start_tx.sh

Lives on: the Hackerman Pi, at ~/Desktop/hackrf project/start_tx.sh (see Hackerman for how to reach that Pi).

What it does

Takes signal.iq (written by generate_signal.py) and feeds it to the HackRF hardware for transmission, using the hackrf_transfer command-line tool.

#!/bin/bash
hackrf_transfer -t signal.iq -f 915000000 -s 2000000 -x 40 -a 1 -R

Walkthrough

hackrf_transfer is the standard command-line tool for driving HackRF hardware (from the hackrf-tools package). Each flag:

Flag	Value	Meaning
-t signal.iq	—	Transmit mode: read samples from signal.iq and send them.
-f 915000000	915 MHz	Center frequency. This matches the frequency called out in the KPP memo for the RFD900x target signal.
-s 2000000	2 MHz	Sample rate. This has to match sample_rate in generate_signal.py — if they don't match, the tone lands at the wrong offset frequency or plays back at the wrong speed.
-x 40	40 dB	TX VGA gain — how much the HackRF amplifies the outgoing signal before transmission.
-a 1	on	Enables the HackRF's onboard RF amplifier.
-R	—	Repeat mode — loops the file continuously instead of transmitting it once.

That last flag, -R, is why Hackerman's notes say the HackRF keeps transmitting until you press Ctrl+C — the script never stops on its own, it just loops signal.iq forever.

Creating/editing this file

The script was created with a heredoc:

cat > start_tx.sh << 'EOF'
#!/bin/bash
hackrf_transfer -t signal.iq -f 915000000 -s 2000000 -x 40 -a 1 -R
EOF
chmod +x start_tx.sh

cat > start_tx.sh << 'EOF' ... EOF writes everything between the two EOF markers straight into start_tx.sh. chmod +x then makes it executable, which is what lets you run it as ./start_tx.sh instead of bash start_tx.sh.