4.8 meter dish for EME (moonbounce)

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The contents on this page will be updated, the design is not fixed yet, it is a work in progress. There are also some things that are not yet designed (mounting of the feedhorn etc). This will be done along the way.

Once ready, the complete design for all the mechanical parts will be published here.

History and background

In 2022 I decided it was time to get back on EME (Moonbounce) again. My old EME dish (taken down in 2004 and now doing its job in Sweden at SM7GVF's place: SM7GVF Dish) was an 8 meter dish, it worked fine and was mechanically ok. During the last 20 years, things have changed a lot, modern 3D design tools have arrived etc., and I have gained a lot more experience in various fields.

I'm not getting any younger! My oldest dish was 5 meter and the last one 20 years ago was 8 meter. The 5 meter dish was easy to handle and construct, the 8 meter was a beast to handle!

To save some time, I have this time, chosen to get a finished "kit" for a 4.8 meter dish. Zdenek, OK1DFC, offers a line of laser cut dish kits, ranging from 1.8 meter to 4.8 meter in diameter.

Zdenek is also in my case, able to manufacture and deliver all the mechanical parts, the tower, elevation mechanism etc. He can even deliver feedhorns for different bands for the dish!

When making parabolic dishes in this size, the biggest job is not the dish itself (although this can also be a monumental task) but rather the mount (Az/El) and the general tower construction. The forces on the dish in a severe storm can be extreme so clever design and careful planning is needed.

For the mount and azimuth/elevation I will be using "Slewing drives". These have gained more and more popularity within the EME community, they are readily available (from China) and the prices are pretty fair. The forces they can stand (according to their datasheets) are pretty impressive. More on this later on.

The tower structure will be designed so it is possible to lower the dish/mount to the ground using an winch. This will make it much easier to assemble and to service the system later on (again, I'm not getting younger ;).

During the design, I have looked a lot around on the internet for ideas and hints of construction, Wolf DF7KB has shared some of his construction as he is using the same 4.8m dish, this has been great inspiration!

Rotor system / slew drive

As the dish itself is already decided, I first focused on the rotor system that is going to move the dish. I know from experience that the rotor can NOT be too strong, the forces that will be seen during a storm are very big. Here in Denmark we see storms that has gusts above 30+ m/s, sometimes peaking to 40+ m/s.

As I have previously installed (and still using) a 3 inch dual axis slewing drive from Coresun in China, I turned to them again for the rotor.

Coresun offers a 7 inch combined Az/El drive (and also a 9 inch). This drive can be delivered with different motor configurations, I selected a stronger motor for the elevation and a stronger/faster motor for the azimuth. The 7 inch drive has a weight of close to 85 Kg, the 9 inch is a massive 135 Kg!

I have always found that Coresun are extremely helpful to work with, they are quick and replies very fast to questions etc. Prices are also very fair from them. On top of that, they have 3D STEP files for all their drives, makes it very easy to do CAD designs using the drives.

The datasheet for the actual drive I use can be downloaded here.

Here is the most important specifications for the drive. As the drive has to "carry" the weight of the dish and feedhorn, the elevation axis has to strong. If possible, I would like to avoid having counterweights.

Elevation

With the stronger elevation motor I got, the elevation axis has an rated output torque of 5250 Nm and a holding torque of 42000 Nm, speed is rated at 0.05 RPM (5 minutes for 90 degrees)

The drive delivered with the "standard" elevation motor has an output torque of 2100 Nm at 0.05 RPM (5 minutes for 90 degrees).

Azimuth

With the upgraded motor, the azimuth (horizontal) axis has (only) an output torque of 870 Nm and holding torque of 10400 Nm, but speed is rated at 0.108 RPM (9.3 minutes for 360 degrees).

With the standard motor, the azimuth axis has an output torque of 820 Nm but a speed of only 0.042 RPM (24 minutes for 360 degrees!).

You can see that the azimuth has a lot less torque compared to the elevation axis, this is not a problem as it should only turn the system in the horizontal plane. What really matters is the holding torque, especially on the elevation axis.

Rotor controller

There are quite a few rotor controllers available, some which are very capable. In this setup, I chose to use my own developed rotorcontroller, the URC. I already run this in my 70cm EME system (with a SVH3 3 inch slew drive) and it has so far proven to be extremely reliable. One of the benefits of using my own controller is that I can add and change features as I see fit (and why make things simpler by buying one, when you can make it more complicated by developing your own ;). The URC controller uses the incremental (Hall) sensors inside the slew drive for both azimuth and elevation using dedicated quadrature decoder hardware inside the used Cortex M7 CPU (NXP i.MXRT1064).

