A clean car install: power, antennas, and where the Pi lives

31 Oct 2026 · 7 min read · car · antennas · power · Raspberry Pi

Power first

A Pi 4 with two RTL-SDRs wants a clean 5 V at up to 3 A. The common failure is a cigarette-lighter USB adapter rated "2.4 A" that sags under load; the Pi shows the lightning bolt, the dongles drop off the bus, and the journal fills with usb_claim_interface -6 and dongles that "stopped sending". Use:

• A USB-C PD car adapter rated 30 W or more, on a short cable, or
• A hard-wired 12 V → 5 V buck converter (5 A) into the Pi's USB-C, fused at the fuse box on an ignition-switched circuit.

Ignition-switched means the Pi loses power when you turn the car off. Raspberry Pi OS survives that fine in practice if the card is good and nothing was writing; the bridge writes nothing but its journal. If you want a clean shutdown, a small UPS HAT with a "power-loss → shutdown" script is the tidy answer.

Antennas: one per radio, outside the metal

A car is a Faraday cage with windows. An antenna on the dashboard hears through the windscreen only; the same antenna on the roof hears 360°, with the roof as its ground plane. Cheap magnetic mounts are fine. What we run:

• Scanner dongle: an 800 MHz mag-mount whip (any "cellular/trunking" 806–896 MHz quarter-wave, ~9 cm). Control channels went from +22 dB inside to +38 dB on the roof.
• rtl_433 dongle: a 433 MHz whip is the compromise for 315/433/915; TPMS is strong at close range anyway, so this one can live inside, on the parcel shelf.
• 1090 MHz (if you run dump1090): its own small whip; aircraft are line-of-sight and loud.

Run the coax through a door seal or a rear-hatch grommet. Don't splice: a $12 mag-mount with 3 m of RG-174 and an SMA plug already is the right cable.

Where the Pi goes

Not on the dashboard. A black Pi case in Florida sun reaches 70 °C and the dongles' tuners drift until the AFC can't follow. Under a seat or in the glovebox, with the dongles on short USB extensions so they aren't pressed against the Pi's CPU, keeps everything under 55 °C. Bluetooth to the phone is fine from the glovebox — it's the same car.

Mount with Velcro or a zip tie through the case's holes; a Pi that slides into the seat rails will eventually unplug something.

Bluetooth, not Wi-Fi

The Pi talks to the phone over BLE and nothing else — no hotspot, no internet. It needs Wi-Fi only to update (companion/pi/update.sh), and it'll join your home network when you're parked at home and do that then. On the road the link is Bluetooth and the phone shares its GPS position with the Pi, which is how the RadioReference matches are ranked by distance.

The kit

The full parts list with prices — Pi, two or four dongles, antennas, mounts, power — is in CAR_KIT.md. Everything in it is a commodity part; nothing is specific to Wavelens.

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