Copyright © 1999 by Dr. H. Paul Shuch, N6TXIntroduction
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.org
originally published in Popular Electronics 16(7):29-32+34-36+79, July 1999
Remember SETI, the electromagnetic Search for Extra-Terrestrial Intelligence? For more than three decades beginning in 1960, this quasi-Government research project sought clear, unambiguous evidence of other technologically advanced civilizations in the cosmos. SETI existed under the auspices of the National Radio Astronomy Laboratory (NRAO), National Atmospheric and Ionospheric Center (NAIC), National Science Foundation (NSF), National Aeronautics and Space Administration (NASA), various other alphabet-soup organizations, and several universities. Three dozen different SETI programs once scanned the skies with the world's greatest radio telescopes, sifting through great buckets of bits with our most massive computers, trying to separate the cosmic wheat from the galactic chaff. When each search came up dry, tax dollars funded the next with still more sensitive receivers, yet more massive antennas, even grander computers. SETI, so the conventional wisdom held, required the kinds of facilities which only Governments could afford (see Figure 1).
Then in 1993 Congress pulled the plug, turning SETI away from the public trough. SETI science was just too expensive. SETI, it began to appear, required the kinds of facilities which not even Governments could afford. By terminating Government-funded SETI, Congress reduced the federal deficit by . . . 0.0006 percent!
But SETI is a science that refuses to die. Driven by humankind's insatiable curiosity, it seeks to answer a fundamental question which has haunted humankind since first we realized that the points of light in the night sky are other suns: "Are we alone?" Today the search continues, privatized by laymen from all walks of life who cannot let that question go unanswered. Around the world, dozens of amateur SETI observatories are springing up, built by radio hams and microwave experimenters and computer hobbyists who hope to make up in strength of numbers what they lack in government funding. Today's SETIzens embrace a new wisdom: that as technology advances, SETI begins to require the kinds of facilities which ordinary citizens can afford.
Ham radio operators call SETI the ultimate DX. This article is about the privatization of SETI, what it takes in nuts and bolts and ones and zeroes to seek out our cosmic companions, and how you can join the Search.
Where Do We Look?
Today's amateur SETI efforts scan the skies in that range of radio frequencies known as the Microwave Window, where photons (the fastest spaceship known to man) can travel relatively impeded through the interstellar medium. Most searches concentrate in the 1.3 to 1.7 GHz band, exactly where the pros started out. This is a spectral region for which much inexpensive equipment already exists, and much amateur radio activity takes place on planet Earth. Though other interesting frequency bands show considerable promise, they generally require equipment that is either too costly or too complex for today's amateurs.
But that is changing even as we speak. The rule is that, since we don't know exactly where ET might be transmitting, there are no wrong frequencies for SETI. So we build the best equipment today's technology will allow, and we search where we can. If we get incredibly lucky, we find the definitive existence proof we seek. If not, we keep on searching, knowing that tomorrow's affordable technology will tune wider, hear farther, dig deeper, and greatly improve the odds. Amateurs are not discouraged by the primitive nature of their stations, because today's private SETI observatory is fully as sensitive as the best NASA had to offer just twenty years ago. And with NASA out of the game, the gap is narrowing!
Strength in Numbers
The giant radio telescopes from the era of NASA SETI , such as we saw in Figure 1, were incredibly sensitive. They dug deep into the noise by zeroing in on an incredibly small portion of the sky, and surveying it for hours on end. But the immensity of the antennas, while making the telescope tremendously powerful, also imparted an important limitation. The typical research-grade radio telescope only sees about one millionth of the sky at a time. Even if it were tuned to exactly the right frequency, at exactly the instant when The Call came in, there would still be a 99.9999% chance it would be pointed the wrong way, and miss the signal completely.
One solution to this dilemma is to build a million such research-grade instruments, pointed in all possible directions. But at a cost of about $100 Million apiece, we've just exceeded the Gross Planetary Product. Isn't there a cheaper way?
