A 14' Dish for AO-40

A year and a half ago I saw an advertisement in the Denver Newspaper saying "Free 14' dish". I inquired about it and was told that it was still avalable. I went over to the site on the other side of Denver from where I live and found that it was indeed quite a dish, but was much more than just I could handle. I contacted a friend, Tom Hooser, whom I taught to fly R/C airplanes, and the two of us went back to the site armed with ladders, wrenches and milk cartons for the hundreds of nuts and bolts in this beast.

Before we started I took several pictures of the dish as it existed. It was a very heavy fiberglass dish with 10 petals making up its reflecting surface. The feed system's mounting arm is referred to as a "figure 7" and the distance from the center of the dish to the Chapparal Feed Horn was 42", making this a very short focal length dish.

Two days before I arrived to take down the dish, the metal shed shown behind the dish had an internal fire that damaged the receiver and motor controls, but I had little interest in them anyway. The dish had not been used in 10 years, and the tracker arm mounts were rusty. This meant that the arm would be of no use moving the heavy fiberglass dish around during disassembly.

The 400 pound dish was far too heavy to take off as a single piece, so after disconnecting the tracker arm, we took the dish petals off the dish support ring one at a time, one from the left side, one from the right side. We filled a 1/2 gallon milk carton with 3/8" bolts just detaching the petals.

Once the pedals were off the support ring, we took the support ring apart and the two of us lifted it off the polar mount. After this the polar mount was disassembled piece by piece, each piece being very heavy. Finally all that was left was a 5 1/2" diameter steel tube going down to China. I told the owner that I would try to get the tube out, but most likely would not be able to remove it. We dug down 3' and still did not come to the concrete base. Then the owner remembered that many years ago he had regraded around the base of the dish, making the concrete pad way down there somewhere. The owner's son said he would bring over a cutting torch some day and cut off the pipe for his dad. At that we loaded my truck and Tom's van and drove the carcass to my house and unloaded everything in my backyard dish cemetery.

I wanted to eventually put up the 14' dish for use on AO-40, but my 10' dish was working well so there was no rush. The 10' dish did have a problem seeing AO-40 when it was East of 145 degrees, because the house was in the way, and even due East, there were several trees blocking the way. I used a 40" wide oval PrimeStar dish and some yagis on top of a 50' tower when the 10' dish couldn't see the satellite. The signal was nowhere as strong as the 10' dish, but it was definitely usable. But I always thought about putting up that 14' dish some day. I set the petals on the ground, face up, so they would not blow over in our occasional high winds. And there they lay for about a year. BIG MISTAKE! Rain, and snow, and dirt accumulated on the front surface, and with their big dirt rings the petals started to look like pepperoni pizza slices. So I stood them up and wired them to a nearby aspen tree, hoping occasional rain storms would make the pepperoni look go away. It didn't.

The old mount would never work as an azimuth/elevation mount, so I dreamed about what kind of a mount it would take to hold this monster. A few years ago I saw a dish mount at a cable TV yard in Green River, Utah and I always thought that this design would be a natural for amateur satellites like AO-40. I called it a "clothesline mount". It was elegantly simple yet very sturdy. The arm ends held the elevation pivots and the mechanical screw at the bottom adjusted the elevation. Azimuth rotation was accomplished by loosening 3 bolts at the bottom of the vertical steel tube and rotating the clothes line around a smaller steel tube set in the ground. The mount was "set and forget", but with a few changes the concept should work fine with full automatic tracking.

The Green River dish was simply U-bolted to the cross arms. By putting 4 bolt flange bearings at the end of the arms, a much smoother elevation bearing would result. Replacing the mechanical screw with the tracker arm from the 14' dish's polar mount would produce all the automated elevation control I desired. Putting a potentiometer or incremental encoder at the end of one arm would enable an elevation bearing readout. Finally, replacing the three holding bolts with a sprockets, chain, Vbelt and gearmotor assembly would produce an automated azimuth control. Another potentiometer or incremental encoder would enable an azimuth bearing readout.

