The design and building of a large dish antenna rotor
Text and pictures by Cliff Bates, KC7PPM
Early 1998, Cliff Bates, aka KC7PPM, wrote a paper on his mistakes while getting started in Radio Astronomy (RA). Entitled, "A basic primer on setting up an amateur radio telescope", this article is available on SARA and on The Amateur Scientist websites, and applies to Earth-Moon-Earth activities (EME), as well as to RA.
This time, became more experimented and ready to embark on a larger scale project, Cliff proposed me to publish a new article dealing with his latest big project, the design and building a large dish antenna of... 4.8 meters (16 feet) in diameter to ping off the Moon among other things. Here is the story of this invention.
Those of us who are interested in RA or EME, usually dream of a bigger antenna some time, in our various quests. Usually this occurs as our Radio Telescope, (RT), and/or EME system gets cleaned up from all the system losses, and internally generated noise. Once this is done, (which is no small feat) the system and the operator can take full advantage of the increased sensitivity and capabilities of a larger antenna.
The only unique thing about this rotor design, over any other rotor system that I've run across, is that it uses a slew thrust bearing off a bucket boom truck. These bearings are able to handle almost any dish we can put on them, and they are very compact. Also if the bearings drive worm gear system is also acquired at the same time, it is also very beefy. And again solves the braking system problem on the rotor system, due to the worm gear drive locking up when not rotating. This setup will handle almost all Mother Natures forces, except tornadoes and hurricanes.
Prior to my big dish idea, I had a 10 foot (3 m) satellite dish, and with the guidance of Dick Flagg, (AH6NM), and who after several hundred emails had helped me clean up my system to a point where we were getting some pretty impressive results for my only having a 10 foot dish. When Dick finished with his final suggestion on the system’s tuneup, almost a year later, that same system was getting a return echo off the moon, with a 125 watt signal on 1296 MHz !
Keep in mind a 10 foot (3 m) dish is considered “the absolute minimum” in dish size at 1296 MHz for doing EME, or moon bounce. That it happened at all was due to Dick’s knowledge. I just followed his directions, and my wife Mary, let me chase my dream without complaint.
Earth-Moon-Earth, (EME) is no small feat, as only approximately one trillionth of the output signal power returns to the receiver (you can hear some EME QSO at the end of the third page).
It is also a perfect test bed for testing your RT system. You don’t need to be an amateur radio (ham) to test your system “as long as you don’t transmit”. You can just listen in on other hams doing EME. And believe me when I say, “listening in” is a whole lot cheaper! If your system can hear a moon return, your on your way to meaningful results with your system.
With my system all polished up, the calling for a bigger dish became what my wife once referred to politely as, a hormonal imbalance. I looked at practically every microwave antenna design ever conceived, and came back to the good ole parabolic dish antenna.
Horn antennas appeared to be a real interesting possibility, and produce a very clean antenna pattern. However when I did the math for the horns dimensions at 1450 MHz. and a gain of at least 33 dB, (considered the minimum for EME), it was almost 30 feet (9 m) long!
Elliptical dish antennas appeared to be the best over all design, and had several advantages over the parabolic dish. They were lighter, and not as subject to ground interference as the parabolic dish. With the antenna horn offset out of the beam path, it also did not shadow the dish. However from the aspect of design and structural considerations it is one of the most difficult to make.
I liked the Cassegrain parabolic dish design, that uses a smaller convex dish at the focal point of the parabolic dish, to refocused the antenna beam back into a horn mounted back in the center of the parabolic dish. This design is strong structurally. Horn and preamp adjustments are much more convenient and accessible. The coaxial cable run to the horn and other equipment is shorter, and less weight is suspended out from the dishes center. It also is less affected by interference from the ground, and has a cleaner beam pattern. Finally, just about every really big radio astronomy dish is of this design. A big sales feature.
The down side is that to make up for the loss of gain from the shadow of the convex dish blocking the parabolic dish, (about 20%) the big dish must be “at least 60 wavelengths” wide for a Cassegrain dish to become really design and cost effective over the simpler parabolic. At "Water Hole" frequencies this is approaching dish diameters of 40+ feet (>12 m). However, as the frequency goes up the Cassegrain dish becomes more and more practical for the amateur.
I finally returned to the parabolic dish antenna when I weighed the difficulty of making an elliptical antenna, against that of making a parabolic, and the antenna gain I would get from all the extra effort involved in making the elliptical.
I found that I could make a parabolic dish bigger and compensate for any loses over the elliptical dish, much easier than I could make an accurately shaped elliptical dish that was as strong as a parabolic.
Besides, other than the 600 foot (180 m) elliptical at Green Bank, every professional big dish I looked at was a parabolic antenna. I figured there was a reason someone with a lot more brains and money than I had decided on a parabolic.
The original antenna size I was contemplating making was 24 feet (7.2 m). This grew to 36 feet (10.8 m) at one point, under the RA theory that bigger is better. It is, until I started crunching the numbers on weight, and more importantly, on how was I going to make a dish of even 24' in diameter, and maintain the accuracy of the dish curvature during its construction.