The SETI League believes there is. Small radio telescopes, the kind that amateurs have been building for years, are perhaps 200 times less sensitive than NASA's Finest. That means they will be somewhat deaf, detecting only the very strongest extra-terrestrial signals. But it also means that each one of them cuts a swath of sky about 200 times wider than its professional counterpart. So it would only take about 5,000 small SETI telescopes, properly aimed and coordinated, to accomplish something NASA never even contemplated: to see in all directions at once, so that no direction in the sky should evade our gaze.
Better still, the cost of the typical amateur SETI station is today on the order of two thousand US dollars. That means the entire global network described above can be built for a total cost about a tenth of that of a single research-grade radio telescope. And that's individual hobbyist's money, not your tax dollars at work. SETI's detractors call it a waste of time and money. And I agree. This is, after all, a hobby for most of us, and isn't "waste of time and money" the very definition of a hobby?
The dream of real-time all-sky monitoring is still a long way off. But it is the vision of The SETI League, to be implemented by its Project Argus search.
Argus was the mythical Greek guard-beast who had a hundred eyes, and could see in all directions at once. Mythology tells us that when Argus died, the gods put his eyes on the tail of the peacock. Though that's a lovely story, we of The SETI League know better. When Argus died, the gods put his eyes in the back yards of 5,000 amateur radio astronomers, all over the world. With their help, we will someday see in all directions at once.
The Typical Station
While no two amateur SETI stations are ever exactly alike, they all have much in common. For example, all use some kind of antenna to scoop up weak photons from space, an amplifier to boost those weak signals, a receiver to shift them down to audio signals, and a computer to sift through the audio noise, searching for patterns which cannot be produced by nature. Figure 2 is a typical block diagram of just such a station.
The sections below provide a general overview of the main elements of a typical amateur SETI station. Further details on each are available in the appropriate chapter in The SETI League Technical Manual available online at http://www.setileague.org, or in hard-copy from The SETI League, Inc., PO Box 555, Little Ferry NJ 07643 USA. While it's unlikely that the average experimenter can build a successful station from either this article or the Tech Manual alone, The SETI League's worldwide network of volunteer Regional Coordinators stands ready to assist any member in getting his or her station on the air.
Though many other antenna types have been used successfully, by far the favored antenna for amateur SETI use is the parabolic reflector ("Dish"). The chief advantage of the parabolic reflector is that it operates over an extremely wide range of frequencies, limited at the low end by its diameter (which must be a respectable multiple of the longest wavelength being received, to provide reasonable gain), and at the high end by its surface accuracy (which must not deviate from the parabolic shape by more than a small fraction of the shortest wavelength being received, to maintain reasonable efficiency). Typical satellite TV dishes generally provide reasonable performance over the 1 to 10 GHz portion of the microwave window.
For reception in the 1.3 to 1.7 GHz "L-band" region which is highly favored for much amateur SETI activity, the optimum dish size is on the order of three to five meters in diameter (see Figure 3). In countries such as the US and Canada, where C-band satellite television distribution has been widely used for two decades, suitable dishes are abundantly available at low to no cost. In other parts of the world they are harder to come by, and enterprising SETI League members have acquired surplus commercial telecommunications dishes, or even built their own from scratch.
The size of the dish and the operating wavelength together determine antenna gain. As a first order approximation, the voltage gain (as a ratio) is equal to the circumference of the reflector, measured in wavelengths. Consider, for example, a three meter dish, which has a circumference of (3 * pi) = about 9.4 meters. At the 21 cm resonant wavelength of neutral hydrogen atoms (corresponding to the popular SETI frequency of 1420 MHz), the voltage gain of this antenna would approach (940/21) ~ 45. Since power ratio equals voltage ratio squared, the power gain of such an antenna would be about 2,000, which equates to +33 dBi of gain. (In fact, since the efficiency of amateur SETI antennas is generally on the order of 50%, the actual gain realized is more like +30 dBi.)
