Yesterday we could test the system at our club. It ran the whole day, and counted 3170 laps in total. Out of these, 801 were made with OpenStint transponders, and the rest with RC3/RC4Hybrid/clone ones. I received no complaints about missed laps from people participating in the test.
As it was an indoor event and the pickup antenna was placed under the carpet. I used a lightly coated 26 AWG (0.14 mm2) wire. The wire separation was 30 cm, and a 330 Ohm termination resistor was soldered onto the remote end. At the business end, I added an openstint-preamp (see github for shematics), which was connected to the HackRF One. The HackRF was connected to a raspberry pi, which streamed the laptimes to a makeshift dashboard people could access with their phone.
I set the amplifier gains of the HackRF with the help of SDRAngel. It's a great tool to do all sort of RF magic with SDRs. To figure out the gains, we only need its' spectroscope though. Open the HackRF device with it, tune to 5 MHZ, set sample rate to 5 MSPS and IF filter bandwidth to 1.75 MHz. Park a car with a well-placed, strong transponder over the loop. Then, start from low values, and increase the LNA gains until you see sidelobes on the spectroscope. It indicates clipping. If seen, back off by 8 dB. Then do the same with the VGA gains (2 dB steps). I ended up starting the decoder with the following parameters:
./openstint -l 24 -v 22 -b
As such, gains are LNA=+24dB, VGA=+22dB, and enabled bias-tee for the optional openstint-preamp. With these settings, the highest signals used the full range of the 8-bit ADCs inside the radio, while weaker transponder placements utilized the lower ~4 bits only. The noise affected the ~1.5 least significant digits, yielding still a good SNR for less-than-ideal transponder placements as well.
The antenna was placed to a slow section of the track. During passings, a typical RC4-hybid registered 55-60 hits, while an OpenStint transponder did 170-180. This is in-line with the expectations, as the openstint transponders produce ~3x more decodable messages as an RC4-hybrid does.
Unfortunately, the decoder also reported (probably) RC3 status messages as passings. I added a patch, so next time (hopefully) this will be not a problem. Unfortunately I'm just guessing though. The real solution would be a monitor mode, similar to what RCHourGlass has, to aid solving these mysteries.