🔧Builder's Day

The longer read

This is a Saturday-morning project. About three hours from box-open to first aircraft contact. About six hours if you also want to set up a permanent roof-mounted antenna and a Raspberry Pi feed to FlightAware.

What you need

• RTL-SDR v3 dongle. $35 from RTL-SDR Blog or NooElec. The v3 has a metal case, a TCXO for frequency stability, and direct-sampling support if you want to listen below 24 MHz later. Don't buy a no-name Chinese clone — the few dollars saved cost you frequency drift and worse local oscillator phase noise.
• Antenna. For first contact, a quarter-wave whip on the supplied magnetic base will work indoors near a window. For real range, build or buy a 1090 MHz collinear or spider antenna. A coat-hanger spider with four 68 mm radials and a 68 mm driven element on an N-connector base, mounted in the attic, will out-perform the supplied antenna by 15 to 20 dB.
• Computer. A Raspberry Pi 4 with 2 GB of RAM is plenty. A Pi Zero 2 W works but runs hot. Any spare Linux laptop also fine.
• SD card. 16 GB minimum.
• 5 V 3 A USB-C power supply (the official Pi supply; do not skimp here).

Software stack

1. Flash Raspberry Pi OS Lite (64-bit) to the SD card. Boot. SSH in.

2. Install readsb (the maintained fork of dump1090):

   sudo apt update
   sudo apt install -y git build-essential librtlsdr-dev libusb-1.0-0-dev
   git clone https://github.com/wiedehopf/readsb.git
   cd readsb && make RTLSDR=yes && sudo make install
   

3. Blacklist the kernel DVB-T driver so it doesn't grab the dongle:

   echo 'blacklist dvb_usb_rtl28xxu' | sudo tee /etc/modprobe.d/blacklist-rtl.conf
   sudo reboot
   

4. Run readsb from the command line first to verify it sees the dongle:

   readsb --device-type rtlsdr --gain max --net
   

   You should see a stream of hex Mode-S messages within seconds. If you don't, it's almost always one of three things: dongle not seated, antenna not on a ground plane, or you're in a Faraday-cage office building. Move to a window.

5. Install tar1090 for a nice live map:

   sudo bash -c "$(wget -O - https://github.com/wiedehopf/tar1090/raw/master/install.sh)"
   

   Then open http://<your-pi-ip>/tar1090/ in a browser on the same network. You'll see aircraft as colored arrows on an OpenStreetMap base layer.

6. Optionally feed to FlightAware and ADS-B Exchange. Each has a one-line installer; both credit you with premium accounts in exchange for your feed. ADS-B Exchange is the one to feed if you want to see military aircraft that other networks filter out.

Antenna build

A spider antenna is the simplest serious 1090 MHz design. You need:

• One N-female chassis-mount connector
• Four pieces of 14 AWG solid copper wire (a #4 bare copper ground wire from any Home Depot is fine), each 69 mm long, for the radials
• One piece of the same wire, 69 mm long, for the driven element
• A small piece of UV-resistant plastic for a weather cap (optional)

Solder the driven element to the center pin of the connector, vertical. Solder the four radials to the four chassis mounting holes (or directly to the connector body), drooping downward at about 45 degrees. Total build time: about 20 minutes. Mount it in the highest spot you can put it — attic, roof, or a PVC pipe lashed to a fence post. The horizon is your only friend at 1090 MHz; every additional foot of antenna height extends your range measurably.

If you don't trust your soldering iron yet, FlightAware sells a pre-made 1090 MHz antenna for about $50 that performs within 1 dB of a good homebrew. Either path works. The homebrew path teaches you what an antenna is.

What you'll see

A clear weekend afternoon from a roof-mounted antenna in central Madison typically yields:

• 100 to 250 simultaneous aircraft at peak
• Coverage out past Memphis, Atlanta, and Nashville on the major airways
• Every commercial flight in and out of HSV, BNA, ATL, BHM
• Cargo flights overhead between Memphis and Atlanta
• Marshall and Redstone test aircraft when they're active (most are visible; some are not)
• The occasional aircraft with no callsign and an unusual altitude profile

You'll start to recognize patterns — the FedEx hub-and-spoke from MEM, the Delta flow into ATL, the Boeing test flights out of Charleston SC. After two weeks, you'll know what's normal traffic and what's interesting. That's the hobby talking.

Where it goes from here

ADS-B is just the beginning. The same dongle reads:

• NOAA weather satellites (137 MHz APT) — beautiful weather images, downloadable with a $5 V-dipole
• AIS ship transponders (162 MHz) on the Tennessee River barges
• Aircraft VOR/ILS (108 MHz to 118 MHz) navigation signals
• Public safety voice traffic (with appropriate trunked-radio decoders)
• HF amateur radio, if you build an HF upconverter or move to an SDRplay

The Huntsville Hamfest in August is the right place to see all of these running on tables, hosted by people who will spend an hour with you for the price of asking. Bring cash. Bring a notebook.