, because I don't have good news for you.  You can find many receivers and transmitters for 433MHz that are used for remote control, such as wireless doorbells, garage door openers, lost model locators, etc.  These are extremely cheap, a couple of dollars literally.  While this tech can be modified to operate with wildlife tags (around 150MHz), we are talking about hacking RF, not to be approached lightly.

If you really need to DIY, I see 2 options: migrate to 433MHz and build/rebuild all your kit (tags, collars, receivers, locators).  Commercial solutions are cheap and available, the work remaining is not much above handyman level.  But it is work, and not just work but also testing.

The other option is to remain with 150MHz (although you can make your own tags) and modify an aviation receiver (it lets you listen in to aircraft talking to each other or to the airport).  These receivers operate at 130MHz, which puts it in spitting range of 150MHz wildlife tags (this matters).  This is the option I'd go for, just because I know it can be done, and I have less sense than curiosity.

You mention you're working with an engineer, here are a couple of pointers:

  • with an SDR (the "digital USB receivers" you mention) you can scan some bandwidth to detect radio pulses. I'm only familiar with the Lotek coded VHF transmitters but they can be received as in the Motus Sensorgnomes. I can point you at the code used there, it does involve a good amount of digital signal filtering/analysis code. One issue with the SDRs is that they're pretty power hungry so you need to plan on more solar panel and battery than you may like, e.g. even rPi Zero + SDR is probably >300mA. Another issue can be interference in that SDRs have a very wideband front-end and so TV, radio, and other stuff can desensitize the front-end.
  • there are integrated receivers (ICs) that can be tuned to 150Mhz and can detect VHF transmissions, it's easy for non-coded ones, a bit more involved for coded ones. Specifically, the Semtech sx1231 or sx1276 series are widely available (often called Hope RF69 or RF96, which are modules with those chips). You need someone comfortable writing a customized driver that uses OOK mode or RSSI detection. The other issue is that while there are many suitable boards available for 433/868/915Mhz (e.g. Adafruit, Sparkfun, LilyGo, and many others) you most likely won't find one for 150Mhz. However, for RX-only the matching isn't so super important if you have a good antenna and the signal isn't crazy weak. From a power point of view these can sip power so you can run one on an 18650 for days and a small 1W-5W solar panel is most likely all you need.

Hope this helps...