Part 2: Radio Astronomy With An Old TV Satellite Dish

Amateur radio operators have several tools to enhance their radio astronomy research...

See Part 1 here

In our previous article we discussed how to set up two dishes to create an interferometer for radio astronomy, and assumed a distance between the dishes at 100 meters. This gave us an angular resolution of about 7 arc minutes. Remember that 60 arc minutes equals 1 degree of resolution. Increasing the distance (baseline) between your two dishes would dramatically improve the angular resolution of your interferometer.

In radio interferometry, the angular resolution θ (the smallest detail you can distinguish on the sky) is fundamentally given by:

θ ≈ λ / B

where:

• λ is the observing wavelength (e.g., ~21 cm or 0.21 m at the hydrogen line frequency of 1420 MHz),

• B is the projected baseline length (the distance between the dishes, projected perpendicular to the line of sight to the source).

More precisely, the fringe spacing (which sets the effective resolution for detecting structure or measuring positions) is roughly λ / B radians, often quoted as ~206265 × (λ / B) arcseconds for small angles.

For reference, the moon is approximately 30 arc minutes across as observed from Earth.

Quick examples at 1420 MHz (λ ≈ 0.21 m)

• At the proposed ~100 m baseline: θ ≈ 0.21 / 100 ≈ 0.0021 radians ≈ ~430 arcseconds (~7 arcminutes). This is already ~30–40 times better than a single 2 m dish’s beam (~7° or ~25,000 arcseconds at the same frequency).

• If you increase to 200 m baseline: θ halves to ~215 arcseconds (~3.6 arcminutes)… A 200 m baseline gives fantastic improvement over a single dish (~7° beam → 3.6 arcmin fringes), enabling you to resolve fine structure in extended sources like solar bursts, Jupiter’s magnetosphere, or galactic hydrogen features.

• At 500 m: ~86 arcseconds (~1.4 arcminutes)… even better.

• At 1 km: ~43 arcseconds… now we’re talking. What can you realistically resolve or detect structure in (at 1420 MHz hydrogen line or nearby continuum)?

  • Solar features: Active regions, prominences, or burst sources on the Sun (which spans ~30–32 arcminutes or ~1,800–1,920 arcseconds). At 43 arcseconds, you could detect fringes from smaller sub-regions or map coarse structure in solar radio bursts/flares — much finer than your single-dish view.

  • Jupiter’s radio emissions: Jupiter’s disk is ~30–50 arcseconds across (similar to the Sun visually). You could potentially see basic structure in its decimetric emission belts or Io-related hotspots, though full mapping would need good signal and multiple baselines.

  • Supernova remnants (SNRs) like Cassiopeia A (~5 arcminutes overall, but with shell structure and compact features). 43 arcseconds lets you probe shell thickness or brighter knots — amateurs with similar resolutions have imaged Cas A structure.

  • Radio galaxies and quasars: Bright ones like Cygnus A (extended lobes ~1–2 arcminutes apart, with hotspots) or 3C sources show resolvable structure. Many extragalactic sources have components separated by tens to hundreds of arcseconds; you’d see them as multiple or extended rather than points.

  • Galactic hydrogen features: Large-scale arms or clouds in the Milky Way (spanning degrees) would show velocity-resolved structure, but finer HI filaments or small clouds (~arcminutes) could start to be distinguished or show internal gradients.

  • Extended nebulae or star-forming regions: If radio-bright (e.g., Orion A molecular cloud complex spans degrees but has clumps on arcminute scales), you’d resolve moderate-scale features better than before.

• At 10 km (very ambitious for amateur setup): ~4.3 arcseconds — approaching what large professional arrays achieve on short baselines. This would be a great project for an amateur radio club, especially in a rural area with less radio interference than what would occur in an urban area.

Longer baselines = finer fringes = higher resolution. This is a core principle of all interferometry (from amateur two-dish setups to the Event Horizon Telescope with Earth-sized baselines). Professional arrays like the VLA or ALMA deliberately reconfigure antennas to longer baselines when they want sharper images of compact sources.

