Mergen-21: Low-Cost 21 cm Hydrogen Line Radio Telescope

A radio telescope I built for my EE401 graduation project at Ozyegin University. It listens at 1420.405 MHz — the hydrogen line — and I used it to map how fast different parts of the Milky Way are rotating.

Science goal: Galactic rotation curve via the tangent-point method, observed from Istanbul, Turkey.

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System Overview

Antenna

• Type: Pyramidal horn, 1.5 mm aluminum sheet, laser-cut & riveted
• Design frequency: 1420.405 MHz (HI 21 cm line)
• Measured S11: -42 dB @ 1.4204 GHz (exceeded simulation by a lot)
• Simulated S11: ~-30 dB near 1.40 GHz (CST Studio Suite)
• Directivity: 16.9 dBi at 1.42 GHz
• 3 dB beamwidth: 22.0° (H-plane), 25.2° (E-plane)

RF Chain

Stage	Component	Function	Gain (dB)	NF (dB)	OIP3 (dBm)
1	ZX60-P162LN+	LNA	19.7	0.7	+29.8
2	ZX75BP-1450-S+	Bandpass filter (~50 MHz passband @ 1450 MHz)	-0.8	0.8	N/A
3	ZX60-V63+	Second amplifier	20.8	3.7	+32.2
Cascade	LNA + BPF + Amp		39.7 (meas.)	0.75 (meas.)	+29.5 (meas.)

Backend

• SDR: ADALM-PLUTO
• Software: GNU Radio for signal acquisition
• Analysis: Python (NumPy, SciPy, Matplotlib, Astropy)

Here's what the GNU Radio receiver flowgraph looks like:

Mechanical

• Horn antenna from laser-cut aluminum sheet metal
• Waveguide-to-coax transition with N-type connector
• A 3D-printed tripod adapter was designed (files in hardware/antenna/3d-print/) but the mount did not work out in practice and wasn't used for observations

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Project Status

Component	Status	Notes
Hardware (antenna, RF chain, LDO)	Complete	Fully characterized; measurement data available
Power supply board	Complete	Gerbers ready; BOM in hardware/ldo-regulator/bom.pdf
Simulations (CST, AWR)	Complete	Exported results in hardware/simulation/
Measurements (VNA, IP3, NF)	Complete	39.7 dB gain, 0.75 dB NF, OIP3 +29.5 dBm (cascade)
GNU Radio flowgraphs	Complete	reciver.grc (main) + 21cm synth/ test flowgraphs
Analysis software	In Progress	Waterfall viewer done; full calibration pipeline in development
First-light observations	Complete	Directional sweeps 2026-04-29; data in observations/data/
Open-source release	In Progress	Final cleanup underway

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Key Results

RF Receiver

• Cascade gain: 39.7 dB (measured @ 1.42 GHz via ZNB8)
• Cascade NF: 0.75 dB (measured; agrees with theory to 0.19 dB)
• Cascade OIP3: +29.54 dBm (-12 dBm tone input; TOI spread 0.3 dB)
• Measurements traceable to R&S ZNB8 VNA & FSVA3044 spectrum analyzer

Antenna

• Measured S11: -42 dB @ 1.4204 GHz
• Simulated directivity: 16.9 dBi
• 3 dB beamwidth: ~23° (H-plane & E-plane similar due to square waveguide)
• Material: 1.5 mm aluminum sheet, laser-cut

System Sensitivity

• System temperature: ~30–40 K (sky + ground + receiver near zenith)
• MDS: ~-150 dBm @ 1 MHz BW (conservative)

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Repository Structure

mergen-21/
├── hardware/                    # All hardware (antenna, RF chain, power, sims)
│   ├── antenna/                 # Horn: CAD, drawings, DXF, STL, photos
│   ├── rf-chain/                # Component datasheets & S-parameters
│   ├── ldo-regulator/           # Dual LDO board (Altium, Gerbers, BOM, STEP)
│   └── simulation/              # CST antenna sims & AWR cascade analysis
├── measurements/                # Lab characterization data
│   ├── rf-chain/vna/            # VNA S-parameters (R&S ZNB8)
│   ├── rf-chain/ip3/            # IP3 / intermodulation
│   ├── rf-chain/nf/             # Noise figure
│   └── antenna/                 # Horn S11 & manufacturing notes
├── software/                    # Data acquisition & analysis
│   ├── gnuradio/                # GNU Radio flowgraphs (.grc)
│   └── analysis/                # Python scripts (waterfall viewer, etc.)
├── observations/                # First-light data & plots (2026-04-29)
│   ├── data/                    # Raw spectra (.dat, NumPy float32)
│   └── plots/                   # Waterfall & sweep plots
└── docs/                        # Build log, status, diagrams

Quick Navigation

• Measurement data? → measurements/
• Hardware files? → hardware/ — antenna CAD, RF chain, LDO, sims
• First-light data? → observations/ — raw spectra + plots
• Running observations? → software/gnuradio/ — flowgraphs + setup
• Build progress? → docs/STATUS.md

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Getting Started

git clone https://github.com/AlpGoXd/mergen-21.git
cd mergen-21
pip install -r software/requirements.txt

Dependencies:

• GNU Radio 3.10+ with PlutoSDR block (gr-iio)
• Python 3.8+ — numpy, scipy, matplotlib, astropy

CST, AWR, and Autodesk Inventor are only needed to re-run simulations or edit CAD. All exported results (S-parameters, STEP, Gerbers, PDFs) are already in the repo.

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Observation Site

• Location: Istanbul, Turkey (~41.0°N, 29.0°E)
• Target: Galactic plane HI emission at various galactic longitudes
• Method: Tangent-point method for rotation curve extraction

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Licensing

• Hardware (antenna, mechanical, RF chain): CERN-OHL-S v2
• Software (GNU Radio, Python): GPL-3.0
• Documentation & Photos: CC BY-SA 4.0

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Why "Mergen"?

Mergen is a figure from Turkic mythology — associated with wisdom, precision, and skilled targeting. Felt like the right name for a telescope.

Acknowledgments

• PICTOR project — reference for radio astronomy data acquisition

Author

Alp Gokalp — Electrical & Electronics Engineering, Ozyegin University (Class of 2026)