Information on RTL-SDR based Itty Bitty Radio Telescope

RTL-SDR Based Itty Bitty Radio Telescope

That is a classic project. The RTL-SDR Based Itty Bitty Radio Telescope (IBT) was developed as an inexpensive demonstration radio telescope rather than a precision astronomical instrument. The original design used:

• An 18-inch (about 45 cm) satellite TV dish.
• A Ku-band satellite TV LNB operating around 12.2-12.7 GHz.
• A satellite finder meter as a logarithmic detector.
• An RTL-SDR (mainly for displaying and recording the detector output).
• A laptop running SDR software.

The principle is straightforward. The Sun emits broadband microwave radio noise. The dish focuses this onto the LNB, which amplifies and downconverts the signal. As the dish is pointed at the Sun, the total received noise increases slightly, and this appears as a rise in the signal level.

There are a few important limitations:

• It measures total power only, not a radio spectrum.
• It is primarily useful for observing the Sun only.
• It cannot detect the 21 cm neutral hydrogen line, because it operates at Ku-band (around 12 GHz), whereas the hydrogen line is at 1420.405 MHz.
• The satellite finder is the limiting component in terms of sensitivity and stability. Later work by the Society of Amateur Radio Astronomers describes improved IF processors and ADCs that significantly enhance the original design.

The IBT is most valuable as:

• a public outreach instrument,
• a teaching aid for demonstrating radio astronomy,
• or a quick way to monitor solar radio emission.

in this original design using a satellite finder, if users wanted data logging they either had to manually record measurements with pen and paper or use an automatic recording voltmeter (we have done the later in past at LRO).

A revised version of the Itty Bitty Radio Telescope can be built with very little extra hardware while giving significantly better performance.

Basic system

Dish (45-90 cm)
        |
     Ku-band LNB
        |
   Bias-T (13 V or 18 V)
        |
      RTL-SDR V4
        |
    Laptop/Raspberry Pi
        |
 GNU Radio or SDR++
        |
 Integration and plotting software

The LNB converts approximately 10.7-12.75 GHz down to about 950-2150 MHz, which an RTL-SDR V4 can receive directly.

Why this is better

Instead of measuring only the output of a satellite finder meter, you:

• Record the full IF spectrum.
• Average thousands of spectra digitally, reducing the noise.
• Reject interference.
• Save data for later analysis.
• Produce drift scans automatically.

With integration times of tens of seconds to a few minutes, the Sun is detected very clearly.

Software

For Windows:

• SDR++ for initial testing. Also can record data over time and integrate over time using IFAverage plug-in.
• GNU Radio Companion for automated measurements.
• Spectrum Lab (which you already use) can log the received power if fed suitable data.

For Raspberry Pi/Linux:

• GNU Radio
• Python with NumPy and Matplotlib
• rtl_power for long integrations

Dish size

A larger dish gives a useful improvement:

• 45 cm: suitable for the Sun.
• 60 cm: noticeably better.
• 80-90 cm: excellent solar instrument.
• 1.2 m: can begin detecting some stronger astronomical microwave sources with long integrations.

Calibration

You can calibrate using:

• Cold sky and hot ground.
• The Sun.
• Atmospheric emission at different elevations (“sky tipping”).

This allows measurements in kelvin rather than arbitrary signal units.

Extensions

For those already experienced with radio interferometry, an interesting next step would be to build two identical microwave telescopes separated by several metres and correlate their outputs. At 12 GHz the wavelength is only about 2.5 cm, so even a modest baseline gives high angular resolution. This would be an excellent educational interferometer for observing the Sun.