Maximum detection range of Lotek nanotags

Hello everyone.

We have a project that will involve setting up Motus stations for the detection of Nathusius’s pipistrelle bats. Due to the small mass of these bats, in terms of tags, we are limited to Lotek nanotags. In particular, the tags that I’ve been testing are the NTQB2-2-M nanotags (0.32g, 150.1MHz, burst interval 7.1 seconds).

The Motus stations we plan to install will need a detection range of at least 2km (and preferably more) to get full coverage of the area we want to monitor. I’ve been trying to optimise a testing setup to get as large a detection range as possible. I’ll go into a lot of detail here; you can skip ahead to the bold part below for a summary of the questions I have. The testing setup is as follows:

  • A 5-element (Laird Y1505) or 6-element (Sirio WY 140-6N ) Yagi antenna. The Laird Y1505 is gamma matched as per the instruction manual. I’ve mostly been using one antenna at a time for measuring. The antenna is attached to a telescopic metal pole, which in turn is attached to a fire escape. This gives the antenna a height of ~9m above the ground, and ~2m above the metal railing of the fire escape.
  • The nanotag is attached to the end of a metal pole, which is attached to a bicycle, putting the tag ~2.5m above the ground. The bicycle is then cycled up to the dike, where I can continue cycling on top of the dike for a few kilometres while keeping line of sight to the antenna.

Antenna output has been analysed using a SensorGnome, a SensorStation, and a Lotek SRX1200. For the SensorGnome/Station, the SDR used was a FunCube dongle pro+. I’ve also looked at raw radio signal from the FunCube using SDRsharp.

During an initial test, the tag could be detected in the raw signal up to a distance of ~1300m, where the signal is barely above the noise level. Of course, this doesn’t mean that any software would also detect the tag, so we tested it with the SRX1200. With the antenna gain set to 71 (as high as it’d go without pelting us with a flurry of false detections) we were able to detect the tag up to a range of ~1100m. We didn’t have the SensorStation at this point, so we unfortunately couldn’t test it yet with these same conditions.

The SensorStation arrived last week, and I’ve been busy trying to at least replicate the 1100m result with it… but to no avail. Over the last few days, my testing has become extremely inconsistent, and I simply cannot figure out why. Using the same setup as before, I’ve gotten a detection range of ~700m with both the SRX1200 and the SensorStation… once. With that same setup, and the pole holding the tag attached to a sign at ~600m, two out of three times the tag was simply undetectable. I checked the raw radio signal as well, and it didn’t show up there either, so the problem must’ve been the tag itself. Other times, I could clearly still see the radio signal (quite a bit stronger than the 1300m case earlier), and the software still wouldn’t detect the tag.

I’ve been trying to tinker with the SensorStation settings in the deployment.txt file, but it seems like the important ones are completely undocumented (regarding a lot of SDR gain/filter settings).

Overall, I’ve got the following questions as a result of the tests I’ve done so far:

*** What is the maximum detection range other people have achieved with Lotek nanotags and any detection software? I’d really like to know what range I can expect under the most optimal of circumstances. Assuming line of sight between tag and receiver is always achieved, does tag/receiver height affect the detection range at all?**

*** What are the allowed values for lna_gain, rf_filter, mixer_gain, if_filter, and if_gain in deployment.txt? Is it ever necessary to change these parameters? **

*** * What value should we use for tuner_gain? The default is 40.2 - does this imply a 40.2 dB amplification? Can we use higher values in environments where we expect less radio noise? How high can the tuner gain realistically go?**

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:nerd_face: :smile: :+1:

You may want to consider modifying your attachment so you do not place metal next to the tag where it can influence the antenna. The recommended set-up is to use a non-conductive pole (e.g. wood, PVC, fiberglass) and to attach the tag to something that simulates the animal body. The reason is that the antenna needs a counterpoise to radiate effectively. The two “popular” methods are a piece of hotdog or a small plastic flask/bottle filled with a sponge and saline solution. Both should be sized/filled to weigh about the same as the target animal. Then attach the tag to that with the antenna pointing in the expected direction, for example, horizontally to simulate a bird in flight.

Those are pretty cool screen shots! I have not been able to capture the signals with SDRsharp, they go by too fast for me. The only app that has worked for me is SDRangel, which has an “interesting” UI… But if SDRsharp works for you all the better!

I have been primarily working with RTL-SDRs, see the thread on RTL-SDR vs. FUNcube, but I may be able to help a little. On Sensorgnome V2 the file that controls some of the SDR detail settings is /etc/sensorgnome/acquisition.json, on SSv3 it lives at /data/sg_files/deployment.txt. In that file there are “plans” for rtl-sdr, funcubePro, and funcubeProPlus. Be sure to edit the plan that pertains to the SDR you’re using (e.g., "devType": "funcubeProPlus"). After editing, you need to restart the main process, on SSv3 that would be sudo systemctl restart sensorgnome and on SGv2 that would be sudo systemctl restart sg-control.

