A BASIC PRIMER ON SETTING UP AN AMATEUR RADIO TELESCOPE Cliff Bates KC7PPM crcwnet.com 1/17/98
This is a basic primer about getting started in Amateur Radio Astronomy (RA), and is mostly based on the school of hard knocks and mistakes I've made over the last 4 years since I've been interested in RA. Hopefully, after reading this, those of you who are up and coming will not repeat my mistakes and thus save yourselves not only a lot of frustration, but considerable money. Many SARA members who are much more knowledgeable than I have published papers on how to observe, record, document and understand what shows up on your chart. This primer is about the basic eqipment you'll need, how to set up your system, and finally how to minimize the dirt on your window to the universe. From an email question having to do with some Ham stuff I met Dick Flagg, (AH6MN), whom come to find out, was also an SARA member who lived in Hawaii. Dick had acquired RA experience while working as a electronic engineer at the University of Florida, and currently is a engineering consulant. Over the years and hundreds of emails Dick has inadvertently become my mentor as my interest wondered more and more toward RA and "The Ultimate DXing". He has had the patience of Jobe, and then some. Though Dick and I have never met face to face, whatever you may find of use in this primer is from Dick's patience,knowledge, and editing. A Little History As luck would have it, just before I got interested in RA I bought a house where they have covanants against antenna's. Mistake #1. Still, I thought if I bought a 10 foot dish and designed it to lay down when not in use, then planted 6 foot arborvitaes around the area who would care. Mistake #2. So I bought a 10' TVRO dish, designed kind of a neat setup to allow the antenna to be raised and lowered down to 2 feet of the ground and spent a week planting a forest of $800 dollars worth of arborvitaes to hide everything when viewed at ground level. That done I started to dig the hole to mount the dish in. I had about 3 shovelfuls of dirt out of the ground when the convanence committee showed up and explained that this was a no-no and I was going to get sued if I didn't knock it off. Several days later I ran into a friend who has an apple orchard and he mentioned that somebody had stole his irrigation pump and he needed to put up a structure to house the pump in. I explained my problem and that I would build him a small building if I could locate my RA stuff in it. So we ended up with a nice 8' by 10' flat roof structure. This is as large a building as we could build without obtaining a building permit. The roof was designed to be removed if it ever became necessary to pull the well pump. I then mounted the 10' dish and a Yaesu AZ-EL rotor up on the roof and decided to try to do RA using the ICOM 7100 scanner audio output into a voltage chart. It worked, but not very well. I could see really big sources, but little else. Dick suggested that if I was really going to do this seriously I needed to put a total power receiver, (TPR), at the receiving end, instead of a scanner. A scanner was too noisy and didn't have the needed sensitivity. As luck would have it Radio Astronomy Sales had just come out with a TPR setup for receiving 1420 Mhz hydrogen line radio emissions. A Total power receiver is defined as, "any receiver which measures the total noise power from the antenna and from the receiver." This explanation when I got started was about as clear as mud. How about instead we say, "a receiver that generates a DC output voltage proportional to the RF input power". RF power is often expressed as equivalent antenna temperature - that is the "temperature of a resistor" would have across the input terminals of the receiver that would generate the same noise level as the antenna is seeing. It is important to remember that the TPR is looking at what the antenna is seeing, and any other noise generating components including the preamp due to its noise figure, lossy coax and connectors. A TPR is simply a radio thermometer, tuned to the thermal radio emissions in a particular wavelength. But when properly setup it is a very sensitive instrument. Anyway, I bought a TPR and I was having a great time seeing all kinds of interesting things on my chart. The only problem was the results of one observation were not reproducible two days in a row! Shortly thereafter in 1995 I went to the SARA Conference at Green Bank. If any of you can possibly attend I would highly recommend it! Especially if your just getting started. For me this established the baseline of what RA was all about. Not only did I get to play with a real 40' RT, but I found that it's results and my results were considerably different, and not just because of the size of the dish. But because of what I was "over looking". My window to the universe was very dirty. We have to keep in mind how weak some of the signals we're trying to receive really are. The sun light falling on a just one square meter surface at the equator generates about 1 horsepower. The radio energy of all the stars falling on all the radio telescopes in the world has about the same energy as a fly taking off. A basic unit used in RA for measuring this radio frequency power is known as the Jansky. One Jansky is