Learn / Guide 11

First receiver: RTL-SDR

How to start spending little: what the RTL-SDR is, a simple dipole, the programs, what it hears and why antenna and noise rule.

intermediate 6 min read

A TV receiver that became a radio for everything

The RTL-SDR was born as a digital TV USB receiver. Antti Palosaari, a Linux digital TV driver developer, realized that its chip, the RTL2832U, could deliver raw signal samples to the computer instead of a TV image. Around 2012, the Osmocom project published the rtl-sdr driver, which is still used today. With this, the computer took over the radio's role: tuning, filtering, and demodulation became software. That is the meaning of SDR, software defined radio.

What it offers:

There are generic versions and improved versions, with a stable oscillator (TCXO), metal case and power supply via cable for an external amplifier. The price difference is small. The TCXO version is worth it, because the generic one misses the frequency and also drifts as it heats up.

What you can hear

• FM stations.
• Air band in AM, from 118 to 137 MHz.
• Radio amateurs on 2 meters and 70 centimeters, and regional repeaters.
• Space Station at 145.800 and 145.825 MHz.
• Radiosondes, 400 to 406 MHz.
• Aircraft on ADS-B, at 1,090 MHz.
• Weather satellites on 137 MHz (analog NOAA satellites went offline in 2025; the space guide states what is left).
• Wireless sensors at 433 MHz, like home weather stations.
• Amateur radio digital voice, such as DMR, with the right software.

What is not possible

• Transmit. Receiver only.
• Shortwave, direct. Below 24 MHz it needs an external converter or a special sampling mode, with lower performance. And shortwaves require a large antenna and a quiet location. For HF, at the beginning, a remote KiwiSDR easily wins.
• Encrypted signals. Cell phones, a good portion of public safety radios and private networks use encryption. It is not decoded, and you should not try.

It also has a limit that does not show on the box. With 8 bits, a very strong signal nearby overpowers weak ones. Anyone living near FM towers feels this: the entire band goes deafer and ghost stations appear where they should not be.

The antenna weighs more than the receiver

Changing receiver helps little. Raising the antenna two meters, or taking it out from inside the house, changes everything.

The simplest antenna that works is the dipole: two pieces of wire or two rods, in line, with the feedline connected in the middle. The size of each leg, in meters, is approximately 71 divided by the frequency in MHz.

Some precautions:

• Polarization. Repeaters, aviation and ADS-B use vertical antennas. Mount the dipole vertically. For 137 MHz satellites, it is common to lay the dipole flat and open the legs into a V shape.
• Height and clear view. In VHF and UHF, line of sight applies. A window is better than a table, and a roof is better than a window.
• Short cable. Thin coaxial cable loses signal, and loses more the higher the frequency. At 1,090 MHz, every meter less helps.

Aircraft can be received with an RTL-SDR and a simple antenna. Range is usually partial, and what limits it is the antenna environment, not the receiver.

Noise: the enemy indoors

The weak signal you want to hear competes with the electrical noise in the house. What matters is the difference between signal and noise, not signal strength.

• Move the receiver away from the computer. A USB extension cable of one or two meters already takes the receiver away from the source and screen.
• Beware of switch mode power supplies, LED lamps and chargers. Turn them off one by one and look at the waterfall display.
• Do not turn the gain to maximum. Raise it until the background noise starts rising along with it, and stop. From then on you are just amplifying noise and risk overloading.
• FM filter. If there are strong FM towers nearby, a filter rejecting 88 to 108 MHz restores sensitivity to the rest.

The programs

All are free. On Windows you need to change the receiver driver with a utility called Zadig before first use. On Linux, sometimes you need to prevent the system from loading the TV driver, which occupies the device.

A first test in four steps

1. Connect an antenna to your receiver and open SDR++. Tune to a familiar FM broadcast station in WFM mode. If you hear it, your setup works.
2. Build a vertical dipole for 125 MHz near a window and look for the nearest airport's automated terminal information service (ATIS) recording in AM.
3. Browse from 144 to 148 MHz in narrow FM and look for repeaters in your region.
4. Shorten the legs to 6.5 cm, point the ADS-B program at the receiver and see how many planes appear. Then change the antenna location and compare.

Plane counts and range respond instantly to every meter of height and every wall that gets in the way.