The remaining problem would be more difficult. I needed to put the dish well up in the air if I wanted to clear the house and trees. I could cut a few trees in the name of "forest fire mitigation", but not a whole forrest. The bottom of the dish would have to be at least 8' in the air to provide complete coverage. A steel tube would be monstrously heavy and still be wobbly with a load like this, and I had no idea how I would get a 14' dish up onto the mount.

Finally I realized what it would take to accomplish my dream. I got a 10' section of Rohn 45 tower, and would use the tower as an external support for the rotating azimuth shaft instead of the usual internal steel tube found on smaller dishes. If I made the base for the Rohn 45 tower tilt over, I could assemble the dish face down on the ground, and then simply winch the tower vertical with the dish attached.

I talked Jim Burr into making the "clothesline mount" and azimuth drive system for me at his JMI telescope manufacturing plant on the West side of Denver. 20 years ago Jim had been on the team that designed the 14' dish. After working on the mount in his spare time for 6 months, Jim dropped the new mount and tower off in my backyard dish cemetery and I laid it all out to see what else I needed to put it up ASAP. I dug two holes and started the process the day after these pictures were taken. The weather stayed warm for about a week, allowing me to get the cement poured and set. The mount is really heavy duty, all right. I would think that it could hold a 20' lightweight dish. The tower will result in the bottom of the 14' dish being about 8' off the ground as I wanted. I have attached several detailed pictures for your viewing pleasure.

This picture shows the base of the Rohn 45 tower. In the foreground is a steel cradle for supporting the tower during erection. The 3 plates in the back will be buried in concrete so the whole dish mount becomes a tiltover system, pivoting on 1" bolts in the cradle and 2 of the plates.

This picture shows the lower thrust bearing and the azimuth motor assemply from a bottom view. The lower thrust bearing is a large 4 bolt flange bearing mounted on a heavily braced adjustable shelf which has to carry the weight of all the steel shafts and the dish. The motor is a heavy duty Dayton 1L480 6 RPM gearmotor from Grainger driving a V belt and Harley Davidson motorcycle sprockets & chain. The stepdown ratio is 13.5 : 1. Both V belt and chain have variable tension adjustments.

The picture on the left shows the lower thrust bearing and azimuth motor system from above. The gearmotor moves back and forth to tension the V belt. A 1/4-20 tensioning bolt pushes the motor to tension the V belt, and 4 motor mounting bolts keep it solidly in place. The V belt is ribbed and the inner surface of the small V belt pulley is roughed to keep the V belt from slipping, but not so tight that it won't slip in case of a jam or severe wind.

This is how the lower portion of the azimuth shaft is built. A heavy stud protrudes from the bottom of a 4" I.D. steel shaft. The stud inserts into the flange bearing that supports the weight of the shafts and dish. An 8" Harley Davidson motorcycle sprocket was trued and welded to the 4" I.D. steel aximuth shaft. At the top end of this shaft is a 4" O.D. shaft used to couple the lower Azimuth tube to the upper azimuth tube and clothesline assembly. By making a 2 piece azimuth shaft, the carrying weight and size is significantly reduced, and is required to be able to slip the azimuth axis through the top trust plate. Two 5/8" x 5" bolts lock the lower and upper sections together after assembling.

This picture shows the upper portion with crossarm. The crossarm is a 37" long piece of 3" O.D. steel pipe. 1" diameter shafts welded inside end caps provide a pivoting surface for the 1" 4 bolt flange bearings on the dish support ring. An adjustable bracket holds the lower mount of the tracker arm. The tracker arm is a very heavy duty version, needed to move the ~300 lb 14' fiberglass dish and support ring. Also shown is the upper thrust bearing assembly that fits over the top of the ROHN 45 tower section.

The azimuth shaft is held in the exact center at the top of the Rohn 45 section by a top plate that fits on the 3 leg tops. 3 solid brass rollers center the shaft (one is adjustable) and 3 pieces of Delrin provide additional support, especially needed during tiltover.

The clothesline arms are placed on the dish support ring in the picture at the left. The two arms have 1" 4 bolt flange bearings which ride on the 1" diameter shafts coming out of the ends of the elevation shaft. An adjustable bracket is used to attach a heavy duty tracker arm to the azimuth shaft. The other tracker arm bracket attaches to the bottom of the dish support ring.

