The Ultimate DX - Radio Astronomy and SETI
The science of radio astronomy and the search for extra terrestrial intelligence has been one of the things that has not caught the interest of the general amateur population. It is an area that as a group we should look into with our usual amateur ability to succeed where others won’t even try. The ingenuity hams have exhibited in the past can be put to good use here in this field. Granted, most of us do not have the funds to match the most modest professional radio telescope. But, in place of funds, there is ingenuity!
With the advances in satellite communications and the availability of excellent low-noise amplifiers in the microwave frequency range, as well as inexpensive parabolic antennas, a lot of the expensive components have come down to more or less tolerable levels. Besides, there are other parts of the rf spectrum we can check out that have not had too much attention paid to them. In the lower ranges, such as from 30 to 1000 MHz, there is a lot of expertise already at our disposal due to the many hams that have put moon bounce on the map on the map of reality. Scatter, both meteor and tropo have further increased the ability for very weak signal detection.
Going still lower, from 1.6 to 30 MHz. there is even more signal capturing ability and even less expense. Ha, you say! There is too much noise and other people talking to hear anything there. Bah! Humbug. There is where the challenge lies. All it takes is a determined person and a solution will be found. After all, if the DX hounds can dig out that elusive ZU3 on 15 meters after the band has croaked for the night, listening for even greater DX should be no problem. On the lower frequencies such as 75/40,the natural QRN level will limit things a bit, but not make things impossible. Ionosphere penetration and absorption will also cause some problems, but cannot be helped. In the LF and VLF ranges, from 5 Hz to 500 KHz, here again we can make observations with the best.
This is where the hams can shine, because we are versatile. We don’t need to get approval from some committee to shift our operations to another frequency. We just do it and that is that. No requests for funding for FY 98. If we cannot afford to buy it, we build it, particularly if it is some off the wall thing that no one else has ever needed before. This is the strength we have. We can do as we please with our equipment, and as long as we don’t strain the household budget out of shape, and don’t cause any interference or break any laws, we can pretty much do as we want. So you say what can a low budget ham system do?
Well keep in mind the first radio telescope ever was built by Karl Jansky, a Bell telephone radio engineer looking for sources of interference on transoceanic radio circuits. He found out that Jupiter was the source of the noise that was giving them fits. This was in the range of 20-27 MHz. When the giant red spot, which is a huge storm on the surface of Jupiter faces Earth, we get static crashes here that can be heard, usually around 21-27 MHz. Grote Reber also did a lot of experimental work with radio astronomy and he was a ham. Most of this work was done in the early 1930’s with Jansky presenting a paper at a meeting of the IRE on “Electrical Disturbances Apparently of Extraterrestrial Origin” which subsequently was published in the October 1933 issue of the Proceedings of the IRE. Unfortunately Karl Jansky died at the age of 44 and all of his records were destroyed.
Grote Reber built his own 30-ft. parabola before such things were even deemed useful. For years he was the only radio astronomer in the whole world, and had published several papers on “cosmic static” and other observations that he had made during the past years. Both Jansky and Reber built their equipment and antennas from scratch, at a time when you really had to build what you needed. Their work was a start into an aspect of radio communication that has grown into a science all to its self. Keep in mind that the planet Pluto was discovered by an amateur astronomer working for nothing as an volunteer aide at one of the large telescopes. We can make a contribution, as amateur radio operators. Remember, until we were forced to 200 meters and down no one thought that the short waves were worth anything.
Another ability that hams have is that we can communicate with a variety of emissions and on a bunch of frequencies. Exchange of data via packet or rtty by a computer operated network would allow a rapid exchange of information between participants. To keep everything synchronized, so that data can be correlated accurately, Heathkit has a WWV radio clock that locks on to the timing pulses that WWV sends allowing accuracy of plus or minus 10 milliseconds. It even has adjustable propagation delay correction which allows the small time delay that takes place from the time that the signal is transmitted until it is received to be corrected for. One of the accessories for this clock is an RS-232 interface which allows accurate timing of data coming into your computer.
