cat-radio/CONTEXT.md

CAT-Radio - Cave Adapted Telephone

Open source through-the-earth digital text messaging system. FOSS clone of Cave-Link.

Overview

Parameter	Value
Frequency	20-140 kHz (VLF), selectable
Mode	Digital text, FSK modulated
Baud rate	10-100 baud (frequency dependent)
TX power	10-30W
Target range	1000m+ through rock
Error handling	CRC-16 + ARQ (retry until success)
Relay	Store & forward

Why This Works

Lower frequency = deeper rock penetration.

Skin depth formula: δ ≈ 503 × √(ρ/f) meters

For 100 Ωm resistivity rock:

• 87 kHz → ~170m skin depth (Nicola voice system)
• 40 kHz → ~350m skin depth
• 20 kHz → ~500m skin depth

Digital + checksums = bad signal just means slower transmission, not errors.

Hardware

MCU

STM32L476RG (NUCLEO-L476RG dev board)

Spec	Value
Core	Cortex-M4 + FPU
Clock	80 MHz
RAM	128 KB
ADC	12-bit, 5 Msps
DAC	12-bit, 2 channels
Power	Ultra low (µA in sleep)

Display + Input

Part	Notes
ILI9341 2.4" LCD	320x240, SPI interface
4x4 membrane keypad	T9-style text input
HC-05 Bluetooth	Optional phone interface

Test Equipment

Part	Purpose
SDRplay RSP1A	View TX signal, debug, spectrum analysis

Power (Relay Node)

• Milwaukee M12 3.0Ah battery (~$30, adapter plate ~$5)
• 3S Li-ion, 11.1V nominal
• ~0.17Ah/day (5mA idle + TX bursts)
• Runtime: ~2.5 weeks on single charge

Antenna Options

Earth Electrodes (best range):

• Two copper/steel stakes, 30-50cm long
• 50-100m wire between them
• Low ground resistance critical
• Surface: lay out wire in field
• Underground: clip to metal, wet cracks, pools

Loop Antenna (portable RX):

• 0.5m diameter, collapsible frame
• 80-100 turns, 0.5mm² enameled wire
• ~50 mH inductance
• Tuned with ~300 pF variable cap
• Electrostatically shielded

Firmware Architecture

┌─────────────────────────────────────┐
│           Application               │
│  - Message compose/display          │
│  - T9 text input                    │
│  - Menu system                      │
│  - Store & forward logic            │
├─────────────────────────────────────┤
│           Protocol                  │
│  - Packet framing                   │
│  - CRC-16-CCITT                     │
│  - ARQ (ACK/retry)                  │
│  - Addressing                       │
├─────────────────────────────────────┤
│           Modem                     │
│  - FSK modulator (DAC + DMA)        │
│  - FSK demodulator (Goertzel)       │
│  - Bit sync / clock recovery        │
│  - Carrier detect                   │
├─────────────────────────────────────┤
│           HAL / Drivers             │
│  - ADC + DMA                        │
│  - DAC + DMA                        │
│  - UART (debug, BT)                 │
│  - SPI (display)                    │
│  - GPIO (keypad)                    │
└─────────────────────────────────────┘

Packet Format

┌────────┬────────┬──────┬────────┬─────────────────┬───────┐
│Preamble│Sync    │ Len  │ Header │     Payload     │ CRC   │
│ 16 bits│ 16 bits│8 bits│ 32 bits│   0-64 bytes    │16 bits│
└────────┴────────┴──────┴────────┴─────────────────┴───────┘
   0xAAAA   0x2D4B   N       ...        message       CRC-16

Header:

• Dst (1 byte): destination address, 0xFF = broadcast
• Src (1 byte): source address
• Seq (1 byte): sequence number for ARQ
• Flags (1 byte): ACK, REQ_ACK, RELAY, URGENT

FSK Modem Specs

Parameter	Value
Center frequency	40 kHz (default)
Mark (1)	center + 50 Hz
Space (0)	center - 50 Hz
Baud rate	50 baud
Modulation	Continuous-phase 2-FSK
Demodulation	Goertzel algorithm

Dev Plan

Phase 1: Modem Proof of Concept

1. Set up STM32CubeIDE
2. Generate 40 kHz sine on DAC
3. View on SDR
4. Add FSK modulation
5. Implement Goertzel demodulator
6. Loopback test (DAC → wire → ADC)

Phase 2: Packet Protocol

1. Implement packet framing
2. Add CRC-16
3. Test encode/decode
4. Add ARQ state machine

Phase 3: User Interface

1. Wire up ILI9341 display
2. Wire up 4x4 keypad
3. Implement T9 input
4. Message display/history

Phase 4: RF Hardware

1. Build RX preamp
2. Build TX amplifier (10W)
3. Build antenna matching network
4. Test with loop antenna
5. Test with earth electrodes

Phase 5: Field Testing

1. Short range through-air test
2. Through-rock test (shallow)
3. Deep cave test
4. Range optimization

Parts List

DigiKey:

Item	Price
NUCLEO-L476RG	$14.85

Amazon/AliExpress:

Item	Price
ILI9341 2.4" SPI LCD	~$10
4x4 membrane keypad	~$3
HC-05 Bluetooth module	~$5

SDRplay:

Item	Price
RSP1A	~$110

Total: ~$145

Phase 1 Test BOM (DigiKey):

Item
40 kHz ultrasonic transducers x2 (muRata MA40S4S/R)
TL072 dual op-amp
Breadboard
Jumper wire kit (M-M)
Through-hole ceramic capacitor variety pack
Through-hole resistor variety pack

Later (RF hardware):

Item	Notes
TL072 / LF353 op-amps	RX preamp
IRLZ44N MOSFETs	TX PA
Matching transformer	Antenna interface
Wire, stakes	Antennas

References

• Cave-Link
• Nicola System
• 3496 Hz DQ Receiver
• Arduino Cave Radio (GitHub)
• Through-the-Earth Communications (Wikipedia)
• Skin Depth Theory

License

TBD - likely GPLv3 or MIT