The picture at the right shows how the Azimuth incremental encoder is attached to the small sprocket shaft, producing a potential accuracy of 1/65 of a degree. The sprocket shaft has a 1/4" hole drilled in it for the incremental encoder's shaft. The body of the encoder will be strapped to the side of the motor bracket.

The picture at the left shows the Elevation incremental encoder, that will produce an elevation accuracy of up to 1/23 of a degree.

Well, after 3 days of hard rock digging, I got a hole dug for the Rohn 45, and the tower is perfectly vertical, suspended over the hole. A smaller hole was dug for a West guy wire post. The other two guy wires go to conveniently spaced pine trees.

In a 75 MPH wind, the 14' dish will produce a 4000 lb pull so the tower will be self supporting and guyed with 1/4" EHS guy lines and turnbuckles. I have about 200 elk wandering around the backyard at times, so the guy wires will pose a serious navigational problem. 4 years ago the bull elk got his horns tangled in a low guy wire and pulled a 300 lb concrete guy post out of the ground and drug it 25 feet before getting free when he snapped a high strength turnbuckle. Fortunately he didn't pull the tower down and walk off wearing my satellite yagis.

When the concrete was hard, the backstay was loosened and the rear 1" bolt removed so the tower could pivot over towards the camera on the two front 1" bolts. A boat winch can be seen mounted on the tree behind the tower.

When completed, the bottom of the 14' dish will come down to the 8' high guy wires seen near the top of the Rohn 45 section. The dish system is high enough so I have complete clearance over the house to the South.

The weather cooperated for several more days; sunshine, 45 degrees, and no new snow. Richard, N0OD and Bill, KU0E came over to help with the concrete work. We filled the two holes in two hours, mixing and pouring 22 bags of Readi-Mix by hand at the rate of a bag every 5 minutes. The tower base took 17 bags of concrete, and the guy wire pad took 5 bags.

I let the concrete harden for a couple of days and then took the forms off the concrete home plate shaped tower base and the square guy wire pad. In a few days I tilted over the tower and put on the azimuth axis and clothesline upper section; finally the 14' dish.

The cream colored 14' dish had gotten dirty from sitting on the ground for a year with a large dark spot on most of the 10 panels, where dirty water had evaporated. If I had put it up like it was, the dish would look like a pepperoni Pizza on end. I told the XYL that I was going to take my dish panels to a local car wash to clean them up. Puts "doing dishes" in a whole new light.

After $10 of soap and spray at the car wash, the panels were still dirty. So Mary Jane got some of her special cleaning chemicals out and cleaned the dish some more. They look more presentable now.

Assembling the 14' Dish

K0DJV came over and we put the 14' dish together and attached the big dish to the tilted over Rohn 45 tower. Things did not start off too well, as the concrete tower base cracked when I initially lowered the tower to begin assembly operations. Post Mortem analysis theorized that the base twisted because the frame was not completely true. Though a bit late for me, I would recommend not bolting the frame and supports directly together on a pivot bolt, but rather set them up so the bolts simply rest through the supports. Some rebar through holes in the supports inside the concrete would be a good addition if I had to do it over again. We changed our minds about the erection process due to the weakened base, but once the tower was upright and guyed, the cracked base became only a cosmetic problem.

The various cables and wires were attached to the tower before the erection process started. One end of the tracker arm was attached to the dish support ring, but the other arm support point was unattached so the dish could shift for itself during the erection process. Similarly, the azimuth gearmotor and sprockets were attached and wired, but the chain was left off until the tower was vertical.

Erecting the 14' Dish

The erection process occured on December 1, 2002. K0DJV and N0OD assisted me in putting up this monster. A week of preparations paid off in letting the erection occur in less than 2 hours.

A 1000 lb boat trailer winch had been attached to a nearby tree, and a 6' high wooden 2x6 gin pole was attached to the base for leverage. Richard, N0OD, turned the winch, while Paul, K0DJV, and I put safety 2x6 supports under the top of the tower as the tower arced upwards. When the dish center was about 8' high, with the tower well supported, the bottom of the dish was pushed forward so the other support for the elevation tracker arm could be attached. I had originally planned to attach the feed sytem to the front of the dish at this point, but working under 700 pounds of diagonal dish and tower did not seem like a real smart idea.

