My study in particular is only looking at species that go under fence holes in Southern Africa. The hole determines species size and juveniles for these species are so small the collar would just fall off. I think a proper study could really determine these collars could be useful if the right study species and setting location was determined. 

We suggest and use Python Locks for urban areas here in Australia. Can also screw the Mini 2 to a tree or phone post with the python lock.

We've deployed passive acoustic monitors in various urban environments without any issues of theft, even in locations where camera traps and other technology has been historically taken or vandalised. I think acoustic sensors generally look less valuable than other technology, and I would recommend removing or covering any stickers or labels on devices that would otherwise make them more appealing! Placing them in hedgerows or on trees with some (but not too much!) foliage will help keep them well hidden too.

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...