defined as,"10 to the minus 26th power, watts per square meter per hertz." Or another way, 0.000000000000000000000000001 watts per square meter per hertz. A fairly large sized astronomical radio source would be 200 Jansky at 1420 Mhz, most are much, much less than that. The name of the game in RA is WATTS/SQUARE METER/ HERTZ. The more square meters of aperature, or area of the antenna, and the wider the receivers bandwidth, the more watts. So, unless your system is very clean of noise, power deviations, temperature changes and signal losses, much of what you'll be seeing on your chart will be from your own equipment. This was what I was overlooking. You'll be able to see the Sun, and other large sources, but the rest will be lost in the noise of your system, or variations generated by your system. This can be extremely frustrating to say the least, and I've been there. In fact I'm there most of the time for one reason or another. It's also "The Challenge" of doing RA. Which Frequency Most of my experience in RA has been in the 1420 Mhz range, though I play quite a bit in the 440 Mhz Ham bands as a Ham Operator. The needed equipment for RA at frequencies below 450 Mhz is usually considerably less expensive and easier to obtain, adjust, and repair than an equal system at 1420 Mhz. Test equipment is also considerably easier on the budget at 400 Mhz. At 1420 Mhz and up, cost, (already out of sight), follows frequency! However, the lower the frequency, the higher the cosmic background noise level. 1420 Mhz is in the pocket of the Water Hole where things are about as quiet, as far as cosmic back ground noise, as they are going to get. Antenna gain is a function of capturing as many wavelengths of a particular frequency as possible. If my 10' dish has a antenna gain of 30 db at 1420 Mhz and has a antenna receiving horn 5 1/2" in diameter, at 400 Mhz to get the same antenna gain I would need a dish 33' in diameter and a antenna horn 20" in diameter! So for two systems of the same gain, what is saved at 400 Mhz in the receiving equipment is lost in the antenna system, and visa-versa at 1420 Mhz. Everything being roughly equal between the two systems as far as total cost per db, 1420 Mhz has a considerably quieter noise level and a smaller sized antenna system. Consequently I prefer 1420 Mhz. Antenna Tracking The 40' dish antenna at Green Bank is fixed in azimuth at 180 deg. and is only moveable in the elevation mode. There is nothing wrong with this type of design, except you have to wait until the object passes through the antenna's beamwidth by the rotation of the earth before it is observable. If this window of opportunity is missed and there is a need to try again, then a AZ-EL rotor is nice to have to follow the target object. They are also expensive, and unless you design your own system, do not track that well. A 10' dish is about as small as you can go and get good results at 1420 Mhz without undue dish blockage, or shadowing by the antenna horn of the dish surface. However for a rotor a 10' dish is one heck of a sail to hold in place. Even with counterweights it is much more than a Yaseu rotor is designed to handle in both weight and wind area. The rotor lasted a year before the wind stripped the gears in the rotor one day. I then installed a Emotator rotor which was about twice as expensive. This is a fairly husky rotor and has lasted 2 years now without a problem. However, it does not track less than 3 degrees in the auto tracking mode. So if you have visions of tracking and object and getting a long steady run on it like the big boys, forget it. You end up with a sawtooth chart as the object drifts partly through your 5 deg. beam, then the rotor advances another 3 degrees to offset the earths rotation. The big boys RT's are usually run in a tracking mode when studying a particular source. But they have a little better system that slowly and continously drives their dish offsetting the earth's rotation. This capability is nice to be able to do, but requires variable speed motors and very accurate positioning equipment of where the dish is in relation to azimuth and elevation. Plus it needs a computer program able to drive the dish accurately. Not impossible, but very expensive and mostly a do it yourself thing better left until you get everything else working at its best. The Emotator rotor is designed to work to 1.5 deg., but here the problem is one of momentum. With a hundred and ten pounds of counter weight, 45 lbs. of antenna, and 15 lbs. of support structure, horn and preamp, when the rotor stops, it stops! It doesn't slow down and gently stop. So with this sudden stoppage the antenna's torque momentum causes the setup to twist slightly and then snap back. This in turn causes the rotor to think its not pointed where it should be and energize again to correct. In short, the whole setup starts hunting back and forth and shaking the building down. Therefore the 3 deg. minimum dead band. Most of the available rotor systems are designed for operation using Yagi antenna's, with say 17 degrees of beamwidth for satellite tracking and such. A 10' dish at 1420 Mhz properly adjusted is looking at 5 deg. of beamwidth. The only way my setup will work on tracking is to manually run the dish around until the object of interest is just