Dish size also determines beamwidth, which dictates the degree of aiming precision required when targeting specific stars. As an approximation, half-pwer beamwidth in radians equals wavelength divided by antenna diameter. Thus, for our prior example of a 3 meter dish operated at 21 cm, the beamwidth is on the order of (21/300) ~ 0.07 radians, or 70 milli-radians, which is about four degrees.
If you choose to obtain a surplus antenna, dish condition becomes an important factor. The main consideration here is surface accuracy. In order to perform up to expectations, a dish's surface cannot deviate from the parabolic by more than a tenth of a wavelenght. At 1420 MHz, that's about 2 cm of allowable surface error. If the surface of the dish is dimpled, dented, or distorted beyond 2 cm, avoid that dish! Look for something which approximates a smooth parabolic curve. If panels are missing or bent, performance is going to suffer.
Next, look at the mounting hardware. If it's rusted, you're going to have trouble getting the dish apart, and more trouble reassembling it. Weight is sometimes a consideration, as is wind loading. If these are concerns to you, a mesh dish may prove more realistic than a solid one.
Many of the accessories which come along with a satellite TV dish will be of limited use for SETI, and therefore you should not pay extra for them. C-band or Ku-band feedhorns and preamps are only useful if you're going to search in C-band or Ku-band (some of our members do; most prefer to scan the popular L-band region.) TVRO receivers are great sources of microwave components, but unless other civilizations utilize exactly the same TV transmission standards we do, they're not particularly useful as SETI receivers. And a motorized mount which tracks the Clarke Geosynchronous) orbital belt is not particularly useful for drift-scan, meridian transit mount radio telescopes, except if modified per the instructions in the Antenna Mounts section of this document.
In the final analysis, your budget will likely be your chief limitation, so go with what you can afford. Any dish at all will receive better than no dish at all!
Additional information on various SETI antenna options, along with vendor recommendations, may be found in the Antennas and Feedhorns chapter of The SETI League Technical Manual.
The Antenna Mount
The beauty of mounting a parabolic antenna for SETI use is that you just can't go wrong. Since we are interested in monitoring the sky for artificial signals from beyond, the antenna merely need be pointed up -- there are stars (with potentially habitable planets) to be found in all directions. So mounting an antenna for SETI use is considerably simpler than, for example, using the same antenna for satellite TV, where it must be precisely aimed at the satellite's location in the sky.
Because there are no wrong directions for SETI, many SETI antennas are simply set on the ground, "bird-bath" style, looking straight up. But a disciplined sky survey, such as The SETI League's Project Argus effort, requires coordinated sky coverage, and that in turn necessitates a limited steering ability for at least some of the antennas in the network.
Where steering of the antennas is desired, we need to consider two degrees of freedom: azimuth (the compass heading to which the antenna points), and elevation (the angle which the antenna's beam makes with respect to the horizon). In terms of celestial coordinates, azimuth of a radio telescope (along with a station's latitude and longitude, and the date and time) determines the Right Ascention (RA) of its target, while elevation (again, along with lat/lon, time and date) determines Declination (Dec). Conversion between terrestrial and celestial coordinates is handled by a spreadsheet found on The SETI League's website.
Since we live on a rotating planet, the Earth herself makes a most cost effective RA rotor, as long as we are willing to be patient and let the proper portion of the sky eventually rotate into view. But since (thankfully!) the Earth doesn't rotate north-to-south, the only way to acheive Dec control is to physically rotate the antenna along a north-south line. This can be accomplished by aligning a satellite TV antenna's position rotor as a vertical (elevation) rotor, as described in an article on The SETI League's website.
Further information about mounting SETI antennas may be found in the Antennas and Feedhorns chapter of The SETI League Technical Manual.
When radio waves strike a dish antenna, the parabolic shape of its reflector directs all the energy to a single point out in front of the dish, called its focus. The purpose of the feedhorn, which is mounted at the focus, facing the reflector, is to scoop up all this energy, and apply it to the LNA and receiver for processing.