The availability of inexpensive computers such as the Radio Shack Color Computer allows one or two computers to be dedicated to doing tasks such as antenna control, signal analysis, data transmission comparatively inexpensively. The COCO is a good computer for this as it already has a pair of A\D converters built in that can handle up to 5 volts. The game ports are the A\D converters and programming them to do what is needed is comparatively simple. It also has an RS-232 port built in and can have up to 4 disk drives or a hard disk by using the Multi-Pak interface. There are several programs for the COCO that allow direct copy of fax, rtty, and cw by going into the cassette port with the audio with no special interface between the COCO and a receiver. Other computers need special cards to accomplish what the COCO can do with its hard ware already built in and it is inexpensive. A programmer could come up with the software to make the COCO handle just about all of the chores that are needed done for a project like this.
Going to an equally important subject, the antennas that are needed for this type of project are going to depend on what frequency range you want to work in. For the 50-800MHz range, the TV antennas cannot be beat. Many are some form of log periodic, and as such, cover the entire spectrum with very few gaps. Also, there are a lot of high-gain wide-band low-noise amps available for very good prices. An array of high gain TV antennas with matching pre-amps could be set up and do a fine job.
For the lower frequencies, 2-30 MHz, the size of an array can get out of hand very quickly. An array with a very high angle of radiation, and minimum side lobes for 75 would be a project for a lot of acreage, so, city dwellers would be limited somewhat, at least on 75. 20 meters and higher would not be as much as a problem. The noise level would be a problem, particularly towards the horizon. Again, an antenna with high front to side ratio would be desirable. It may be difficult, but that has not stopped the determined hams from getting something set up and running if they wanted to badly enough.
Now, suppose there were about 15 or 20 kilo hams scattered all over the world that had set up radio telescopes on frequency ranges from 10 KHz to 10 GHz? And, everyone was tied together by packet or a landline network with autodial modems with software that would allow auto calling of other stations monitoring the same frequencies? Now, suppose an anomaly showed up that deserved a closer look. If the software told all stations listening to the 200-300 MHz range that there was something on 275.564 MHz, horizontal polarization, with slow flutter possibly due to polarization changes, modulated by unknown means, doppler shift of -500 Cps, followed by antenna aiming co-ordinates, then every station receiving the message could set itself up to hear the unknown signal. No operator need be present since the computer could handle everything, aiming the antenna, tuning the receiver, turning on the tape recorder, or whatever means used to save the signal. By using the RS-232 ports that a lot of the newer receivers have, it would be easy to do this.
To carry this a little further, suppose all of the data gathered was then sent via packet, AMTOR, or whatever to a central point at regular intervals, then dumped to a mainframe at a large radio telescope, or university every hour. Since the data was all synchronized with WWV, or WWVB, and the times of data collection sent along in the, data co-ordination should not be difficult for a mainframe to do. Granted, there is a lot more to it than this, but it is food for thought.
Well you say, I can’t compete with super low noise paramps, and the 120 ft. parabolas the professional radio telescopes have. Don’t bet on it. How many of you have been listening on some band and heard a rare DX station pop up and been able to work him/her before the pileup with all of the big guns hit the frequency? It has happened to a lot of us, so don’t say it is impossible. Besides we are not competing with the big guys, but working with them. Besides, 10,000 small ears can hear a lot and tell the big ones what, when, and where.
We have the technology and the ability and the curiosity, I should hope. All we need is to put our collective intelligence and abilities to work. If anyone decides to get involved in this sort of thing, antenneX would be glad to see that the info and other data gets out to the others. But remember the famous words of Spiro Agnew, “The nattering nabobs of negativism” will be telling you that you cannot do anything that would be of any note. Don’t believe them, because as many a great thinker and brilliant person will tell you “Along the road to wisdom and enlightenment at every intersection stand a thousand of the disciples of ignorance, ready to stop anyone who seeks knowledge.”