outside the beamwidth, let it drift through, then step ahead of it and repeat the observation. I cheated a bit in the tracking mode by mounting a small TV camera on the back of the horn in a weather tight PVC pipe container. The camera is very small, about 1 1/2" square, and will see down to .5 lux. The light from a full moon will overpower it, and the sun will burn out the chip during sun shots if it is not protected with a arc welding filter I have cut out to fit in front of the lens. Still it will see the brighter stars and planets and gives a view of the night sky for some idea of what the antenna is seeing. It also helps when you get that big jump on the chart and you look over at the TV monitor and see those flashing lights of a 747 at 33 thousand going through the beam. With a bigger lens, or connected to a telephoto lens you could see quite a bit more. However, don't forget to consider the extra weight and leverage such a lens will bring with it hanging out there beyond the focal point, as well as the extra stress added to your elevation system. If your needs are limited by expenses, (and I don't know anyone's who isn't), go for a fixed azimuth and a manual or powered elevation system. Put the rest of the rotor expense funds into the BIGGEST dish you can afford. Preferably sized so that it doesn't allow any sunlight to fall anywhere on your property! Antenna Dishes Where can you get a large dish? Of course you can buy them, 12' dishes are common enough. But if you have the property and the experience to handle moving 12' to 16' dishes, first look around where you live. Cable TV services, newspapers, big businesses, hotels, telephone companies. Many are getting rid of the big dishes and going to the smaller ones. Inquire if they are going to update that paint faded thing on the roof to a newer lower profile model of an up and coming company. If they say they are, tell them you belong to SARA, what SARA is, that you could make good use of it, and will remove it for nothing. Don't expect them to help you in anyway to do it! They will pay a scrap dealer to do it rather than send their employees out to help you do it. Also the scrapper is interested in the metal, not the dishes condition when it's down. You on the other hand are interested in a first rate dish surface, and some of these dishes are extremely well made, good for use up to 6 Ghz. With that kind of surface it has to be taken apart carefully, any bending or dents are bad news. Find out what is to be removed and what isn't. Then take everything that is supposed to be removed, don't leave the junk you don't want. They did you a favor, you return it. Most of these dishes are of the fixed type, extremely well built and cost thousands new. Once you've got your prize dish at home it'll be up to you to next come up with a way to move it either in the AZ and/or EL modes. Let alone a mount to put it on that will stand up to the considerable forces of Mother Nature trying to topple it. When mounting the dish, never, never let it rest on one edge as it will deform it. Use enough people to control it into the mount, but not so many as their falling all over each other. Dishes are made light, just enough to handle winds to say 45 MPH. Beyond that their life expectancy goes down as wind speed increases. TVRO dishes will fold up like an umbrella in a high wind. And the same with snow or ice buildup. Also if the snow or ice doesn't fold it up the extra weight may spring it out of shape. Consequently if it looks like snow is coming I lower the dish and put a infrared heat lamp at the horn and shine it onto the dish. This seems to handle the problem. Feed Antennas The purpose of the feed antenna is to collect energy reflected from the surface of the dish. Ideally the feed antenna beam pattern is designed to see just below the edge of the dish, or 10 DB down. If the feed antenna see's beyond the edge of the dish it will see behind the dish and/or off to the side, called spillover or high sidelobes. We only want to see what is in front of the dish of course, so a little of the dish collecting area is sacrificed to assure that what is showing on the chart recorder is from the sky and not from the ground. An antenna feed horn at the focal point is the only way to go at 1420 Mhz's and higher. A TVRO dish usually has a F/D ratio, (Focal Length to Dish Diameter), of 0.35 to 0.4. In this F/D area a cylindrical waveguide makes a reasonably effective feed antenna. Actually I make my own feed horns out of coffee cans. At area's around 1296 to 1420 Mhz these coffee can horns work pretty well. When I come up with a good design with the right length and probe location, I take it to the metal smith and have one made up in copper. If it works really good I'll silver plate it inside with a silver paste plating compound. I seriously doubt that plating improves anything by very much, but after all we're after every bit of signal we can get. Also silver oxide is conductive, copper oxide is not. See the yearly ARRL Radio Handbook on antenna horns for design formulas. Aligning the feed horn is critical. First the horn has to be centered in the dish. This can be done by using the sun's shadow or, more accurately, by drilling a hole in the very center of the dish just the right size of a laser pointer. The