The most common feedhorn for amateur SETI use is a metal pipe, closed off at the end farthest from the dish, forming a shorted cylindrical waveguide (see Figure 4). The horn contains a small metallic probe, connected to the center pin of a coaxial connector, to collect the energy and apply it to the input connector of the Low Noise Amplifier (LNA). The horn may be surrounded by a metal ring, used to improve the efficiency of energy collected from the surface of the dish, or to block interference from entering the feed from beyond the periphery of the dish, as described in yet another SETI League website article.
The chief drawback of the cylindrical waveguide feedhorn is that its large physical size actually blocks a part of the dish surface from view of its incoming signals, effectively reducing the size (and hence the gain) of the parabolic antenna. This blockage loss is most severe for small dishes, becoming almost negligible at the popular 1.3 to 1.7 GHz SETI frequencies when the dish diameter exceeds about four meters.
An alternative to the waveguide feedhorn is the helical feed, consisting of about three turns of heavy wire in a corkscrew shape, with a circumference of one wavelength at the operating frequency, and a spacing between turns of a quarter wavelength. A helix feed doesn't block the aperture of the dish to the extent that a waveguide horn does, but is more prone to interference from signals off to the side of the antenna. Both helix and waveguide feedhorn designs have been used successfully by SETI League members.
Additional information on various SETI antenna feeds, along with vendor links, may be found in the Antennas and Feedhorns chapter of The SETI League Technical Manual.
The Low Noise Amplifier
The Low Noise Amplifier, or LNA (see Figure 5), is sometimes called a preamplifier, or preamp. Its function is to turn an impossibly weak signal into a merely ridiculously weak one. The critical parameters to consider in selecting an LNA are its frequency response, gain, and noise temperature.
Frequency response determines that portion of the electromagnetic spectrum over which a particular LNA will boost the received signal, with minimum distortion or added noise. You should select an LNA with a frequency range consistent with your particular SETI station requirements. For example, C-band Satellite TV LNAs cover the frequency range of 3.7 to 4.2 GHz. Thus they are not suitable for use in SETI stations designed to monitor the 1.4 GHz hydrogen line. Some LNAs incorporate filtering, which reduces the overall range of frequencies amplified, but which can help to reduce out-or-band interference.
Gain, measured in deciBels (dB), is a measure of how much the LNA boosts the incoming signal. Although in many things "if a little is good, a lot is better," this is not the case for preamplifier gain. In fact, excess LNA gain can actually reduce the sensitivity of your SETI receiver. The rule of thumb is that the gain of the LNA should equal the sum of the microwave receiver's noise figure (in dB) plus the RF cable insertion loss (also in dB), plus an additional ten dB. For the average SETI station with a short coaxial cable between the LNA and the receiver, twenty dB of preamp gain is usually about right. If a very long or unusually lossy RF cable is used, a 30 dB gain LNA might be more appropriate.
Noise temperature is a measure of how much additional noise the LNA adds to your SETI system. Since any actual signal has to compete with a variety of natural and artificial noise sources, the lower the noise temperature, the better. The LNAs commonly used for amateur SETI typically have between 35 Kelvin and 100 Kelvin of internal noise. Noise is sometimes expressed not in Kelvins, but as Noise Figure (in dB) or Noise Factor (a unitless power ratio). The SETI League provides a Microsoft Excel ® spreadsheet for conversion between these various noise units. Sometimes you can reduce the noise temperature of an LNA by thermally cooling it. An additional spreadsheet allows you to calculate the improvement acheived by lowering an LNA's ambient temperature.
Many commercial LNAs are provided with a choice of coaxial input and output connectors. Most SETI League members prefer to standardize on the coax connector known as Type N, since this is the connector used on most feedhorns and microwave receivers. To minimize losses, the LNA should be mounted directly on the output connector of the antenna feedhorn, with the appropriate coaxial adapter (probably a Type N male-to-male barrel adapter).
