The torque required depends mostly on three things:
- Weight of the dish
- Distance of its centre of gravity from the elevation axis
- Wind load
If the dish is perfectly balanced around the elevation axis, the motor only needs to overcome friction and inertia, which is quite small. If it is not balanced, the torque rises quickly.
1. Basic torque from dish weight
The torque equation is:
\tau = r \times F
where
- = distance from rotation axis (m)
- = weight force (N)
\tau = r \cdot m g
2. Example for a typical 150 cm mesh dish
Typical values:
- Dish diameter: 1.5 m
- Weight: 8–12 kg (mesh type)
- Centre of mass from pivot: ~0.7–0.8 m if unbalanced
Assume:
Force:
F = mg = 10 \times 9.81 \approx 98\,N
Torque:
\tau = 0.75 \times 98 \approx 73\,N\,m
So the elevation motor must supply roughly:
≈ 70 N·m
or
≈ 700 kg·cm
3. Compare with the Yaesu G-5500 satellite rotator.
The Yaesu G-5500 elevation rotor is rated roughly:
- ≈1400 kg·cm
- ≈137 N·m
So:
- Unbalanced dish: ~70 N·m → about 50% of rotor capacity
- Balanced dish: typically <10 N·m
That’s why most builders add a counterweight bar so the dish’s center of gravity sits on the elevation axis.
4. Wind torque (often the real limit)
For a 1.5 m dish:
- Area ≈ 1.77 m²
- Effective wind area (mesh) ≈ 0.7–1.0 m²
At 30 mph wind (13 m/s) the torque can reach roughly:
80–120 N·m
This is close to the G-5500 limit, which is why many dish systems:
- balance the dish carefully
- park the dish when wind increases
- use mesh instead of solid reflectors
✅ Practical rule used by radio astronomers
For a 150 cm dish:
- Balanced mount: 5–15 N·m required
- Unbalanced mount: 60–80 N·m
- Wind torque: up to ~100 N·m
So the G-5500 can drive a 150 cm dish, but only if it is very well balanced.