feed horn mouth is then covered with white paper and the laser pointer put through the hole. The feed horn supports are then adjusted to center the laser beam in the center of the horns mouth. Fine tuning distance from the dish surface to the feed horns mouth is unfortunately by trial and error. One way is to again use the sun as a noise source. The feed horn is first adjusted to the focal point of the dish and then the antenna is centered on the sun. A reading is taken, which can be either voltage off the receiver S meter, speaker output, or your RT's TPR DC output. It is then quickly moved a 1/4" either in or out and another read is taken, and so on, until the highest level of output is found. This is best done around high noon on a cloudless day, as the suns radiowaves are attenuated by the atmosphere and clouds at 1420 Mhz as it moves toward the horizon and the atmosphere is thicker and more water vapor is encountered. By the way, while taking the readings don't shadow the dish with your body or any other object or the readings will be affected. Though simple, the cylindrical feed horn is not perfect in its beam pattern, as it tends to create sidelobes. To help control the sidelobes of the feed horn it is sometimes recommended that a feed horn collar be applied. This is a round metal disk, twice the diameter of the wavelength which is being received, with a hole cut in its center the same diameter as the feed horn. The collar is slid over the mouth of the feed horn and assists in keeping the sidelobes confined to the dish. Keep in mind that at 1420 Mhz, a 10' dish with a feed horn collar 16" in diameter can cause sever problems of over shadowing the dish surface and actually create worse side lobe problems. That is a flaring of the antenna's beamwidth to the side like a flower petal, instead of the ideal search light beam pattern for which we are always striving. Probably a 12' dish is about as small as you can get with a feed horn collar at 1420 Mhz, and that is kind of iffy. At higher frequencies and/or bigger dishes, collars work well. Coaxial Cable and Connectors Only use the best coax cable you can afford, #9913 or better. Semi or hardline is even better, but very expensive. RG-58 should not be used as it is extremely lossy at this frequency. Don't even think of using Radio Shack cable. Kinks or dents in the cable should be cut out. These will change the cables impedance and kill your signal. Good enough doesn't apply in this field, we're after perfection. Radio waves beyond a 1000 Mhz turns into strange stuff the higher up you go. #9913 coax cable at HF radio frequencies of say 28 Mhz looks like pure water to the radio waves shining through it. The same cable at 1420 Mhz appears more like muddy water when viewed by the radio waves trying to shine through it. Consequently some SARA members add a down converter at the antenna to lower the frequency, and thus the losses in the cable to the receiver on long cable runs. Down converting becomes cost effective around 2200 Mhz as coaxial cable becomes less and less effective and waveguides come into the picture at big time cost. Connectors for cables should be of the "N" type, again the best you can afford. Buy these from a good electronic supplier who also sells good cable. The cost of good connectors will help you minimize their use, as each one causes a loss of signal. Avoid 90 deg. connectors. Only very few of these are worthwhile and consequently are very expensive. 90 deg. connectors are a special breed of cat. Mistake #3. One of the worst problems I've had was caused by a cheap 90 deg. connector that was bad. Finally I spent $500 for a wattmeter and a 1420 Mhz slug for it to finally find the problem. The connector looked great outside, but inside it was trash when viewed by the signal at 1420 Mhz. Follow the directions to the letter on putting "N" type connectors on the cable or the impedance will be changed and the signal degraded. If you use #9913 cable, get connectors for that cable size. Don't use RG-8 connectors on #9913 cable as they are slightly larger and don't hold onto #9913 cable well. Besides that the connector pin doesn't fit on the #9913 cable center conductor. Seal all the connectors with black silicone that comes in the form of a tape roll. This is one of the few things Radio Shack makes we can use. Mistake #4. I thought this really wasn't necessary as I live in a low humidity, low rain area. I paid dearly for it. I ended up not only replacing the connectors, but moisture had gotten into the cable as well. Once moisture gets in the cable it degrades it and a new cable run is called for. Taping the connectors is much cheaper. Preamps Mount the preamp as close to the horn as possible without further blocking or shadowing the dish. At 1420 Mhz and higher inches of coaxial cable make a difference. Until the signal is amplified from the horn by the preamp, every connector and inch of cable we put between the horn and preamp is creating a loss of signal. Down East Microwave makes a fairly good receive only preamp for 1420 Mhz. It is a 2 stage preamp that is small, light weight and has a reasonable noise factor. Radio Astronomy Supply has come out with a 2 stage 1420 Mhz preamp that is supposed to be the best on the market with the lowest noise factor