An additional consideration is how to get the appropriate operating potential to the LNA. Most LNAs operate from a DC power supply, typically in the +12 VDC range. Some designs require that this operating voltage be applied via the center-conductor of the RF cable, and some LNA vendors give you a choice between internal and separate DC feed. DC feed via the transmission line requires that the microwave receiver be designed to provide this voltage, or that an accessory called a DC Inserter or Bias Tee be connected into the signal path ahead of the receiver, and tied in to an appropriate power supply. Although this is the scheme commonly used to power the antenna-mounted circuitry in commercial satellite TV receivers, most SETI experimenters consider direct DC feed through the coax to cause more problems than it solves. It is generally prefered to run a separate DC cable (such as a telephone cable, speaker cable, or lamp cord) outside to the LNA, and to apply the required DC potential to it inside the SETI station. (Caution: double-check the polarity applied to this cable, as reversing the positive and negative power supply leads can damage the LNA.)
Although most commercial (and many home-built) LNAs are metal-boxed to provide good shielding against Radio Frequency Interference (RFI), few are provided in weather-proof enclosures. To prevent damage from exposure to the elements, I like to put my LNAs in plastic Tupperware ® sandwich boxes. It is necessary to drill or punch holes in the plastic for the input coax adapter, output cable, and power wiring. Be sure to seal these openings with room-temperature vulcanizing (RTV) silicon rubber, which you can obtain in a tube from most any hardware store.
Information on various commercial LNAs available in kit or assembled form, along with vendor links, may be found in the Preamplifiers and Filters chapter of The SETI League Technical Manual. For the experienced microwave experimenter schematics, component selection criteria and do-it-yourself information are also provided.
The RF Cable
The most common SETI station configuration would place the microwave receiver, signal analysis computer and related accessories inside the house, with the antenna and LNA mounted outside, some distance away. To connect the two halves of a SETI station, we use an RF cable.
RF stands for radio frequency. The cables we use are usually coaxial (i.e., "coax" cable), and we prefer those with low loss at radio (specifically microwave) frequencies. The stuff used for cable TV is cheap (pennies per meter) but pretty lossy in the 1.4 to 1.7 GHz region of the spectrum typically used for amateur SETI. The kind you buy for, say, CB radio antennas is a little better, and a bit more costly. If you have a local Radio Shack store or similar, you can probably find what they call low-loss coax . It's larger (perhaps 1 cm diameter) than the CB or TV type, costs maybe a dollar or more per meter, and may go under such part numbers as Belden 9913, RG-8 Polyfoam, etc. It may take special connectors (the ones most of us use are called "Type N"), which require some experience to properly install.
For any type of coax, the longer, the lossier. So we try to keep our antennas near the radio room. If this is not practical, we can do several things: use more gain in the preamp (to boost the weak signal before it suffers cable loss); mount the whole receiver, or just the downconverter, outside on the dish (pumping a lower frequency through the cable is more efficient); or use specialized cables such as hardline or Andrew Heliax ® (which can cost upwards of tens of dollars per meter).
The Microwave Receiver
The microwave receiver takes a small, selected portion of the radio spectrum, and converts it to audio for signal analysis. Selection of the appropriate receiver leaves more to the discretion of the experimenter than any other portion of the amateur SETI system. Four distinct options present themsleves. In descending order of cost, they are:
Whichever receiver scheme is selected, present practice suggests operating it in single sideband mode (either USB or LSB), and leaving it fixed-tuned, rather than scanning it across the spectrum. The reason for avoiding frequency scanning is that the Earth is turning the antenna continually, so that the spatial dimension of the observation is always changing. Only by holding frequency constant for at least one rotational period of the Earth (that is, one day) can we avoid the problem of "too many variables."
