Hydrogen Line Radio Astronomy and Wow!

M. Mruzek, Michigan USA

Figure 1: Big Ear Radio Telescope at Ohio State University, USA

This is a radio astronomy observation report of recent hydrogen line data collection from a region in the constellation Sagittarius. Specifically, we are observing the sky location where the Wow! signal was detected in 1977 by Ohio State's "Big Ear" radio telescope. (~ 19 Hr 25 Min RA by -27 Deg DEC) These observations are being made using a 3 meter (10 foot) TVRO parabolic antenna operating in drift scan mode. The amateur radio telescope for this study is located near Macon, Michigan USA. (~ 42N LAT, -84W LONG). Experimental results are presented near the end of this report.

Background Information

Figure 2: Radio Telescope Equipment Setup

The radio telescope's signal path is shown in Figure 2. Observations are approximately centered at a frequency of 1420 MHZ. Other software packages are also used for analysis, as detailed below in our descripton of the experimental setup.

Low Noise Amplifier for Hydrogen Line Studies

Figure 3: SAWbird+ H1 Low Noise Amplifier for Hydrogen Line at 1420 MHZ

The most important amplifer in a radio astronomy telescope is the low noise amplifier (LNA) located at the mast of the receiving antenna. It is the LNA that most often establishes the noise figure of the amplifier chain. We experimented with several different LNA designs, including homebuilt and commercial. We found the SAWbird+ H1 amplifier to work very well, and it is a relative bargain from Amazon at about $45. We mounted the LNA in a PVC enclosure at the dish feed horn. The cable run from the LNA to our observatory consisted of about 25 feet of coaxial cable.

Figure 4: HUAZHU Broadband RF Line Amplifier Module

The coaxial cable run to the observatory terminated in a broadband RF Line Amplifier Module. The HUAZHU module is advertised to provide 32 dB of gain. We mounted the line amplifer inside a die cast aluminum enclosure to minimize extraneous RF noise. Power of +5VDC is provided by a benchtop supply. These amplifers are available on Amazon for about $15.

Figure 5: K&L Tunable Bandpass Filter

The output from the line amplifier is routed to a tunable bandpass filter set for 1420 MHZ. We are using the bandpass filter as a precaution to minimize radio frequency interference (RFI).

Figure 6: RTL-SDR Software Defined Radio

We are using the RTL-SDR software defined radio. The SDR can be characterized as inexpensive ( $34 on Amazon) and well-supported by various software packages, including SDR# from AIRSPY, and GNU Radio derivatives for radio astronomy such as VIRGO.

Figure 7: Feed horn and LNA on the 3 meter satellite dish

The hydrogen line frequency is relatively close to the same frequencies radio amateurs use for EME communication (Earth-Moon-Earth). We found a useful construction article for building a feed horn from a 3# coffee can. At first this sounds impractical, but the approach works well. (See a link to the article in our list of references.)

Figure 8: LNA at feed horn in PVC enclosure with aluminum foil shielding

The main challenge with mounting the LNA at the feed horn of the dish was creating a weatherproof enclosure. We opted to place the LNA inside a PVC pipe enclosure attached with silicone sealant to the side of the coffee can. The LNA was wrapped in aluminum foil to minimize RF interference. The +5VDC power was provided to the LNA by a micro-usb connector and cable. Note: The output signal of the LNA has a +5V DC component, which was OK in our setup because the line amplifier has I/O blocking capacitors.

Figure 9: Spectrum and Waterfall for Taurus A

The software we use for general purpose observation of hydrogen lines at 1420 MHZ is SDR# from AIRSPY. The spectrum and waterfall display in SDR# will indicate the presence of a strong source, as shown in Figure 9. The manufacturer of the SDR dongle (RTL-SDR.COM) maintains an excellent set of webpages about the device, with many pages devoted to radio astronomy projects. There is also a forum that can provide assistance, as needed.

Figure 10: Taurus A IF Average Display

The SDR# software is excellent. The authors of the software welcome the development of plugins to enhance functionality. One such plugin is specifically for doing radio astronomy. The IF Average Plugin allows for averaging of the IF signal to remove noise. The plugin also allows background noise subtraction, data storage and data acquisition scheduling. Unbelievably, all this software is free!

We completed a sky map at 1420 MHZ using the 3M Satellite TVRO dish on 8/30/2023. We drift-scanned declinations from +22 to -28 degrees. This represents the range of movement possible when the dish is aligned North-South. Each rectangle in the sky map represents a region of 5 degrees height, by 1 hour angle width. Inside each rectangle is a number representing the magnitude of the observed hydrogen peak height on the display screen, as measured using a ruler with millimeter units. The readings were placed in an Excel spreadsheet, with the cells conditionally formatted such that the brightness corresponds to the cell's numerical entry.

Figure 11: Sky Map at 1420 MHZ using 3M Satellite Dish

The sky map results show the general outline of our galaxy. Taurus A is visible near the top left, and the galactic center is visible near the bottom right. The bright spot near the bottom left is unknown, although we note that the dish is pointing very close to the horizon at those declinations.

Figure 12 is a view of the transit of the region near the galactic center using IF Average plugin for SDR#. Figure 13 is another view of the transit using H-Line software on a Linux machine.

Figure 12: Transit of the region near the Galactic Center

Figure 13: Transit of the region near the Galactic Center

Experimental Setup for the Wow! Observations

The first challenge was to correctly aim the TVRO satellite antenna at the Region of Interest (ROI). To facilitate aiming in declination we installed a remotely controlled linear actuator for up-down mechanical motion, as shown in Figure 14. The actuator is manually controlled using a double-pole double-throw knife switch with snubbing diodes. The angular position of the dish is monitored at the dish with a digital angle meter with magnetic base. Additionally, an angle measurement transducer manufactured by WITMOTION (Model SINVT-232 with 0-5VDC output) was installed to provide a signal to the remote control room.

