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I'm currently trying to find a way to build a bat detector on a budget (good commercial ones are ridiculously expensive), and came across this video building one using SDR: https://www.youtube.com/watch?v=-ikeMSn35T0

Now, I'm pretty clueless about radio, and that's the first time I've heard about SDR... These SDR dongles seem very affordable, so now I'm wondering if I can use one of those, together with my phone (like so https://www.youtube.com/watch?v=COUb3Twd3B4), and a mems microphone to build that bat detector.

My main question now is: Typical radio frequencies seem to be much higher than bat frequencies (which are 20-120 kHz, most interesting for me is the 30-60 kHz range). Do I have any chance to pick those up with an SDR dongle? Or would I need something more involved?

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If you have an SDR stick that can do direct sampling, it might work.
Direct sampling mode bypasses the tuner (which converts the TV signals at hundreds of MHz down to ~10s of MHz for sampling by the RTL chip). This lets the chip see the baseband, DF-50 MHz, which it wasn't designed for, but it does work. The sticks don't have an amplifier so it won't be very sensitive in this band.

I tried it out quickly with my RTL-SDR-Blog stick v3.1, it looks quite usable! A sweep from 10 kHz to 100 kHz looks like this:

enter image description here

This is with a very crude signal generator so lots of harmonics. Input signal (fundamental) was about 1.5 mV RMS or -43 dBm. The Q branch less noisy than the I. RTL AGC on. IQ correction on. Sampling 2.4 MHz. You can see it is somewhat weaker below 20 kHz, but it certainly works there.

The RTL-SDR-Blog does say:

If you want to improve the performance at LF/MF (below 500 kHz) and do not require the bias tee, then you can remove the bias tee inductor at L13.

The tuner is quite sensitive. Your bat receiver will need some solid filtering and shielding to prevent the tuner from receiving FM radio, AM radio, computer USB noise, etc. Make sure nothing over a few MHz gets into the input. Keep the internal tuner RF gain at minimum, rather amplify and filter the AF. It is quite tempting to see if you could power the microphone directly from the bias T output of the SDR stick, this would simplify things enormously. An electret condensor might just about do the trick.

By the way, this stick works quite well plugged into my phone, but it drains the battery. (There are Y cables that let you power the USB device from external power). I use an app called SDR-touch. I assume when set to SSB mode, it would demodulate bat calls just fine, but you'd only be listening to ~ 10 kHz of the spectrum at a time of course.

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  • $\begingroup$ Thank you! That's quite cool. Might give it a try once I figure out what most of these words mean. $\endgroup$
    – Giogina
    Aug 15, 2023 at 14:44
  • $\begingroup$ TL;DR there's some chance that an RTL-SDR with bias T, connected directly to an electret microphone, might work for detecting audio and ultrasound. Choose the microphone carefully, or get several, they're cheap. Jangling keys generates ultrasound. Start with something like SDR++ on a computer, then move to the phone. Good luck! $\endgroup$
    – tomnexus
    Aug 15, 2023 at 16:52
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Not all SDR receivers are the same. Some of them work well at very low frequencies (for instance the SDRplay RSPduo used in your first video, as well as the single-receiver RSPdx, are both specified to work down to 1kHz). Others work poorly or not at all in that frequency range (for instance, most of the $20-30 RTL2832 "dongles", which aren't meant for frequencies below 25MHz).

If you find one that will work down below 100kHz, and has software for your phone, then you're in business. We're not a product recommendation site, so my advice is basically: look around, pay attention to spec sheets, and read reviews to get an idea of what's suitable. You're looking for VLF support and the availability of smartphone software.

It's also worth looking into high-quality USB sound cards; at these frequencies, you don't actually need the frequency downconversion abilities of an SDR, you just need a sufficiently fast ADC. If you can find a 192kHz audio interface without excessive lowpass filtering, then that would let you listen up to around 90kHz with a simple audio spectrogram app. I don't know what devices support that, or if they're ultimately cheaper than a suitable SDR, but it seems like something other bat-listeners would know.

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  • $\begingroup$ Thank you! I'll keep looking into the possibilities. $\endgroup$
    – Giogina
    Aug 15, 2023 at 14:28

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