In terms of changing the devParams, I have only experience tweaking the ones for RTL-SDR. You mention the tuner_gain: that this setting is for RTL-SDR and not FCD. The FCD plan has gains for various stages and not being intimately familiar with FCDs I can’t comment on the default settings. The filter settings control the frequency band filters that the FCD can switch in/out and these are extremely unlikely to be wrong.

In terms of detection sensitivity: a Lotek tag emits bursts of 4 short (2.5ms) RF pulses with a spacing of up to ~100ms which encodes part of the tag ID, and these bursts are repeated every few seconds (7.1 in your case). Each pulse needs to rise at least 6dB above the surrounding noise (that’s the minsnr setting), where surrounding is ~20kHz in frequency and some ms in time (I don’t know the value). You will see the raw pulses in the Web UI (the SGv2 UI shows SNR, which I find very helpful in this context). In order to show a tag detection in the Web UI two things need to happen: you need to have uploaded the tag definition to the SS/SG and it needs to see at least two pulse bursts with the correct burst interval (it accepts 2x the interval as well).

When you compare distance results with other people remember that most probably use a 9-element Maple yagi, which has more gain (but also a much narrower beam). The height above ground does affect the range due to what’s called the fresnel zone (Fresnel zone - Wikipedia).

This is what comes to my mind at the moment… I hope others will chime in!
Welcome to the forum!

Hi Thorsten, thanks for the reply!

On the topic of what to attach the tag to:

We’ve heard before that it’s best to attach a tag to a dead animal of whatever species you’re interested in. The biomass would increase the signal strength emitted by the tag+animal combination compared to just the tag by itself. For want of dead bats, we did try a few other “biomass” objects, such as a damp sponge, and we’ve tried wrapping the tag in leaves as well. In a “from the office” test with the antenna on my desk and the tag attached to the wall (2m or so between the two), the leaves in particular seemed to improve the signal a lot. For longer distances, however, we didn’t get any improvements by putting the leaf-wrapped tag on a pole. If the tag needs a ground to function properly, I’d personally have expected the pole-bicycle setup to have acted as one. I have noted that pinching the tag between my fingers does significantly increase the signal strength.

The metal pole is a bit baffling to me as well. You’d expect it to mess with the signal emitted by the tag and to cause all sorts of interferences, but so far, it’s given us the best results. I’ve tried attaching the tag to both a wooden pole and a plastic tube, and in both cases, the signal only got worse.

In all cases, I’ve had the tag attached in a vertical configuration, and the yagi’s polarisation plane has been vertical. I’ve read that this maximises the signal strength. From a physics perspective it doesn’t make the most sense to me for the yagi elements to be perpendicular to the electric field from the tag - how would this field induce standing waves in the elements - so it makes sense to test out the horizontal configuration as well.

On the topic of tuner_gain: that implies I’ve been trying to test out a setting that changes exactly nothing. That definitely explains why I haven’t gotten much out of it. I’ll have to look into the FCD settings, then.

polarization - Which way is a yagi polarized? - Amateur Radio Stack Exchange looks reasonable to me. The yagis in your two photos are horizontally polarized, that’s not great if you run your tests with vertically oriented tag antennas. In the end you should match the orientation of the antennas on the flying bats, although I’m not sure how much it really matters.

This is what I would have expected. The metal pole is a decent counterpoise. Swap it for wood/PVC and you loose that. The goal here is to get something realistic, not to get the best.

Assuming the metal pole to be a better counterpoise than a bat in flight, this creates quite the problem for the project! I attached the tag to a small container filled with salted water and bits of sponge, with the container being attached to a wooden pole. As a reference measurement, I also used one tag attached to the metal pole, as I’ve used before. At the same distance from the antenna (maybe ~20m horizontally, with a ~7m distance vertically) the metal pole got signal strengths of -19.5dB to -20.5dB with noise levels of -47dB to -49dB, whereas the sponge container got -31dB to -33dB, with noise levels of ~-50.5dB. The detection range was abysmal, going maybe as far as 200m. Frustratingly, I also again wasn’t able to reproduce 600m detections with the metal pole, even though we’ve gotten those detections before (using, as far as I can tell, as close to the same setup as I can manage).

If getting detection ranges of ~600m with a metal pole as counterpoise is already inconsistent, then surely detecting tags attached to actual bats will only be worse. This frustratingly clashes with another report, where ranges over 5km were achieved with a tag glued to a dead songbird (and over 2km with a 5-element yagi). That’s why I’m really looking for more range measurements from other parties, to see if everyone’s able to get >2km detection ranges with these nanotags.