The optical astronomers can see a lot of what is in the sky, and by using long exposures can capture an enormous amount of low intensity light with their cameras. CCD systems can get a lot more of the low light images, as can almost any of the light intensifier devices. Many extremely beautiful images are portrayed in the pages of Sky and Telescope and Astronomy every month. Pick up any issue of either magazine and look through it. The pictures are very beautiful, many of them are by amateur astronomers and many of them are deserving of being framed and hung in a portrait gallery.
Unfortunately, radio telescopes are not too good at rendering color photographs of what they see, unless a Cray supercomputer is used to integrate a lot of data from a lot of telescopes into a composite image. That is a little beyond the means of most of us. So, we must be capable of doing what we can and getting the most out of the equipment that we have.
There is a group already using radio astronomy and is called the Society of Amateur Radio Astronomers (SARA), an international organization existing to assist its members in observing the radio sky. They have a monthly journal and quite capable and dedicated in their intent to foster an interest in radio astronomy. About half of the members are hams and so there is already an existing group of hams that are into the exploration of the sky.
All that is needed is to link everyone up and then maybe we can get our own version of an amateur radio integrated radio source image. It is a point to ponder. Anyway we can not yet get the integrated images without a lot of cooperation and a lot of hams, but we are working on that.
“Well, what are we going to listen for, and where?” said he. Suppose we start in a part of the spectrum where a lot of good low noise equipment is already available, 30 to 900 MHz. There is off the shelf equipment available in the form of pre-amps, receivers, feedlines and antennas. Just check the ads in 73, CQ, and QST, for example and you will see the equipment is there. It just depends on how much you want to spend. Remember, a radio telescope is just a receiving system with a different type of output transducer, usually a chart recorder instead of a speaker. However a speaker is usually in the circuit somewhere. So with that in mind, read on and we will start getting into the good stuff.
Starting with the antenna, there are a lot of choices, from commercial to homebrew. There are so many different types it is hard to make a choice. Some of the more prominent choices are the helix, the log periodic, and the parabola.
The helix is a good antenna to use as it has a good bandwidth, is easy to construct, and can be made in both left and right-hand polarization. At the higher frequency ranges it is small enough to place several on a az-el mount and have both left and right polarization with equal gain, feeding separate receiving systems so that simultaneous polarization can be monitored. The gain at the higher frequency ranges can be increased by making the antenna larger physically, since it is so small and less sensitive to Faraday rotation than a yagi.
The helix bandwidth is on the order of 2:1 and the feed point impedance remains nearly constant over the frequency range of the antenna. There have been several construction articles in the ham magazines on how to make your own helix array. This antenna is also the product of John Kraus, W8JK, who is the inventor of this little gem. Ohio State first used an array of 96 helix antennas for one of their first radio telescope antennas. Sometimes helix arrays will turn up surplus at reasonable prices. It is a good antenna for any type of space communications project.
The log periodic has excellent bandwidth and good feed point impedance over the frequency range that it is designed for. It is linearly polarized, and by using two antennas and by proper phasing of the feed lines polarization can be switched 90 degrees as well as combinations of left hand and right hand polarization to meet signal polarization changes.
The log periodic has been used as feed point antennas for large parabolas due to the large bandwidths that characterize this antenna. The beam widths are not as narrow as other antennas of this type, but by properly designing an array and getting the phasing lines installed correctly, reasonable beam width can be obtained. Many of the popular TV antennas are log periodic and they are pretty reasonable
The best antenna system for the radio astronomy ham to use at the uhf and higher is the parabola. Parabolas have constant efficiency over their design range of frequencies, excellent front to back and front to side ratios, and narrow beam widths, but that depends on the frequency to be used and size of the size of the dish.
For example a 10-ft. parabola has a gain of 21.5 db with a beam width of 7.25 degrees at the 1 db down points at 500 MHz with an antenna efficiency of 50%. Go to 2Gc and the gain goes to 33.5 db with a beam width of 1.85 degrees at 1 db down points. That is some gain for an antenna that will fit in your back yard! Of course, the surface tolerance is what determines the upper frequency limits of the parabola.
