Line Radio Astronomy Experiments
Mruzek, Michigan USA
Figure 1: Helical Antenna for Radio Astronomy at 1420 MHZ
This page describes radio astronomy experiments to observe stellar
line signals at 1420 MHZ. These experiments were performed in the
summer of 2023 near Macon, Michigan USA. (42N, -84W). The initial goal
was to detect the hydrogen line of several well known sources.
project continued with sky mapping at the same frequency using a 3
meter (10 foot) TVRO parabolic antenna in drift scan mode. The
observations began by using a simple helical antenna wound for 1420
MHZ, as shown in Figure 1. The helical antenna was found to be quite
suitable and responsive when using the signal processing
flow shown in Figure 2.
Figure 2: Radio Telescope Equipment Setup
The helical antenna for 1420
MHZ was designed using a helical antenna calculator. (See link
below) Multiple experiments in the configuration of the helical
antenna were made: including number of turns, reflector shape and
impedance matching. We found a 10 Turn Helical wound with 1/4"
diameter soft copper tubing on a nominal 2" schedule 40 PVC pipe worked
very well. The final version of the reflector is a 6" diameter aluminum
disk. The simplicity of the helical antenna combined with low
fabrication costs and compact size make it an ideal first antenna for
hydrogen line work. The primary reason for our transition to a
parabolic TVRO satellite dish was because we already had the 3
meter dish in our backyard antenna farm.
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.
4: HUAZHU Broadband RF Line Amplifier Module
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. These amplifers
are available on Amazon for about $15.
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 in our amplifier chain primarily because
we have it available in our observatory. We eventually plan to
test removal of the filter to gauge impact (after we complete our
sky mapping project). We suspect it is not critical to performance, an
important consideration since the list price of a new bandpass filter
of this type is over $3000.
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
Transitioning from a helical
antenna to the dish antenna created a need for a feed horn. Fortunately
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 almost ridiculous, but the approach actually
works surprisingly well. (See the article link below.)
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
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
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.
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 11: Sky Map at 1420 MHZ using 3M Satellite Dish
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
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
Link to the K&L Tunable Bandpass Filter
Article describing construction of coffee can feed horns for L Band: Feed
Information from SETI LEAGUE on feed horns for Hydrogen Line Astronomy:
Horns (Additional Information)
Helical Antenna Design Calculator: Helical
SDR# from AIRSPY (Software to operate the SDR dongle): SDR#
IF Average Plugin for SDR# (Ideal for Radio Astronomy Observations): IF