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Unit 15: Radio Capstone

Build a real transceiver and talk to someone far away, on a radio we built.

Sessions: 6–10 · Cost: $60–150 · Badges: Tiny Parts, Signal Spotter, (new) 🌎 First Contact Prerequisites: Unit 13 (licensed!), Units 6–8

Pick one track. Both are real radios.

QRP Labs QDX (digital modes, FT8) or QCX-mini (CW). - Excellent manuals, and a huge support community. - Real contacts across the continent on 5 W. - License note: a Technician has only narrow HF privileges: CW on parts of 80/40/15 m, and CW, voice and data on part of 10 m. FT8 on 20 or 40 m needs a General. - So either the parent is the control operator on General-class frequencies, - or it's a great reason for the 11 y.o. to upgrade to General.

Job 8 y.o. 11 y.o. Parent
Parts inventory Leads
Toroid winding (there are several) Winds them: patient, counted work, just like the crystal radio coil Checks the turn counts
Soldering Through-hole parts Everything else SMD rework if needed
Alignment and test Follows the manual, with a dummy load and the Unit 8 scope Supervises RF
First contact Logs it Operates Control operator if needed

Simpler warm-up kits: the 4 State QRP Group Pixie or Cricket, CW transceivers that take about an afternoon.

Track 2: homebrew 2 m handheld (SA818 module)

Everything except the RF core is ours.

flowchart LR
    kp[Keypad + display] --> mcu[Pico / ESP32]
    mcu -- "UART AT commands<br/>(freq, CTCSS, squelch)" --> sa[SA818-V<br/>VHF module]
    mcu -- PTT, H/L power --> sa
    mic[Mic + preamp] --> sa
    sa --> amp[Audio amp] --> spk((Speaker))
    sa -- RF --> lpf[Low-pass filter<br/>we build it] --> ant[Antenna]
    batt[(Li-ion + regulator)] --> mcu & sa
  • The SA818-V is a ~1 W VHF FM transceiver module, set up over UART with AT commands, e.g. AT+DMOSETGROUP=....
  • Use its datasheet for the exact command format and supply range; the documentation varies between vendors.
  • The low-pass filter is not optional. These modules are known for weak harmonic suppression.
  • Design a 5- or 7-pole LPF for 2 m with a filter calculator, and build it Manhattan-style or on an etched board.
  • Verify it with a tinySA (look for the harmonic at about 292 MHz) and a NanoVNA.
  • The rules require spurious emissions to meet §97.307. Measure; don't assume.
  • Case: 3D printed, with the 8 y.o.'s design.
  • What it teaches: everything. UARTs, audio, power, RF filtering, test equipment, and why commercial radios cost what they do.

After the capstone

  • Upgrade to a General license (the 11 y.o.).
  • Parks on the Air (POTA): operate a homebuilt radio from a state park. Kids love it, and it's a family camping trip.
  • ARRL Kids Day, and school club talks: "we built this".