A low-cost home-built horn antenna for 21 cm astronomy
R. N. Patel and N.A. Patel 5 October 2013.
The horn antenna is constructed from cardboard-foam board (usually used for posters), and aluminum foil stuck with adhesive. The edges are stuck from outside using duct tape and from inside, using aluminum reflective tape. A cardboard mailing tube is attached to the bottom of the horn for mounting it on a telescope-mount.
The mouth of the horn connects to a rectangular waveguide (using duct tape and aluminum tape. The waveguide feed is constructed from an empty gallon can (cross-section dimensions: 6.5"x4"). (These cans usually sold as containers for olive oil or paint thinners; we purchased a new can from McMaster Carr). There is a quarter wavelength antenna stub (using copper wire, connected to an N-type connector, screwed to the top of the can). The N-type to SMA connector directly feeds to a pair of Low Noise Amplifiers (purchased from mini-circuits). The thin twisted pair of cables carry 4V DC power to these amplifiers. The blue cable is a flexible RF cable passing the amplified signal to a microstrip filter (200 MHz bandwidth, centered roughly at 1400 MHz).
Spectra are acquired using a python script which is a wrapper for the librtlsdr library. Raw samples from the RTL-SDR USB dongle receiver are acquired as a stream of bytes, and power-spectral density plots are computed using the psd function in python.
Front view of the rectangular waveguide feed showing the stub antenna and the first LNA.
RTL SDR USB dongle receiver (tuning range of 70 -1700 MHz). Purchased from Amazon for $20.
S11 reflection loss response curve of the waveguide feed showing the dip of about -30 dB exactly at 1420 MHz.
S11 reflection loss response curve of the full horn antenna with the waveguide feed. About 16 dB at 1420 MHz.
Total-power spectra of an Ecosorb absorber at room temperature fully covering the horn aperture, and zenith sky, showing a step of about 6 dB. The narrow spectral features are due RF interference. The central spike is either due to LO leakage within the USB receiver, or an unbalanced DC offset before the down-conversion.
Three total-power spectra at slightly different frequencies around 1420 MHz, each with 4 minutes integration in the direction of Cygnus. The 21 cm Hydrogen line appears at around 1420.5 MHz along with several RFI spikes.
Frequency switching is done by acquiring two spectra with about 1 second integration at two different frequencies (separated by 1 MHz) (see bottom panel), and averaging the difference of these spectra (middle panel). The top panel shows the final averaged (and folded) spectrum. This effectively subtracts the gain variation (but RFI problems remain). The frequency switching method was first invented by Purcell and Ewen in their original 1951 detection of this hydrogen line.
Frequency-switched spectra showing the improvement in RFI using ferrite choke cores around USB cable from receiver to computer. (Hydrogen line detected towards Scutum constellation).
Spectrum showing severe RFI caused by laptop power supply (Macbook, switching power supply). Blue curve is with the power supply, red curve is with power supply unplugged).
The same rectangular waveguide feed was also tried out on a larger horn made following the SETI League's Horn Of Plenty design. This horn was made from styrofoam sheets purchased from Lowes. These are sold as 4'x8' panels, for thermal insulation. An older type of insulation board used to have aluminum foil already attached but these are now hard to find. Home Depot sells these under the brand name of Dow Tuff-R. We could not find these readily, so we used the styrofoam with non-conducting aluminum mylar-like thin coating, on which we stuck aluminum foil using 3M spray adhesive. The horn has similar flare angle as before, but has longer dimension along the aperture of about 1.15 m (Compared to 0.74m for the cardboard foam horn made earlier). This large styrofoam horn works quite well- it is large but very light and can be easily mounted on a Dobsonian style alt-az mount (we have yet to make). The following are some images showing this larger horn, with the last figure showing spectra from Cygnus (red), Cassiopeia (green), and Cepheus (blue).
References.
1) The original horn antenna used to detect the radio emission from Hydrogen by Ewen and Purcell in 1951, is currently located at NRAO Greenbank observatory. See this webpage (and references therein). The dimensions of our rectangular waveguide feed are very close to the original waveguide.
http://www.nrao.edu/whatisra/
2) An alternative to horn antenna and dishes, is to use a yagi antenna as described by Peter East here: (this project uses the same mini circuits LNAs that we use).
3) Our python scripts for acquiring spectra in both total power and frequency switching modes, use the Python wrapper for the librtlsdr (C driver for the RTL2832U based SDR USB dongle receivers). This is python package is available here:
4) The basic idea of constructing our horn antenna for 21 cm astronomy came from these two references: the SETI League project's "Horn of Plenty":
and an article on horn antennas for 23-cm Earth-Moon-Earth experiment, by Thomas Henderson:
5) Details about the USB dongle receiver can be found here:
6) Use of Software Defined Radio for 21 cm radio astronomy is described by Marcus Leech, who is also responsible for much of the related software for the GNU Radio platform:
