Can You Build a DIY Radio Telescope to Detect Dark Matter?

DIY Horn Antenna for 21‑cm Galactic Observations

A pyramid‑shaped horn antenna, originally used in 1951 to detect the 1420.4 MHz neutral hydrogen line, is constructed from two 10 ft × 2 ft waterproof roof panels and a one‑gallon paint‑thinner can. The panels form the base, the can serves as the horn’s mouth, and the assembly is held together with tape and bolts. The resulting aperture is comparable in size to an F‑type fuel tank and provides an angular resolution of roughly 20°.

Receiver Chain

The signal chain consists of a Nooelec SAWBird+ H1 module (two low‑noise amplifiers plus a 1420 MHz resonant filter) connected to an RTL‑SDR V4 USB dongle. This combination supplies sufficient gain and band‑pass filtering to record the weak 21‑cm emission from Galactic hydrogen clouds.

Observation Procedure

Use Stellarium to locate six Galactic longitudes (15°, 30°, 45°, 60°, 75°, 90°) along the Galactic plane, each representing a different line‑of‑sight distance from the Galactic centre.

Point the horn antenna at each longitude in turn. Because the beam width is ~20°, the pointings are spaced to minimise overlap.

Record raw I/Q data with SDR# while the IF Average plugin stacks several minutes of signal, thereby enhancing the faint 1420 MHz line.

Data Analysis

Each stacked spectrum is imported into Microsoft Excel and modeled as a sum of Gaussian components, each representing emission from an individual hydrogen cloud. The Gaussian with the largest red‑shifted centre frequency is taken as the cloud with the maximum line‑of‑sight velocity for that pointing.

The measured frequency shift Δf is converted to a radial velocity v_r using the Doppler formula: v_r = c * (Δf / f_0) where c is the speed of light and f_0 = 1420.405751 MHz. The tangent‑point geometry (galactic longitude ℓ, solar radius R_0 ≈ 8.2 kpc) yields the orbital speed v_orb and galactocentric radius R for each cloud:

R = R_0 * sin(ℓ)</code>
<code>v_orb = v_r + v_⊙ * sin(ℓ)

Here v_⊙ is the Sun’s circular speed (≈ 240 km s⁻¹). This produces six (R, v_orb) data points.

Comparison with Published Rotation Curve

The derived points are plotted against the Milky Way rotation curve published by Sofue (2025) ( https://www.ioa.s.u‑tokyo.ac.jp/~sofue/papers/sofcomp/2025-pasj-Inner-Rot-Curve-MilkyWay.pdf ). The inner two points lie slightly below the curve; applying a simple curve‑fit in Excel reduces the discrepancy but does not eliminate it. All six points show orbital speeds that remain roughly constant (≈ 220–240 km s⁻¹) as R increases from ~2 kpc to ~8 kpc.

A Keplerian decline (v ∝ R⁻¹ᐟ²) would predict a marked reduction in speed with radius, which is not observed. The flatness of the measured rotation curve therefore implies the presence of an additional, non‑luminous mass component—dark matter—distributed throughout the Galactic disc and halo.

Key Outcome

Using inexpensive, home‑built hardware and standard software tools, the experiment reproduces the Milky Way’s flat rotation curve and provides observational support for dark matter without requiring professional observatory facilities.