And that wasn’t even considering the dish having to continue to maintain that accuracy while hanging up in the sky with gravity trying to sag it, the wind blowing on it, and snow falling in it. The more I crunched the numbers, the more reality set in, and the smaller the dish started getting.
Three things should be paramount in consideration of construction of any big dish :
1. Dish curvature accuracy. Without it, your wasting your efforts and money.
2. Wind loading, and the ability of the dish to not only continue to maintain its proper position, but also to maintain the shape of the dishes accuracy while under stress.
3. Dish and its support structure.It does no good to have the biggest dish on the block if it can not focus that tiny little signal from space into that little can at the dish’s focal point.
Here are some cruel facts on dish design :
1. If the dish diameter is doubled, the accuracy of the dish curvature becomes four times harder to maintain.
2. The force of the wind increases 4 times with a doubling of the wind velocity.
3. Dish accuracy is also directly related to frequency of operation. The higher the frequency of operation, the finer the tolerance for focusing the wavelengths to the focal point. And consequently the more difficulty in construction, and maintaining that tolerance under everyday weather conditions.
4. A solid 16 foot (4.8 m) dish facing into a 60 mph (100 km/h) wind, mounted on a 20 foot (6 m) tower exerts approximately 30,000 lbs (~13.6 ton) of force at the base of the foundation, trying to blow it over.
On my second pilgrimage to Green Bank for the SARA members yearly 3 day get together, which I might interject is an absolute MUST if your interested in radio astronomy. (For 3 days NRAO at Green Bank opens its grounds to SARA members to wonder about, to ask questions, to touch, peer inside, and drool over some of the finest equipment and moveable structures in the world. It is one of the finest programs offered by a government agency to amateur observers that I know of.)
As I wondered around the grounds looking at all the dishes, each unique in design and the cutting edge of technology when it was built, I noticed that each seemed to be grossly overbuilt.
These were not your typical scaled up version of a large satellite dish antenna on a post! The foundations for these dishes where massive, and so was the backing support structure of the dishes themselves.
One of the technicians happened to be in one of the shops working on a receiver for the new 600 foot (180 m) dish that was under construction. I asked him why things were so overbuilt. He explained the dishes were designed to survive 100 mph (160 km/h) winds, lightning, rain and light snow.
Though Green Bank very seldom gets a heavy snow, it does get its share of thunder storms that attempt to fill the dishes with water, and lightning tries to weld the rotor and elevation bearings into a solid mass. All the dishes are programed to automatically stop and observation and point vertically to reduce their wind resistance at a wind speeds above 35 mph (56 km/h).
He also pointed out the dish drainage system on a 150 footer (45 m) across the way, and the flexible ground straps around the elevation bearings. But he said, the reason for the overbuilding is for two other reasons as well.
One of the biggest problems at Green Bank is ice storms. Ice can build up in the bowls of the dishes to a point where the weight of the ice can permanently distort the dish if it wasn’t backed up by extra structure.
The second reason is due to the dishes attempting to distort from their sheer weight and increased wind resistance when they tilt to track a source during an observation. The weight of the bowl is no longer balanced equally throughout the bowl structure as it was when vertical, and consequently it wants to sag.
We are dealing here with trying to receive an incredibly weak signal sources. If the dish is damaged by ice buildup, or sags when tilted, the antenna is worthless if it can not completely focus that incredible tiny bit of energy from a source into that horn antenna.In fact he said, one of the reasons you see so many dishes is, that many of these very capable older dishes bowls can not focused closely enough to the higher frequencies researchers are using today.
He went on to say that the way the designers dealt the structural stiffness and bowl tolerances on the new 600 foot (180 m) elliptical dish, was to cover the inside of the dish with 2500 individual adjustable panels. These panels are continuously scanned by a laser, and adjusted to always keep the dish focused as it is blown by the wind, changes in size due to temperature, or as it is tilted to track the observation source. Otherwise the 17,000,000 lb. (~7700 ton) structure of the 600 footer would be useless beyond probably 450 MHz, simply because of the distortion from its weight alone. Let alone all the other Nature variables involved. But with the adjustable panels, research can go well into the gigahertz range, and plans are to do research from 450 MHz to 40 GHz., without dish sagging limiting us.
With this information, and several rolls of film taken of the large dishes at Green Bank, I came back home with a dream of building a larger dish.
One of the first orders of business was to find out how to structurally design and build one. One of the first books I bought was “Satellite Antenna Construction”, by Micromod of Los Angles, Ca., Copyright 1995. (I don’t know if it is still available in print.) It’s a pretty worth while soft cover book on actually building dishes from 10.5 to 30 feet (3.1-9 m) in diameter. It also included drawings, parabolic formulas, and other tables for building the dishes.
The second book I recommend is “Structural Engineering of Microwave Antennas For Electrical, Mechanical, and Civil Engineers”, by Roy Levy. Copyright 1996, IEEE Press. This book I borrowed from the librarian at Green Bank until I found my own copy. It about covers everything in dish design you’ll run into. It is expensive, but worth it if your serious about building your own dish structure from the ground up, and all the things to consider.