If the surface tolerance of the dish varies more than a certain percent of a wavelength at the frequency of interest, then gain starts to suffer as scattering of the signal from the uneven surface causes the reflected signal to not hit the focal point of the dish.
By changing the frequency of the feed point elements, the frequency of operation can be changed easily. This is what the larger radio telescopes do on the lower vhf and uhf frequency bands, allowing them to search large parts of the spectrum with minimum down time to switch feed point frequency elements. Just as a point of interest, a 200-ft. dish has a gain of 55.25 db with a beam width of .15 degrees at 1296 MHz. Talk about a bird vaporizer! 1500 watts at 1296 MHz would cook any goose that flew into the beam, and scorch off the feathers too. Not for civilized areas, that is for sure.
There are other antennas that are adequate for this, but these are the ones with the most bandwidth available to the general ham public. Any moon bounce type array would be adequate for radio astronomy and the pre-amps and the feed lines used for moon bounce are adequate too. The mono-band moon bounce arrays will do fine for searching the part of the spectrum they are set up for. So you moon bouncers think about using your rigs for radio astronomy when you are not attacking the moon with your kilowatts. The reward you might get from radio astronomy and SETI may beat any QSL you could get from moon bounce. But, only time will tell.
So on to the feed line. The best way to find a good feed line is to get a catalog from some place like Belden and go through the list of cable specs. Look for the cable with the lowest loss at the frequency of interest. A lot of the stuff made for the cable TV industry will do what we want and the connectors are available at a reasonable price. For example, RG-11/U shows a loss of 5.2 db per hundred feet at 900 MHz in the Belden master catalog 885. That isn’t too bad and with a good pre-amp at the antenna end, that loss can be eliminated or reduced a lot.
For the home brew types there is an alternative to coax…twin lead. Both Channel Master and Belden list twin lead that has a loss per hundred feet of 4.3 db for Belden 8275 and 3.6 db for Channel Master Jumbo Polyclad 9555-500. This is wonderful, but there is a burr under the saddle. When the stuff gets wet, the loss doubles, taking the loss to the level of coax. There is a way to get around this, put the twin lead in pvc plumbing pipe. This also will allow the use of the famous ladder line, which has even less loss and the whole mess can be buried. By burying the feed line, the possibility for interference is cut way back as is damage to the feedline. All you have to do is to bring the twin lead up into a header made of larger diameter pvc and put your balun there. If you wanted to really get professional, you can seal up everything and use dry nitrogen to pressurize the whole thing. The alternative to this is to use a high quality aquarium pump and pump the air though a section of heated copper tubing and then through a jar of desiccant to dry out the air. A lot of pressure is not needed, just enough to keep moisture from entering the system in the event of a small leak. There are pressure switches that can trip with the loss of a few ounces, turning on the air pump.
It will take some digging through parts catalog, but the hardware is there to do this. It would be possible by using this method of pvc enclosure to come up with lower loss than some of the very expensive heliax. It is food for thought. This is also a good trick to use for your regular vhf/hf system to get the last little bit of signal to and from your antenna. Just be sure to use twin lead that is rated for your transmitter power. Coax can be used for short runs to tie elements of the antenna together, but for any long runs at all, low loss twin lead protected from the elements is better than coaxial cable. Look at it this way, the loss in a long run of coax could mean another pre-amp would be needed at the shack to just to make up for the losses in the coax, as well as having a pre-amp at the antenna. Then, the system noise can be made worse than necessary due to the addition of one pre-amp amplifying the noise of the other, as well as the signal, bringing up the system noise floor.
Now that you have hardware information for the antenna and feed line, it is time to figure out what you want to do. Pre-amp choices go up to whatever you want to pay, and picking a pre-amp is usually done by the old cost/performance formula. You pays your money and you takes your chance. Generally speaking, the lower the noise figure and the higher the gain, the higher the cost.
