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Editor's Note: The following message was posted to the SETI Email Discussion List on 22 August 1997. It documents one member's thoughts and efforts, and is reproduced here to help others grapple with the many decisions involved in building a working SETI station. As Dick's station begins to take shape, we hope to update this page.
Greetings everyone,
I am finally starting work on my Radio Telescope after 2 years of procrastination. So far the only progress made is a section of Rohn 25G in about 2 square yards of concrete for my 10' TVRO dish. Perhaps my procrastination is going to have some benefits. Over the last 2 years, I've read every book & article relating to radio Astronomy and SETI that I could find. I've joined the SETI League and the Society of Amateur Radio Astronomers (SARA). In 1996, I made the trek to the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia with the SARA group and actually met Grote Reber (the father of Radio Astronomy). I also got to "play" with a 40 foot Radio Telescope and saw the 150 foot scope demonstrated by visiting astronomers. Yes _ with a 150 foot scope, the "bump" of the Hydrogen line is very apparent on a spectrum analyzer. I also met Paul Shuch for the first time and heard his presentation on SETI. I was impressed enough with Paul to ask him to speak at a Banquet & Seminar given by our local Amateur Radio Club, The Keystone VHF Club, in York, Pennsylvania. Paul graciously accepted our invitation, and stimulated the imaginations of over 50 members of our club_ that's just about the most people I've ever been able to get together for any event from our organization.
Since then, I've scoured the Internet for information and have been a subscriber to this reflector for the last 6 months. I've been exposed to many points of view and have found the multitude of opinions & ideas enlightening. I finally feel that I am armed with enough knowledge to "be dangerous".
My idea for a Radio Telescope is to use as many "off the shelf" components as possible. Build where necessary, but don't re-invent. The basis for my scope' is an existing 10 foot TVRO dish that I've owned for the last 11 years (we finally got the cable).
The Dish - With a little help from a friend with a welder, I am modifying the existing mount so that the positioner arm drives the dish in elevation (rather then along the Clark belt). Basically, this mod entails rotating the mount about 90 degrees and welding a metal plate on the back. Three heavy duty U-bolts will be placed in the metal plate to hold the mount to a thick walled 2 ½" steel pipe. This pipe will extend 10 feet into the Rohn 25 with only a 18 inches or so coming out of the top of the tower section. There will be two thrust bearings in the tower section, one at the top where the pipe comes out, and one near the bottom. While this may not seem heavy duty enough for some of you, the pipe only extends from the tower a short distance, it should be able to take quite a bit of bending torque without being damaged. The whole works is also in a location fairly well protected from high winds. Rotating this pipe will move the dish in azimuth.
For the drive, I have procured a heavy duty electric motor with an integral gearbox. The motor runs on a DC from about 3V to 9V. At it's highest speed, it turns about 1 RPM. I was intending to couple this to the pipe through a heavy duty 11 to 1 ratio gear box and chain/sprocket. I currently have all of these materials but am rethinking this process. I am wondering if perhaps a propeller pitch motor coupled directly to the bottom of the pipe might not be simpler and more robust. With this two axis drive system in place, I can move the dish to any point in the sky desired. Since my positioner arm for elevation has a sensing potentiometer in it (not a reed switch), I can get elevation read out directly in the shack. I will have to mount & couple a potentiometer to the pipe to get position back for azimuth. I also have a TAPR Track box and the Kansas City Tracker that I hope to integrate with this system for automatic pointing. I feel that the dish mounting is going to be the hardest part of the whole project. While a simple stationary Drift Scan mount would suffice, I like the idea of being able to look at one spot in the sky for an extended period of time when desired.
The Feedhorn - Jeffrey Lichtman of Radio Astronomy Supplies currently sells a linear polarized 1.420 Ghz feedhorn with a choke ring. It also includes an N-fitting for output. I should be able to modify the existing C-band LNB mount to support this with little effort. The price for SETI League members is $150 and $160 for others. You can check out Jeffrey's home page at http://www.nitehawk.com/rasmit/ras.html.
The LNA - We have a couple of choices here (probably even more). Jeffrey Lichtman has a two stage LNA with a gain of 28 dB and a noise figure of .35 dB that looks interesting, price is $180. Down East Microwave also has a comparable amp that is a few dollars cheaper. Has anyone had experience with these devices? Opinions? Probably a strip line filter on the input would be desirable but will raise the noise figure. I live "in the boonies" so I hope to get away with no filtering.
The feedline - While probably not necessary, I am going to use 7/8" hardline back to the shack (I already have it). The run will be about 80 feet. I am also going to power the LNA with a separate wire rather then trying to decouple through the hardline.