The dish

As mentioned above, I chose to get a "finished" dish for this system. The previous two dishes I have had for EME (5 meter and 8 meter) I had made myself. It was a time consuming job, especially the 8 meter dish took quite some time to construct.

This time I ordered a 4.8 meter laser cut dish kit from Zdenek OK1DFC at an very affordable price. Zdenek offers kits for 1.8, 2.4, 3.2 and 4.8 meter dish. The components are all done in laser cut hard aluminium, in my case, the dish came with stainless steel screws. The dish is delivered complete minus the mesh for the surface and the feedhorn(s).

Zdenek also helped me acquire the mesh for the surface, in this case 6.35 x 6.35 mm stainless steel mesh.

On Zdenek's webpage there are some information for the dish, more info are available from him as well.

Mechanics

The mechanics for a system like this needs to be able to stand up again wind and weather.

Although the dish is not that big, the forces acting on it are still very high during a storm.

I have designed the complete system in Autodesk Inventor, this makes it easy to check everything for correct fit, you can calculate forces, weights and so on. Below is a number of screenshots from the design phase, once everything is finished and put into production and tested, all the drawings, DXF files, weld drawings etc. will be published here.

I was lucky that Zdenek, OK1DFC, who delivers the laser cut kit for the dish, also are able to have the mechanics produced for me.

Tower

The tower is made so it "folds" over. This enables me to work on the rotor system etc. while standing on the ground.

The tower consists of two parts, the "upper tower" and the "lower tower". When the upper tower is not folded over, it is secured to the lower tower using 8 bolts and two steel plates. Removing these 8 bolts enables me to fold the tower using a winch. Both parts are made of 150 x 150 x 8 mm square steel tubes.

The lower tower bolts to a concrete base using four M20 hot galvanized threaded rods (1 meter long). These threaded rods are held in position by two steel plates, one of the will be removed after concrete has cured, the other will be embedded in the concrete.

The base plate of the lower tower is 15 mm thick.

The lower tower also holds a winch, this is used to lower and raise the upper tower. The winch holds a 6 mm stainless steel cable, the cable goes around two ABS wheels, one in the upper tower and one in the lower tower. This divides the force needed by the winch with a factor of two.

The upper tower holds the SVH7 rotor unit. The upper tower can tilt down around a 20 mm steel rod in the lower tower.

The total weight of the two tower parts is 183 Kg.

The lower tower is 90 Kg and the upper tower is 93 Kg.

Elevation system

The mechanics for the elevation consists of two "arm assemblies" and a square frame for the attachment to the back of the central hub of the dish.

At the center, a 60 mm tube are installed. This enables the complete dish to rotate once the 8 bolts holding it are removed. That way it is possible to assemble/disassemble the dish while it is "hanging" on the tower. It also makes it easy to make modifications, repairs etc. to the rim of the dish.

The complete weight of the elevation mechanics (excluding the Az/El drive) as shown below is 61 Kg.

Feedsupport

The idea for the feed support I got from Wolf DF7KB. Wolf uses two "clamps" that connects the central 60 mm tube to a 60 x 3 mm aluminum tube that will hold the feedhorn(s).

On the picture above is shown a W2IMU horn, this is not what I will be using for this dish (the W2IMU was the only horn I already had made a CAD model of ;)

Actual feedhorn to be decided on later in the process.

Each of the two aluminium clamps weighs 2.5 Kg.

Tilting tower

The tower is made in two parts, "lower tower" and "upper tower". I have done this so that the tower can be folded over. Even though the complete tower is not very high (approximately 2.5 meter) it is still very nice to be able to access everything from ground level. It also makes it somewhat easier at the initial installation, the rotor (slew drive) alone weighs 85 Kg, quite a bit easier to handle at ground level than 2.5 meter in the air!

Folding of the tower is be done with a winch (with 1:2 gearing) when the eight bolts that holds the two tower parts together. Below are a few screenshots of the details.

I have designed two ABS wheels, one for the lower and one for the upper tower. These will steer the stainless steel cable (6 mm thick).

Foundation for tower

The foundation for the tower will be made of concrete, I still need to calculate the needed amount/size. The anchor bolts will be done with four 1 meter long M20 threaded rods, hot zinc galvanized. The rods are spaced 360 mm from each other.

Two steel plates (4 mm thick each) will be used as spacers, these will make sure the distance between bolts will fit the plate at the bottom of the lower tower section. Paul G8AQA recommended to cover the top 150 mm of the threaded rods in the concrete with "bitumastic paint" which indeed seems like a very good idea!

The concrete foundation will have an additional steel cage also.

Carsten Groen's (OZ9AAR) personal place