Radio-frequency interference (RFI)

The 1400 to 1427 MHz band around the hydrogen line is allocated to passive use only, so there are no licensed transmitters in it. That doesn’t make it clean. Strong signals just outside the band can overload a receiver, harmonics and intermodulation products can land inside it, and the noisiest transmitter is usually within a few metres of your antenna.

Where it comes from

Above and below the protected band, in or near the water hole:

• Aeronautical and surveillance radar, roughly 1215 to 1400 MHz in many countries, pulsed and strong.
• Satellite navigation, including GPS L1 at 1575.42 MHz and other GNSS signals between about 1559 and 1610 MHz.
• Mobile satellite downlinks in the 1525 to 1559 MHz range, and satellite phone systems around 1610 to 1626.5 MHz.
• Cellular and broadcast services at the edges of L-band, which vary by region.

Close to home: switching power supplies, LED drivers and dimmers, USB 3 cables and hubs, computer monitors, and the SDR’s own host computer. These are broadband and often the dominant problem for a backyard station.

Recognising it

Natural hydrogen emission is broad (many channels wide), changes smoothly with pointing, and moves in frequency with the LSR correction as the Earth moves. Interference usually doesn’t. Typical signs:

• a spike one or two channels wide, which you’ll only see with fine spectral resolution
• something that stays at the same frequency regardless of where the antenna points
• something that appears at the same clock time every day rather than four minutes earlier each day like the sky
• a raised noise floor across the whole band, which points to overload

Reducing it

1. A SAW-filtered LNA at the feed rejects out-of-band signals before they can overload anything. This fixes most problems.
2. Keep the receiver and computer away from the antenna, put ferrite chokes on USB and power leads, and run on a linear supply or battery for a test.
3. Turn things off one at a time while watching a live spectrum.
4. Observe above about 20° elevation, where less ground-level interference reaches the main beam.

Ruling out terrestrial and satellite sources is the larger part of what any SETI station does, ours included. The first test is the one in the list above: does it track the antenna? LAB-007 describes how we applied it to the signal we publish, LAB-003 notes a stretch where our own interference exposure went up after an LNA swap, and LAB-005 covers what the protected band does and doesn’t protect.