Computer Monitors and 1420 MHz Radio Frequency Interference (RFI)
Here’s a more detailed breakdown:
📡 1. 1420 MHz band characteristics
1420.405 MHz corresponds to the neutral hydrogen line, which is in the L-band. Emissions here are usually weak natural signals — ideal for radio astronomy but also sensitive to local interference. Even tiny emissions from nearby electronics can swamp the signal.
🖥️ 2. Typical emissions from computer monitors
- Modern LCD/LED monitors usually operate with switching power supplies and internal clocks at frequencies well below 1 GHz, typically tens to hundreds of MHz.
- However, these clocks can produce harmonics (integer multiples of their base frequency) that extend into the GHz range.
- Poorly shielded or cheaply made equipment can leak measurable RF noise.
For example:
- A 71 MHz switching circuit can produce a 20th harmonic at 1.42 GHz.
- Internal data interfaces (like LVDS or DisplayPort cables) can radiate wideband noise up to several GHz.
- HDMI or USB 3.0 cables can act as antennas.
🧪 3. Detectability by a small radio telescope
A small hydrogen line telescope (say, a Yagi or horn + LNA + SDR) can indeed pick up this kind of interference, but it will usually look like:
- Narrow spikes if it’s harmonic leakage.
- A “broadband noise floor” increase if it’s general switching noise.
- Intermittent or modulated signals if the monitor refresh or cable activity changes.
It will not look like a Doppler-broadened hydrogen line from the galaxy — but can mask or distort that line if close enough.
🛡️ 4. Ways to check and mitigate
- Turn off the monitor while observing: if the spike disappears, it’s the culprit.
- Move the telescope or the monitor — even 2 m distance can make a big difference.
- Use ferrite chokes on HDMI, DisplayPort, and power cables.
- Keep the telescope and frontend electronics physically separated from computers and displays.
- Run the telescope on battery power and control it remotely to reduce local RFI.
- A spectrum waterfall over time (e.g., in GNU Radio or SDR#) can make spurious lines stand out clearly.
🛰️ 5. Bottom line
- Computer monitors are not intentional emitters at 1420 MHz.
- But they can leak noise or harmonics detectable by sensitive receivers.
- If you are observing hydrogen at 21 cm, place your monitor and computer several meters away from the feed and LNA, or use a long cable / remote session.
- It’s good practice to do an RFI survey of your site with everything switched on and then off.