Small Amateur Radio Telescope
by Dennis Allen
Last month we got a 12' satellite dish from Dan Seeley. So the question on everyone's mind: what will we be able to accomplish with this dish? Well, depends on what objects we want to go after and what equipment we can afford. In amateur radio astronomy we could do Solar observations, Jupiter observations, Meteor observations, Galactic observations, or even SETI (search for extraterrestrial intelligence) observations.
Solar Observations: We could detect solar flares at the VLF (very low frequency) 30-80 KHz range or in the VHF (very high frequency) 1-30 MHz range. We'd need only simple ham radio equipment. With the satellite dish, we'd be able to pick up solar burst activity at 80-890 MHz frequency range.
Jupiter Observations: We could detect radio noise storms from Jupiter. at the 18-24 MHz range These storms are believed to be caused by the movement of the Jovian moons Io and Ganymede through the magnetic field of Jupiter, which in turn causes great electrical storms on the planet, Again, a simple short wave radio equipment and loop antenna.
Meteor Observations: By turning into a blank signal, say an marginally received aircraft beacon at 75 MHz, we could pick up in-falling meteors as "ping" sounds. We'd need ham radio equipment and a directional (Yagis) antenna.
Galactic Observations: With short wave equipment and a directional antenna, we could study solar flares. Perhaps we could study some of the more powerful radio sources such as Cassiopeia A or Cygnus A at the 80-100 MHz range. We could also study the galactic arms and the center of the Milky Way.
SETI Observations: You heard of the 21 centimeter band? This is the radio wavelength created by an excited hydrogen hydroxyl molecule. At 1420 MHz, it's the hole of silence where almost no Cosmic static is generated. This so-called "water hole" is an ideal place to observe in general (or look for ET). At this frequency, however, we'd need a satellite dish.
Oh, FYI. When we talk about the 21 cm Band, it's the wavelength (meters) = 300 / frequency (MHz). Example: 300 / 1420 MHz = .21 meters or the 21 centimeter band. The 21 cm band is also called the L-Band (1420 MHz or 1.4 GHz). Other bands include the 23 cm band (1300 MHz), the 2-meter band (148 MHz), the C-Band (4 GHz), and the Ku-Band (12 GHz).
Other Observations: A satellite dish is viable only above 400 MHz. In areas such as the "water hole", it might be possible to observe Doppler shifts in the Milky way or detect HEPs (high energy pulses) from the galactic center. These HEPs are mysterious pulses, possibly generated by flare stars or black hole radiation. Given the right equipment, we could observe pulsars, supernova remnants, gamma ray bursts, or other blackbody radiation (radiation that an object would absorb if it were a perfect absorber).
The basic radio telescope has an antenna, a pre-amplifier, bandpass filter, a mixer/oscillator, an IF (intermediate frequency) amplifier, square-law detector, and DC amplifier. The antenna, of course, is the TVRO (TV Receive Only) satellite dish. Signals from the antenna are sent to the pre-amplifier. The pre-amplifier (also called the LNA or low noise amplifier) boosts the weak in-coming signal. The bandpass filter (white box) allows only selected ranges of frequencies to pass to the mixer. The mixer/oscillator lowers the frequency for the IF amplifier (avoids signal feedback to the antenna). The signal is boosted by the IF amplifier (also does some bandpass filtering). The square-law detector allows passage of the signal in one direction by throwing out the other half (otherwise the highs would cancel the lows). The DC processor removes receiver noise and other fluctuations before sending the signal on to either a recorder or an A/D (analog/digital) converter and computer.
It'll be up to Dan to assemble our radio telescope. He might obtain the individual components separately. He might opt to get a TPR (total power receiver), an all-in-one receiver that has most of the components built-in. Radio Astronomy Supplies seems to be the main supplier of RA components. They also have $1500-$2500 all-in-one receivers. rfspace.com has an interesting receiver called a SDR-14 which runs about $1000. If Dan assemblies the individual components (gets the signal to the computer), we might be able to get the SDR-14 SpectraView software directly from www.moetronix.com.
In radio observations, you aim the dish ahead of the desired object, recording the object as it drifts across your field of view. The hard part is finding the object and getting ahead of it before the observation. If Dan can get four-way control, we'll be able to find objects easier. And if he can train the RA drive to track in sideral time, we'll be able to extend our observing time.
But don't hope for images any time soon. I'm told our dish will have a five degree field of view. By optical standards, that's huge and will result in low resolution. Radio astronomy in general is like seeing the sky through a soda straw (and an opaque straw at that). If we can make enough accurate sweeps of a section of sky perhaps we'll be able to create some sort of image. Eventually.
So, will we get to observe galactic Doppler shifts or hear ET? Again, depends on the equipment. But you have to start somewhere. And even if we don't see pulsars, at least we'll know why we can't see them. I liken this project to a beginner getting his first telescope. Images off the Internet are a thousand times better then anything you can see in your small scope. But your scope sees the real thing. A picture is like taking someone's word. Same thing with Radio Astronomy. We might end up with just lines on a graph, but they'll be OUR lines.
In writing this article, I found several sources of information. "Radio Astronomy Projects" by William Lonc, and "Amateur Radio Astronomy Systems, Procedures, and Projects" by Jeffery M. Lichtman were useful. Also found the following web site helpful:
Frequently Asked Questions About Radio Astronomy
Oh, if you want to hear actual radio signals, check out the INSPIRE VLF radio receiver at NASA's Marshall Space Flight Center in Huntsville, AL.
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This web page was last updated 10/15/19