around. I have both kinds, but I personally think the RAS one is slightly better than the DEM one. Both are relatively broad banded and can be used for other things besides RA, such as SETI. However keep in mind the bandwidth gain of each is peaked at, and the noise factor is the lowest at 1420 Mhz. In short, both will work over a pretty broad range, but less so and noisier than at 1420 Mhz. Mistake #5. It is also a good idea to use the old black silicone tape again on the preamp as well. Both preamps come in a weather resistant case. Notice the word "resistant", not "weather proof." Taping the seam, screw holes, and power feed thru will keep the moisture out of the box. The stuff inside is extremely sensitive to moisture and if it gets in you will find you have your own universe in a box, as it turns into a noise source and there is no reclaiming it. Again I found this out the hard way. While you are at it with the black silicone tape, cut two holes in the sides of a Zip Lock Bag for your coax cables to go thru. Put this over the preamp and run the cable in and screw them on the preamp. Next seal the coax where it goes through the bag with black silicone and leave the open end of the bag down and open for things inside to breathe. This rain shield lasts for quite a long time and yet is easy to work with. Mistake #6. I cooked one of my preamps last summer on a hot day doing sun shots. It seems that a dish "really" does focus infrared as well as microwave energy, even with holes in it! Shading the preamp will also keep the preamps noise level down, besides protecting it from a heat stroke during hot sunny days. First Receiver When it comes to the type receivers you will need, I would suggest first you get a very good quality scanner such as ICOM or other top brand. The reason is you need a test source for many things. Testing of the antenna horn, coaxial cables, preamp, and such. A scanner produces a sound, a TPR produces a voltage reading on a chart. It is very difficult to determine from a chart recorder what it is you are receiving. However your ear can quickly determine whether that pulsar signal your seeing on your chart is coming from "out there" or is a radar, satellite, pager system, or a high voltage line insulator flashing over, to name just a few. There are a few other good uses for a scanner. I use radars in the 1200 to 1350 Mhz band to determine attenuation in the atmosphere caused by inversions, or water vapor. Most FAA air traffic control radars run at a fixed power level and it is amazing how atmospheric conditions can affect their signals. If the atmosphere is affecting these radars, they are affecting signals coming from "out there" as well. Especially the lower the source is to the horizon and the more atmosphere it has to shine through. If you have any doubts about these affects having an effect on your observations, turn your RT toward the sun and watch what happens when a cloud goes through your beam. The scanner can also be used to tell whether a large source is coming toward earth or receding by scanning at 15 second intervals and 50 Khz steps from 1400 to 1440 Mhz in the "AM" mode and recording the output, (S-meter, or audio output), to a voltage chart to show doppler shift. A word of caution! Receiver stability is a must here. So allow the scanner temperature to stablize and then keep it out of drafts. These doppler shifts are very slight. If the equipment isn't stable what you see will be variations of your equipment due to temperature changes. A minimum size dish to detect doppler shift is 10'. Finally if you are into SETI you can use your scanner for that as well. I don't want to discourage anyone in pursuing this field as I think it is an interesting challenge, and very worth while effort. I have read several articles on how to use ICOM's 25 to 2000 Mhz scanners to automatically scan the heavens and record any ET events as we sleep. Receiving A Possible "WOW" Signal Let me say that if you have a 10' dish as I have, a very good preamp, good cable, and a Yaseu 736R transceiver, which is considerably quieter than my ICOM 7100 scanner. And in France you have a Ham running 600 watts at 1296 Mhz into a 25' dish, bouncing his morse code signal off the moon and back to earth, EME, (Earth-Moon-Earth), I can very faintly hear him and then not get a very accurate copy of what he is sending. One trillionth, (that's correct with a "T"), of the power he sends out at his antenna will fall back to me! Besides that, it took me a year of polishing my system to hear him. This is the same system I use for RA except for the receiver. I therefore have real doubts when the next "WOW" signal is heard that it is going to break the squelch on any of the hundreds of ICOM scanners listening during the night and record the event!! But using a good FFT program such as Mike Cooks FFTDSP 42, the same Ham in France left a nice track of his signal down the computer screen. So there is hope. And the FFTDSP42 program does automatically save it's screens while everyone is asleep. However the squelch of the receiver has to be off for it to see anything. I think a better way would be to have one computer step the scanner though the interested frequencies using a computer/scanner interface. A stereo recorder continuously monitoring what