The hardest part is getting the dish and tower to 60 degrees. The picture at the right gives you an idea of how massive this dish system really is. At this point we were out of reach for the 2 x 6 supports, so a secondary "come along" had been attached to the end of the loose guy wire on the right to pull along with the boat winch until vertical was achieved.

The top of the dish is about 20' from the base of the tower, and my XYL always goes shopping when I put up a tower. When she drove in the driveway and saw the dish and tower up, she was surprised at its impressive size.

Once the tower and dish were vertical, a "530" sized Harley Davidson chain was attached between to the two azimuth sprockets. Inside the house a Satellite Tracker Senior had been attached to a 100' long 12 conductor cable of #18 wires. Now the dish could be rotated in azimuth, and raised or lowered in elevation.

On the rear of the dish, in the center, is a small motor driving a spool of 1/16" wire rope for raising and lowering the feed system without resorting to a scoffold or very tall ladder to reach the feed system to work on it. The wire rope goes up the back of the dish to the top, over a pulley and through the inside of some 1" aluminum tubing towards the front center of the dish. The 1" tubing is hinged at the top of the dish so that it will float as the dish feed system is lowered towards the bottom of the dish. When a need arises to lower the feed system, a small box holding three 6V lead batteries in series is brought out to the dish and connected to the motor leads. One polarity raises the dish feed; the other polarity lowers the dish feed for servicing. The bottom two feed support arms pivot on parallel plates bolted to the dish rim. A special center mounting bolt on the tri band circular patch holds the two feed support arms at the correct feed angle. A third arm stub at the top is used as a stop when raising the feed system to the proper place in front of the dish.

              .      

The Azimuth drive panel has several possible adjustments so the exact tension of the chain and the V belt can be made. The slotted shiney bent plate slides sideways on the fixed black panel to tension the chain. The motor slides sideways on a slotted plate attached to the shiney bent plate to tension the drive belt. All 12 of the #18 wires coming from the Satellite Tracker Senior in the house terminated at the azimuth drive panel.

The feed system is a tri band circularly polarized patch feed for 2401 MHz, 1269 MHz, and 435 MHz, described in the feed section of this webpage. The AIDC 3731 2401 MHz down converter is attached to the back of the 435 MHz reflector. About 110 feet of RG 6 carries the 145 MHz signal into the ham shack. 30' of Ecoflex 15 coax carries the 1269 MHz signal from the base of the tower to the 1269 patch feed and another 30' of Ecoflex 15 coax carries the 435 MHz signal from the base of the tower to the 435 MHz patch feed. 80' of 1/2" Andrew hardline brings the 1269 MHz signal to the tower base and 80' of Aircom brings the 435 MHz signal. There is about 3 db of loss in these cable sets, so about 1/2 the power I run in the station actually makes it to the antenna. The base of the tower is now a logical place for a high powered amplifier and preamp for EME. Ecoflex 15 is available from SSB for $1.90 / foot and the special solderless type N connectors are $15 each. Echoflex 15 is a 5/8" diameter 50 ohm coax with a 3 db loss / 100' spec, about the same as the 1/2" hardline and is so flexible it can be bent in a 7" diameter circle. This coax isn't cheap, but why go to a lot of effort and rig expense just to lose your signal warming coax?

Was it worth it?

As I have told others, it was 30 db of effort for 3 db of gain! I now have an antenna system for AO-40 that will work the satellite anywhere, under any adverse condition. It has about 21 db of gain on 435 MHz, 32 db of gain on 1269 MHz, and 37.5 db of gain on 2401 MHz. I rarely run over 5 watts on 435 MHz or 1 watt on 1269 MHz. The AO-40 middle beacon comes in at 20 over S9 most of the time.The AO-40 telemetry signal is copied perfectly with squint angles well over 40 degrees. The Satellite Tracker Senior tracks AO-40 within 1/5th of a degree. On 12/17/2002 I used this dish system to send and receive the first successful Digital SSTV picture by satellite. The feed system can be easily exchanged for one optimized for EME, Radio Astronomy, or even SETI.

Robert W0LMD