The bandwidth of the receiver's audio stages will typically be the limiting factor, as far as instantaneous frequency span concerned. Many SSB receivers cover as little as 3 kHz of spectrum at a time, which is an inefficient way to search for ETI. Advanced SETI experimenters sometimes modify their receivers for up to 22 kHz of instantaneous IF and audio bandwidth, while custom-built receivers can cover several hundred kHz all the way up to a few MHz of spectrum at a time.
Information on various commercial and kit receivers, recommended modifications, and vendor contact information may be found in the Receivers and Converters chapter of The SETI League Technical Manual.
Even the simplest of today's personal computers is thousands of times more powerful than the ones NASA used to put men on the moon. Of course, the objective of SETI is not to reach the moon, but rather to reach much farther out into space for intelligently generated signals. To do so, we employ a technique known as Digital Signal Processing, or DSP.
The first step in the DSP process is to feed the receiver's audio output into the computer, in a form which the computer can recognize -- that is, as binary data. We need an analog to digital converter (ADC) to accomplish this, and the ADC of choice for amateur SETI is the PC Sound Card. Just about any SoundBlaster ® compatible audio card will work with The SETI League's signal analysis software. These cards sample an audio waveform 44,000 times per second. One of the rules of information theory is that to digitize a signal, it must be sampled no less than twice for every cycle at its highest frequency. With 44 KSPS (kilo-samples per second) sound cards, this means we can digitize and analyze audio components out of our receiver up to 22 kHz in frequency. 22 kHz is a rather narrow bandwidth, which even a 486-class computer can analyze in real time, with excellent resolution. The typical DSP program chops the received audio band up into 2048 individual channels, each about 10 Hz wide, analyzing and displaying all those channels simultaneously, in real time. Thus, the computer turns the SETI station into a 2048-channel receiver.
The required software, developed by SETI League members, typically runs under the Microsoft DOS or Windows operating systems. It is shareware, offered at low or no cost to all participating SETI league members via the Software pages of The SETI League website. Its job is to identify signals which exhibit the hallmarks of artificiality, characteristics which distinguish intelligently generated from natural phenomena, and then to help determine whether those characteristics might have come from some terrestrial source. Our civilization pollutes its own radio environment, so we need to sift through any detected signals rather thoroughly in order to rule out manmade interference from our own transmitters, aircraft, spacecraft and orbiting relay stations. Figure 9, for example, shows the first candidate signal received by The SETI League in May of 1996. It turned out to be interference from a classified military satellite, a difficult conclusion for the human observer to arrive at, but a trivial identification task for our computers.
In addition to analyzing signals, some SETI League computers also control the station. Remember the computer-controlled microwave receivers discussed above? They can often be tuned by software, driven from the PC's serial, parallel or USB port. Antennas can similarly be computer-aimed, if they are equipped with software-driven azimuth and elevation rotors. Some SETI computers make lights ring and bells flash whenever they detect something interesting. And the most advanced of the computers used by SETI League members also dial into the internet when an interesting candidate signal is received, automatically alerting other participants that their assistance in signal verification is required.
More SETI computer information may be found in the Software chapter of The SETI League Technical Manual.
Putting It All Together
When I built my first amateur dish more than twenty years ago, I was going it alone. That was frustrating, because I had to learn from my own mistakes (which were legion). Today there's assistance. The non-profit, membership-supported SETI League exists to help you become one of those 5,000 active Argus observers. Though we are only 1000 members strong at present, we're still a young organization, just three years into our search. Our volunteer Regional Coordinators in over 50 countries on six continents have already helped more than five dozen of our members to put stations on the air. Our extensive website and various books and articles are already attracting hundreds of like-minded enthusiasts into the SETI community. To come on line with us, email us your postal address, call our Membership Hotline at 1(800) TAU-SETI, or write us at The SETI League, Inc., PO Box 555, Little Ferry NJ 07643 USA for a free brochure. Together, we may well end humanity's isolation in the Universe.
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this page last updated 28 December 2002
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