comes out of the sound card in one channel, as well as recording the frequency voice chip as the scanner is changed through the frequency range, all the while recording through a cheaper shortwave receiver WWV at 10 Mhz in the other channel for a time reference. Then if something unusual shows up on another computer running and continuously recording the FFT program during the night, the time of the event could be noted and the stereo recorder advanced to the time hacks of WWV and the sounds of the event and at what frequency could be studied. The Bambi Group, (they have a Web Page under SARA Links), is apparently using this approach. Unfortunately I have been unsuccessful in getting their freeware software to run. However it looks very promising and a lot of work has gone into it. Checking Out The System Back to the RA stuff. Using Ham EME morse code signals as a test source can be an excellent way of testing out your RT system. If you can hear these kinds of signals you have a VERY GOOD system. A change of the antenna horn may or may not be needed. It will depend on the bandwidth of your 1420 Mhz horn. A properly designed horn will have maximum signal strength at 1420 Mhz and will drop off to either side in a curve. Very much like a bell curve. However a 1296 Mhz horn is easy to make out of the trustly coffee can. Simply take two 36 ounce cans and solder the open ends together. This will take a large soldering gun. This produces a horn that is to long for the frequency, so we then cut the can very carefully with a very fine blade saw to a length of 10 1/8". Be sure the cut is square to the bottom of the can. Don't use tin snips as they distort the metal too much. The probe is made out of #14 solid wire 2.128" long soldered to a female "N" type chassis connector. From the bottom end of the can measure out 2 3/4" and lightly center punch the mark. Drill a hole 3/8" in diameter and place the chassis connector, probe end first, in the hole. Drill out the 4 little screw mounting holes. I use very small sheet metal screws. Keep screw length to a minumum inside the can. The wire probe picks up the signal, not the sheet metal screws! Align and adjust the horn as recommended before and your in business. The best time for listening is during a EME contest. When these are going to occur is published in the Ham magazines and there are several Ham EME Web Pages. What Total Power Receivers Can Do To really do RA with any seriousness a TPR is required. The only commerically available one I know of for the amateur radio astronomer is through Radio Astronomy Sales. I purchased mine several years ago and I have returned it twice for updates in the circuitry. It has worked extremely well, is highly sensitive, and is "on" continuously for temperature stability. The gain is adjustable between 5 and 50, as is the integration time from .01 to 10 seconds. It also comes with an offset adjustment to adjust the receiver for various conditions. To give those of you who may not have used one before an idea of how sensitive these units are I'll give a couple of examples. I was making an adjustment on the rotor one night and had the dish in the down position. It had snowed the night before and down inside the shack I could hear the chart recorder pen slide suddenly zipping back and forth. I stopped what I was doing thinking I'd shorted something. The chart pen continued zipping back and forth. I looked out to where the dish was pointed toward a mountain side 5 miles away and I could see a small red light of a snow mobile heading up the mountain. I thought, "No way!", and climbed down and went inside. You could see the ignition trace as the driver got on and off the gas. Still disbelieving, I stood and watched until the red light went out at the ski resort. The pen settled down immediately. Unfortunately the unit has the same attraction to piston powered airplanes, relay contacts, or any spark no matter how small, such as produced in a mercury switch in a thermostat. In the summer to do a sun shot at high noon I have to put 6 db of attenuation in the antenna feed line and set the gain to its lowest setting which is 5. At that setting the sun will drive the unit to 10 Vdc. The units voltage output limit is 11.2 Vdc. Normally while doing RA I run the unit at its highest setting which is 50 with no attenuation. At a gain setting of 20 it has no problem detecting the moon. A full moon will drive my 6" chart recorder pen, when set on a 0 to 2 volt scale, 2" across the chart at a gain setting of 50. Not to shabby for amateur equipment and a 10' dish. I noticed that Radio Astronomy Supply has a new version of the TPR out now that has several built in features that it didn't have before. And appears more convenient to not only operate, but set up and test. If this unit is anywhere as sensitive as my older model, and I don't see why it shouldn't be, it'll be like getting your drivers license and taking a tour of the neighborhood in a top fuel dragster compared to using a scanner! I have seen some plans now and then for building your own TPR, but most I don't believe will come close to one of the RAS units. I'm not selling RAS stuff, I have no financial interest in it. To me it is just a fact. Some of you may want to try your hand at designing and building your