Carsten Groen's (OZ9AAR) personal place
Interests and experiences
4.8 meter dish for EME (moonbounce)
Quick links to sections below:
- History and background
- Design of system:
- Actual hardware:
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.
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 past 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 (160 km/h).
As I have previously installed (and still using) a 3 inch dual axis slewing drive from Coresun in China for my (small) 70cm EME system, 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 which makes it very easy to do CAD designs using their 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).
Gearing on motor/planetary gear is 680.3:1, slewdrive 60:1 for a total of 680.3 * 60 = 40818 * 2 pulses (hall sensor sees two pulses each rotation of DC motor) => 81636. With x4 quadrature this is 326544 pulses for 360 deg. => 907.067 pulses/deg. I measured the speed on the drive delivered, at 24V (100% PWM duty cycle) the elevation moves 0.36 degrees/second. This means approximately 250 seconds (4m10s) for 90 degree movement (with no load).
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).
Gearing on motor/planetary gear is 234:1, slewdrive 71:1 for a total of 234 * 71 = 16614 * 2 pulses (hall sensor sees two pulses each rotation of DC motor) => 33228. With x4 quadrature this is 132912 pulses for 360 deg. => 369.2 pulses/deg. I measured the speed on the drive delivered, at 24V (100% PWM duty cycle) the azimuth moves 0.6 degrees/second. This means approximately 600 seconds (10m0s) for 360 degree movement (with no load).
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. The URC is using dedicated quadrature decoder hardware inside the used Cortex M7 CPU (600 MHz NXP i.MXRT1064) for the decoding (only proper way of doing this). In addition to the 600 MHz CPU, the URC also has 32 MByte of SDRAM, 4 MByte of Flash memory (for code) and 128 MByte Flash filesystem.
I also prepared the URC controller with RS-232. RS-485 and CAN Bus interfaces, one of this will be handy should I ever want to change to absolute encoders for any reason.
The dish
As mentioned earlier, I opted to acquire a "ready-made" dish for this particular system. While I had previously designed and built dish systems for EME of 5 meters and 8 meters myself, it was a rather time-intensive endeavor, especially constructing the 8 meter dish.
This time, I decided to purchase a 4.8 meter dish kit that was laser cut from Zdenek OK1DFC at a highly competitive price point. Zdenek provides kits for dishes ranging from 1.8 to 4.8 meters in size, all crafted from laser-cut hard aluminum components, with stainless steel screws included in the package. The dish is shipped as a complete unit, lacking only the mesh surface and the feedhorn(s).
Zdenek also assisted me in obtaining the necessary stainless steel mesh for the dish surface, with a dimension of 6.35 x 6.35 mm.
Additional information about the dish can be found on Zdenek's website, and further details can be obtained directly from him.
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.
Tower
The tower is designed to fold over, providing convenient access to the rotor system and other components while standing on the ground.
Comprising two main sections, namely the upper and lower towers, the structure is secured using 8 bolts and two steel plates when not folded over.
The upper tower can be easily folded using a winch by removing these bolts. Both sections are constructed using 150 x 150 x 8 mm square steel tubes.
The lower tower is anchored to a concrete base using four 1-meter long M20 hot galvanized threaded rods. These rods are hold in place by two steel plates that remains embedded in the concrete.
With a 15 mm thick base plate, the lower tower also houses a winch for lowering and raising the upper tower. The winch operates with a 6 mm stainless steel cable running around two ABS wheels, one situated in each tower, effectively halving the force required for winching.
Atop the upper tower sits the SVH7 rotor unit, which can tilt downwards around a 20 mm steel rod housed in the lower tower.
Collectively, the two tower parts weigh 183 Kg, with the lower tower weighing 90 Kg and the upper tower 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 (the central hub of the dish has 61 mm holes in the top and bottom plates). 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 2 mm galvanized steel 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 done with a winch (with 1:2 gearing of the wire) when the eight bolts that holds the two tower parts together are removed. 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) and will provide 1:2 gearing of the winch/wire.
Foundation for tower
The foundation for the tower will be made of concrete. 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.
Rotor/Slewdrive
The slewdrive arrived from Coresun in China (3 days express delivery).
As usual, excellent service from Coresun and Jason, all arrived in perfect condition and very well packed.
Speed of the drive was tested at 24V (100% PWM duty cycle) under no load (just sitting on the bench):
- Elevation moves 0.36 degrees/second. Approximately 250 seconds (4m10s) for 90 degree movement.
- Azimuth moves 0.6 degrees/second. Approximately 600 seconds (10m0s) for 360 degree movement.
Threaded rod assembly for concrete foundation
The four M20 1 meter long threaded rods arrived. I had two stainless steel plates lasercut. Two plates are also currently being made together with the rest of the mechanics for the tower, but as I will have the concrete poured shortly (size 100x100x140 cm, approx. 3400 Kg), I needed the plates in a hurry. Got the plates from Scandcut in Sweden, fast service and very reasonable prices.