Figure 14: Linear Actuator for Declination Adjustment

Angular Sensor for Declination Measurement

Figure 15: Analog output angle sensor for declination measurement

We found it was especially convenient to operate the linear actuator with a low voltage DC power supply and series resistor. The dish motion was relatively slow, making it easy to adjust accurately. Calibration runs were made to correlate dish motion in declination to the voltage output from the transducer. The right ascension of the dish is adjusted using a simple turnbuckle, which is visible in Figure 14. Finally, radio observation of solar transits were made to accurately determine the dish aim in both declination and right ascension. Thereafter, aiming the dish at the ROI became a simple matter of a declination offset after a precise solar noon transit.

Figure 16: Radio-Eyes sky map showing ROI near the Galactic Center (+W!)

The ROI for these observations is not well-placed in the sky for our observing location. The ROI is 21 degrees above the southern horizon at transit. The proximity of the sun during seasonal changes is also a concern. Additionally, the proximity of the Galactic Center, as can be seen in Figure 16, creates a significant hydrogen background radio enviroment.

Search Strategy

We endeavored to create a spectral plotting strategy capable of displaying real-time hydrogen spectrum "time slices" during a drift scan transit. We also included provisions to permanently record the raw I/Q data from the SDR, for subsequent analysis as warranted.

We found an integration period of 240 seconds produced a reasonable hydrogen line spectral plot. During a drift scan transit we record 5 spectral plots of 240 seconds duration each, for a total observation time of 20 minutes. This 20 minute set is centered on the ROI. Since we are looking for changes during the transit, we record a background plot immediately prior to the start of the 20 minute observation window, and plot the resulting differences during transit. (Background subtraction)

During each transist we also record 890 seconds (~ 15 Min) of raw I/Q data centered on the ROI. Recording raw I/Q data generates large files requiring significant data storage capability. We have a dedicated Terabyte sized hard drive for the purpose.

The software observation tool we are using for data collection and plotting is Virgo: A Versatile Spectrometer for Radio Astronomy. Our computer platform is a Dell Desktop operating Linux as provided by the Zorin Lite distribution package.

Figure 17: Excerpt from the Virgo Spectrometer webpage description

Virgo software observations are invoked using a command line interface in Linux terminal. For example, the following command line invokes our background acquisition:

virgo -rf 20.7 -f 14204e5 -b 25e5 -c 4096 -t 24 -d 240 -S spectrum

rf 20.7 means set the RF gain of the SDR at 20.7
f 14204e5 means observe at a center frequency of 1420.4 MHz
-b 25e5 means use an instantaneous bandwidth of 2.5 MHz
-c 4096 means use 4096 channel bins (FFT size)
-t 24 means use 24 seconds integration time for each slice
-d 240 means collect data for 240 seconds total duration
-S spectrum means name the output file "spectrum"

The complete data acquisition sequence is automated by using a Bash script in Linux terminal. The script is invoked at the start of a 7 day observing session. Bash scripting is a powerful and convenient tool for automation of data collection.

Figure 18 shows an excerpt from our Bash script for data acquisition. Figure 19 shows a typical calibration data set. Figure 20 shows a typical time slice data set. In Figure 20 note the "Calibrated Spectrum" graph. This graph is the calculated difference between the calibration run and the current run. The negative spike in the graph indicates reduced signal at a particular frequency. This is not surprising, since we are moving away from the powerful galactic center. We note this change is not easily detected when only comparing the "Average Spectrum" graphs.

Figure 18: Excerpt from our Bash script for data acquisition

Figure 19: A typical calibration plot from the Virgo Spectrometer

Typical data

Figure 20: A typical data slice plot during ROI transit. Note the "Calibrated Spectrum" graph.

Comments

Routine data collection for these observations began on October 7th, 2023 and continued until December 26th, 2023. Thereafter, the data collection was restarted on August 13th, 2024 and is currently in progress. Calibrated spectrums from each transit are generally reviewed within 24 hours.

We have found the calibrated spectrums are reasonably consistent from transit-to-transit. No unusual differences or changes in any of the data sets has been detected. We continue to record and analyze data in an ongoing observation program. We have not detected any event that would require additional investigation using the raw I/Q data also being recorded.

TVRO Satellite Dish for Hydrogen Line Observations

References:

Radio Astronomy, 2nd Edition by John D. Kraus (CALL W8JK), published by Cygnus-Quasar Books, 1986.

Radio Astronomy Handbook, 4th Edition by Robert M. Sickels, published by Bob's Electronic Service, 1992-1993.

The Satellite Experimenter's Handbook, 2nd Edition by Martin Davidoff (CALL K2UBC), published by ARRL, 1990.

Hardware & Suppliers:

Link to the SAWbird+ H1 low noise amplifier from NOOELEC: Low Noise Amplifier

Link to the K&L Tunable Bandpass Filter (Pasternack): Filter

Link to the WITMOTION Inclinometer: Angular Transducer

Websites:

Article describing construction of coffee can feed horns for L Band: Feed Horns

Information from SETI LEAGUE on feed horns for Hydrogen Line Astronomy: Feed Horns (Additional Information)

How to install Linux on a Desktop Computer: How to Install Linux

SDR# from AIRSPY (Software to operate the SDR dongle): SDR#

Radio-Eyes software for Radio Astronomy: Radio-Eyes Software

IF Average Plugin for SDR# (Ideal for Radio Astronomy Observations): IF Average Plugin