RTL-SDR with OnAir: what it can and cannot receive

What it can do#

• DAB, FM and narrow modes. An RTL-SDR tunes roughly 24 MHz to 1.7 GHz at up to about 2.4 Msps. FLARM, for example, needs at least 1 Msps, so an RTL-SDR works. ADS-B asks for 4 Msps but decodes from 2 Msps (about 6 dB less sensitive), which an RTL-SDR can reach.
• Not TV channels. Wide television channels need a radio that samples faster.

HF (below 24 MHz)#

An RTL-SDR picks the way down by itself ("direct sampling: Auto"):

• The RTL-SDR Blog V4 and V4 Lite use the up-converter in the Blog library; OnAir never switches direct sampling on there. On macOS OnAir bundles the Blog librtlsdr (0.2.6) so the V4's HF input works.
• Every other dongle samples its Q input directly. A Blog V3 has its HF input on Q; a stock dongle has no HF input, so the signal may be weak or absent unless it is modified.
• The radio settings can still pick the I input or switch direct sampling off. Over rtl_tcp "Auto" sends nothing and the server's own library decides.

Setup#

1. Windows: install the WinUSB driver with Zadig.
2. Plug it in and choose it in the source list.

Sharing it over the network#

• Use rtl_tcp to put the dongle on another computer; OnAir connects again by itself when the connection drops and counts the samples that were missed. Servers of other radios that speak rtl_tcp (rsp_tcp, airspy_tcp, hackrf_tcp) get their full rates.
• Share my radio (View > Data feeds) makes OnAir itself an rtl_tcp server for the radio it is running.