own. I say have at it. That is how things are improved, and SARA members could use the competition. Recorders Now we need a recording device to keep a record of the observation. Chart recorders are nice things to have and a fairly good one will be expensive. It should be able to record over various voltage ranges from 200 millivolts to 12 volts, as the TPR output is varied over different voltage outputs depending on the gain and what you are observing. If possible it should have two recording pens. I've found that having two pens is worth while. An immediate backup pen is available if one starts to go dry, or as a second channel for tracking outside or equipment temperature. This ability at times is very helpful when comparing unusual recordings to temperature changes. I also use mine for adding time hacks on the hour. This saves a lot of effort in going over a long chart figuring out what time it was when that interesting observation happened, verses counting off and marking inches or centimeters of chart speed per hour. Chart paper is permanent, can be photocopied easily, and is not subject to erasing the wrong computer file accidentally. On the downside chart recording paper is a little costly. My 6" recorder uses folding paper charts that run about $5.00 a chart. Though a chart lasts quite a while if run at a slow chart speed of say 6 cm. per hr. I often rerun a chart using a different color pen sometimes if the observation isn't critcal or I'm testing something, to save paper. How long a chart pen lasts depends on how fine a scale you run it on. In other words if it is drawing a straight line it seems to last forever. If it is set to a sensitive scale showing each and every deviation of the signal, resulting in a wide tracking path, it will exhaust itself much sooner. Probably within the length of the chart. However I haven't found much use for a chart looking like an electrocardiogram. They may look impressive to show, but as far as what the overall trend of the source is it is worthless in my opinion. I prefer the pen to be set at a sensitivity level that it responds mostly to the source, not background static. Finally I see very few used chart recorders for sale unfortunately. But this maybe also be an indication that in the digital world, the old pen chart recorder still has its place. If you do find a good used one, be sure you can still get chart paper for it. I've found that just about every brand of recorder uses a different size paper, or has a different size chart feed system. Very few have interchangeable chart paper with other brands. An alternative is to use a digital software program through a computer interface and your detectors output to view and/or record on your computer the observations. This works pretty well and is considerably less expensive than a chart recorder. It has the potential benefit of also being shipped around as a text file on email to others, besides being able to be used for data processing directly for sky charts and other processes you might want to do. Radio Shack makes a digital multimeter, (Cat. No. 22- 168A), that I have used that comes with the computer interface built into the multimeter and the software for your PC. It comes with several recording programs and has a print capability of your observations. I does a pretty fair job of recording, especially for the price and the fact that your getting a multimeter in the deal as well. On the downside of digital recording, if the computer is in use recording it is not available for other things. If you live where electrical power is very expensive, leaving your machine on all night or for several nights to record may not be appealing. The data files generated can and do become fairly large. In an 8 hour run, recording output reads of the mulitmeter every 5 seconds I generate a text file in the neighborhood of 250 K. In a few nights of running you have a million bytes, and so on, until storage becomes a bit of a problem. Running the recording at 1 second intervals of course increases the output size of the files 5 times. But there are times when you will want greater detail in the recording for closer examination. Finally is the battery problem. The 9 V battery in the multimeter lasts about 4 days of 8 hour recordings. This expense somewhat offsets the chart paper expenses. A battery eliminator is a consideration here. However I have not tried it as the eliminators output voltage is not polished and will follow the power line variations. This in turn may produce outside influences affecting the multimeter reads, and for the moment I have enough problems with those from other causes.