Trade-offs and practical realities for your low-cost setup

• Bigger is better for resolution, but only up to a point for your targets:

  • Bright, compact sources like solar bursts, Jupiter’s emissions, or strong point-like radio galaxies (Cygnus A) benefit hugely from longer baselines — you can resolve finer structure or see clearer fringes.

  • Extended sources like the galactic hydrogen plane or large supernova remnants may “wash out” (fringes disappear) on very long baselines because the source fills many fringe cycles. Shorter baselines are better for mapping large-scale features.

  • A good strategy: Start with 50–200 m (easy cable run, manageable phase stability), then experiment with wider spacings if your property allows.

• Challenges that grow with distance:

  • Cable loss: Coax attenuation at 1420 MHz is high (~10–20 dB per 100 m typical RG-213/9913). You’d need low-loss cable (e.g., LMR-400 or hardline), amplifiers at each dish, or downconversion to IF before transmission.

  • Phase stability: Over longer distances, temperature changes, wind, or ground movement cause phase drifts that smear fringes. GPS-disciplined oscillators or frequent calibration on a strong source (like the Sun) become essential.

  • Synchronization: For offline correlation, accurate timing (better than ~1 ns) is needed; longer baselines demand better clocks.

  • RFI pickup: Longer cables can act as antennas for interference.

  • Alignment/tracking: Dishes must point identically; baseline orientation affects fringe direction.

Amateur hydrogen-line interferometers often start with short baselines (4–20 m) to prove fringes on the Sun, then push to 50–200 m for galactic work. Some SARA members have achieved usable results at hundreds of meters with careful phase referencing.

Increasing the baseline distance will further improve resolution (make it finer/better). Decreasing it would make resolution coarser (worse), though it might improve sensitivity to extended emission and make the setup easier mechanically. For most exciting science (resolving details in Jupiter, solar active regions, or compact galactic sources), go longer if you can manage the engineering hurdles.

The Amateur Radio Community

So, let’s discuss how amateur radio operators might connect two or more dishes wirelessly at these greater distances, using off-the-shelf or homemade equipment.

Connecting two dishes wirelessly via licensed amateur radio equipment is fully feasible and aligns perfectly with ham radio principles. As licensed operators, you can use amateur allocations in the microwave bands (e.g., 2.4 GHz, 3.4 GHz, or 5 GHz segments) for high-speed digital data links to transfer digitized samples or spectra between the remote dish stations and your central processing point. This avoids any unlicensed WiFi gear, keeps everything legal under Part 97 rules, and leverages the ham community’s expertise in point-to-point microwave networking.

The key is: you don’t transmit the raw 1420 MHz radio signal wirelessly (that would be expensive and phase-unstable). Instead, you digitize the signal right at each dish and send only digital data over a cheap wireless Ethernet/WiFi bridge.

This eliminates hundreds of dollars in low-loss coax, amplifiers, and phase-calibration headaches. Your 100 m baseline stays easy, and you preserve full phase information for clear fringes on the Sun, Jupiter, or hydrogen.

The core idea is this: digitize the 1420 MHz hydrogen-line signal locally at each dish using an RTL-SDR (or better ham SDR), then send the IQ data (or processed spectra) over a ham-band digital radio link instead of coax. This enables your interferometer to achieve the full resolution benefit of the baseline while staying low-cost and amateur-focused.

Why This Fits Licensed Amateur Radio Operators

• Legal & Purposeful: Data transmission for your radio astronomy experiment qualifies as an amateur radio communication (telemetry, control, and scientific data exchange). Use identified callsigns, proper identification, and stay within band plans/power limits.

• Ham Microwave Tradition: Many hams build high-speed point-to-point links using modified commercial gear or ham-specific equipment on 2.4 GHz (2390–2450 MHz), 3.4 GHz (3300–3500 MHz), or 5 GHz (5650–5925 MHz) bands. These are popular for “Hinternet” networks, mesh systems, and experimental data backbones.

• No Raw RF Transmission of 1420 MHz: You’re not relaying the astronomy RF signal wirelessly (which would be inefficient and regulated differently). Instead, you’re transmitting digital payload data on a separate ham band.

Lowest-Cost Ham Radio Equipment Setup (~$200–500 total add-on for both stations, beyond your existing dishes/feeds/LNAs)

Focus on affordable, widely available ham-compatible gear. Start simple with offline recording if live streaming feels complex.