The Power divider - Upon entering the shack with the feedline, I'm going to try something a little different. Since I want to do some Radio Astronomy as well as SETI, I am going to attempt to drive two separate receive systems. I will be building the power divider myself. It will be frequency selective but should cover everything near the Hydrogen line.
Receiver #1 - For radio Astronomy a wide receive bandwidth is necessary. There is a small company called Science Workshop that makes what is known as "The Poor Man's Spectrum Analyzer". While their analyzer lacks the calibration accuracy of the multi thousand dollar units, it covers from below 100 Mhz all the way through 1.7 Ghz with a sensitivity just about as good as it's expensive HP cousins. You have to supply an inexpensive Oscilloscope (freq response < 100 Khz) for the display. As well as serving as a Spectrum Analyzer, the ramp generator may be switched off and this analyzer will serve as a wide band receiver tunable throughout the same frequency range. It is not super stable in frequency for narrow band signals (eg: 5Khz FM). If I tune in a local 2 meter repeater, it eventually drifts out of the passband. The thing is, it's not meant for high performance narrow band reception, it's a Spectrum Analyzer, and it does that job well! It also will serve quite well in the capacity of a wide band receiver (<250Khz bandwidth). If you take the IF outputs of the Varactor tuners and drive a detector circuit, and then from that, a sensitive DC amplifier, followed by a Strip Chart Recorder (or a computer A/D converter) _. Bingo, an instant Radio Astronomy receiver tunable anywhere between below 100 Mhz through 1.7 Ghz. Want to try and see the bump on the Hydrogen line? You even have a Spectrum Analyzer to make the attempt!
You can also use this device to do Radio Astronomy at C-Band frequencies. Keep your existing C-Band LNB powered up and drive the analyzer with it's output. Since the TVRO satellite 3 Ghz outputs are all downconverted to about 950-1500 Mhz, you will be effectively be looking at the entire satellite C-band spectrum, if you are aimed at a TVRO satellite, you will see data from all of it's transponders. If you are looking away from the Clark Belt, you will be looking at a chunk of the sky with a wide band receiver! This little gem is available from Science Workshop located in Bethpage, New York (516) 796-1693, WEB: http://www.science-workshop.com. The price will amaze you, it kit form, you can build the complete unit for less then $250. I've had mine for over 10 years now and I'm upgrading it for up to 1.7 Ghz operation. Check out Science Workshop's WEB page!
Receiver #2 - For SETI, narrow bandwidth and good frequency stability is necessary. With the availability of computer programs such as Mike Cook's FFTDSP and Dan Fox's SETIFOX that do Fast Fourier Transformations to detect coherent signals under the noise, receiver drift cannot be tolerated. For this reason, I await with baited breath the release of the SETI League's 1420 Mhz receive converter from Down East Microwave. This is crystal controlled and has an advertised noise figure of approximately 1.5 dB. It's IF output is on the 4 Mhz of the 2 Meter Amateur band. Using this converter will allow your existing 2 Meter all mode receiver to provide plenty of selectivity and certainly serve as a good I.F. amp. I was even thinking about using the downconverter as a wide band receiver for Radio Astronomy but with the IF output in the 2 meter band, the potential for interference is great, especially if I am trying to listen to something over 250 Khz wide! Price in kit form should be around $130. Hopefully, I will be able to run both of these receivers at the same time off the outputs of the power divider. With 28 dB of gain from the LNA and 7/8" hardline, I shouldn't need any post amplification. My main concern is RF from either receiver getting into the other.
Signal Processing - I am currently using Mike Cook's software FFTDSP42u for audio processing. I also have a 8 bit A/D converter connected to my PC and am using a package called "Datalog" that I obtained from Radio Astronomy Supplies. This makes very nice strip charts and also creates a text file of the sampled data that is easily imported into a variety of database programs and spreadsheets. I have used Mike's FFTDSP to detect the Mars Global Surveyor during the Mars Relay Flight Test. I also use it to look at the Philadelphia 432 Mhz CW beacon. This beacon is located about 100 miles from my location and , Mike's software clearly shows reflections of this signal off aircraft with the associated Doppler. I've used the Datalog software for a variety of purposes; from logging enhancements on 2 meters & 70 CM to plotting signal strength profiles of high altitude balloons carrying Amateur Television (ATV) from over 400 miles away.
While I have a good bit of the hardware/software necessary for this project, I'm sure that I'll run into some set backs. My first priority is to get the dish re-mounted before the onset of Winter, after that I can put the other stuff together even with a foot of snow on the ground. Some questions that I have are:
73,
Dick Goodman, WA3USG
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