System Stablity My problem right now, and since I've gotten seriously interested in RA, is not how to make my system even more sensitive but continually battling the forces that block the use of the sensitivity I already have. Heat, voltage deviations, noise, and interference! The view through a telescope is only as clear as the glass your looking though. Temperature Stability One of the big problems I've been battling is temperature stability. When I first got my TPR I had it setting out and it didn't take long to notice that as the room temperature cycled a few degrees from the heater going on and off, so did the pen on my chart recorder. And I don't mean slightly. A 10 deg. change in TPR temperature will change the output up to 8 volts! Not a good thing when the TPR is set at it most sensitive setting and the chart recorder is on a 5 volt scale. So I put the entire setup in a large Coleman Picnic Cooler, drilled a hole large enough to get the antenna feed, output and power wires in. I also drilled another 1/4" hole opposite the TPR's offset adjustment and extended a wooden dowel out of the cooler attached to offset knob to adjust it. Just popping the lid for 10 seconds to make a quick offset adjustment took the unit a minimum of 10 minutes to recover from it's lost temperature. This helped considerably with the stability problem. The temperature in the cooler averages about 85 degrees F from the total power receivers own internal heat. As long as the temperature inside the cooler is higher than the room temperature things are fairly stable. However I began to worry about the heat shortening the life of the equipment. I had a small Sears refrigerator in the shack to keep me supplied with pop and such as there is no drinking water. I decided to put the TPR in the refrigerator where it would be cooled and have a fairly stable environment. So I drilled a hole in the side of the refrigerator, being careful not to drill through a cooling coil. (Wouldn't recommend doing this to the wife's refrigerator.) All in all it didn't work out that great in my opinion. I noticed that I ended up with a very slow sine wave on the chart output as the temperature inside the refrigerator changed as it cycled on and off about every hour and a half. I have since moved the TPR back to the picnic cooler where it seems to be more stable, and the heat apparently is low enough that it hasn't shortened the life of anything. Along this line it has been suggested to me that putting dry ice in the cooler and cooling the TPR down to approximately -120 deg. F. might handle the problem at a reasonable cost. I haven't tried this yet as I don't want to rush into it and create Mistake #7, and some really expensive repairs. Cooling the TPR down to -120 deg. F will probably put quite a bit of strain on the circuit boards and components. I also don't see or hear of any amateurs doing any super cooling, so this again sends up another flag of caution to me. I think I'll wait on this one until the next SARA conference and talk to some of the Green Bank engineers about it first. However for those of you who prefer to go where no amateurs have gone before, dry ice is reasonable to purchase and can be had at some of the larger super markets if you ask the manager. Voltage Stability The next source of problems in my system is getting an accurate voltage reference. As the total power receivers temperature changes, so does the electrical components inside vary in their abiltiy to conduct electricity. The same holds true if the power supplied to the TPR is varied. It will see a change in reference as to where it was set and will duly record it on your chart as a bump, a dip, or a slow sine wave. You of course not being plugged into the wall outlet think everything is cool and the TPR is seeing something "out there" and congraduate yourself on how sensitive your equipment is. Not to be outdone, the preamp is out there by the horn subjected to much greater temperature deviations that are also affecting its components in the same manner that those in the TPR are experiencing, but on a lesser scale because it isn't as sensitive. Added into these variations are any preamp supply voltage changes, which instantly affects the preamps output level. All these little variations are then fed to the TPR, (which from our earlier definition is looking for any outside, as well as internal changes), they are greatly amplified, and then deposited on your chart as garbage! Yes, the world of RA is a cruel, cruel world! The only way to handle this problem is to obtain the best regulating power supply you can afford or make, to polish out the dips and surges in the supply line voltage. The extra expense is well worth the effort in your systems stability and consequently in the reliability of your observations. My system requires 15 Vdc for the preamp and 24 Vdc for the TPR. The wattage requirements of the power supplies to supply the preamp and TPR is relatively low and this helps considerably in lowering cost. The temptation to add other loads needing these voltages to these power supplies is tempting, but these two power supplies should only supply the preamp and receiver. Other loads are subjected to the same outside and internal influences as the preamp and receiver, and consequently will change the stable voltage the preamp and receiver require and show up on the chart. Finally I would recommend one last piece of test equipment, and that is a "good" 50 ohm resistive load rated for 1420 Mhz. Installed in a thermos bottle to protect it from the winds and temperature changes and connected to the cable connector that would attach to the antenna horn probe, it will quickly show any nasty influences in the system below the antenna. Once you have a stable, reliable system the sky has no limit. It is a good hobby, challenging, and never ending. Take it from one who gets bored easily, and has yet to with this one. Cliff Bates KC7PPM crcwnet.com 1/17/98