Now that your SDR is working, you can explore parts of the spectrum used for broadcast, weather, aircraft, marine, and shortwave signals. Some are easy to receive, some are more advanced, and some raise legal or ethical considerations depending on where you live.
88-108 MHz
Confirm hardware is working
162.4-162.5 MHz
Test NFM reception
1090 MHz
First digital decode
3-30 MHz
Explore distant signals
Signals grouped by purpose and ease of reception
Shortwave numbers stations are mysterious voice or tone broadcasts that have long attracted listener interest. While their exact purpose remains unconfirmed, they are widely theorized to be used for secure communication with field agents using one-time pad encryption.
Reported active for decades · 4625 kHz
Continuous buzzing with rare voice interruptions. Purpose is officially unconfirmed.
Believed linked to MI6 · Ceased 2008
Historical station using musical intervals and groups of five numbers.
Historical Mystery · Various HF
Noted for broadcasting specific audio clips followed by data bursts.
Note: Reception of shortwave stations varies significantly based on solar conditions, time of day, and location.
🟢 Generally Legal to Receive
Public broadcast (FM, AM), NOAA weather, ADS-B aircraft, AIS ships, amateur radio bands.
🟡 Check Local Regulations
Public safety dispatch, walkie-talkies—legal to hear in many areas, but never act on info or interfere.
🔴 Restricted / Illegal
Intercepting encrypted comms, decoding cellular calls (ECPA), intentional privacy breaches.
Safety First: Monitoring over-the-air signals is an educational hobby, but never transmit unless you are authorized. Intercepting and using private or encrypted information for gain or action crosses critical legal and ethical lines.
1
RTL-SDR v3
~$30 on Amazon
2
Telescopic Antenna
Included with RTL-SDR kit
3
SDR++ Software
Free — tune to 98.1 FM first!
Test what you learned about listening to the airwaves
What is the international distress and calling frequency for Marine VHF?