Per Dish/Remote Station (×2):

• Raspberry Pi 4/5 (~$35–60 each) — Runs local digitization and data handling.

• RTL-SDR dongle (~$25–35 each) — Receives 1420 MHz (many hams use these for weak-signal work).

• GPS module (e.g., inexpensive u-blox NEO series, ~$15–25 each) — For precise timing/position stamps (essential for correlation; hams use GPSDOs for frequency discipline too).

• Small enclosure/weatherproof box for outdoor mounting.

Wireless Ham-Band Data Link (shared pair for the baseline):

• Use modified commercial point-to-point gear tuned to amateur segments — very common in the ham community.

  • Recommended low-cost option: Pair of Ubiquiti airMAX devices (e.g., NanoStation or Rocket series) configured for the 3.4–3.5 GHz amateur band (M3 models for export/international, but many US hams retune/modify for 3.3–3.5 GHz allocation). These provide 50–100+ Mbps over 100–500 m with directional antennas (~$80–150 per pair used/new).

  • Alternative: 5 GHz ham segment gear (e.g., Ubiquiti NanoBeam or Rocket M5 series modified/tuned to 5650–5925 MHz amateur portions). Widely available used on eBay or ham flea markets (~$50–100 per pair).

  • Even cheaper starter: Broadband-Hamnet (now AREDN) firmware on compatible routers (e.g., older Ubiquiti or MikroTik units) for mesh-style links on 2.4 GHz or 5 GHz ham bands. AREDN is free ham-specific firmware designed exactly for this — high-speed data over amateur microwave.

• Mount directional antennas (small dishes or panels) on poles near each dish for line-of-sight.

Examples of typical ham microwave point-to-point setups (similar to what you’d adapt):

Software (all free/open-source, ham-friendly):

• On each Pi: GNURadio or rtl_sdr tools to capture timestamped IQ data (use GPS for sync).

• Transfer: UDP streaming over the ham link for real-time, or record to SD card and fetch later (offline mode — easiest to start).

• Correlation: Python scripts or GNURadio flowgraphs on your central station (hams share interferometer code on GitHub; search “ham radio VLBI” or “RTL-SDR correlator”).

• Control: Use the link for remote Pi access (SSH/VNC) to aim dishes or monitor.

Offline “Zero Live Bandwidth” Version (Highly Recommended to Start):

• Record timestamped data locally at each dish.

• Use the ham microwave link only for remote control/monitoring (very low bandwidth needed).

• Physically transfer files or use the link sparingly to copy — fringes still computable perfectly.

Trade-offs & Ham-Specific Notes

• Range & Reliability: These links handle 100 m easily (even kilometers with clear LOS); hams routinely span miles.

• Power & Licensing: Keep ERP within Part 97 limits (typically no issue at low power for short links). Use your callsign in any identification if needed.

• Phase/Timing: GPS provides ns-level sync — sufficient for your baseline. Hams often add cheap GPS-disciplined oscillators if pushing precision.

• Community Resources: Check AREDNmesh.org for firmware/setup guides, TAPR.org or DCC papers for microwave data experiments, SARA (Society of Amateur Radio Astronomers) forums for hydrogen-line specifics, and QRZ/eHam for used gear.

This keeps your entire system amateur-radio-centric: licensed links, ham gear, and experimental spirit. It’s cheaper and more educational than commercial unlicensed WiFi, and you can expand to longer baselines or more dishes later. This project opens up an entire universe (yeah, pun intended) of new ways to enjoy amateur radio, and at the same time contribute to science and research.

With the marked decrease in Earth’s magnetic field, and the subsequent issues that less protection creates from both cosmic rays and solar events like solar flares and CMEs, having eyes on the sun is especially important, as we can not only observe and report solar events, but also track and plot over time any increases in the signals that occur as a result of the weakening field strength. This is not only a great project, but critical research as well.

Additional Resources

https://www.youtube.com/@radio-astronomy/videos

http://www.arrl.org/resources/nets/client/netdetail.html?mfind=3055

https://radiojove.gsfc.nasa.gov/