Receive bandwidth of the pre-amp is going to depend on the type of listening that you are going to do. If you are going to listen for the large high power natural sources of interstellar and intergalactic signals, such as the pulsars, and other things of that nature, a good bandwidth might be advisable, as is a remote tuning capability. SETI systems could benefit from a narrow bandwidth, remote tunable pre-amp that could be set up to track with a scanning receiver for maximum sensitivity. SETI system operators have a problem in that no one knows what they are listening for, or where, which rather complicates things somewhat. That is a challenge that even beats getting 6 band DXCC with 100 milliwatts and an underground antenna. Finding the first signal from another species would definitely rattle cages from the Kremlin to ARRL Hq. Wonder if we should QSL with IRC’s? Wonder where their QSL managers are? Only time will tell. Stamp collectors will come unglued.
At this point we will stop with the equipment digging and think about what type of antenna system you may want to set up. One antenna system is the az-el mount with tracking capabilities. This will enable long observations of a given object since the antenna will track the signal source from horizon to horizon as the earth rotates in the same manner that optical telescopes operate. The other type is the one used by the OSU radio telescope. It can only scan in elevation from north to south as the rotation of the earth scans the antenna pattern across the sky. So, by aiming this antenna at a set elevation from the north or south horizon, and increasing the elevation daily by a few degrees, the entire sky could be mapped over a period of time.
The other antenna system is very long baseline interferometry. This system uses a pair or more of antennas separated by a specific horizontal distance, called the base line, and the distance is preferably a distance greater than 15 wavelengths. The antennas are on an east-west line and the system operates on the premise a signal from a distant source at each antenna operates separately with both an in phase and an out of phase component in sequence. This is due to the time difference between the signal’s arrival at the separate antennas. Combining the two signals in a zero-degree phase shift combiner results in alternating patterns of phase cancellation and phase addition so the antenna major lobe is broken up into a lot of smaller lobes. By increasing the baseline the narrower the lobes get, and the resolution of the array increases. VLBI with antennas thousands of miles apart is made possible by synchronizing the receiver local oscillators and the recorded data with atomic clocks, making a beam width of 0.0001 arc-second possible.
At the level of the technology available to the general ham population today, we can not quite get that close on the timing, but sooner or later some ham somewhere will come up with a solution. One that everyone can afford, is user friendly, guaranteed not to bite, and housebroken. That is the type of software and equipment we need. So keep it in mind those who would be designers and programmers for this kind of project. As far as timing is concerned, the Heathkit Most Accurate Clock is as close as most of us can afford, and is within 10 milliseconds. There is an RS-232 interface as an option and a step in the right direction.
One side note to VLBI is that the human ear determines direction by the same method of phase detection. Only we can tell direction to within 3 degrees, depending on the frequency of the sound. Of course the lower the audio frequency the longer the wavelength, the shorter the baseline is (the distance between the ears), and therefore the lower the resolution. It is difficult to tell where a very low frequency sound is coming from, but it becomes easier as the frequency increased.
For an example, you can get the general direction of a thunderstorm at night, but, unless you see the lightning, you can not pinpoint the exact location. A cricket chirp can be located easily, as can a dripping faucet. Incidentally, a dog cannot locate the position of a sound as accurately as a human, and, a cat is worse than a dog at sound locating.
So now you are aware of some of the major radio astronomy methods, and if you want to pursue this as an addition to your regular hamming activities, it would be an interesting addition to your experiences on the ham bands. With the advent of computer controlled az-el antennas and receivers like the Icom R-7000 that covers 25-2000 MHz, with CAT capabilities, things could get very interesting particularly if we use the extensive linking capabilities for data exchange that we already have. By adding a main frame for compiling the collected data, the distinct possibilities of an interesting discovery increase.
We have the capabilities and most of the hardware and anything not readily available could be homebrewed. With satellite packet bbs in operation, data can be exchanged worldwide, with some delay. CompuServe is an expensive alternate, but we should be able to use our own systems for this.
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So now that the ideas are running around in your head, it is time for brainstorms and coming up with systems that will allow us to chase the ultimate DX. Share your ideas with others. This is a project for international cooperation.
For more information on radio astronomy write;
Society of Amateur Radio Astronomers, Jeff Lichtman, 37 Crater Lake Dr., Coram, N.Y. 11727
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 16th March 2024